Gravitational Microlensing by Neutron Stars and Radio Pulsars: Event Rates, Timescale Distributions, and Mass Measurements

TL;DR

This paper studies gravitational microlensing caused by neutron stars and radio pulsars, estimating event rates, timescale distributions, and how such events can be used to measure pulsar masses with high precision.

Contribution

It provides new estimates of microlensing event rates by neutron stars and demonstrates the potential for precise pulsar mass measurements through microlensing observations.

Findings

Event rate of ~0.2 per year for all-sky surveys.

Neutron star microlensing events are shorter than previously thought.

Neutron stars contribute significantly to short-duration microlensing events.

Abstract

We investigate properties of Galactic microlensing events in which a stellar object is lensed by a neutron star. For an all-sky photometric microlensing survey, we determine the number of lensing events caused by $\sim10^{5}$ potentially-observable radio pulsars to be $\sim0.2\ \rm{yr^{-1}}$ for $10^{10}$ background stellar sources. We expect a few detectable events per year for the same number of background sources from an astrometric microlensing survey. We show that such a study could lead to precise measurements of radio pulsar masses. For instance, if a pulsar distance could be constrained through radio observations, then its mass would be determined with a precision of $\sim10\%$. We also investigate the time-scale distributions for neutron star events, finding that they are much shorter than had been previously thought. For photometric events towards the Galactic centre that last $\sim15$ days, around $7\%$ will have a neutron star lens. This fraction drops rapidly for longer time-scales. Away from the bulge region we find that neutron stars will contribute $\sim40\%$ of the events that last less than $\sim10$ days. These results are in contrast to earlier work which found that the maximum fraction of neutron star events would occur on time-scales of hundreds of days.