X-Ray Spectrum of RBS 315: Absorption or Intrinsic Curvature

X-ray absorption of high-redshift quasars is enigmatic, because it remains unclear where in the universe the absorbing gas is. If absorption occurs near the high-z host, it could help us understand early stages of galaxy formation. If it is in the intergalactic medium (IGM), it provides a unique way to probe this elusive baryon component. We report on observations of one of the brightest X-ray sources at a high redshift, RBS 315 (z = 2.69). Despite several previous analyses, no definite conclusion as to the source of the curvature in its spectrum, whether absorption or intrinsic, could be reached. We present observations by XMM-Newton’s European Photon Imaging Camera (EPIC) and Reflection Grating Spectrometers (RGS) as well as NuSTAR and Swift’s X-Ray Telescope (XRT). The XMM-Newton spectra of this source are of unprecedented quality. A purely statistical analysis of the charge-coupled device (CCD) spectra yields no clear results—the spectrum is as likely to be photoelectrically absorbed as it is to be curved at the source, and no constraint on the position of the absorber can be obtained. Assuming absorption governs the spectral curvature, the lack of absorption lines in the grating spectra indicates the absorber is not well localized in redshift space, and could be dispersed over the cosmological scales of the IGM. Intrinsic curvature, however, can not be unambiguously ruled out.