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SKA-AAMID Science Requirements

Document number: SKA-TEL-MFAA-0200009

Context: SE-R

Revision: C

Authors: S.A. Torchinsky/J.W. Broderick/A. Gunst/A.J. Faulkner

Date: 2016-10-03

Document Classification: Public

Status: Released

Name Designation Affiliation Signature

Authored by:

S. A. Torchinsky Project

Scientist

AAMID

Date: 2016-10-03

Owned by:

J. W. Broderick Project

Scientist

AAMID

Date: 2016-10-03

Approved by:

Wim van

Cappellen

Project lead AAMID

Date: 2016-10-03

Released by:

Date: 2016-10-03

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DOCUMENT HISTORY

Revision Date Of Issue Engineering Change

Number

Comments

A 2015-12-14 - First draft by J. Broderick

2016-02-25 science motivation, FoV, requirements list, etc

2016-03-04 improved text

2016-03-15 refining requirements

2016-03-25 Refining requirements, and text for science cases

2016-03-29 Requirements split into “required” and “goal”

2016-04-22 Refining requirement and goals, and science cases

1.19 2016-04-29 Version submitted for SRR

1.20 2016-10-03 Final version after SRR OAR action items

DOCUMENT SOFTWARE

Package Version Filename

Wordprocessor LibreOffice Oct 2015 SKA-TEL-MFAA-0200009-SE-SRS-A-SKA-AAMID_Science.odt

Wordprocessor Google docs Feb 2016 SKA-TEL-MFAA-0200009-SE-SRS-A-SKA-AAMID_Science

Other

ORGANISATION DETAILS

Name Aperture Array MID Frequency Consortium

Registered Address ASTRON

Oude Hoogeveensedijk 4

7991 PD Dwingeloo

The Netherlands

+31 (0)521 595100

Fax. +31 (0)521 595101

Website www.skatelescope.org/mfaa/

Copyright

Document owner Aperture Array MID Frequency Consortium

This document is written for internal use in the SKA

project

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TABLE OF CONTENTS

Table of Contents

1. Introduction..................................................................................................................................5

1.1. Purpose of the document..........................................................................................................5

1.2. Scope of the document..............................................................................................................5

2. References....................................................................................................................................6

2.1. Applicable documents...............................................................................................................6

2.2. Reference documents................................................................................................................7

2.3. Reference papers.......................................................................................................................7

3. Scientific motivation.....................................................................................................................9

3.1. The history of hydrogen in the Universe and the mystery of dark energy.................................9

3.2. Galactic structure and star formation......................................................................................10

3.3. The structure of the Universe..................................................................................................10

3.4. The dynamic radio sky.............................................................................................................11

3.4.1. Transients and variables........................................................................................................11

3.4.2. Pulsar astronomy..................................................................................................................12

3.5. The origin and evolution of cosmic magnetism.......................................................................13

3.6. Additional science....................................................................................................................14

4. Definition of ‘Field of View’........................................................................................................14

4.1. Optical Field of View................................................................................................................14

4.2. Processed Field of View...........................................................................................................14

4.3. Use of the term FoV in science documentation.......................................................................15

4.4. The FoV advantage of SKA-AAMID, and future upgrades.........................................................15

5. Science Requirements.................................................................................................................15

5.1. General considerations............................................................................................................16

5.2. Galaxy evolution and cosmology probed by neutral hydrogen................................................19

5.3. Galactic HI................................................................................................................................21

5.4. The transient radio sky.............................................................................................................21

5.5. Strong-field tests of gravity with pulsars..................................................................................23

5.6. Galaxy evolution and cosmology probed by radio continuum.................................................24

5.7. The origin and evolution of cosmic magnetism.......................................................................26

5.8. Widefield SETI..........................................................................................................................27

6. Recommendation for SKA-AAMID parameters...........................................................................27

7. Progressive science operations with the SKA-AAMID telescope.................................................29

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LIST OF TABLES

Table 1. Key SKA-AAMID parameters that define the performance envelope of the system...... 32

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LIST OF ABBREVIATIONS

AAMID Aperture Array MID Frequency

ASTRON Netherlands Institute for Radio Astronomy

AD-n nth document in the list of Applicable Documents

ALIGO Advanced Laser Interferometer Gravitational-Wave Observatory

BAO Baryon Acoustic Oscillations

DM Dispersion Measure

DRM Design Reference Mission

FoV Field of View

FRB Fast Radio Burst

FWHM Full Width Half Maximum

HI Neutral Hydrogen

IM Intensity Mapping

LOFAR Low Frequency Array

LSST Large Synoptic Survey Telescope

MFAA MID Frequency Aperture Array

PSF Point Spread Function

RD-N nth document in the list of Reference Documents

RF Radio Frequency

RM Rotation Measure

RMS Root Mean Square

SETI Search for Extraterrestrial Intelligence

SKA Square Kilometre Array

SKA1 Square Kilometre Array Phase 1

SKA2 Square Kilometre Array Phase 2

SKAO SKA Office

1. Introduction

1.1. Purpose of the document

This document describes the top level requirements for the SKA-AAMID telescope as determined by

the SKA key science projects. These include parameters such as operating frequency range,

instantaneous bandwidth (total processed bandwidth), field of view (or survey speed, as

appropriate), sensitivity, dynamic range, polarization purity etc. Moreover, through the definition of a

set of science requirements, this document serves as input to a number of other documents

contained within this review package (particularly SKA-TEL-MFAA-0200005: ‘SKA-AAMID System

Requirements’ and SKA-TEL-MFAA-0200008: ‘MFAA Requirements’).

1.2. Scope of the document

This document covers the science requirements for the complete SKA-AAMID telescope (Fig. 1). We

first provide an overview of the key science cases in Section 3, and then, in Section 4, discuss the

various concepts of FoV and how they need to be carefully differentiated for SKA-AAMID. Section 5

contains the list of science requirements, with the salient features of the telescope being

summarised in Section 6. Lastly, Section 7 describes a phased approach to the implementation of

SKA-AAMID, including possibly early operations in a hybrid combination with the SKA dishes.

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Figure 1: An artist’s conception of an AAMID station. The inset image shows the AAMID pathfinder

instrument EMBRACE currently operating at Nancay, France.

2. References

2.1. Applicable documents

The following documents are applicable to the extent stated herein. In the event of conflict between

the contents of the applicable documents and this document, the applicable documents shall take

precedence.

Id Title Code Issue

AD-1 SKA Request for Proposals SKA-TEL-SKO-0000020 01

AD-2 Statement of work for the study, prototyping and design on

an SKA element

SKA-TEL-SKO-0000021 01

AD-3 Statement of Work for the Study, Prototyping and

Preliminary Design of an SKA Advanced Instrumentation

Programme Technology

SKA-TEL-SKO-0000022 01

AD-4 SKA Pre construction Top Level WBS SKA-TEL-SKO-0000023 01

AD-5 SKA System Engineering Management Plan SKA-TEL-SKO-0000024 1

AD-6 The Square Kilometre Array Intellectual Property Policy SKA-GOV-0000001 1.3

Draft

AD-7 Draft Consortium Agreement PD/SKA.26-4 Draft

AD-8 Document Requirements Description SKA-TEL-SKO-0000029 1

AD-9 SKA Document Management Plan SKA-TEL-SKO-0000026 2

AD-10 SKA Product Assurance and Safety Plan SKA-TEL-SKO-0000027 1

AD-11 Change Management Procedure SKA-TEL-SKO-0000028 1

AD-12 SKA Interface Management Plan SKA-TEL-SKO-0000025 1

AD-13 SKA1 Level-0 Science Requirements SKA-TEL-SKO-0000007 2

AD-14 The Radio Frequency Environment At Candidate SKA Sites WP3-010.020.000-R-001 1.2

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2.2. Reference documents

The following documents are referenced in this document. In the event of conflict between the

contents of the referenced documents and this document, this document shall take precedence.

