Two fast X-ray transients in archival Chandra data

TL;DR

This paper reports the discovery of two new X-ray transients in archival Chandra data, analyzes their properties, possible origins, and estimates their occurrence rates, contributing to understanding rare high-energy transient phenomena.

Contribution

The study identifies two new X-ray transients, characterizes their temporal and spectral features, and discusses their potential origins, including a possible tidal disruption event and a nearby stellar flare.

Findings

XRT 110103 shows a rapid rise and decay, no optical/infrared counterpart, similar to a possible tidal disruption event.

XRT 120830 exhibits a rapid X-ray flare with a likely infrared counterpart, consistent with a stellar flare from a nearby dwarf.

Estimated all-sky rate of similar transients is approximately 1.4 x 10^5 per year.

Abstract

We present the discovery of two new X-ray transients in archival Chandra data. The first transient, XRT 110103, occurred in January 2011 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 preexisting survey data or in an observation taken by the SIRIUS instrument at the Infrared Survey Facility 2.1 yr after the transient, providing limiting magnitudes of J>18.1, H>17.6 and Ks>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*10^5 per year over the whole sky with a peak 0.3-7 keV X-ray flux greater than 2*10^-10 erg cm^-2 s^-1 . The second transient, XRT 120830, occurred in August 2012 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 Ks=15.37+/-0.06 which shows an average proper motion of 74+/-19 milliarcsec per year compared to archival 2MASS observations. The JHKs 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.