Title: New X-ray bound on density of primordial black holes,
Abstract: We set a new upper limit on the abundance of primordial black holes (PBH)
based on existing X-ray data. PBH interactions with interstellar medium should
result in significant fluxes of X-ray photons, which would contribute to the
observed number density of compact X-ray objects in galaxies. The data
constrain PBH number density in the mass range from a few $M_\odot$ to $2\times
10^7 M_\odot$. PBH density needed to account for the origin of black holes
detected by LIGO is marginally allowed.