Id Title Code Issue

RD-1 Management Plan AAMID-TEL.MFAA.MGT-AAMID-MP-001 2.0

RD-2 Delivered Item and Document List AAMID-TEL.MFAA.MGT.DMT-AAMID-LI- 001 2.0

RD-3

The Square Kilometre Array Design Reference

Mission: SKA mid and SKA lo ‐ ‐

DRM 1.0

2.3. Reference papers

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Abdalla F.B. et al., 2015, Proc. AASKA14, 017

Anderson G.E. et al., 2014, MNRAS, 440, 2059

Ansari R., et al., 2008, arXiv:0807.3614

Arshakian T. G., Beck R., 2011, MNRAS, 418, 2336

Bates S.D. et al., 2013, MNRAS, 431, 1352

Bhat N.D.R. et al., 2004, ApJ, 605, 759

Bignall H. et al., 2015, Proc. AASKA14, 058

Blyth S. et al., 2015, Proc. AASKA14, 128

Braun R. et al., 2009, ApJ, 695, 937

Braun R. et al., 2015, Proc. AASKA14, 174

Brentjens M.A., de Bruyn A.G., 2005, A&A, 441, 1217

Bridle A.H., Schwab F.R., 1999, Synthesis Imaging in Radio Astronomy II, 180, 371

Brown M.L., Battye R.A., 2011, MNRAS, 410, 2057

Brown M.L. et al., 2015, Proc. AASKA14, 023

Bull P. et al., 2015a, Proc. AASKA14, 024

Bull P. et al., 2015b, ApJ, 803, 21

Chu Q. et al., 2016, MNRAS, 459, 121

Condon J.J. et al., 2012, ApJ, 758, 23

Cordes J.M., Lazio T.J.W., 2002, arXiv:astro-ph/0207156

Cordes J.M. et al., 2004, New Astron. Rev., 48, 1459

de Blok E. et al., 2015, Proc. AASKA14, 129

Dewdney P., Landecker T., 1991, IAU Coll. ASP Conf. Series, Vol. 19, pg. 415

Faulkner A. et al., 2010, SKA Memo Series, 122

Fender R.P., Bell M.E., 2011, Bull. Astron. Soc. India, 39, 315

Fender R. et al., 2015a, Proc. AASKA14, 051

Fender R.P. et al., 2015b, MNRAS, 446, L66

Fernández X. et al., 2013, ApJ, 770, L29

Fomalont E.B., 1999, Synthesis Imaging in Radio Astronomy II, 180, 301

Fomalont E., Reid M., 2004, New Astron. Rev., 48, 1473

Gaensler B. et al., 2015, Proc. AASKA14, 103

Hassall T.E. et al., 2013, MNRAS, 436, 371

Haverkorn M. et al., 2015, Proc. AASKA14, 096

Heald G. et al., 2015, Proc. AASKA14, 106

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Heidmann J. 1966, L'Astronomie, 80, 157

Heymans C. et al., 2013, MNRAS, 432, 2433

Hopkins A.M., Beacom, J.F., 2006, ApJ, 651, 142

Janssen G. et al., 2015, Proc. AASKA14, 037

Jarvis M. et al., 2015a, Proc. AASKA14, 018

Jarvis M. et al., 2015b, Proc. AASKA14, 068

Johnston-Hollitt et al. M., Proc. AASKA14, 092

Kaplan D.L. et al., 2015, ApJ, 814, L25

Keane E.F., Petroff E., 2015, MNRAS, 447, 2852

Keane E. et al., 2015, Proc. AASKA14, 40

Keane E.F. et al., 2016, Nature, 530, 453

Keith M.J. et al., 2013, MNRAS, 429, 2161

Kloeckner H.R. et al., 2015, Proc. AASKA, 027

Kramer M. et al., 2004, New Astron. Rev., 48, 993

Kulkarni S.R. et al., 2015, arXiv:1511.09137

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Lee K.J. et al., 2014, MNRAS, 441, 2831

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LSST Science Collaboration et al., 2009, arXiv:0912.0201

Maartens R. et al., 2015, Proc. AASKA14, 016

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Macquart J.-P., Koay J.Y., 2013, ApJ, 776, 125

Macquart J.-P., 2014, PASA, 31, e031

Macquart J.-P. et al., 2015, Proc. AASKA14, 055

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van der Laan H., 1966, Nature, 211, 1131

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3. Scientific motivation

Aperture Arrays are the enabling technology that will lead to the promise of the SKA as a

transformational survey machine. The Mid-Frequency Aperture Array (MFAA) is particularly well

suited to perform the neutral hydrogen ‘billion galaxy survey’. This will enormously advance our

understanding of galaxy formation and evolution, and help determine the nature of dark energy. In

addition, MFAA will accommodate a range of survey-related science projects profiting from wide field

of view, large bandwidth, and the unique multi beaming capabilities. These include all-sky transient

monitoring, pulsar searches and bulk timing, as well as wide-field radio continuum surveys, both in

total and polarized intensity.

3.1. The history of hydrogen in the Universe and the mystery of dark energy

The earliest identified science driver for the SKA is the mapping of HI, including the measurement of

the HI mass distribution at cosmological epochs (e.g. Heidmann 1966; Rawlings et al. 2004), and

mapping HI in the Galaxy (e.g. Landecker & Dewdney 1991). Indeed, an all-sky ‘billion galaxy survey’,

using the redshifted HI 21-cm line, is at the heart of the SKA2 science case (e.g. Abdalla & Rawlings

2005; Staveley-Smith & Oosterloo 2015). SKA-AAMID, with its wide field of view and exquisite

sensitivity, is the ideal survey instrument to obtain this unique, high-legacy source of information.

By tracing the HI of individual galaxies over the entire sky and at different redshifts, SKA will be able

to detect baryon acoustic oscillations (BAO), which constitute a preferred clustering scale in the

distribution of matter on cosmological scales, and its evolution as a function of cosmic time (e.g.

Abdalla et al. 2015; Bull et al. 2015a). This will enable a direct measurement of the effects of dark

energy (e.g. Rawlings et al. 2004; Abdalla & Rawlings 2005); in particular, below a redshift of 2, dark

energy will start to dominate the Universe's expansion, and BAO detections will enable important

distinctions between different cosmological models. The optimal redshift range for this study is

between z = 0.5 – 2, corresponding to a frequency range of 450 to 1000 MHz. A large processed FoV

of order hundreds of square degrees will be critical in accessing large numbers of galaxies within a

few years of observing time. Angular resolution on the order of 5 arc seconds is needed to beat the

confusion limit. With these data, SKA-MFAA will provide constraints on the Equation of State of the

Universe, and may well indicate that Einstein's theory of General Relativity, together with a

Cosmological Constant, is insufficient to explain dark energy.

The method of intensity mapping (IM) has been proposed to make a statistical detection of the BAO

in the HI mass distribution (e.g. Peterson et al. 2006; Ansari et al. 2008; Santos et al. 2015a; Bull et al.

2015b). It is likely to detect the first peak in the power spectrum, and a modest-sized SKA-AAMID

initial station can achieve this result. Moreover, HI emission observations are not affected by, for

example, dust extinction, which may complicate competing optical/NIR projects such as the Euclid

space mission (Laureijs 2012). Only the full SKA-AAMID can make a detailed map of the HI

distribution in an unprecedented cosmic volume. It will enable measurements of the higher-order

features in the power spectrum, which are necessary to properly match the signature found in the

Cosmic Microwave Background (Abdalla & Rawlings 2005).

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3.2. Galactic structure and star formation

Star formation is a complex process involving interactions on length scales spanning many orders of

magnitude. The study of star formation in the Milky Way requires observations of regions as small as

the Solar system, while at the same time, the matter fuelling the star formation is flowing in and out

on Galactic scales. As a result, it is necessary to understand the structure in this flow of material,

including the large scale of the ‘Galactic fountain’ and throughout the many length scales of

turbulence down to Solar system sized over-densities where the stars are forming (e.g. McClure- Griffiths et al. 2015).

Neutral hydrogen is the primary material for star formation, arriving from the intergalactic medium,

as well as from nearby galaxies such as the Magellanic System. SKA-AAMID is ideally suited to trace

the HI at angular scales spanning many orders of magnitude, with short baselines within stations and

out to long baselines on the order of 100 km.

Neutral hydrogen can also be measured in absorption by observing along lines of sight towards

distant strong continuum sources, such as quasars. In this way, we probe cool gas, which has formed

pockets after splitting to form the Cool Neutral Medium, or fast moving High Velocity Clouds. This

method also allows us to measure the local spin temperature, which is a crucial parameter for

understanding the dynamics leading to star formation. With SKA-AAMID, we will have access to

many sources in the large FoV to probe the Galaxy for cool gas in multiple directions.

The Milky Way is soon entering a phase of evolution seen in other galaxies called the ‘Green Valley’

during which there is no longer star formation (Mutch et al. 2011). It remains unclear why star

formation is quenched, and in-depth studies of the Milky Way will teach us more about this

important phase of evolution, and the role it plays in galaxy evolution.

The Magellanic System, including the Large and Small Magellanic Clouds (LMC and SMC), provides

material to the Milky Way, but it is also the site of star formation (Olszewski et al. 1996). Studies of

the Magellanic clouds provides us with a nearby laboratory for observing star formation in a low

metallicity environment, giving us clues about the earliest star formation in the history of the

Universe. For these studies, the telescope must be in the southern hemisphere in order to observe

the LMC and SMC, which are at a declination of approximately -70∘

. The Karoo site is far enough

south to observe the LMC/SMC, but they reach a maximum elevation of only 48 ∘

so observing time is

somewhat limited. This could be improved by having access to the sky at elevations below 45∘

.

3.3. The structure of the Universe

A radio continuum survey comprising over three billion galaxies will also be carried out with SKA- AAMID (an order of magnitude more than with SKA1; e.g. Prandoni & Seymour 2015; Norris et al.

