GRB lensing parallax: Closing primordial black hole dark matter mass
  window
  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.
