  Stars that pass within the Roche radius of a supermassive black hole will be
tidally disrupted, yielding a sudden injection of gas close to the black hole
horizon which produces an electromagnetic flare. A few dozen of these flares
have been discovered in recent years, but current observations provide poor
constraints on the bolometric luminosity and total accreted mass of these
events. Using images from the Wide-field Infrared Survey Explorer (WISE), we
have discovered transient 3.4 micron emission from several previously known
tidal disruption flares. The observations can be explained by dust heated to
its sublimation temperature due to the intense radiation of the tidal flare.
From the break in the infrared light curve we infer that this hot dust is
located ~0.1 pc from the supermassive black hole. Since the dust has been
heated by absorbing UV and (potentially) soft X-ray photons of the flare, the
reprocessing light curve yields an estimate of the bolometric flare luminosity.
For the flare PTF-09ge, we infer that the most likely value of the luminosity
integrated over frequencies at which dust can absorb photons is $8\times
10^{44}$ erg/s, with a factor of 3 uncertainty due to the unknown temperature
of the dust. This bolometric luminosity is a factor ~10 larger than the
observed black body luminosity. Our work is the first to probe dust in the
nuclei of non-active galaxies on sub-parsec scales. The observed infrared
luminosity implies a covering factor ~1% for the nuclear dust in the host
galaxies.