2015). When combined with the HI survey discussed in Section 3.1, this survey will ensure major

advances in state-of-the-art precision cosmology. By probing the distribution and evolution of large- scale structure over ultra-large volumes, key parameters associated with dark matter, dark energy,

modified gravity and primordial non-gaussianity will be able to be determined to an accuracy well

beyond that which is possible with SKA1. Moreover, SKA2 will surpass optical/near-infrared projects

such as Euclid and the Large Synoptic Survey Telescope (LSST; LSST Science Collaboration et al. 2009),

although there will be valuable and important synergies by combining data from several facilities

(e.g. Maartens et al. 2015). SKA-AAMID alone can conduct a unique test of the cosmological principle

of isotropy by confirming if the direction of the matter dipole matches that of the dipole evident in

the Cosmic Microwave Background (e.g. Maartens et al. 2015; Jarvis et al. 2015a). Owing to the

expected significant improvements in sensitivity and survey speed of SKA-AAMID over SKA1-MID, it

may even be possible to directly measure the expansion rate of the Universe in real time, by

detecting very small frequency shifts in the HI signal of galaxies over the expected 50-year lifetime of

the SKA-AAMID facility (Kloeckner et al. 2015).

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The effects of weak gravitational lensing of distant galaxies is an important part of the SKA2 science

case; this can be investigated by measuring the morphological curvature in the SKA-AAMID large

sample of galaxies. This in turn gives a measure of the dark matter in the Universe, and, in general, a

probe of late time cosmology (e.g. Heymans et al. 2013). The analysis will not only complement

optical surveys having completely different systematic error effects, but the radio survey has a

number of important advantages which will surpass optical surveys of cosmic shear (Brown et al.

2015). The combination of the radio continuum survey with the HI spectroscopic survey will allow a

clear separation of populations between the lens galaxies and the lensed galaxies, thus reducing a

source of systematic error. The spectroscopic survey will also provide rotation curves of lensed

galaxies which can be compared to the morphological major axis of the galaxy. In the absence of

lensing, the galaxy major axis and the axis of rotation are orthogonal, so a departure from

orthogonality is an independent measure of gravitational lensing which can be used to reduce

systematic error in large samples of lensed objects. Polarization can also be used as a

complementary probe since polarization angle is not affected by gravitational lensing (Brown &

Battye 2011). Detailed polarization maps of lensed objects studies permits a determination of the

original unlensed morphology of the object which is effectively a direct measure of the mass of the

lens object.

An SKA-AAMID continuum survey will detect almost all star-forming galaxies that are seen in the

optical with LSST (Norris et al. 2015). The HI survey will be highly complementary: optical-quality

imaging (i.e. arcsec resolution) of galaxies the size of our Milky Way will become routine out to a

redshift of 1, when the Universe was about 40 per cent of its current age (e.g. Blyth et al. 2015).

More locally, highly-detailed HI studies of the interstellar medium will be possible for hundreds of

nearby galaxies (e.g. de Blok et al. 2015), at a resolution currently only possible for just a handful of

very close objects (e.g. Braun et al. 2009). Furthermore, on larger scales, huge reservoirs of gas in the

‘cosmic web’ will be able to be seen in unprecedented detail (e.g. Popping et al. 2015). By combining

these features, the life cycle of gas in galaxies, particularly how gas initially enters galaxies to enable

star formation to take place, as well as the interplay between the star formation rate and black hole

growth/accretion, can be explored across nearly 13 billion years of cosmic time (e.g. Jarvis et al.

2015b; Smolcic et al. 2015; McAlpine et al. 2015). This is particularly important at z = 2, where the

star formation rate was at its peak, over an order of magnitude higher than in the present-day

Universe (e.g. Hopkins & Beacom 2006). SKA-AAMID has the frequency coverage, angular resolution

and sensitivity to study this crucial epoch for the first time in both continuum and HI.

3.4. The dynamic radio sky

3.4.1. Transients and variables

Our understanding of the dynamic radio sky will be revolutionized with SKA-AAMID. The

implementation of mid-frequency aperture array technology will result in enormous, multiple FoVs;

these will be combined with unprecedented sensitivity, high time resolution, as well as the precise

localization accuracy offered by long baselines. The near-real-time reduction of the resulting

continuous, commensal, all-sky data streams, as well as extremely rapid responses to external

triggers from other facilities, will allow a highly-detailed census of luminous cosmic explosions: the

sites of extremes in density, pressure, temperature and gravity, far beyond what is possible in a

terrestrial laboratory (e.g. reviews by Fender & Bell 2011; Fender et al. 2015a). The SKA will then

truly be a radio sky monitor, shedding light on a rich, virtually unexplored parameter space spanning

over 25 orders of magnitude in luminosity, and over time-scales from nanoseconds to years (e.g.

Cordes et al. 2004; Pietka et al. 2015).

It is crucial to note that the key metric for transient studies is detection rate, and that this is linearly

proportional to FoV (e.g. Macquart 2014). Thus, aperture arrays are essential for making

transformational advances in this science area. While the low-frequency aperture array will already

be deployed in SKA1, a mid-frequency aperture array will operate in a frequency regime that is (i)

less affected by scattering (standard frequency dependence ν-4; e.g. Bhat et al. 2004); (ii) in which the

sky temperature is lower, resulting in better instantaneous sensitivity; and (iii) where the angular

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resolution is finer for a given maximum baseline, thus allowing more precise localization of a

transient event. Hence, SKA-AAMID will be the facility of choice for surveying the transient and

variable radio sky, with a particular ‘sweet spot’ for bright, coherent emission from ‘fast’ transients,

including the most rare, extreme outbursts in the Universe.

Another critical advantage of aperture arrays for transient science is that they are ‘software

telescopes’ with no moving parts. Hence they have the ability to react within seconds to external

robotic triggers, or internal triggers from the SKA itself. It is this capability for rapid response that is

absolutely essential for high-impact, cutting-edge science (e.g. Anderson et al. 2014; Fender et al.

2015b; Petroff et al. 2015; Kaplan et al. 2015). On the other hand, with dish arrays, the technique of

sub-arraying will be required to cover large parts of the sky at any moment, carrying a significant

sensitivity penalty. This is not nearly as efficient as the aperture array solution.

The discovery of 10,000 or more ‘fast radio bursts’ (FRBs; e.g. Lorimer et al. 2007, Thornton et. al.

2013) at cosmological distances will be a unique probe of the dark energy equation of state, as well

as the magnetic fields in the intervening intergalactic medium (e.g. Macquart & Koay 2013; Zhou et

al. 2014; Macquart et al. 2015). At the time of writing, 17 FRBs had been discovered, all but one at

1400 MHz, with the remaining event being detected at 800 MHz (Petroff et al. 2016). Clearly, much

will be learned about these enigmatic sources with the current generation of radio facilities (e.g.

recent results by Keane et al. 2016; Spitler et al. 2016), as well as SKA1. However, it is interesting to

note that SKA-AAMID will cover the frequencies where FRBs have already been detected, and that, at

least in some cases, FRBs perhaps may not be able to be detected as easily with low-frequency

aperture arrays (Kulkarni et al. 2015). Hence, with a field of view of at least several hundred square

degrees at 1000 MHz, SKA-AAMID is in a very strong position to be the prime facility for discovering

FRBs in sufficient numbers to do world-class cosmology.

In addition, SKA-AAMID will be very sensitive to more slowly varying, incoherent synchrotron

transients, which will be detected at rates of up to 1000 per week (Burlon et al. 2015; Fender et al.

2015a), allowing us to move from current standard individual source studies to detailed population

statistics. While one can argue that frequencies of a few GHz and above offer the best prospects of

detections (assuming, for example, a van der Laan 1966 model for a given outburst), commensal and

dedicated SKA-AAMID surveys around 1000 MHz will still be vital in transforming our knowledge of

the complex zoo of slow transients (e.g. forecasts by Metzger et al. 2015), as well as variability

statistics in general (e.g. as discussed by Bignall et al. 2015 in the context of AGN and extreme

scattering events). As just one example, an all-sky survey with an rms noise level of a few μJy beam-1

and at arcsecond resolution could be performed on a daily basis, helping to open up entirely new

parameter space, as discussed above.

3.4.2. Pulsar astronomy

Pulsar astronomy with SKA2 will have a profound impact on our understanding of the fundamental

tenets of General Relativity. SKA-AAMID will be able to find the vast majority of the expected 40,000

visible pulsars in our Galaxy, enabling more than a ten-fold increase on the known pulsar population

of today (e.g. Smits et al. 2009; Keane et al. 2015). A definitive census of the complex zoo of neutron

stars will then be possible, including new discoveries at the extremes of parameter space such as the

‘holy grail’ pulsar-black hole binary systems. Such systems may be so rare that there are only a few

existing in the entire Milky Way; the only way to find them is to detect close to all the active pulsars

in the Galaxy. Studies of the equation of state of ultra-dense matter, and strong-field tests of

relativistic gravity, will benefit substantially from this new paradigm. For example, completely novel

tests of the basic characteristics of a black hole – mass, charge and spin – as well as the presence of

an event horizon, will be possible (e.g. Kramer et al. 2004).

The wide field of view, large instantaneous bandwidth, and multi-beaming capabilities make SKA- AAMID an unrivalled pulsar finding and timing machine. Excluding a few degrees on either side of the

Galactic Plane, pulsar surveys are most effective over the approximate frequency range 550 - 900

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MHz (Smits et al. 2008,2009). Surveys near and along the Galactic plane can be carried out at slightly

higher frequencies: approximately 1100-1400 MHz. A large field of view makes the survey time

efficient, while the large bandwidth enables an accurate estimate of dispersion measure (DM) as well

as the potential effects of scatter broadening. The multi-beaming allows highly efficient monitoring

and timing, while eliminating instrumental effects in the calibration by observing multiple pulsars

simultaneously. Moreover, a southern location for the telescope is ideal for surveying along the

Galactic plane.

