The soft X-ray spectrum from NGC 1068 observed with LETGS on Chandra

Abstract

Using the combined spectral and spatial resolving power of the Low Energy Transmission Grating (LETGS) on board Chandra, we obtain separate spectra from the bright central source of NGC 1068 (Primary region), and from a fainter bright spot 4 to the NE (Secondary region). Both spectra are dominated by line emission from H- and He-like ions of C through S, and from Fe L-shell ions, but also include narrow radiative recombination continua, indicating that most of the soft X-ray emission arises in low-temperature (kT few eV) photoionized plasma. We confirm the conclusions of Kinkhabwala et al. (2002), based on XMM-Newton RGS observations, that the entire nuclear spectrum can be explained by recombination/radiative cascade following photoionization, and radiative decay following photoexcitation, with no evidence for hot, collisionally ionized plasma. In addition, this model also provides an excellent fit to the spectrum of the Secondary region, albeit with radial column densities a factor of three lower, as would be expected given its distance from the source of the ionizing continuum. The remarkable overlap and kinematical agreement of the optical and X-ray line emission, coupled with the need for a distribution of ionization parameter to explain the X-ray spectra, collectively imply the presence of a distribution of densities (over a few orders of magnitude) at each radius in the ionization cone. Relative abundances of all elements are consistent with Solar abundance, except for N, which is 2-3 times Solar. The long wavelength spectrum beyond 30 A is rich of L-shell transitions of Mg, Si, S, and Ar, and M-shell transitions of Fe. The velocity dispersion decreases with increasing ionization parameter, as deduced from these long wavelength lines and the Fe-L shell lines.