Confirming the Detection of an Intergalactic X-Ray Absorber toward PKS 2155–304

Abstract

We present new observations of PKS 2155-304 with the Chandra Low Energy Transmission Grating (LETG), using the Advanced CCD Imaging Spectrometer (ACIS). We confirm the detection of an absorption line plausibly identified as O VIII Lyα from a warm-hot intergalactic medium associated with a small group of galaxies along the line of sight, as originally reported by Fang and coworkers. Combining the previous observations by Fang and coworkers with five new long observations on the same target, we increase the total exposure time by a factor of 3 and the total counts per resolution element by a factor of 5. The measured line equivalent width is smaller than that observed by Fang and coworkers, but still consistent at 90% confidence. We also analyze the XMM-Newton observations on the same target, as well as observations using the combination of the Chandra LETG and the High Resolution Camera (HRC). These observations have been used to challenge our reported detection. While no line is seen in either the XMM-Newton or the Chandra LETG+HRC data, we find that our result is consistent with the upper limits from both data sets. We attribute the nondetection to (1) the higher quality of the Chandra LETG+ACIS spectrum and (2) the rather extended wings of the line-spread functions of both the XMM-Newton Reflection Grating Spectrometer (RGS) and the Chandra LETG+HRC. We discuss the implication of our observation on the temperature and density of the absorber. We also confirm the detection of z ~ 0 O VII absorption, and, comparing with previous Chandra analysis, we obtain much tighter constraints on the line properties.