Probing isolated compact remnants with microlensing

TL;DR

This paper investigates the potential of using microlensing to detect isolated compact remnants like neutron stars and black holes in the Milky Way, which are otherwise difficult to observe directly.

Contribution

The study models the distribution of ICoRs including a bulge component and assesses their microlensing signatures, highlighting their significant contribution to long-duration events.

Findings

ICoRs contribute about 6-7% to microlensing events.

Black holes account for 30-40% of events longer than 100 days.

Microlensing is a promising method to probe Galactic ICoRs.

Abstract

We consider isolated compact remnants (ICoRs), i.e. neutrons stars and black holes that do not reside in binary systems and therefore cannot be detected as X-ray binaries. ICoRs may represent $\sim\,5$ percent of the stellar mass budget of the Galaxy, but they are very hard to detect. Here we explore the possibility of using microlensing to identify ICoRs. In a previous paper we described a simulation of neutron star evolution in phase space in the Galaxy, taking into account the distribution of the progenitors and the kick at formation. Here we first reconsider the evolution and distribution of neutron stars and black holes adding a bulge component. From the new distributions we calculate the microlensing optical depth, event rate and distribution of event time scales, comparing and contrasting the case of ICoRs and "normal stars". We find that the contribution of remnants to optical depth is slightly lower than without kinematics, owing to the evaporation from the Galaxy. On the other hand, the relative contribution to the rate of events is a factor $\sim\,5$ higher. In all, $\sim\,6-7$ percent of the events are likely related to ICoRs. In particular, $\sim\,30-40$ percent of the events with duration $>\,100$ days are possibly related to black holes. It seems therefore that microlensing observations are a suitable tool to probe the population of Galactic ICoRs.