Capability of Cherenkov telescopes to observe ultra-fast optical flares

The large optical reflector (∼100 m2) of a H.E.S.S. Cherenkov telescope was used to search for very fast optical transients of astrophysical origin. Forty-three hours of observations targeting stellar-mass black holes and neutron stars were obtained using a dedicated photometer with microsecond time-resolution. The photometer consists of seven photomultiplier tube pixels: a central one to monitor the target and a surrounding ring of six pixels to veto background events. The light curves of all pixels were recorded continuously and were searched offline with a matched-filtering technique for flares with a duration of 2 μs–100 ms. As expected, many unresolved (<3 μs) and many long (>500 μs) background events originating in the earth’s atmosphere were detected. In the time range 3–500 μs the measurement is essentially background-free, with only eight events detected in 43 h; five from lightning and three presumably from a piece of space debris. The detection of flashes of brightness ∼0.1 Jy and only 20 μs duration from the space debris shows the potential of this setup to find rare optical flares on timescales of tens of microseconds. This timescale corresponds to the light crossing time of stellar-mass black holes and neutron stars.