Hyperaccreting black holes in galactic nuclei
  The rate at which matter flows into a galactic nucleus during early phases of
galaxy evolution can sometimes exceed the Eddington limit of the growing
central black hole by several orders of magnitude. We discuss the necessary
conditions for the black hole to actually accrete this matter at such a high
rate, and consider the observational appearance and detectability of a
hyperaccreting black hole. In order to be accreted at a hyper-Eddington rate,
the infalling gas must have a sufficiently low angular momentum. Although most
of the gas is accreted, a significant fraction accumulates in an optically
thick envelope with luminosity $\sim L_{\rm Edd}$, probably pierced by jets of
much higher power. If $\dot M > 10^3 \dot M_{\rm Edd}$, the envelope spectrum
resembles a blackbody with a temperature of a few thousand K, but for lower
(but still hyper-Eddington) accretion rates the spectrum becomes a very dilute
and hard Wien spectrum. We consider the likelihood of various regimes of
hyperaccretion, and discuss its possible observational signatures.