An exciting new era of astrophysics has recently begun, with the direct detection of gravitational

waves with the Advanced Laser Interferometer Gravitational-Wave Observatory (ALIGO; Abbott et al.

2016). SKA2 will also be a gravitational wave detector, using an ensemble of ultra-stable, isolated

millisecond pulsars, well distributed on the sky (Janssen et al. 2015; also see e.g. Verbiest et al.

2016). Long-term, very-high-precision observations of such a pulsar timing array will allow the

detection of long-period gravitational waves from, for example, galaxy collisions and the subsequent

mergers of their central supermassive black holes, or from more exotic phenomena such as cosmic

strings. This will allow further crucial tests of gravity in the strong-field regime, and new constraints

for models of galaxy evolution. Moreover, the nHz frequencies of these gravitational waves will

complement the much higher frequencies accessible to facilities such as ALIGO.

At present, insufficient suitable candidates for the timing array are known, and the dedicated pulsar

survey described above is essential to establish a dense network of new candidates. These sources

then need to be monitored for an extended period (e.g. once every two weeks for about 6 months;

Smits et al. 2009) to determine which are most suitable for high precision timing, which is best done

at frequencies of at least a few GHz (Janssen et al. 2015). However, SKA-AAMID is not only ideal for

finding pulsars, but also for the initial ‘bulk timing’ consisting of multiple observations of new

discoveries to obtain basic parameters: spin period, binary parameters, spin down etc. It will also be

a vital facility for tracking dispersion measure variations in the high-precision timing experiment,

owing to its large bandwidth, and there will be completely novel pulsar timing experiments only

possible with SKA-AAMID (e.g. semi-continuous timing using dedicated beams).

3.5. The origin and evolution of cosmic magnetism

The often overlooked contribution of cosmic magnetic fields is beginning to be ameliorated. This is

due both to the advent of new analysis techniques (e.g. Faraday rotation measure (RM) synthesis;

Brentjens & de Bruyn 2005), as well as telescopes with improved sensitivity and large instantaneous

bandwidths of a 1000 MHz or more. Nonetheless, SKA2 will be required to enter an era of very-high- precision magnetism science, in which the origin and evolution of cosmic magnetic fields can be fully

explored from the distant to local Universe.

SKA-AAMID has a number of features that make it very attractive for magnetism studies. The most

important is a wide bandwidth at and below approximately 1000 MHz: this ensures excellent

resolution in Faraday depth (i.e. RM) space, where such a resolution is inversely proportional to the

λ

2

baseline covered by the telescope. An AAMID telescope operating between 450 – 1450 MHz has

the requisite Faraday depth resolution, sufficient sensitivity to extended Faraday depth structures, as

well as the ability to detect very large absolute Faraday depths, making it an excellent telescope to

study a wide range of magnetised phenomena, both extragalactic and within the Milky Way (e.g.

Gaensler et al. 2015; Haverkorn et al. 2015). Apart from increasing the λ 2 coverage, the lower

operating frequencies would allow progressively higher-redshift sources to be targeted (e.g.

Arshakian & Beck 2011; Gaensler et al. 2015). SKA-AAMID also has sufficient angular resolution to

reduce the deleterious effects of beam depolarization.

Key SKA2 projects such as a dense, all-sky RM grid of at least 40 million radio sources (Johnston- Hollitt et al. 2015), high-resolution Faraday tomography of nearby galaxies (Heald et al. 2015), and

studies of the faint polarized sky at the level of a few μJy or less (Taylor et al. 2015) all require survey

speed that only SKA-AAMID can deliver. In addition, mid frequencies offer an ideal compromise

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between the usually competing effects of continuum brightness level (assuming a canonical spectral

index α = -0.7) and internal Faraday depolarization (e.g. Sokoloff 1998).

3.6. Additional science

Many science cases for the SKA will benefit from the unique capabilities of mid-frequency aperture

array technology. While we have described a number of key science areas above, an additional

example that would utilize the very wide FoV is commensal, ultra-sensitive searches for

extraterrestrial intelligence (SETI; e.g. Siemion et al. 2015). More locally, Solar imaging in the

frequency range 450 – 1000 MHz is currently poorly explored, and SKA-AAMID will have the

combination of baseline coverage, field of view and modes of operation that would allow it to be

significantly more flexible for such a task than the SKA1 dishes (Nakariakov et al. 2015).

Furthermore, through the exploration of previously uncharted parameter space, SKA-AAMID will

undoubtedly also result in many new discoveries (e.g. Wilkinson et al. 2004; Wilkinson 2015). In fact,

most telescopes have made unexpected discoveries completely outside the domain for which the

instrument was designed (e.g. review by Torchinsky 2009). The 'unknown' covers not just time- domain science: for example, the combination of multi-beaming, along with unprecedented

instantaneous FoV, enables detailed studies of structures that span tens of degrees in angular size on

the sky. Thus, from our own Galaxy out to cosmological distances, SKA-AAMID has the immense

potential to transform our knowledge of the Universe, while fully realizing the philosophy of the SKA

as a scientific facility for exploring the unknown.

4. Definition of ‘Field of View’

The term ‘Field of View’ (FoV) is used throughout SKA documentation, but the definition is not

always clear.

There are two definitions, which can be qualified by ‘optical’ and ‘processed’.

4.1. Optical Field of View

The optical field of view is the maximal solid angle on the sky in which beams can be formed

instantaneously. Since the processed field of view (see below) is not necessarily contiguous, the

optical field of view is mostly larger than, or equal to, the processed field of view.It is determined by

the electromagnetic response of the frontend receptor. For dishes, the optical FoV is a function of

the ratio of focal length to dish diameter. This determines how far from the centre of the focal spot

one can place a detector before there is unacceptable aberration. For aperture arrays, the optical

FoV is determined by the antenna response (point spread function), and also the maximum

acceptable scan angle (angle measured from zenith). In general, the FWHM of the smallest antenna

element determines the optical FoV. If we group antenna elements together either by analogue or

digital summing, then this grouping becomes the smallest antenna element, and the optical FoV is

limited by the FWHM of this grouping. It is possible for an aperture array FoV to be constrained by

the maximum scan angle, rather than the FWHM of the antenna element. The optical FoV is

frequency dependent.

4.2. Processed Field of View

The processed field of view is the sum of all solid angles on the sky that can be instantaneously

observed and processed. It is determined by the digital processing capacity of the backend

(correlator or digital beamformer). There is a processing bandwidth, which can be shared between

the number of beams formed and the bandwidth of each beam. In general, it is the processed FoV

which puts the limitation on FoV, and this is the one of interest. However, it is possible to have a

processed FoV which exceeds the optical FoV by forming beams outside the optical field of view. In

these cases, performance will be compromised. For example, for a LOFAR station, one can form a

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maximum of 488 digital beams, each with a bandwidth of only 200 kHz (van Haarlem et al. 2013).

However, tiling the sky with this many beams would put many of them well below the 45° scan angle

limit, and those beams would have severely compromised performance because of the optical

limitation of scan angle and antenna response.

4.3. Use of the term FoV in science documentation

The term FoV is used throughout SKA science documentation, and in general, this refers to the

processed FoV. However, there may sometimes be confusion. For example, in SKA-AAMID

documentation, one advantage which is often mentioned is the ability to "form multiple fields of

view". In fact, in that case, the ‘field of view’ is determined by the PSF of the analogue summed

antenna elements. We split the signal and perform analogue beamforming on the copies, and

continue with digital processing of the multiple copies of the analogue signal. This is effectively a

method to increase the processed FoV. One might ask the question, why do analogue summing, only

to split the signal again and digitize? We do this to achieve a processed FoV which is larger, but not

as large as what would be achieved by digitizing the antenna elements. In this way, we have a larger

processed FoV, but with fewer digitizers than required to process the entire optical FoV which would

require the digitization of each individual antenna element. If we have sufficient "multiple fields of

view" to cover the entire optical FoV, then there is no sense in making multiple fields of view, and we

should just digitize the individual elements.

4.4. The FoV advantage of SKA-AAMID, and future upgrades

The main advantage of SKA-AAMID is its large optical FoV. While it may be true that we do not

currently have the capacity to produce a processed FoV equal to the optical FoV, the SKA is expected

to operate for 50 years and more. SKA-AAMID provides the opportunity for future upgrades to

increase the processed FoV without changing the frontend hardware. This is not the case for dish- based solutions which will be limited by a much smaller optical FoV compared to SKA-AAMID.

5. Science Requirements

The full SKA telescope is envisaged to be a facility approximately one order of magnitude more

sensitive than SKA1, with additional significant upgrades to the FoV (at least 20 times larger) and

maximum baseline (as large as 3000 km). We now describe a range of preliminary science

requirements for an AAMID component in SKA2. It is important to stress that the final requirements

will naturally evolve depending on the scientific outcomes and lessons learned from SKA1.

