Speaker
Description
Pulsar Timing Arrays (PTAs) are collecting years of data by constantly monitoring the radio emission from the most stable millisecond pulsars (MSPs) across the sky in order to detect variation between the observed time of arrival of each pulse and the one expected from the timing model (i. e. the timing residual) that can be induced by gravitational waves (GWs). This technique, called pulsar timing, is the only one that allows to ``hear'' ultra-low frequency GWs, that may be generated by sources relevant for cosmology (e. g. cosmic strings) and astrophysics (e. g. super-massive black hole binaries) and form the gravitational wave background (GWB). In spite of the fact that PTAs should already have the potential to detect the GWB, by finding a quadrupolar correlation in the timing residuals, the goal has not been achieved yet.
The GW-induced timing residual depends on the difference between the metric perturbation at the position of the observer (called Earth term) and the metric perturbation at the position of the pulsar (called pulsar term). This implies that the GW-induced timing residual of pulsars located in almost the same position is expected to be approximately equivalent. Such effect can be only observed in a globular cluster (GC), where tens of MSPs are confined within a very small distance from the center.
We suggest the possibility of including GC MSPs in PTAs. Currently, these MSPs cannot be timed with a precision adequate for the detection of GW-induced variations on timing residual, mainly because of the effects induced on the MSPs dynamics by the gravitational potential. Nevertheless, we expect that the situation will change in the near future thanks to new powerful detectors, like the Five hundred meter Aperture Spherical Telescope (FAST), the MeerKAT radio telescope and the Square Kilometre Array (SKA). SKA, in particular, will certainly play a crucial role in ultra-low frequency GW detection. With its unprecedented sensitivity it will be possible to time significantly better the GC MSPs, opening, within the next decade, the possibility of adopting the strategy proposed in this work.
The interest in GC MSPs is well motivated by the fact that each of them respond in a very similar way to a GW, that would give an additional ``smoking gun'' for the GW detection. Moreover, the globular cluster distance is often known with great precision and this allows to take into account the pulsar term which, in current analysis, is treated as an additional noise source because of the difficulties in the determination of the precise distance of the isolated pulsars. It has to be noticed that including the pulsar term in the analysis is relevant since it depends on the angular coordinates of the GW source and must be considered in order to estimate its position. Finally, since in some cases the inter-pulsar distance is extremely small, the random contribution to the timing residuals can be reduced by an opportune averaging procedure.
| Reasearch area | Gravitational Waves |
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