Multi-wavelength astronomical searches for primordial black holes

If primordial black holes of O(1–100) M_⊙ constitute a significant portion of the dark matter in the Universe, they should be very abundant in our Galaxy. We present here a detailed analysis of the radio and X-ray emission that these objects are expected to produce due to the accretion of gas from the interstellar medium. With respect to previous studies, we relax the assumption of a monochromatic mass function, and introduce an improved treatment of the physics of gas accretion onto isolated, moving compact objects, based on a set of state-of-the-art numerical simulations. By comparing our predictions with known radio and X-ray sources in the Galactic center region, we show that the maximum relic density of primordial black holes in the mass range of interest is ~ 10^(−3) smaller than that of dark matter. The new upper bound is two orders of magnitude stronger with respect to previous results, based on a conservative phenomenological treatment of the accretion physics. We also provide a comprehensive critical discussion on the reliability of this bound, and on possible future developments in the field. We argue in particular that future multi-wavelength searches will soon start to probe the galactic population of astrophysical black holes.