The ASCA X-ray spectrum of ARP 102B and evaluation of simple models for its associated, metastable Fe II absorber

We have observed the broad-line radio galaxy Arp 102B with ISCHIA in order to determine the absorbing column density toward its X-ray source and measure its X-ray spectrum. The ultimate goal was to constrain the properties of the medium responsible for the metastable Fe II absorption lines observed in the ultraviolet spectrum of this object. The 0.5-10 keV X-ray spectrum is best described by a simple power-law model of photon index 1.58 ± 0.04 modified by photoelectric absorption with an equivalent hydrogen column density of (2.8 ± 0.3) × 10²¹ cm^-2. An Fe Kα line is not detected with an upper limit to its equivalent width of 200 eV, assuming that its FWHM is 60,000 km s^-1. Using the column density measured from the X-ray spectrum and the observed spectral energy distribution as constraints, we explore simple (single-zone, constant-density) photoionization models for the absorber for a wide range of densities and ionization parameters in an effort to reproduce the strengths of the ultraviolet absorption lines. We find that densities of at least 10 ¹¹ cm^-3 are needed. However, a single ionization parameter cannot explain all of the observed lines. An ionization parameter between 10^-2.5 and 10^-3.5 is needed to explain the Mg and Fe lines and the soft X-ray absorption, but the observed lines from Si, C, Al, and H require different density-ionization parameter combinations. According to the models, such an absorbing medium must be located very close to the source of ionizing radiation (within 5000 gravitational radii) and must be very compact. As such, the properties of this absorbing medium differ from those of more luminous quasars but are reminiscent of the absorber in the Seyfert galaxy NGC 4151. We suggest that the absorber is in the form of thin sheets or filaments embedded in an outflowing wind that overlays the accretion disk of Arp 102B. This picture is consistent with all of the available constraints on the central engine of this object. In an appendix, we present the X-ray spectrum of the source MS 1718.6+4902, which happened to fall within the field of view of the ROSAT Position Sensitive Proportional Counter and the ISCHIA Gas Imaging Spectrometer during the observations of Arp 102B.