We present the discovery, photometric, and spectroscopic follow-up
observations of SN 2010X (PTF 10bhp). This supernova decays
exponentially with τ d = 5 days and rivals the current
recordholder in speed, SN 2002bj. SN 2010X peaks at M r =
-17 mag and has mean velocities of 10,000 km s-1.
Our light curve modeling suggests a radioactivity-powered event and an
ejecta mass of 0.16 M sun. If powered by Nickel, we show that
the Nickel mass must be very small (≈0.02 M sun) and that
the supernova quickly becomes optically thin to γ-rays. Our
spectral modeling suggests that SN 2010X and SN 2002bj have similar
chemical compositions and that one of aluminum or helium is present. If
aluminum is present, we speculate that this may be an accretion-induced
collapse of an O-Ne-Mg white dwarf. If helium is present, all
observables of SN 2010X are consistent with being a thermonuclear helium
shell detonation on a white dwarf, a ".Ia" explosion. With the 1 day
dynamic-cadence experiment on the Palomar Transient Factory, we expect
to annually discover a few such events.
