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. CandidateTDEareidentifiedbysearchingforsoftX-rayflaresfromnon-activegalaxiesinrecentXMM-Newtonslewdata. Results. In April 2014, X-ray emission was detected from the galaxy XMMSL1 J074008.2-853927 (a.k.a. 2MASX 074007858539307), a factor 20 times higher than an upper limit from 20 years earlier. Both the X-ray and UV flux subsequently fell, by factorsof70and12respectively.Thebolometricluminositypeakedat Lbol ∼2×1044 ergss−1 withaspectrumthatmaybemodelled 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.