  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 (EPIC and RGS) as well as NuSTAR and Swift/XRT. The
XMM-Newton spectra of this source are of unprecedented quality. A purely
statistical analysis of the CCD spectra yields no clear results - the spectrum
is as likely to be photo-electrically 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.
