Structure of the circumnuclear region of Seyfert 2 galaxies revealed by Rossi X-ray Timing Explorer hard X-ray observations of NGC 4945

NGC 4945 is one of the brightest Seyfert galaxies on the sky at 100 keV, bu t is completely absorbed below 10 keV; its absorption column is probably th e largest that still allows a direct view of the nucleus at hard X-ray ener gies. Our observations of it with the Rossi X-Ray Timing Explorer (RXTE) sa tellite confirm the large absorption, which for a simple phenomenological f it using an absorber with solar abundances implies a column of (4.5(-0.4)(0.4)) x 10(24) cm(-2). Using a more realistic scenario (requiring Monte Car lo modeling of the scattering), we infer the optical depth to Thomson scatt ering of similar to 2.4. If such a scattering medium were to subtend a larg e solid angle from the nucleus, it should smear out any intrinsic hard X-ra y variability on timescales shorter than the light-travel time through it. The rapid (with a timescale of similar to 1 day) hard X-ray variability of NGC 4945 discovered by us with RXTE implies that the bulk of the extreme ab sorption in this object does nor originate in a parsecsize, geometrically t hick molecular torus. Instead, the optically thick material on parsec scale s must be rather geometrically thin, subtending a half-angle less than 10 d egrees, and it is likely to be the same disk of material that is responsibl e for the water maser emission observed in NGC 4945. Local number counts of Seyfert 1 and Seyfert 2 galaxies show a large population of heavily obscur ed active galactic nuclei (AGNs) which are proposed to make up the cosmic X -ray background (CXRB). However, for this to be the case, the absorption ge ometry in the context of axially symmetric unification schemes must have th e obscuring material subtending a large scale height-contrary to our infere nces about NGC 4945-implying that NGC 4945 is not a prototype of obscured A CNs postulated to make up the CXRB. The small solid angle of the absorber, together with the black hole mass (of similar to 1.4 x 10(6) M-circle dot) from megamaser measurements, allows a robust determination of the nuclear l uminosity, which in turn implies that the source radiates at similar to 10% of the Eddington limit.