Direct Detection of Hawking Radiation from Asteroid-Mass Primordial Black Holes

Light, asteroid-mass primordial black holes, with lifetimes in the range between hundreds to several millions times the age of the Universe, are well-motivated candidates for the cosmological dark matter. Using archival COMPTEL data, we improve over current constraints on the allowed parameter space of primordial black holes as dark matter by studying their evaporation to soft gamma rays in nearby astrophysical structures. We point out that a new generation of proposed MeV gamma-ray telescopes will offer the unique opportunity to directly detect Hawking evaporation from observations of nearby dark matter dense regions and to constrain, or discover, the primordial black hole dark matter.

Figure 1

The effective area, as a function of energy, of existing and proposed MeV gamma-ray telescopes. Thin lines mark existing telescopes and thick lines mark proposed ones. The effective area of MAST (not shown) ranges from $\sim 7\times {10}^4–{10}^5{\mathrm{cm}}^2$ over $E_{\gamma }=10\mathrm{MeV}–60\mathrm{GeV}$.

Figure 2

Photon emission from light black hole evaporation. We consider Hawking temperatures of 20, 3, 0.3, and 0.06 MeV (from top left to bottom right), corresponding to masses $M=5.3\times {10}^{14},3.5\times {10}^{15},3.5\times {10}^{16},1.8\times {10}^{17}\mathrm{g}$. The thick blue lines show the spectra computed in this work; the dashed red curves correspond to the primary (thin lines) and secondary-plus-primary (thick lines) output from BlackHawk. We also show contributions from ${\pi }^0$ decay (magenta dotted lines) and from final state radiation off of electrons and muons (dot dashed yellow and magenta lines) and charged pions (dotted green lines). In the two lower panels we also show results from adopting the geometric optics approximation for the gray-body factors, while in the top-left panel we show the results of Ref. [55], which did not include final-state radiation.

Figure 3

New bounds on the PBH abundance based on COMPTEL observations of the Milky Way. Assuming the PBHs follow an Einasto rather than NFW distribution gives a slightly stronger bound. Existing bounds collected in Ref. [7] from INTEGRAL observations of galactic diffuse emission [22], CMB [8, 9], EDGES 21 cm [17], Voyager 1 [19], the 511 keV gamma-ray line [20, 21], the extragalatic gamma-ray background [16] and dwarf galaxy heating [18] are also shown.

Figure 4

Discovery reach for PBHs with future MeV gamma ray telescopes. In the top row, the PBH density is assumed to track an Einasto density profile (left) and an NFW profile (right) fit to the Milky Way’s DM distribution. In the lower panels we consider the Draco dwarf spheroidal and M31 as targets. For all targets, we assume a 5° circle around the central region, and an observation time of ${10}^8\mathrm{s}$.