Exploratory Study of the X-Ray Properties of Quasars With Intrinsic Narrow Absorption Lines

TL;DR

This study investigates the X-ray properties of quasars with intrinsic narrow absorption lines, finding their X-ray characteristics similar to normal quasars and supporting a high-latitude location of the NAL gas in accretion-disk winds.

Contribution

It provides the first systematic X-ray survey of intrinsic NAL quasars, comparing their properties to BAL and normal quasars, and tests models for the NAL gas location.

Findings

Intrinsic-NAL quasars have X-ray properties similar to normal quasars.

No significant excess X-ray absorption was found in intrinsic-NAL quasars.

Results support NAL gas being located at high latitudes above the accretion disk.

Abstract

We have used archival Chandra and XMM-Newton observations of quasars hosting intrinsic narrow UV absorption lines (intrinsic NALs) to carry out an exploratory survey of their X-ray properties. Our sample consists of three intrinsic-NAL quasars and one "mini-BAL" quasar, plus four quasars without intrinsic absorption lines for comparison. These were drawn in a systematic manner from an optical/UV-selected sample. The X-ray properties of intrinsic-NAL quasars are indistinguishable from those of "normal" quasars. We do not find any excess absorption in quasars with intrinsic NALs, with upper limits of a few times 10^22 cm^-2. We compare the X-ray and UV properties of our sample quasars by plotting the equivalent width and blueshift velocity of the intrinsic NALs and the X-ray spectral index against the "optical-to-X-ray" slope, alpha-ox. When BAL quasars and other AGNs with intrinsic NALs are included, the plots suggest that intrinsic-NAL quasars form an extension of the BAL sequences and tend to bridge the gap between BAL and "normal" quasars. Observations of larger samples of intrinsic-NAL quasars are needed to verify these conclusions. We also test two competing scenarios for the location of the NAL gas in an accretion-disk wind. Our results strongly support a location of the NAL gas at high latitudes above the disk, closer to the disk axis than the dense BAL wind. We detect excess X-ray absorption only in Q0014+8118, which does not host intrinsic NALs. The absorbing medium very likely corresponds to an intervening system at z=1.1, which also produces strong absorption lines in the rest-frame UV spectrum of this quasar. In the appendix we discuss the connection between UV and X-ray attenuation and its effect on alpha-ox.