NGC 4945: The Brightest Seyfert 2 Galaxy at 100 keV

Abstract

Seyfert 2 galaxies are generally heavily absorbed in soft X-rays, plausibly because of obscuration of the Seyfert nucleus by a thick molecular torus. With a high torus column, 10²³ ≥ N_H ≥ 10²⁵ cm^-2, the observed flux in the "classical" 2-10 keV X-ray band can be strongly suppressed, while at higher energies the photoelectric absorption becomes less important and the true nuclear luminosity can be seen. The high-energy bandpass of the Ginga X-ray telescope, extending to 20 keV, allowed several such objects to be discovered, and an extrapolation of the obscured nuclear spectra out to 100 keV suggested that NGC 4945 should be among the very brightest radio-quiet Seyferts in the sky at 100 keV, second only to NGC 4151. OSSE data reported here confirm this prediction, and we analyze these, together with archival Ginga and ASCA data, in order to present a broadband spectrum from 0.6-500 keV of this object.