SOFT X-RAY ANALYSIS AND MULTIWAVELENGTH MODELING OF X-RAY SELECTED ACTIVE GALACTIC NUCLEI

Abstract

ABSTRACT We survey the broadband spectral properties of ~500 X-ray-selected Active Galactic Nuclei (AGN) observed with the Einstein X-ray Observatory in order to better understand the X-ray emission characteristics of this population as compared to previously studied optically- and radio-selected populations. We further select a subsample of X-ray-selected AGN which possess strong components of soft X-ray emission below ~0.5 keV. This sample is used to investigate in greater detail this ultrasoft emission and to search for possible correlations between it and emission at other wavelengths. These ultrasoft sources are used for multiwavelength accretion disk modeling in a test of the current black hole plus accretion disk paradigm of AGN energy generation. We find the mean spectral index of the AGN between 0.1 and 0.6 keV to be softer, and the distributions of indices wider, than previous estimates based on analyses of the X-ray spectra of optically-selected AGN. We confirm a correlation between radio and hard X-ray luminosity, but find that the data do not support a correlation between radio and soft X-ray spectral slope. We report evidence for physically distinct soft and hard X-ray components. Imaging photometry in the J, H, and K infrared bands is carried out for a sample of eight AGN. The infrared images show the AGN to be primarily point sources, with little extended emission from the host galaxy. The photometric measurements represent part of a multiwavelength database to be used for accretion disk modeling. We report observations of six AGN in the ultraviolet region with the International Ultraviolet Explorer satellite. The ultraviolet spectra from these observations are used for measurement of the strengths of the emission lines present (e.g., Ly-alpha, CIV, and CIII]). The data also allow a measurement of the shape of the ultraviolet continua of these AGN to be used further in multiwavelength accretion disk modeling. Observations of a sample of six optically faint, X-ray-selected AGN with the ROSAT Position Sensitive Proportional Counter are reported. The spectra of the AGN are all fit satisfactorily with single power-law models and their spectral indices (with one exception) agree with those estimated from their Einstein IPC observations. Tests of short term variability are carried out, with all sources showing constant count rates over the period of their ROSAT observations. Some sources do show significant changes in count rate when compared to previous Einstein data taken a decade earlier. The measurements of the soft X-ray spectra of these objects are combined with the infrared and ultraviolet data discussed above to carry out multiwavelength accretion disk modeling. Accretion disk modeling is carried out on a sample of five Seyfert I galaxies possessing strong optical-ultraviolet-soft X-ray emission components. It is found that while bare accretion disk models cannot account for the soft X-ray emission observed in these objects, the addition of a hot (kTe~120 keV), Comptonizing component enables the soft X-ray emission to be accounted for within the black hole plus accretion disk paradigm.