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 4 − 5.5 × 10 −13 erg s −1 ) from 2000 to 2008, APM 08279 was found in a fainter state in the latest X-ray exposures ( f 2−10 2.7 × 10 −13 erg s −1 ), which can likely be ascribed to a lower X-ray activity. Moreover, the 2019 data present a prominent Fe Kα 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 cut = 99 +91 −35 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.

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“…NuSTAR (Harrison et al 2013), with its broad spectral coverage, has led to a number of studies that have shown that the primary emission of Seyfert galaxies can be well approximated as a cutoff power law with a photon index Γ = 1.7-2.0 and a high energy rollover at E C =50-300 keV (Fabian et al 2015(Fabian et al , 2017Tortosa et al 2018;Middei et al 2019;Baloković et al 2020). At higher luminosities, the high energy cutoffs measured in the four most distant quasars with L 2−10 keV > 10 45 erg s −1 are 106 +102 −37 keV and 66 +17 −12 keV in 2MASSJ1614346+470420 and B1422+231 (Lanzuisi et al 2019), 99 +67 −35 keV in B2202-209 (Kammoun et al 2017) and 99 +91 −35 keV in APM 08279+5255 (Bertola et al 2022). However, the reprocessing of such a continuum radiation of the AGN from the circumnuclear material makes the measurement of the photon index and of the cutoff energy degenerate with other parameters, like the amount of radiation reflected by circumnuclear matter, which produces a Compton hump at ∼ 30 keV Matt et al (1991); George & Fabian (1991).…”