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 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.

Data for this object are available within the Open TDE Catalog at https://tde.space/tde/XMMSL1 J0740-85