Nonetheless, the values described henceforth allow important initial constraints to be placed on the

overall design of SKA-AAMID.

To derive the requirements, we have used references in the literature, chapters from the SKA Science

Book ‘Advancing Astrophysics with the Square Kilometre Array’ (summary chapter Braun et al. 2015),

as well as the SKA Design Reference Mission (DRM, document RD-3) and the latest version of the

SKA1 Level 0 science requirements (document AD-13).

Note that henceforth, the term ‘beamformed’ refers to high-time-resolution, pulsar-like observing

modes. Multiple beams may be formed, and each beam is the result of either the coherent (‘tied- array’) or incoherent summation of the output of all SKA-AAMID stations, each producing high-time- resolution output. Furthermore, the term ‘imaging’ refers to interferometric observations, where a

number of beams are used to tile a sky region of interest.

In addition, although the following specifications are stated for a number of particular durations, this

is not a limitation on observation length, but merely a time-scale for a given performance. Moreover,

unless otherwise stated, the requirements are necessary within the entire MFAA tuning range of 450

to 1450 MHz. Finally, HI requirements often use an upper limit of 1425 MHz, to account for the

possibility of nearby blueshifted HI.

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5.1. General considerations

AMSC_R1001:

SKA-AAMID shall be sited in the Karoo Region of South Africa, at a specific location as decided by the

SKA Members.

This location, the site of SKA1-MID, is ideal for the full range of Galactic and extragalactic science

cases. It will allow sharing of infrastructure already on the ground during SKA1, as well as the

possible hybrid combination of SKA-AAMID and the SKA dishes (Section 6). Moreover, the telescope

will be at a similar latitude to a number of current premier optical/NIR facilities, as well as the

planned next generation of these telescopes, including LSST, the European Extremely Large Telescope

(E-ELT), and the Giant Magellan Telescope (GMT). The latitude of the Karoo also allows the telescope

access to a number of current and planned deep fields with multi-wavelength ancillary data (e.g. RD- 3, Prandoni & Seymour 2015). Lastly, the near-pristine radio-frequency interference (RFI)

environment (e.g. AD-14) is also vital for numerous scientific objectives.

AMSC_R1002:

SKA-AAMID shall provide continuous frequency coverage of both perpendicular polarization states

from 450 - 1450 MHz. It is desirable to extend the frequency coverage to the (continuous) range 400 -

1500 MHz.

This fundamental requirement is mandatory to ensure that cutting edge, mid-frequency science can

be carried out with the telescope.

AMSC_R1003a:

SKA-AAMID shall have an instantaneous, possibly non-contiguous, processed FoV ≥ 100 deg2

at 1000

MHz, where this processed FoV is synthesized from multiple beams. A ν-2 frequency profile for the FoV

is assumed for the other operating frequencies.

AMSC_R1003b:

The (possibly non-contiguous) instantaneous bandwidth per beam shall be ≥ 500 MHz.

AMSC_R1003c:

The product of processed FoV and bandwidth shall be constant, allowing larger FoVs to be realised by

using smaller bandwidths.

The flexibility of choosing observing configurations for SKA-AAMID ties together FoV, instantaneous

bandwidth, and the number of beams. Apart from detections of transients, a wide FoV is vital for

sufficiently high survey speeds for a range of other science topics. Wide instantaneous bandwidths

are also important for a number of reasons, namely sensitivity, survey speed (e.g. covering a large

redshift range in extragalactic HI studies), spectral index determination (particularly for time-variable

phenomena), interference rejection, coherent de-dispersion and scatter-broadening measurements

for pulsars, as well as high Faraday depth resolution using the RM synthesis technique.

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AMSC_R1004a:

The total optical FoV shall be ≥ 200 deg2

at 1000 MHz, with a ν-2 frequency profile for the FoV being

assumed for the other operating frequencies.

AMSC_R1004b:

SKA-AAMID shall be capable of multi-beaming, providing simultaneous access to large, possibly non- contiguous and distantly separated parts of the sky.

Requirement AMSC_R1004a is based on the recommendation in RD-3 for transients science. A very

large FoV significantly increases the detection rate of transients (e.g. Macquart 2014; Macquart et al.

2015), particularly for the rarest, brightest events. Moreover, a FoV of this size would allow the

sample of 10 000 FRBs needed for cosmological studies to be found within 2-3 years of on-sky

telescope time, based on the current best-estimate rates (e.g. Keane & Petroff 2015).Multi-beaming

significantly enhances the flexibility of SKA-AAMID. Specific science examples include bulk timing of a

suite of pulsars spread across the sky, and wide-field searches for transients and variables. Being able

to carry out independent observing programmes in parallel is also highly desirable.

AMSC_R1005:

SKA-AAMID shall provide access to all local azimuthal angles, 0° – 360°, and all local elevation angles,

45° – 90°, to ensure a minimum total accessible sky coverage of approximately 25 000 deg2

. There is

a desire to observe at elevation angles 30° – 45° to extend the sky coverage to approximately 30 000

deg2

for at least a subset of observing frequencies .

This requirement is necessary for all-sky science cases such as cosmology, as well as searches for

transients and pulsars. The Large and Small Magellanic Clouds will also be accessible (Declinations of

-69.8° and -72.8°, respectively), allowing important studies on topics such as gas accretion and star

formation. The suggested extension of the scan angle to elevations as low as 30° would ensure that

the entire Southern hemisphere is accessible to the telescope; for example, observing the South

Celestial Pole would be advantageous for (semi-)continuous searches for transients. Furthermore,

the full ecliptic would be accessible for Solar imaging, and there would also be an increased sky

overlap with a number of next-generation facilities operating in the Northern hemisphere.

AMSC_R1006:

SKA-AAMID shall provide nominal sensitivity at all local azimuthal angles, 0 – 360°. Sensitivity will be

optimized at zenith (Declination ≈ -30°). Sensitivity degradation away from zenith will remain within

5 per cent of the theoretical performance loss of the system due to the changing geometric

foreshortening of the aperture.

This requirement provides an initial performance benchmark to ensure reasonable sensitivity levels

away from zenith, in turn allowing both all-sky surveys and individual deep pointings to be carried

out in an acceptable period of time. Optimizing the sensitivity at zenith is advantageous because a

significant fraction of the Galaxy passes close to or directly overhead; in addition, there is also good

coverage of high Galactic latitudes, including the South Galactic Pole, which is an excellent candidate

deep field location for extragalactic projects.

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AMSC_R1007:

SKA-AAMID shall support simultaneous imaging and beamformed observations.

This requirement is mainly linked to the transients science case. Depending on the sub-class of

transient, and the degree of scatter broadening, having both interferometric and beamformed data

offers the most complete picture of an outburst (e.g. Hassall et al. 2013). Another important example

is detecting an FRB in beamformed data, and using interferometric data to localize it (e.g. Macquart

et al. 2015). Furthermore, commensality is absolutely key to transient science, and so this observing

mode should be used for all observations, where possible.

AMSC_R1008:

This requirement was deleted based on conclusions of the System Requirements Review (SRR), 2016

July 13-14.

AMSC_R1009:

SKA-AAMID shall be capable of making continuum images, with any specific integration time in any

specific direction within its sky coverage, with an effective noise level that is within a factor of 2 of

the thermal noise level expected theoretically based on telescope sensitivity, bandwidth, integration

time and the weighting scheme used in the imaging process.

This requirement is adapted from AD-13, and allows a range of science goals to be met, particularly

for deep imaging.

AMSC_R1010:

SKA-AAMID shall provide an absolute photometric accuracy ≤ 1–2 per cent, and an internal (relative)

systematic photometric accuracy ≤ 1 per cent.

This requirement is adapted from AD-13. Following AD-13, photometric accuracy is defined as the

fractional error, ΔS/S, relative to an adopted celestial flux standard for the integrated brightness of

compact objects (smaller than the PSF FWHM). The absolute requirement is based on the latest

state-of-the-art flux density scales (e.g. Perley & Butler 2013). The internal consistency of flux

densities, over both time and frequency, across a range of observing runs, is vital for many different

science applications, particularly studies of variability.

AMSC_R1011:

SKA-AAMID shall use an absolute astrometric reference frame that is accurate to ≤ 10 μas. The

internal (relative) systematic astrometric accuracy shall be ≤ 1 per cent.

This requirement is adapted from AD-13. The absolute requirement is based on Fomalont & Reid

(2004); also see Paragi et al. (2015). Following AD-13, astrometric accuracy is defined as the

fractional error in the position, Δρ/Δθ, for the centroid of compact objects (smaller in size than the

PSF FWHM, Δθ) relative to the adopted celestial reference frame. The internal consistency of

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astrometric positions, over both time and frequency, across a range of observing runs, is vital for

many different science applications, for example proper motion studies.

AMSC_R1012:

This requirement was deleted based on conclusions of the System Requirements Review (SRR), 2016

July 13-14.

AMSC_R1013:

SKA-AAMID shall provide a frequency accuracy ≤ 10-11 over a 10 year interval.

