Searching for Primordial Black Holes in the Radio and X-Ray Sky

We model the accretion of gas onto a population of massive primordial black holes in the Milky Way and compare the predicted radio and x-ray emission with observational data. We show that, under conservative assumptions on the accretion process, the possibility that $\mathcal{O}(10)M_{\odot }$ primordial black holes can account for all of the dark matter in the Milky Way is excluded at $5\sigma$ by a comparison with a Very Large Array radio catalog at 1.4 GHz and at $\simeq 40\sigma$ by a comparison with a Chandra x-ray catalog (0.5–8 keV). We argue that this method can be used to identify such a population of primordial black holes with more sensitive future radio and x-ray surveys.

Figure 1

Upper limits on the fraction of DM in PBHs of a given mass $M$, arising from the nonobservation of bright x-ray (blue shaded regions) and radio (red) BH candidates at the GC. We assume a conservative value of $\lambda$, regulating the departure from the Bondi accretion rate: $\lambda =0.02$. The dotted gray line corresponds to $30M_{\odot }$ PBH, and the hatched gray region is unphysical ($f_{\mathrm{DM}}>1$).

Figure 2

Example of the distribution of $30M_{\odot }$ PBHs detectable by VLA in the ROI, for one Monte Carlo realization. The colored background depicts the column gas density. The size of the black points is proportional to the PBH velocity in the range $0.3–3\mathrm{km}/\mathrm{s}$ (for detectable PBHs).