AAS 205th Meeting, 9-13 January 2005
Session 62 Great Observatories Origins Deep Survey
Poster, Tuesday, January 11, 2005, 9:20am-6:30pm, Exhibit Hall

## [62.04] Examining the Nature of the Optically Variable, X-ray Undetected GOODS Sources

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.

Bulletin of the American Astronomical Society, 36 5
© 2004. The American Astronomical Society.