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 × 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.
Examining the Nature of the Optically Variable, X-ray Undetected GOODS Sources / Steffen, A. T.; Brandt, W. N.; Grogin, N. A.; Koekemoer, A. M.; Paolillo, Maurizio; Schneider, D. P.; Schreier, E. J.; THE GOODS, Team. - STAMPA. - 36:(2004), pp. 1448-1455. (Intervento presentato al convegno American Astronomical Society Meeting 2005 tenutosi a San Diego nel 2004).
Examining the Nature of the Optically Variable, X-ray Undetected GOODS Sources
PAOLILLO, MAURIZIO;
2004
Abstract
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 × 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.I documenti in IRIS sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.