This requirement is taken from AD-13, and mainly relates to the real-time cosmology experiment

outlined in Kloeckner et al. (2015). Following AD-13, frequency accuracy is defined as the fractional

error in the frequency, Δν/ν, relative to an adopted frequency standard over a specified time interval.

AMSC_R1014:

SKA-AAMID shall provide timing accuracy ≤ 10 ns over a 10 year interval.

This requirement is taken from AD-13, and is particularly relevant for high-precision pulsar timing

(e.g. RD-3; Janssen et al. 2015). Following AD-13, timing accuracy is defined as the RMS time error,

Δτ, relative to an adopted time standard over an indicated time interval.

5.2. Galaxy evolution and cosmology probed by neutral hydrogen

AMSC_R2001:

SKA-AAMID shall achieve a Stokes-I sensitivity ≤ 5 μJy beam-1 RMS from 450-1425 MHz, over a

bandwidth of 10 kHz, with 2-15 arcsec angular resolution, over at least 25 000 deg2 and up to a

maximum of 30 000 deg2

, within two years on-sky integration.

This requirement will allow one of the original primary science visions of the SKA to be achieved: a

billion galaxy HI survey out to a redshift of 2 and beyond. Such a survey will enable unprecedented

studies of galaxy evolution, as well as world-class precision cosmology, such as through the detection

of baryon acoustic oscillations. This requirement is based on Santos et al. (2015b) and RD-3.

Following Santos et al. (2015b), a minimum detection is assumed to be at the 10σ level.

AMSC_R2002:

SKA-AAMID shall achieve a Stokes-I sensitivity ≤ 1.5-5 μJy beam-1 RMS from 450 – 650 MHz, over a

bandwidth of 10 kHz, with 2 arcsec angular resolution, over a selection of fields with total area 30-50

deg2

, within 1000 hours on-sky integration per field.

This requirement will enable a fiducial HI mass of M* = 5 × 109

solar masses (or lower) to be reached

in deep fields at high redshift (RD-3), assuming averaging is carried out over a feature that would

have a velocity width of 50 km s-1 in the local Universe, as well as the velocity width - redshift scaling

assumptions used by Abdalla & Rawlings (2005). For consistency with AMSC_R2001, a minimum

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detection is assumed to be at the 10σ level, and the lower and upper flux densities stated in the

requirement correspond to the required values at 450 and 650 MHz, respectively.

AMSC_R2003:

SKA-AAMID shall achieve a Stokes-I sensitivity ≤ 20 μJy beam-1 RMS from 450 – 1425 MHz, over a

channel width of 2 kHz, with an angular resolution ≤ 1 arcsec, over at least 10 000 deg2

, within two

years on-sky integration.

This requirement will allow a wide field, blind HI absorption survey to be performed to optical depths

of e.g. 0.01/0.001 (10 mJy / 100 mJy background continuum sources; assuming 5σ detections), and

to redshifts beyond 2 (RD-3; Morganti et al. 2015).

AMSC_R2004:

SKA-AAMID shall achieve a Stokes-I sensitivity ≤ 15 μJy beam-1 RMS from 1400 – 1425 MHz, over a

bandwidth of 5 kHz (velocity resolution ∆v ≈ 1 km s-1), with 1-10 arcsec angular resolution, over an

area of approximately 20 deg2

, within 10 hours on-sky integration.

This requirement will allow highly-detailed studies of the interstellar medium in nearby galaxies,

including at 50 pc resolution for galaxies within 10 Mpc, as well as their immediate ambient

environments, to column density limits of ≤ 1017-19 cm-2 (e.g. de Blok et al. 2015; column density

calculation using equation 3 in Fernández et al. 2013). Note that the requirement refers to an

observation of each individual galaxy, and assumes a minimum 5σ detection level.

AMSC_R2005:

SKA-AAMID shall achieve a Stokes-I sensitivity ≤ 1.7 μJy beam-1 RMS from 1400 – 1425 MHz, over a

bandwidth of 50 kHz (∆v ≈ 10 km s-1), with 10 arcsec angular resolution, over at least 25 000 deg2 and

up to a maximum of 30 000 deg2

, within two years on-sky integration.

This requirement will allow detections of diffuse gas filaments in the nearby cosmic web with a

spatial resolution of a few kpc and better, at column densities ≤ 1018 cm-2 (e.g Popping et al. 2015;

column density calculation using equation 3 in Fernández et al. 2013). A minimum detection level of

5σ is assumed.

AMSC_R2006:

Between 450 – 1425 MHz, SKA-AAMID shall have a velocity resolution ≤ 2 km s-1

.

This is required for sufficient velocity resolution for the full range of extragalactic HI science cases

(e.g. RD-3). Note in particular that the proposed real-time cosmology experiment outlined in

Kloeckner et al. (2015) would require velocity resolution as fine as 2.9 cm s-1

.

AMSC_R2007:

SKA-AAMID shall achieve a spectral dynamic range ≥ 60 dB from 450 – 1425 MHz, over a bandwidth

≤ 10 kHz.

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This requirement is necessary for deep HI detections (e.g. RD-3). Following AD-13, spectral dynamic

range is defined as the ratio of peak brightness to the residual instrumental spectral response, I/ΔI, at

the source location over a specified spectral interval. The minimum bandwidth requirement follows

from AMSC_R2001 and AMSC_R2002.

AMSC_R2008:

SKA-AAMID shall achieve a bandpass stability of 1 in 106

for integration times of up to 1000 hr per

field.

This requirement is necessary to unambiguously detect HI emission/absorption at the tens of μJy

level. Note that we assume the same definition of bandpass stability as in AD-13: the residual

fractional error in the brightness, ΔI/I, as function of frequency for a specified frequency interval over

a specified time.

5.3. Galactic HI

AMSC_R3001:

SKA-AAMID shall achieve a Stokes-I sensitivity ≤ 20 μJy beam-1 RMS from 1415 – 1425 MHz, over a

bandwidth of 1.4 kHz (∆v ≈ 0.3 km s-1), with 5 arcsec angular resolution, over at least 25 000 deg2 and

up to a maximum of 30 000 deg2

, within six months on-sky integration.

This requirement will allow the all-sky Galactic HI emission survey for mapping the structure of

turbulence in the interstellar medium down to length scales on the order of the size of the Solar

system (McClure-Griffiths et al. 2015).

AMSC_R3002:

Between 1415 and 1425 MHz, SKA-AAMID shall have a velocity resolution of 0.3 km s-1

.

This is required to resolve narrow HI absorption lines in the Galactic cold neutral medium which have

line widths on the order of 2.5 km s-1 (McClure-Griffiths et al. 2015).

5.4. The transient radio sky

AMSC_R4001:

SKA-AAMID shall achieve a Stokes-I sensitivity ≤ 1 mJy beam-1 RMS from 450 – 1450 MHz, over a

bandwidth of 100 MHz, with ≤ 2 arcsec angular resolution, over the full instantaneous processed FoV,

for an integration time of 1 ms.

This level of sensitivity for coherent, fast transients will allow for a far more complete understanding

of transient parameter space (e.g. Cordes et al 2004; RD-3). Note that the total instantaneous

bandwidth may be much larger than 100 MHz, but the requirement then ensures that a radio

spectrum can be determined. De-dispersion of the signal is assumed. High angular resolution is

needed for accurate transient localization.

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AMSC_R4002:

SKA-AAMID shall achieve a Stokes-I sensitivity ≤ 5 μJy beam-1 RMS from 450 – 1450 MHz, over a

bandwidth of 100 MHz, with ≤ 2 arcsec angular resolution, over at least 25 000 deg2 and up to a

maximum of 30 000 deg2

, within 24 hours on-sky integration.

Deep, high-cadence, all-sky surveys will allow for a far more complete understanding of transient

parameter space (e.g. Cordes et al 2004; RD-3). Note that the total instantaneous bandwidth may be

much larger than 100 MHz, but the requirement then ensures that a radio spectrum can be

determined. High angular resolution is needed for accurate transient localization.

AMSC_R4003:

The SKA-AAMID data streams (imaging and beamformed) shall be automatically searched for

astrophysical transients, with unambiguous detections being reported with a latency of less than 30

seconds at most, and preferably less than 10 seconds.

Low-latency capabilities are mandatory for optimizing this area of science (e.g. Fender et al. 2015a;

Chu et al. 2016). Rapid dissemination of alerts maximises the global scientific return via multi- wavelength follow up.

AMSC_R4004:

SKA-AAMID shall be able to respond to both human- and robotically-generated alerts on a time-scale

of 5 seconds at most, and preferably less than 1 second, and begin beamforming in the appropriate

direction(s) on this time-scale.

This requirement is essential for high-impact science (e.g. Fender et al. 2015a). SKA-AAMID will then

be able to fully participate in multi-facility campaigns to understand the most exciting, novel

astrophysics from outbursts, which is often at most very shortly after the event itself.

AMSC_R4005:

SKA-AAMID shall be able to store raw voltage data over the full optical FoV and for a (possibly non- contiguous) bandwidth ≥ 250 MHz, for a period ≥ 120s.

