The 500 ks Chandra observation of the z = 6.31 QSO SDSS J1030 + 0524

¹ INAF – Osservatorio di Astrofisica e Scienza dello Spazio di Bologna, via Gobetti 93/3, 40129 Bologna Italy
email: riccardo.nanni7@unibo.it
² Dipartimento di Astronomia, Università degli Studi di Bologna, via Gobetti 93/2, 40129 Bologna, Italy
³ INAF – Osservatorio Astrofisico di Arcetri, Largo E. Fermi 5, 50125 Firenze, Italy
⁴ Institut de Ciències del Cosmos (ICCUB), Universitat de Barcelona (IEEC-UB), Martí i Franquès 1, 08028 Barcelona, Spain
⁵ ICREA, Pg. Lluís Companys 23, 08010, Barcelona, Spain
⁶ Department of Astronomy & Astrophysics, 525 Davey Lab, The Pennsylvania State University, University Park, PA 16802, USA
⁷ Institute for Gravitation and the Cosmos, The Pennsylvania State University, University Park, PA 16802, USA
⁸ Scuola Normale Superiore, Piazza dei Cavalieri 7, 56126 Pisa, Italy
⁹ Leiden Observatory, Leiden University, PO Box 9513, 2300 RA Leiden, the Netherlands
¹⁰ Dipartimento di Fisica e Astronomia, Università di Firenze, Via G. Sansone 1, 50019 Sesto Fiorentino, Italy
¹¹ Dipartimento di Fisica “Ettore Pancini”, Università di Napoli Federico II, via Cintia, 80126 Napoli, Italy
¹² INFN – Unità di Napoli, via Cintia 9, 80126 Napoli, Italy
¹³ Agenzia Spaziale Italiana - Science Data Center, Via del Politecnico snc, 00133, Roma, Italy
¹⁴ INAF – Istituto di Radioastronomia, via P. Gobetti 101, 40129 Bologna, Italy
¹⁵ Università degli Studi di Ferrara, Via Savonarola 9, 44121 Ferrara, Italy
¹⁶ Space Telescope Science Institute, 3700 San Martin Dr., Baltimore MD 21210 USA
¹⁷ Johns Hopkins University, 3400 N. Charles Street, Baltimore MD 21218 USA
¹⁸ Kapteyn Astronomical Institute, University of Groningen, Postbus 800, 9700 AV Groningen, The Netherlands
¹⁹ European Southern Observatory, Alonso de Córdova 3107, Casilla 19 Santiago 19001, Chile
²⁰ Physics Department, University of Miami, Coral Gables FL 33124, USA
²¹ Yale Center for Astronomy and Astrophysics, PO Box 208121, New Haven CT 06520, USA
²² Department of Physics, Yale University, PO Box 208121, New Haven CT 06520, USA

Received: 24 January 2018
Accepted: 15 February 2018

Abstract

We present the results from a ~ 500 ks Chandra observation of the z = 6.31 QSO SDSS J1030 + 0524. This is the deepest X-ray observation to date of a z ~ 6 QSO. The QSO is detected with a total of 125 net counts in the full (0.500A0–7 keV) band and its spectrum can be modeled by a single power-law model with photon index of Γ = 1.81 ± 0.18 and full band flux of f = 3.95 × 10⁻¹⁵ erg s⁻¹ cm⁻². When compared with the data obtained by XMM-Newton in 2003, our Chandra observation in 2017 shows a harder (ΔΓ ≈ −0.6) spectrum and a 2.5 times fainter flux. Such a variation, in a timespan of ~ 2 yr rest-frame, is unexpected for such a luminous QSO powered by a > 10⁹M_⨀ black hole. The observed source hardening and weakening could be related to an intrinsic variation in the accretion rate. However, the limited photon statistics does not allow us to discriminate between an intrinsic luminosity and spectral change, and an absorption event produced by an intervening gas cloud along the line of sight.

We also report the discovery of diffuse X-ray emission that extends for 30″ × 20″ southward of the QSO with a signal-to-noise ratio (S/N) of approximately six, hardness ratio of HR = 0.03^+0.20_−0.25, and soft band flux of f_0.5– keV = 1.1^+0.3_−0.3 × 10⁻¹⁵ erg s⁻¹ cm⁻² , that is not associated to a group or cluster of galaxies. We discuss two possible explanations for the extended emission, which may be either associated with the radio lobe of a nearby, foreground radio galaxy (at z ≈ 1 – 2), or ascribed to the feedback from the QSO itself acting on its surrounding environment, as proposed by simulations of early black hole formation.