Gamma-ray burst lensing parallax: Closing the primordial black hole dark matter mass window

Abstract

The primordial black hole (PBH) comprising full dark matter (DM) abundance is currently allowed if its mass lies at 10-16M⊙≲M≲10-11M⊙. This lightest mass range is hard to be probed by ongoing gravitational lensing observations. In this paper, we advocate that an old idea of the lensing parallax of gamma-ray bursts (GRBs), observed simultaneously by spatially separated detectors, can probe the unconstrained mass range, and that of nearby stars can probe a heavier mass range. In addition to various good properties of GRBs, astrophysical separations achievable around us - r⊕- AU - are just large enough to resolve the GRB lensing by the lightest PBH DM.