摘要：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 (f2−10 ' 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 (f2−10 ' 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 (Ecut = 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.
关键词：accretion;accretion disks;black hole physics;quasars: absorption lines;quasars: supermassive black holes;quasars: individual: APM 08279+5255;X-rays: general
