Structure of the circumnuclear region of Seyfert 2 galaxies-clues from RXTE observations of NGC 4945

Details Structure of the circumnuclear region of Seyfert 2 galaxies-clues from RXTE observations of NGC 4945 Structure of the circumnuclear region of Seyfert 2 galaxies-clues from RXTE observations of NGC 4945

Dec 2001

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2001aipc..599.1023z&link_type=abstract

X-RAY ASTRONOMY: Stellar Endpoints,AGN, and the Diffuse X-ray Background. Edited by Nicholas E. White, Giuseppe Malaguti, and G

Astronomy and Astrophysics

Astronomy

Active And Peculiar Galaxies And Related Systems, Galactic Nuclei, Circumnuclear Matter, And Bulges, Radiative Transfer, Scattering, X-Ray Sources, X-Ray Bursts

Scientific paper

NGC 4945 is a nearby (~3.7 Mpc) Seyfert 2 galaxy. Our line of sight to the center is heavily absorbed (NH~3×1024 cm-2, i.e. τT~2), so that the primary hard X-rays are observed only above ~10 keV (Done, Madejski & Smith 1996; Figure 1). The mass of the central black hole, M~1.4×106 Msolar and the source inclination (i~90°, i.e. edge-on) are known thanks to observations of the water maser emission (Greenhill et al. 1997). According to unification schemes of Seyfert galaxies, the absorber could be envisioned as the molecular/dusty torus, geometrically thick (half-opening angle ~45°), extending on ~parsec scale (Antonucci 1993). In such a geometry any intrinsic source variability would be smeared out due to distribution of path lengths of photons scattered within the absorber before reaching an observer. Our recent RXTE observations revealed, however, strong variability of the hard X-rays (E>10 keV) on timescale of days (Figure 2). This discovery rules out the ``classical'' torus as the optically thick absorber in NGC 4945. The geometry of the absorber has to be such that the contribution from photons scattered towards our line of sight is negligible, or the spatial scale of the absorber is very small. This first possibility leads to a geometrically thin, disk-like absorber, i.e. a torus with a large opening (funnel) angle. Any geometrically thick, torus-like absorber would have to be located at distances << 1 light day ~104 Rg, where its density would have to be >>1013 cm-3 in order for the absorber to remain neutral, as observed. Adopting the disk-like absorber geometry we estimate the true source luminosity to be >=1.5×1043 erg s-1, i.e. L>=0.08 LEdd. .

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