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A. T. Steffen, W. N. Brandt (PSU), N. A. Grogin (JHU), A. M. Koekemoer (STScI), M. Paolillo (U. of Naples), D. P. Schneider (PSU), E. J. Schreier (STScI), GOODS Team
Deep X-ray surveys using Chandra have found the highest density of AGN on the sky (~5000 deg-2) and are thought to be the most efficient means of finding AGN. However, a recent two-epoch variability analysis of the HDF-N found comparable sky densities of sources with variable nuclei, which strongly suggest the presence of an AGN. There is only a small amount of overlap between the optically variable (OV) and X-ray selected AGN, which indicates that the majority of these optically variable selected AGN are underluminous in the X-ray band.
In this poster we investigate the nature of the X-ray faint, optically variable GOODS source population. We employ X-ray stacking techniques to measure the average X-ray photometric and spectroscopic properties of these sources. A comparison between the (stacked) X-ray undetected and X-ray detected OV sources shows the hardness ratios of the two populations differ with redshift. The X-ray undetected OV population appears to get harder with redshift, while the hardness ratios of the X-ray detected OV sources remain relatively unchanged. This is consistent with the X-ray undetected OV population being comprised of moderately Compton thick sources (NHI ~2 \times 1024 cm-2) where, at higher redshifts, the >10 keV photons that can penetrate this column can be seen in the hard (2-8 keV) X-ray band. In addition, we combine the optical spectra, when available, to look for the presence of high ionization lines that are indicative of AGN. We fail to find high-ionization lines in the stacked optical spectra of the X-ray undetected, optically variable sources, but we do find weak NeIII, NeV, and MgII lines in the stacked X-ray detected, OV population.
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Bulletin of the American Astronomical Society, 36 5
© 2004. The American Astronomical Society.