This will ensure proper localization and follow-up of fast transients, including the rise phase of a

given transient event. As an example, current FRB searches at Parkes make use of a 120 s ring buffer

(e.g. Petroff et al. 2015). The maximum detectable DM would be approximately 6500 pc cm-3, which

would generally be comfortably adequate for most purposes (e.g. Cordes & Lazio 2002; Petroff et al.

2016).

AMSC_R4006:

The temporal resolution of SKA-AAMID beamformed data shall be on a time-scale as short as 100

nanoseconds. It is also desirable to have a temporal resolution as short as nanoseconds.

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This will allow the fullest sampling of transients parameter space (e.g. Cordes et al. 2004; RD-3), with

an ultra-short time-scale of nanoseconds corresponding to very high brightness temperature events

such as giant pulses from the Crab Pulsar (e.g. Staelin & Sutton 1970).

AMSC_R4007:

SKA-AAMID shall have sufficient instantaneous uv coverage as to allow the localization of fast

transients in imaging data to a precision ≤ 0.1 arcsec for signal-to-noise ratios ≥ 5, and an

integration time of 50 ms. It is also desirable to be able to make similar images for integration

times < 50 ms.

This requirement is necessary for fast transient science, such as FRB studies (e.g. Macquart et al.

2015). Following Fomalont (1999), being able to synthesize a 1 arcsec beam on these time-scales

would allow a formal localization as precise as 0.1 arcsec for a 5σ detection.

5.5. Strong-field tests of gravity with pulsars

AMSC_R5001:

SKA-AAMID shall achieve a Stokes-I 1400 MHz limiting luminosity of 0.1 mJy kpc2

(or equivalent over

the frequency range 450 – 1450 MHz) at distances ≥ 5 kpc, over a bandwidth ≤ 100 MHz, over at

least 25 000 deg2 and up to a maximum of 30 000 deg2

, within two years on-sky integration.

This requirement will allow SKA-AAMID to detect the vast majority of the visible pulsar population of

the Galaxy (but excluding the population at the Galactic Centre). The luminosity requirement is

based on the work presented in Lorimer et al. (2006) and Smits et al. (2009), as well as RD-3. At a

distance of 5 kpc, this corresponds to a detectable 1400 MHz flux density requirement of 4 μJy (0.8

μJy beam-1 RMS for a 5σ detection) . Alternatively, assuming 10 kpc (e.g. as in RD-3) gives a 1400 MHz

flux density requirement of 1 μJy (0.2 μJy beam-1 RMS). Pulsar surveys will typically have several

hundred MHz bandwidth or more that could be used to meet the sensitivity requirements (e.g. RD-3;

Smits et al. 2009), but ideally detections would be made using e.g. no more than 100 MHz

bandwidth, to allow a spectral index to be determined. Note that higher frequencies are more

appropriate for lower Galactic latitudes, where (i) the dispersion measure will generally be larger; (ii)

the effects of scattering can be significant at lower frequencies; and (iii) the sky temperature is

higher, particularly at lower frequencies. Assuming an average pulsar spectral index of -1.4 (Bates et

al. 2013), this would result in sensitivity requirements that are generally less stringent at lower

frequencies.

AMSC_R5002:

Using the full instantaneous bandwidth from 450 – 1450 MHz, SKA-AAMID shall carry out initial bulk

timing of all pulsars discovered by the telescope, taking no more than approximately 250 h on-sky

integration over a six month period to achieve a signal-to-noise ratio ≥ 10 per pulsar over a total sky

area of about 15 000 deg2

.

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The observing strategy necessary for such a programme is discussed in Smits et al. (2009), as well as

Lorimer et al. (2006). For example, individual pointings will typically last several minutes, reaching

1400 MHz luminosities of about 0.1 – 0.5 mJy kpc2

(i.e. approximately 0.3 μJy beam-1 RMS at 10 kpc).

AMSC_R5003:

SKA-AAMID shall have the capability to track dispersion measure variations of magnitude ≤ 10-4 pc

cm-3, associated with very-high-precision pulsar timing experiments, using the full bandwidth range

450 – 1450 MHz,

This requirement is based on the recommendations in RD-3 for the pulsar timing array (also see e.g.

Janssen et al. 2015). Examples of dispersion measure variations for the current generation of pulsar

timing array candidates can be found in e.g. Keith et al. (2013); also see e.g. Lee et al. (2014).

AMSC_R5004:

SKA-AAMID shall achieve a polarization purity ≤ -40 dB, on axis.

This requirement follows the recommendation in RD-3, to ensure sufficiently precise pulsar timing.

Polarization purity is defined using AD-13 as a starting point: the ratio of the residual instrumental

polarized response peak to Stokes I brightness to, ΔP/I, at the source location. Here, the specified

value applies to the target pointing directions at the centres of individual beams.

AMSC_R5005:

SKA-AAMID beamformed modes shall have a frequency resolution ≤ 10 kHz per channel.

This requirement follows the recommendation in RD-3, to minimize dispersion smearing.

AMSC_R5006:

SKA-AAMID shall have beamformed observational modes with a temporal resolution between 100 ns

– 50 μs. It is desirable to have temporal resolution better than 100 ns.

The stated minimum temporal resolution follows the recommendation in RD-3 for a general pulsar

survey (using a dense array core), and would also potentially allow for sub-millisecond pulsars to be

discovered, if they indeed exist. Also, following RD-3, 100 ns temporal resolution would allow the

pulse profiles of millisecond pulsars to be resolved by at least a factor of a few hundred, allowing the

best candidates for the pulsar timing array to be identified.

5.6. Galaxy evolution and cosmology probed by radio continuum

AMSC_R6001:

SKA-AAMID shall achieve a Stokes-I sensitivity ≤ 100 nJy beam-1 RMS from 450 – 1450 MHz, over a

bandwidth ≤ 300 MHz, with angular resolution ≤ 0.5 arcsec at all frequencies and as good as 0.2

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arcsec above 1000 MHz (with the possibility of extending this resolution to as low as 600 MHz), over

at least 25 000 deg2 and up to a maximum of 30 000 deg2

,within 6 months on-sky integration.

This requirement allows a wide field, high-resolution radio continuum survey to be carried out, in

which over three billion galaxies will be detected, allowing unprecedented studies of star formation

across cosmic time, as well as cosmological constraints to be derived from both weak and strong

lensing (e.g. Jarvis et al. 2015a,b; Brown et al. 2015; Patel et al. 2015; McKean et al. 2015; Norris et

al. 2015). A minimum detection level of 5σ is assumed. Depending on how the survey strategy is

refined, a canonical spectral index of -0.7 would result in sensitivity requirements that are less

stringent by a factor of approximately 2 at 600 MHz (the lowest frequency for the survey assumed in

Prandoni & Seymour 2015) than at 1450 MHz.

AMSC_R6002:

SKA-AAMID shall achieve a Stokes-I sensitivity ≤ 5 nJy beam-1 RMS from 450 – 1450 MHz, over a

bandwidth ≤ 300 MHz, with angular resolution ≤ 0.5 arcsec at all frequencies and as good as

0.2 arcsec above 1000 MHz (with the possibility of extending this resolution to as low as 600 MHz),

over a selection of fields with total area 30 – 50 deg2

, within 1000 hours on-sky integration per field.

This requirement will enable highly-detailed studies of star formation (e.g. to a precision of 25 solar

masses per year at z 7) and AGN activity in selected ultra-deep fields (e.g. RD-3, Jarvis et al. ∼

2015b). A minimum detection level of 5σ is assumed. Depending on how the survey strategy is

refined, a canonical spectral index of -0.7 would result in sensitivity requirements that are less

stringent by a factor of approximately 2 at 600 MHz (the lowest frequency for the observations

assumed in Prandoni & Seymour 2015) than at 1450 MHz. The suggested angular resolution of 30

mas by Jarvis et al. (2015b) could be achieved by a hybrid combination of AAMID stations and dishes

(Section 6).

AMSC_R6003:

SKA-AAMID shall achieve an imaging dynamic range ≥ 70 dB from 450 – 1450 MHz, over a

bandwidth ≤ 300 MHz.

This requirement is necessary for ultra-deep radio continuum imaging. Following AD-13, imaging

dynamic range is defined as the ratio of peak to RMS brightness, I/ΔI, where the RMS fluctuation

level is measured within the entire station beam(s),but may exclude an area of 5x5 PSF FWHM

centred on the brightness peak.

AMSC_R6004:

For radio continuum surveys and deep imaging conducted over the frequency range 600 – 1450 MHz,

where the theoretical noise level is ≤ 100 nJy beam-1, the measured RMS noise level in SKA-AAMID

images shall not be limited by source confusion.

The confusion estimates presented in Condon et al. (2012) imply that an angular resolution better

than a few arcsec is generally needed, which is consistent with requirements AMSC_R6001 and

AMSC_R6002. However, more specific requirements may result from continuum observations carried

out with the SKA pathfinders, as well as SKA1.

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5.7. The origin and evolution of cosmic magnetism

AMSC_R7001:

SKA-AAMID shall achieve a Stokes Q and U sensitivity ≤ 100 nJy beam-1 RMS from 450 – 1450 MHz,

with (a possibly non-contiguous) instantaneous bandwidth ≥ 500 MHz, with 2-3 arcsec resolution,

over at least 25 000 deg2 and up to a maximum of 30 000 deg2

, within six months on-sky integration.

