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

Abstract

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 PG 1634+706 - one of the most luminous (an X-ray luminosity $\sim 10^{46}$ erg/s) 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 spectrum to be measured more 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 $\Gamma\approx 1.9$ 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.