The properties of the X-ray corona in the distant ($z=3.91$) quasar APM 08279+5255

Abstract

We present new joint XMM-Newton and NuSTAR observations of APM 08279+5255, a gravitationally-lensed, broad-absorption line quasar ($z=3.91$). After showing a fairly stable flux ($f_{2-10}\simeq 4-5.5\times 10^{-13}\mathrm{erg}{\mathrm{s}}^{-1}$) from 2000 to 2008, APM 08279+5255 was found in a fainter state in the latest X-ray exposures ($f_{2-10}\simeq 2.7\times 10^{-13}\mathrm{erg}{\mathrm{s}}^{-1}$), which can likely be ascribed to a lower X-ray activity. Moreover, the 2019 data present a prominent Fe K$\hat{I}\pm$ emission line and do not show any significant absorption line. This fainter state, coupled to the first hard X-ray sampling of APM 08279+5255, allowed us to measure X-ray reflection and the high-energy cutoff in this source for the first time. From the analysis of previous XMM-Newton and Chandra observations, X-ray reflection is demonstrated to be a long-lasting feature of this source, but less prominent prior to 2008, possibly due to a stronger primary emission. The estimated high-energy cutoff ($E_{\mathrm{cut}}=99_{-35}^{+91}$ keV) sets a new redshift record for the farthest ever measured and places APM 08279+5255 in the allowed region of the compactness-temperature diagram of X-ray coronae, in agreement with previous results on high-$z$ quasars.

Accepted for publication in A&A. 14 pages, 4 figures, 6 tables