Gravitational Microlensing of the Galactic Centre $\gamma$-Ray Excess: A New Test for Point-Like or Extended Emission?

TL;DR

This paper proposes using gravitational microlensing to distinguish whether the gamma-ray excess at the Galactic Centre is due to dark matter or unresolved point sources like pulsars, though current detection sensitivity is insufficient.

Contribution

It introduces a novel method to test the origin of the gamma-ray excess via microlensing-induced variability, highlighting the sensitivity requirements for future gamma-ray observations.

Findings

Microlensing can differentiate between dark matter and point sources based on variability.

Current gamma-ray detectors lack the sensitivity to detect microlensing effects.

Significant improvements in detector sensitivity are needed for this method to be feasible.

Abstract

We present a potential test of the origin of the $\gamma$-ray Galactic Centre Excess (GCE). We demonstrate how gravitational microlensing by stellar mass objects along the line of sight to the Galactic Bulge can distinguish between the possibility of extensive emission due to dark matter self-annihilation from more prosaic astrophysical sources, namely millisecond pulsars. Such an astrophysical origin would result in emission from a population of small, currently unresolved point-like sources - in contrast to the expected smoother emission resulting from dark matter annihilation. Given that the scale of gravitational microlensing, that is, the Einstein radius for stellar mass lenses, and hence, the degree of induced magnification, is sensitive to the size of the emitting region, such microlensing will induce time variability in the emission of astrophysical sources, whereas $\gamma$-ray emission from dark matter annihilation will effectively be immune to such influences. However, we find that detecting microlensing-induced variability requires significantly greater sensitivity than that of current or planned $\gamma$-ray detectors. For a small population of bright GCE sources, more than an order-of-magnitude increase in effective area over Fermi-LAT would be required, with events remaining extremely rare. For a large population of faint sources, events would occur multiple times a year, but would only be detectable with a four-order-of-magnitude improvement. Whilst microlensing might not be a definitive test of the origin of the GCE, in future observations, it may prove useful in determining the properties of any point-like source population.