The X‐Ray Reflectors in the Nucleus of the Seyfert Galaxy NGC 1068

Abstract

Based on observations of the Seyfert nucleus in NGC 1068 with ASCA, RXTE, and BeppoSAX, we report the discovery of a flare (increase in flux by a factor of $1.6) in the 6.7 keV Fe K line component between observations obtained 4 months apart, with no significant change in the other (6.21, 6.4, and 6.97 keV) Fe K line components. During this time, the continuum flux decreased by $20%. The RXTE spectrum requires an Fe K absorption edge near 8.6 keV (Fe xxiii-xxv). The spectral data indicate that the 2-10 keV continuum emission is dominated ($ 2 3 of the luminosity) by reflection from a previously unidentified region of warm, ionized gas located d0.2 pc from the AGN. The remaining $ 1 3 of the observed X-ray emission is reflected from optically thick, neutral gas. The coronal gas in the inner narrow-line region and/or the cold gas at the inner surface of the obscuring '' torus '' are possible cold reflectors. The inferred properties of the warm reflector are size (diameter) d0.2 pc, gas density ne10 5:5 cm À3 , ionization parameter % 10 3:5 ergs cm s À1 , and covering fraction 0:003ðL 0 =10 43:5 ergs s À1 Þ À1 < ð=4Þ < 0:024ðL 0 =10 43:5 ergs s À1 Þ À1 , where L 0 is the intrinsic 2-10 keV X-ray luminosity of the AGN. We suggest that the warm reflector gas is the source of the (variable) 6.7 keV Fe line emission and the 6.97 keV Fe line emission. The 6.7 keV line flare is assumed to be due to an increase in the emissivity of the warm reflector gas from a decrease (by 20%-30%) in L 0. The properties of the warm reflector are most consistent with an intrinsically X-ray-weak AGN with L 0 % 10 43:0 ergs s À1. The optical and UV emission that scatters from the warm reflector into our line of sight is required to suffer strong extinction, which can be reconciled if the line of sight skims the outer surface of the torus. Thermal brems-strahlung radio emission from the warm reflector may be detectable in Very Long Baseline Array radio maps of the NGC 1068 nucleus.