  The relatively nearby (distance=24.1 Mpc) elliptical galaxy NGC821, hosting a
central massive black hole but inactive at all wavelengths, was observed with
Chandra for a total exposure of 230 ksec, to search for nuclear emission and
gas available for accretion. Within its optical image, 41 sources were
detected, with spectral properties typical of low mass X-ray binaries (LMXBs).
The fractions of LMXBs in the field and in globular clusters were determined,
together with their X-ray luminosity function (XLF) down to L(0.3-8
keV)=2\times 10^{37} erg/s. At the galactic center a source of L(0.3-8
keV)=6\times 10^{38} erg/s was detected for the first time, slightly extended.
Its spectral shape is quite hard (\Gamma=1.49^{+0.14}_{-0.13}), without
intrinsic absorption. It is surrounded by three sources with spectral shape
typical of LMXBs and luminosities on the brightest end of the XLF. One is
consistent with being pointlike; the others could be the superposition of few
point sources and/or truly diffuse emission, with one resembling a jet-like
feature. Diffuse emission was detected out to R\sim 30'', and comes mostly from
unresolved LMXBs, with a minor contribution from other types of stellar
sources. Different lines of investigation consistently provide no evidence for
hot gas. Hydrodynamical simulations show that stellar mass losses are driven
out of NGC821 in a wind sustained by type Ia supernovae, but also hot accreting
gas within a very small inner region. A companion paper presents further
observational results from $Spitzer$ and the VLA, and possible accretion
modalities for this central massive black hole.
