X-ray Properties of the Luminous Quasar PG 1634+706 at z = 1.337 from SRG and XMM-Newton Data

In the fall of 2019, during the in-flight calibration phase of the SRG observatory, the
onboard eROSITA and Mikhail Pavlinsky ART-XC telescopes carried out a series of observations of
PG1634+706—one of the most luminous (an X-ray luminosity ∼1046 erg s−1) quasars in the Universe at
z < 2. Approximately at the same dates this quasar was also observed by the XMM-Newton observatory.
Although the object had already been repeatedly studied in X-rays previously, its new observations allowed
its energy spectrumto be measuredmore accurately in the wide range 1–30 keV (in the quasar rest frame).
Its spectrum can be described by a two-component model that consists of a power-law continuum with a
slope Γ ≈ 1.9 and a broadened iron emission line at an energy of about 6.4 keV. The X-ray variability of the
quasar was also investigated. On time scales of the order of several hours (here and below, in the source
rest frame) the X-ray luminosity does not exhibit a statistically significant variability. However, it changed
noticeably from observation to observation in the fall of 2019, having increased approximately by a factor
of 1.5 in 25 days. A comparison of the new SRG and XMM-Newton measurements with the previous
measurements of other X-ray observatories has shown that in the entire 17-year history of observations
of the quasar PG 1634+706 its X-ray luminosity has varied by no more than a factor of 2.5, while the
variations on time scales of several weeks and several years are comparable in amplitude.