XMMSL2 J144605.0+685735 : a slow tidal disruption event
  Aims. We investigate the evolution of X-ray selected tidal disruption events.
Methods. New events are found in near-real time data from XMM-Newton slews and
are monitored by multi-wavelength facilities. Results. In August 2016, X-ray
emission was detected from the galaxy XMMSL2 J144605.0+685735 (a.k.a. 2MASX
14460522+6857311), a factor 20 times higher than an upper limit from 25 years
earlier. The X-ray flux was flat for ~100 days and then fell by a factor 100
over the following 500 days. The UV flux was stable for the first 400 days
before fading by a magnitude, while the optical (U,B,V bands) have been roughly
constant for 850 days. Optically, the galaxy appears to be quiescent, at a
distance of $127\pm{4}$ Mpc (z=$0.029\pm{0.001}$) with a spectrum consisting of
a young stellar population of age 1-5 Gyr, an older population and a total
stellar mass of ~6 x $10^{9}$ solar masses. The bolometric luminosity peaked at
L bol ~ $10^{43}$ ergs s$^{-1}$ with an X-ray spectrum that may be modeled by a
power-law of $\Gamma$~2.6 or Comptonisation of a low-temperature thermal
component by thermal electrons. We consider a tidal disruption event to be the
most likely cause of the flare. Radio emission was absent in this event down to
< 10$\mu$Jy, which limits the total energy of a hypothetical off-axis jet to E
< 5 x $10^{50}$ ergs. The independent behaviour of the optical, UV and X-ray
light curves challenges models where the UV emission is produced by
reprocessing of thermal nuclear emission or by stream-stream collisions. We
suggest that the observed UV emission may have been produced from a truncated
accretion disk and the X-rays from Compton upscattering of these disk photons.
