Letter to the Editor

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

¹ INAF – Istituto di Astrofisica Spaziale e Fisica Cosmica Milano, Via A. Corti 12, 20133 Milano, Italy
e-mail: andrea.deluca@inaf.it
² INFN, Sezione di Pavia, Via A. Bassi 6, 27100 Pavia, Italy
³ Institut für Astronomie and Astrophysik Tübingen, Eberhard-Karls Universität Tübingen, Sand 1 72076, Germany
⁴ INAF – Osservatorio Astronomico di Palermo, Piazza del Parlamento 1, 90134 Palermo, Italy
⁵ Center for Astrophysics and Space Science, University of California San Diego, 9500 Gilman Drive, La Jolla, CA 92092, USA
⁶ Combient Mix AB, Kyrkogatan 22, 41115 Gothenburg, Sweden
⁷ Scuola Universitaria Superiore IUSS Pavia, Piazza della Vittoria 15, 27100 Pavia, Italy
⁸ Dipartimento di Fisica e Astronomia, Università degli Studi di Bologna, Via P. Gobetti 93/2, 40129 Bologna, Italy
⁹ INAF – Osservatorio di Astrofisica e Scienza dello Spazio di Bologna, Via P. Gobetti 93/3, 40129 Bologna, Italy

Received: 21 November 2019
Accepted: 7 January 2020

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 E_X, 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 L_X, qui <  10²⁷ erg s⁻¹. The flare peak luminosity (L_X, peak = 6.3 × 10²⁹ erg s⁻¹), flare duration (τ_decay ≈ 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 E_X, 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.