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

DE LUCA, AndreaAndreaDE LUCASTELZER, BEATEBEATESTELZERBurgasser, Adam J.Adam J.BurgasserPizzocaro, DanieleDanielePizzocaroRanalli, PieroPieroRanalliRaetz, StefanieStefanieRaetzMARELLI, MARTINOMARTINOMARELLINovara, GiovanniGiovanniNovaraVIGNALI, CRISTIANCRISTIANVIGNALIBELFIORE MONDONI, AndreaAndreaBELFIORE MONDONIESPOSITO, PAOLOPAOLOESPOSITOFRANZETTI, PAOLOPAOLOFRANZETTIFUMANA, MarcoMarcoFUMANAGILLI, RobertoRobertoGILLISALVATERRA, RubenRubenSALVATERRATIENGO, ANDREAANDREATIENGO2022-02-182022-02-1820200004-6361http://hdl.handle.net/20.500.12386/31415We 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 ˜ 2 × 1033 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 LX, qui < 1027 erg s-1. The flare peak luminosity (LX, peak = 6.3 × 1029 erg s-1), flare duration (τdecay ≈ 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 ˜ 2.5 × 1032 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.STAMPAenEXTraS discovery of an X-ray superflare from an L dwarfArticle10.1051/0004-6361/201937163https://www.aanda.org/articles/aa/full_html/2020/02/aa37163-19/aa37163-19.html2020A&A...634L..13DFIS/05 - ASTRONOMIA E ASTROFISICAERC sectors::Physical Sciences and Engineering::PE9 Universe sciences: astro-physics/chemistry/biology; solar systems; stellar, galactic and extragalactic astronomy, planetary systems, cosmology, space science, instrumentation