Setting Upper Limits on the Local Burst Rate Density of Primordial Black Holes Using HAWC

Primordial Black Holes (PBHs) are black holes that may have been created by density fluctuations in the early Universe and could be as large as supermassive black holes or as small as the Planck scale. It is believed that a black hole has a temperature inversely proportional to its mass and will thermally emit all species of fundamental particles via Hawking Radiation. PBHs with initial masses of ~5×10^14 g should be expiring today with bursts of high-energy gamma radiation in the GeV – TeV energy range. The High Altitude Water Cherenkov (HAWC) Gamma-Ray Observatory is sensitive to the high end of the PBH burst spectrum. With its large instantaneous field of view of ~2 sr and a duty cycle above 95%, the HAWC observatory is well suited to perform an all-sky search for PBH bursts. We performed a search on ~2.6 years of HAWC data by optimizing a previous transient search to the PBH burst energy spectrum. As this search yielded a null detection, we placed competitive upper limits at the 99% confidence level on the local PBH burst rate density. Previous studies predicting HAWC’s ability to place such a limit were surpassed, as well as limits placed by similar detectors.