XMMSL1 J074008.2-853927: a tidal disruption event with thermal and non-thermal components^⋆

¹ XMM SOC, ESAC, Apartado 78, 28691 Villanueva de la Cañada, Madrid, Spain
e-mail: richard.saxton@sciops.esa.int
² Dept. of Physics and Astronomy, University of Leicester, Leicester LE1 7RH, UK
³ QianNan Normal University for Nationalities, Longshan Street, Duyun City of Guizhou Province, PR China
⁴ Max Planck Institut für Radioastronomie, Auf dem Huegel 69, 53121 Bonn, Germany
⁵ Universidad de Chile, Observatorio Astronomico Nacional Cerro Calan, Santiago, Chile
⁶ Harvard-Smithsonian Center for Astrophysics, 60 Garden St., Cambridge, MA 02138, USA
⁷ Australia Telescope National Facility, CSIRO Astronomy and Space Science, PO Box 76, Epping, NSW 1710, Australia

Received: 27 May 2016
Accepted: 29 September 2016

Abstract

Aims. We study X-ray bright tidal disruption events (TDE), close to the peak of their emission, with the intention of understanding the evolution of their light curves and spectra.

Methods. Candidate TDE are identified by searching for soft X-ray flares from non-active galaxies in recent XMM-Newton slew data.

Results. In April 2014, X-ray emission was detected from the galaxy XMMSL1 J074008.2-853927 (a.k.a. 2MASX 07400785-8539307), a factor 20 times higher than an upper limit from 20 years earlier. Both the X-ray and UV flux subsequently fell, by factors of 70 and 12 respectively. The bolometric luminosity peaked at L_bol ~ 2 × 10⁴⁴ ergs s^-1 with a spectrum that may be modelled with thermal emission in the UV band, a power-law with Γ ~ 2 dominating in the X-ray band above 2 keV and a soft X-ray excess with an effective temperature of ~86 eV. Rapid variability locates the X-ray emission to within <73 R_g of the nuclear black hole. Radio emission of flux density ~1 mJy, peaking at 1.5 GHz was detected 21 months after discovery. Optical spectra indicate that the galaxy, at a distance of 73 Mpc (z = 0.0173), underwent a starburst 2 Gyr ago and is now quiescent. We consider a tidal disruption event to be the most likely cause of the flare. If this proves to be correct then this is a very clean example of a disruption exhibiting both thermal and non-thermal radiation.