Two fast X-ray transients in archival Chandra data

We present the discovery of two new X-ray transients in archival Chandra data. The first transient, XRT 110103, occurred in 2011 January and shows a sharp rise of at least three orders of magnitude in count rate in less than 10 s, a flat peak for about 20 s and decays by two orders of magnitude in the next 60 s. We find no optical or infrared counterpart to this event in pre-existing survey data or in an observation taken by the Simultaneous-3color InfraRed Imager for Unbiased Survey (SIRIUS) instrument at the Infrared Survey Facility similar to 2.1 yr after the transient, providing limiting magnitudes of J > 18.1, H > 17.6 and K-s > 16.3. This event shows similarities to the transient previously reported in Jonker et al. which was interpreted as the possible tidal disruption of a white dwarf by an intermediate-mass black hole. We discuss the possibility that these transients originate from the same type of event. If we assume these events are related a rough estimate of the rates gives 1.4 x 10(5) per year over the whole sky with a peak 0.3-7 keV X-ray flux greater than 2 x 10(-10) erg cm(-2) s(-1). The second transient, XRT 120830, occurred in 2012 August and shows a rise of at least three orders of magnitude in count rate and a subsequent decay of around one order of magnitude all within 10 s, followed by a slower quasi-exponential decay over the remaining 30 ks of the observation. We detect a likely infrared counterpart with magnitudes J = 16.70 +/- 0.06, H = 15.92 +/- 0.04 and K-s = 15.37 +/- 0.06 which shows an average proper motion of 74 +/- 19 mas yr(-1) compared to archival 2MASS observations. The JHK(s) magnitudes, proper motion and X-ray flux of XRT 120830 are consistent with a bright flare from a nearby late M or early L dwarf.