EXTraS discovery of an X-ray superflare from an L dwarf

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 E-X,E- F similar to 2x10(33) erg, placing it in the regime of superflares. No quiescent emission is detected, and from 2.5 Ms of XMM-Newton data we derive an upper limit of L-X,L- qui< 10(27) erg s(-1). The flare peak luminosity (L-X,L- peak=6.3x10(29) erg s(-1)), flare duration (tau(decay)approximate to 2400 s), and plasma temperature (approximate to 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 similar to 2100 K. However, the absence of any other flares above the detection threshold of E-X,E- F similar to 2.5x10(32) erg in a total of similar to 2.5 Ms of X-ray data yields a flare energy number distribution inconsistent with the canonical power law dN/dE similar to 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.