Where are all the pulsars at the Galactic Centre?
As stable, regularly pulsing rotators, pulsars are superb instruments for testing theories of gravity. An important test of General Relativity is the behaviour of bodies in extremely strong gravitational fields. The detection of a pulsar in the gravitational field of a black hole therefore potentially constitutes an exceptional test of theories of gravity in the strong field regime.
Astronomers have long sought to find pulsar-black hole binary systems by conducting large surveys of the Galactic pulsar population. But there is another way to find such systems: find a supermassive black hole and then search for orbiting pulsars. For this reason, the region immediately surrounding the four million solar mass black hole at the Galactic Centre, Sgr A* has been the site for a number of deep pulsar surveys over the past several decades. Yet, despite these searches, none of these searches has uncovered a single regular pulsar.
Where are all the pulsars at the Galactic Centre? In their current paper, CAASTRO Associate Investigator Jean-Pierre Macquart (Curtin University) and colleague Nissim Kanekar (National Centre for Radio Astrophysics, Pune / India) argue that the high stellar density in the central parsec around the Galactic Centre is likely to result in a pulsar population dominated by millisecond pulsars (MSPs). Earlier pulsar searches have been largely insensitive to such an MSP population, accounting for the lack of pulsar detections. We estimate the best search frequency for such an MSP population, taking into account new information on the scattering environment towards Sgr A* provided by the recently-detected magnetar near the Galactic Centre. The optimal search frequency is near 8 GHz for pulsars with periods 1-20ms, assuming that the pulsars have a luminosity distribution similar to those in the rest of the Milky Way. We find that 10-30 hour integrations with the Green Bank Telescope or the Very Large Array would be sufficient to detect MSPs at the Galactic Centre.
Interestingly, observations of the Galactic Centre at X-ray and GeV energies have, just within the last few months, independently suggested evidence for such an MSP population (Perez et al. 2015, O’Leary et al. 2015).
J.-P. Macquart and N. Kanekar in the Astrophysical Journal (2015) “On Detecting Millisecond Pulsars at the Galactic Center”