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 ^ { -1 } .
At the galactic center a source of L ( 0.3–8 keV ) = 6 \times 10 ^ { 38 } erg s ^ { -1 } 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 ^ { \prime \prime } , 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 .