Femtolensing by Dark Matter Revisited

TL;DR

This paper critically reevaluates femtolensing of gamma-ray bursts as a method to detect small dark matter objects, highlighting limitations due to source size and wave effects, and discusses future observational prospects.

Contribution

It provides a comprehensive analysis of femtolensing constraints, accounting for source size and wave optics, and revises previous bounds on primordial black hole dark matter.

Findings

Most GRBs are unsuitable for femtolensing due to their large sizes.

Previous femtolensing bounds on primordial black holes are not robust.

Future observations could still constrain small dark matter objects if fast-variable GRBs are identified.

Abstract

Femtolensing of gamma ray bursts (GRBs) has been put forward as an exciting possibility to probe exotic astrophysical objects with masses below $10^{-13}$ solar masses such as small primordial black holes or ultra-compact dark matter minihalos, made up for instance of QCD axions. In this paper we critically review this idea, properly taking into account the extended nature of the source as well as wave optics effects. We demonstrate that most GRBs are inappropriate for femtolensing searches due to their large sizes. This removes the previous femtolensing bounds on primordial black holes, implying that vast regions of parameter space for primordial black hole dark matter are not robustly constrained. Still, we entertain the possibility that a small fraction of GRBs, characterized by fast variability can have smaller sizes and be useful. However, a large number of such bursts would need to be observed to achieve meaningful constraints. We study the sensitivity of future observations as a function of the number of detected GRBs and of the size of the emission region.