On the recurrence times of neutron star X-ray binary transients and the nature of the Galactic Centre quiescent X-ray binaries

TL;DR

This paper challenges the assumption that neutron star low-mass X-ray binaries (LMXBs) have short recurrence times, showing many have long recurrence periods exceeding a decade, which impacts the interpretation of quiescent X-ray sources in the Galactic Centre.

Contribution

It demonstrates that a significant fraction of neutron star LMXBs have long recurrence times, suggesting many are yet undiscovered and affecting population estimates.

Findings

About half of known transient NS LMXBs have recurrence times over a decade.

Systems with recurrence times exceeding a century likely dominate the NS LMXB population.

Many long-recurrence NS LMXBs remain undiscovered with current instruments.

Abstract

The presence of some X-ray sources in the Galactic Centre region which show variability, but do not show outbursts in over a decade of monitoring has been used to argue for the presence of a large population of stellar mass black holes in this region. A core element of the arguments that these objects are accreting black holes is the claim that neutron stars (NSs) in low mass X-ray binaries (LMXBs) do not have long transient recurrence times. We demonstrate in this paper that about half of the known transient LMXBs with clear signatures for NS primaries have recurrence times in excess of a decade for outbursts at the sensitivity of MAXI. We furthermore show that, in order to reconcile the expected total population of NS LMXBs with the observed one and with the millisecond radio pulsar (MSRP) population of the Galaxy, systems with recurrence times well in excess of a century for outbursts detectable by instruments like MAXI must be the dominant population of NS LMXBs, and that few of these systems have yet been discovered.