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

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) similar or equal to 4-5.5 x 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) similar or equal to 2.7 x 10(-13) erg s(-1)), which can likely be ascribed to a lower X-ray activity. Moreover, the 2019 data present a prominent Fe K alpha 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.