Search for Primordial Black Holes with SGARFACE

The Short GAmma Ray Front Air Cherenkov Experiment (SGARFACE) uses the Whipple 10 m telescope to search for bursts of $\gamma$ rays. SGARFACE is sensitive to bursts with duration from a few ns to $\sim$20 $\mu$s and with $\gamma$-ray energy above 100 MeV. SGARFACE began operating in March 2003 and has collected 2.2 million events during an exposure time of 2267 hours. A search for bursts of $\gamma$ rays from explosions of primordial black holes (PBH) was carried out. A Hagedorn-type PBH explosion is predicted to be visible within 60 pc of Earth. Background events were caused by cosmic rays and by atmospheric phenomena and their rejection was accomplished to a large extent using the time-resolved images. No unambiguous detection of bursts of $\gamma$ rays could be made as the remaining background events mimic the expected shape and time development of bursts. Upper limits on the PBH explosion rate were derived from the SGARFACE data and are compared to previous and future experiments. We note that a future array of large wide-field air-Cherenkov telescopes equipped with a SGARFACE-like trigger would be able to operate background-free with a 20 to 30 times higher sensitivity for PBH explosions.