A Full Year's Chandra Exposure on SDSS Quasars from the Chandra Multiwavelength Project

TL;DR

This study analyzes a large sample of SDSS quasars observed by Chandra, revealing no evolution in X-ray properties up to redshift 5, and identifying correlations between X-ray and optical characteristics.

Contribution

It provides the first comprehensive analysis of X-ray spectral properties and evolution of SDSS quasars over a broad redshift range using extensive Chandra data.

Findings

No evidence for evolution of X-ray photon index or optical/UV to X-ray flux ratio up to z~5.

Approximately 10% of quasars are obscured, possibly up to one-third considering non-detections.

Gamma hardness correlates with X-ray luminosity and X-ray loudness, indicating increased non-thermal emission.

Abstract

We study the spectral energy distributions and evolution of a large sample of optically selected quasars from the Sloan Digital Sky Survey (SDSS) that were observed in 323 Chandra images analyzed by the Chandra Multiwavelength Project (ChaMP). Our highest-confidence matched sample includes 1135 X-ray detected quasars in the redshift range 0.2<z<5.4, representing some 36Msec of effective exposure. Spectroscopic redshifts are available for about 1/3 of the detected sample; elsewhere, redshifts are estimated photometrically. With 56 z>3 QSOs detected, we find no evidence for evolution out to z~5 for either the X-ray photon index Gamma or for the ratio of optical/UV to X-ray flux alpha_ox. About 10% of detected QSOs are obscured (Nh>1E22), but the fraction might reach ~1/3 if most non-detections are absorbed. We confirm a significant correlation between alpha_ox and optical luminosity, but it flattens or disappears for fainter AGN alone. Gamma hardens significantly both towards higher X-ray luminosity, and for relatively X-ray loud quasars. These trends may represent a relative increase in non-thermal X-ray emission, and our findings thereby strengthen analogies between Galactic black hole binaries and AGN.