  Recent X-ray surveys by {\it Chandra} and {\it XMM-Newton} have revealed a
population of X-ray bright, optically normal galaxies (XBONGs) at moderate
redshifts. We propose that many XBONGs are powered by an inner radiatively
inefficient accretion flow (RIAF) plus an outer radiatively efficient thin
accretion disk. The absence of optical/UV activity in XBONGs is explained by
the truncation of the thin disk near the black hole, while the relatively
strong X-ray emission is explained as inverse Compton emission from the hot
RIAF. As an example, we show that the spectra of two XBONGs can be fit fairly
well with such a model. By comparing these two sources to other accreting black
holes, we argue that XBONGs are intermediate in their characteristics between
distant luminous active galactic nuclei and nearby low-luminosity nuclei.

Astrophysics