  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. Candidate TDE are identified by searching for soft
X-ray flares from non-active galaxies in recent XMM-Newton slew data. 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~2E44 ergs/s
with a spectrum that may be modelled with thermal emission in the UV band, a
power-law with slope~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.
