XMMSL1 J074008.2-853927: A tidal disruption event with thermal and non-thermal components

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 Lbol ∼ 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 Rg 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.