Coronal properties of the luminous radio-quiet quasar QSO B2202$-$209

Abstract

We present an analysis of the joint $XMM$-$Newton$ and $NuSTAR$ observations of the radio-quiet quasar QSO B2202$-$209. Using an optical observation from the Hale Telescope at the Palomar Observatory, we revise the redshift of the source from the previously reported $z=1.77$ to $z=0.532$, and we estimate the mass of the central black hole, $\log (M_{\mathrm{BH}}/M_{\odot })=9.08\pm 0.18$. The X-ray spectrum of this source can be well described by a power-law of photon index $\hat{I}“=1.82\pm 0.05$ with $E_{\mathrm{cut}}=152_{-54}^{+103}\mathrm{keV}$, in the rest frame of the source. Assuming a Comptonisation model, we estimate the coronal temperature to be $k{T}_{\mathrm{e}}=42\pm 3\mathrm{keV}$ and $k{T}_{\mathrm{e}}=56\pm 3\mathrm{keV}$ for a spherical and a slab geometry, respectively. The coronal properties are comparable to the ones derived for local AGN, despite a difference of around one order of magnitude in black hole mass and X-ray luminosity ($L_{2-10}=1.93\times 10^{45}\mathrm{erg}{\mathrm{s}}^{-1}$). The quasar is X-ray loud, with an unusually flat observed optical-to-X-ray spectral slope $\hat{I}{\pm }_{\mathrm{OX}}=1.00\pm 0.02$, and has an exceptionally strong optical [O III] line. Assuming that both the X-ray emission and the [O III] line are isotropic, these two extreme properties can be explained by a nearly edge-on disk, leading to a reduction in the observed UV continuum light.

8 pages, 4 figures, accepted for publication in MNRAS