X-ray observation of ULAS J1120+0641, the most distant quasar at z = 7.08

¹ INAF, Osservatorio Astronomico di Brera, via Brera 28, 20121 Milano, Italy
e-mail: alberto.moretti@brera.inaf.it
² INAF, Osservatorio Astronomico di Bologna, via Ranzani 1, 40127 Bologna, Italy
³ INAF, via E. Bassini 15, 20133 Milano, Italy
⁴ Dipartimento di Astronomia, Università degli Studi di Bologna, via Ranzani 1, 40127 Bologna, Italy

Received: 14 November 2013
Accepted: 31 January 2014

Abstract

We probe the emission mechanism of the accreting super massive black holes in the high redshift Universe and, to do this, we study the X-ray spectrum of ULAS1120+064, the highest redshift quasar detected so far at z = 7.085, which has been deeply observed (340 ks) by XMM-Newton. Despite the long integration time, the spectral analysis is limited by the poor statistics, with only 150 source counts being detected. We measured the spectrum in the 2−80 keV rest-frame (0.3−10 keV observed) energy band. Assuming a simple power law model, we find a photon index of 2.0 ± 0.3 and a luminosity of 6.7 ± 0.3 × 10⁴⁴ erg s^-1 in the 2−10 keV band, while the intrinsic absorbing column can only be loosely constrained (N_H < 10²³ cm^-2). Combining our measure with published data, we calculate that the X-ray-to-optical spectral index α_OX is 1.8 ± 0.1, in agreement with the α_OX-UV luminosity correlation that is valid for lower redshift quasars. This is the second time that a z > 6 quasar has been investigated through a deep X-ray observation. In agreement with previous studies of z ~ 6 AGN samples, we do not find any hint of evolution in the broadband energy distribution. Indeed from our dataset, ULAS 1120+0641 is indistinguishable from the population of optically bright quasar at lower redshift.