This requirement will enable an all-sky grid of Faraday rotation measures to be constructed, with a

density of several thousand per square degree, an order of magnitude denser than in SKA1.

Applications include measurement of the magnetic fields in a significant number of intervening

galaxies out to high redshifts, as well as highly detailed measurements of the structure of the

magnetic field in the Milky Way (e.g. RD-3; Johnston-Hollitt et al. 2015; Gaensler et al. 2015;

Haverkorn et al. 2015). A minimum detection level of 10σ is assumed for sufficiently accurate RM

synthesis (e.g. RD-3); note that the full 1000 MHz bandwidth would yield the optimal point spread

function in Faraday depth space.

AMSC_R7002:

SKA-AAMID shall achieve a Stokes Q and U sensitivity ≤ 10 nJy beam-1 RMS from 450 – 1450 MHz,

with (a possibly non-contiguous) instantaneous bandwidth ≥ 500 MHz, with ≤ 3 arcsec resolution,

over a selection of fields with total area 30 – 50 deg2

, within 1000 hours on-sky integration per field.

This requirement will allow the origin and evolution of cosmic magnetism to be explored at very faint

flux density levels in selected ultra-deep fields (RD-3; Johnston-Hollitt et al. 2015; Gaensler et al.

2015; Taylor et al. 2015). A minimum detection level of 10σ is assumed for sufficiently accurate RM

synthesis (e.g. RD-3); note that the full 1000 MHz bandwidth would yield the optimal point spread

function in Faraday depth space.

AMSC_R7003:

SKA-AAMID shall achieve a Stokes Q and U sensitivity ≤ 20 nJy beam-1 RMS from 450 – 1450 MHz,

with (a possibly non-contiguous) instantaneous bandwidth ≥ 500 MHz, with 0.5-1.5 arcsec resolution,

over an area of approximately 20 deg2

, within 100 hours on-sky integration.

This requirement is necessary for significant improvements in magnetic field tomography of nearby

galaxies (e.g. Heald et al. 2015), compared with SKA1. In particular, the evolution of the three- dimensional magnetic field structure in galaxies will be investigated at a spatial resolution of as good

as 10 pc for nearby galaxies, 100 pc for all galaxies within 50 Mpc, and on scales of 1 kpc out to a

redshift of about 0.1. Note that the requirement refers to an observation of each individual galaxy.

The final resolution chosen across the frequency range will depend on the required sensitivity to

extended emission (see discussion in Heald et al. 2015).

AMSC_R7004:

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SKA-AAMID shall achieve a polarization purity of at least -25 dB, and preferably as good as -30 dB,

over the full processed FoV.

This requirement follows the recommendations in RD-3 and Taylor et al. (2015), to ensure sufficiently

precise studies of the faint polarized radio sky. Polarization purity is defined as in AMSC_R7004.

AMSC_R7005:

SKA-AAMID shall be able to routinely achieve an observed-frame Faraday depth precision ≤ 1 rad m-2

,

and to be able to detect observed-frame Faraday depths up to a maximum amplitude of 10 000 rad

m-2

.

This requirement will allow SKA-AAMID to be sensitive to a wide range of magnetized phenomena.

The first part can be achieved by utilizing the full frequency coverage, 450 - 1450 MHz; for example,

the formal RM error on a 10 sigma detection would be 0.43 rad m-2 For the second part of the

requirement, the channel width at 450 MHz would need to be 90 kHz, using the relevant formula in

Brentjens and de Bruyn (2005); at 1450 MHz, a channel width of 3 MHz would suffice.

5.8. Widefield SETI

AMSC_R8001:

SKA-AAMID shall achieve a sensitivity ≤ 190 mJy beam-1 RMS per polarization from 450 – 1450 MHz,

over a bandwidth ≤ 0.01 Hz, within 10 min on-sky integration.

This requirement is for narrowband SETI, in particular airport radar-like signals from stars within

60 pc (Siemion et al. 2015). Following Siemion et al. 2015, a minimum detection is assumed to be at

the 12σ level.

6. Recommendation for SKA-AAMID parameters

Using the science requirements from Section 4, key parameters leading to an overall recommended

design are summarised in Table 1.

Table 1: Key SKA-AAMID parameters that define the performance envelope of the system.

Parameter Value

(required)

Value

(goal)

Science experiment(s) that

drive(s) the requirement

Lowest frequency

(MHz)

450 400 High-z HI

Highest frequency

(MHz)

1450 1500 Range of experiments

Instantaneous

sensitivity at zenith

10 000 15 000 HI / continuum deep fields

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Aeff/Tsys (m2

/K)

Optical FoV (deg2

) 200

(at 1000 MHz)

250

(at 1000 MHz)

Transients

Processed FoV (deg2

)

and instantaneous

bandwidth (MHz)

[product; can be

traded off against each

other]

≥100

(at 1000 MHz;

500 MHz

bandwidth per

beam)

≥100

(at 1000 MHz;

1000 MHz

bandwidth per

beam)

Range of experiments

Total instantaneous

bandwidth (MHz)

1000

(per processed

beam)

1000

(per processed

beam)

Range of experiments

Minimum baseline (m) 20 1 Correlations within the core

station for HI intensity

mapping; Galactic HI

Maximum angular

resolution (arcsec)

0.5 0.2 Continuum surveys, including

weak/strong lensing

Scan angle (deg; from

zenith)

±45 ±60 Cosmology; transients; pulsar

searches; Galactic HI (Small

and Large Magellanic Clouds)

Overall sky coverage

(deg2

)

25 000 deg2 30 000 deg2 Cosmology; transients; pulsar

searches

Native temporal

resolution (s)

0.15

(interferometric;

standard imaging

mode)

0.05

(interferometric;

fast imaging mode)

Range of experiments;

possible fast imaging mode

for transients

10-7 – 10-5

(beamformed)

10-9 – 10-5

(beamformed)

Pulsars; transients

Native frequency

resolution (kHz)

1.4

(interferometric)

1.4

(interferometric)

Galactic HI

≤10

(beamformed)

≤10-5

(beamformed;

correlator in zoom

mode with narrow

bandwidth)

Pulsars; transients. The goal

of ≤10-5 kHz (≤ 0.01 Hz) is for

real-time measurement of

space-time expansion, and

SETI.

Frequency agility Option for non- contiguous

bandwidth

Option for non- contiguous

bandwidth

Transients; pulsars; cosmic

magnetism; commensal

observing

Imaging dynamic range 70 75 Continuum deep fields

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(dB)

Spectral dynamic range

(dB)

60 65 HI BAO and galaxy evolution

Polarization capability Full Stokes Full Stokes All experiments

Polarization purity (dB) -25

(off axis)

-40

(on axis)

-30

(off axis)

-40

(on axis)

Pulsar timing ; cosmic

magnetism

Collecting area

distribution

50% collecting

area within 5km

50% collecting

area within 5km

Pulsars; transients; HI

intensity mapping

Array reconfiguration

time (s)

<5 <1 Transients

7. Progressive science operations with the SKA-AAMID telescope

Phased approach to AAMID implementation: initial operations with SKA1-MID

Several key science projects are optimally addressed with a large fraction of the collecting area in the

core. These include pulsar searches, fast transient searches, the Galactic HI survey, and the Intensity

Mapping experiment for the detection of the BAO signature. SKA-AAMID is perfectly suited to these

surveys, providing an extremely large FoV, and the possibility to correlate very short baselines for

sensitivity to extended emission.

The large FoV necessitates a significant digital processing capability, especially for surveys at high

angular resolution. The combination of large FoV and high angular resolution results in a very large

number of beams on the sky. In order to remain within processing limitations, only a subset of the

FoV can be processed with high angular resolution. As a result, much of the RF capability of SKA- AAMID is not exploited when doing surveys with the longest baselines. It is natural to have an early

implementation SKA architecture that is composed of AAMID stations in the core, and with the SKA1-

MID dishes in the outer stations. However, future upgrades to processing capacity will eventually

catch up with the RF capability, delivering the full potential of SKA-AAMID as the ultimate survey

machine.

A hybrid architecture uses the advantages of both frontend technologies. The AAMID core provides

a very large FoV, while the outer dishes increase the angular resolution for mapping within a subset

of the AAMID FoV. The dishes must point to lower elevations to match the core pointing, but this is

not an issue in general as they will be able to slew well below, for example, a 45 degree elevation

angle (e.g. RD-3).

A further consideration is the exploitation of existing instruments. When SKA-AAMID is being

installed, there will already be SKA1-MID operating. In the frequency overlap region, the first AAMID

station could begin operations immediately as part of SKA1, providing a large core to the existing

infrastructure. This will significantly augment the capability of SKA1-MID by providing increased

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sensitivity to the overall instrument, and, by immediately beginning a transient survey. SKA1 with

AAMID will be a powerful instrument for transient detections and immediate follow-up observations.

In the nearer term, an AAMID technology demonstrator can be installed and running together with

MeerKAT. This combination would already be the most powerful transient survey instrument in

operation.

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