The primordial black hole (PBH) comprising full dark matter (DM) abundance is currently allowed if its mass lies between $10^{ -16}M_{ \odot} \lesssim M \lesssim 10^{ -11} M_{ \odot}$. 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 --- $r_\oplus \text{ --}$ AU --- is just large enough to resolve the GRB lensing by lightest PBH DM.
Categories: astro-ph.CO gr-qc hep-ph