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

¹ Dipartimento di Fisica e Astronomia “Augusto Righi”, Università degli Studi di Bologna, Via P. Gobetti 93/2, 40129 Bologna, Italy
e-mail: elena.bertola2@unibo.it
² INAF–OAS, Osservatorio di Astrofisica e Scienza dello Spazio di Bologna, Via P. Gobetti 93/3, 40129 Bologna, Italy
³ European Space Agency (ESA), European Space Astronomy Centre (ESAC), 28691 Villanueva de la Cañada, Madrid, Spain
⁴ Department of Physics and Astronomy of the College of Charleston, Charleston, SC 29424, USA
⁵ INAF – Osservatorio Astronomico di Roma, Via Frascati 33, 00040 Monte Porzio Catone, Roma, Italy

Received: 11 November 2021
Accepted: 15 April 2022

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+5255 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 = 99₋₃₅⁺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.