Chronometry of a Pulsar Timing Array


Scheme of a pulsar (© NRAO)

Pulsars are fast-moving, stable magnetized neutron stars that emit a beam of electromagnetic waves in the manner of a headlight. The millisecond pulsars are considered almost perfect clocks and the ultra-precise timing of their radio transmission not only makes it possible to understand the pulsar system itself but also the detection of gravitational waves in the frequency band of the nanoHertz. Some pulsars are timed with an accuracy of less than 100 ns over an observation period of about twenty years. 

The pulse sequences emitted by the pulsars are regularly observed by radio telescopes. Using complex instrumentation, it is possible to measure the arrival times of the radio pulses at the telescope. It is thus possible to obtain a large number of astrophysical information on the pulsars: position in sky, parallax (distance), clean motion, rotation period and its variations. For orbiting pulsars with at least one companion, it is possible to constrain the orbital parameters and measure the effects of general relativity affecting orbital motion. Constraints can also be made on the equation of state of the matter composing the neutron stars by measuring the mass of the pulsars in multiple systems.

The arrival times also contain information on the propagation of radio waves through the interstellar medium and on the deformations of the space-time between the pulsar and the Earth caused by the gravitational waves. The effect of gravitational waves on arrival time measurements for a given pulsar is extremely low and difficult to separate from other types of disturbances such as instabilities in intrinsic rotation or instrumental problems. However, gravitational waves influence the signal of all the pulsars in a coherent manner and it is therefore possible to detect the passage of a gravitational wave by a correlated analysis of several pulsars. The chronometry of a network of millisecond pulsars for the search of gravitational waves is called "Pulsars Timing Array" (PTA).

Gravitational Wave Detection Scheme
by a network of pulsars
(© D. Champion)

The two main sources of gravitational waves that can be observed by PTA are cosmological bottoms and binaries of supermassive black holes, both as individually observable sources and in the form of a background of unresolved binary sources. Most models describing the evolution of supermassive black holes provide for detection of gravitational waves emitted by binary systems in the coming years.

The PTA observation system allows access to the very low frequencies and is thus complementary to the spatial gravity waves detectors (eLISA) and to the ground (Virgo, LIGO, etc.).

EPTA at APC lab

The PTA group in the APC laboratory works in close collaboration with the LPC2E (Orléans) and the radio astronomy station of Nançay, of which Grand RadioTelescope produces almost 50% of the European data. It participates in the European effort through EPTA (European PTA) collaboration, which includes 5 radio telescopes Nançay (France), Effelsberg (Germany), Jodrell Bank (United Kingdom), Westerbork (Netherlands) and Sardinia Radio Telescope). The APC is also a member of the international collaboration IPTA (International PTA). The main contribution of the PCA concerns the data analysis aspects.


EPTA : http://www.epta.eu.org/
IPTA : http://www.ipta4gw.org/