EXTraS discovery of an X-ray superflare from an L dwarf
Abstract
We present the first detection of an X-ray flare from an ultracool dwarf of spectral class L. The event was identified in the EXTraS database of XMM-Newton variable sources, and its optical counterpart, J0331-27, was found through a cross-match with the Dark Energy Survey Year 3 release. Next to an earlier four-photon detection of Kelu-1, J0331-27 is only the second L dwarf detected in X-rays, and much more distant than other ultracool dwarfs with X-ray detections (photometric distance of 240 pc). From an optical spectrum with the VIMOS instrument at the VLT, we determine the spectral type of J0331-27 to be L1. The X-ray flare has an energy of EX,F ~ 2x1033 erg, placing it in the regime of superflares. No quiescent emission is detected, and from 2.5 Msec of XMM data we derive an upper limit of LX,qui < 1027 erg/s. The flare peak luminosity LX,peak = 6.3x1029 erg/s, flare duration taudecay ~ 2400 s, and plasma temperature (~16 MK) are similar to values observed in X-ray flares of M dwarfs. This shows that strong magnetic reconnection events and the ensuing plasma heating are still present even in objects with photospheres as cool as ~2100 K. However, the absence of any other flares above the detection threshold of EX,F ~2.5x1032 erg in a total of ~2.5 Ms of X-ray data yields a flare energy number distribution inconsistent with the canonical power law dN/dE ~ E-2, suggesting that magnetic energy release in J0331-27 -- and possibly in all L dwarfs -- takes place predominantly in the form of giant flares.