Searching for single-frame rapid X-ray transients detected with Chandra

We propose a new method to identify rapid X-ray transients observed with focusing telescopes. They could be statistically significant if three or more photons are detected with Chandra in a single CCD frame within a point-spread-function region out of quiescent background. In the Chandra archive, 11 such events are discovered from regions without point-like sources, after discrimination of cosmic rays and background flares and control of false positives. Among them, two are spatially coincident with extended objects in the Milky Way, one with the Small Magellanic Cloud, and another one with M31; the rest have no or a dim optical counterpart ($\gtrsim 20$ mag), and are not clustered on the Galactic plane. Possible physical origins of the rapid transients are discussed, including short gamma-ray bursts (GRBs), short-lived hypermassive neutron stars produced by merger of neutron stars, accreting compact objects in the quiescent state, magnetars, and stellar flares. According to the short GRB event rate density, we expect to have detected $2.3_{-0.6}^{+0.7}$ such events in the Chandra archive. This method would also allow us to reveal quiescent black holes with only a few photons.