The X-Ray Spectral Properties and Variability of Luminous High-Redshift Active Galactic Nuclei

We perform a detailed investigation of moderate- to high-quality X-ray spectra of 10 of the most luminous active galactic nuclei (AGNs) known at z > 4 (up to z ~ 6.28). This study includes five new XMM-Newton observations and five archived X-ray observations (four by XMM-Newton and one by Chandra). We find that the X-ray power-law photon indices of our sample, composed of eight radio-quiet sources and two that are moderately radio-loud, are not significantly different from those of lower redshift AGNs. The upper limits obtained on intrinsic neutral hydrogen column densities, NH ≲ 1022-1023 cm-2, indicate that these AGNs are not significantly absorbed. A joint fit performed on our eight radio-quiet sources, with a total of ~7000 photons, constrains the mean photon index of z > 4 radio-quiet AGNs to Γ = 1.97, with no detectable intrinsic dispersion from source to source. We also obtain a strong constraint on the mean intrinsic column density, NH ≲ 3 × 1021 cm-2, showing that optically selected radio-quiet AGNs at z > 4 are, on average, not more absorbed than their lower redshift counterparts. All this suggests that the X-ray production mechanism and the central environment in radio-quiet AGNs have not significantly evolved over cosmic time. The mean equivalent width of a putative neutral narrow Fe Kα line is constrained to be ≲190 eV, and similarly we place constraints on the mean Compton reflection component (R ≲ 1.2). None of the AGNs varied on short (~1 hr) timescales, but on longer timescales (months to years) strong variability is observed in five of the sources. In particular, the X-ray flux of the z = 5.41 radio-quiet AGN SDSS 0231-0728 dropped by a factor of ~4 over a rest-frame period of 73 days. This is the most extreme X-ray variation observed in a luminous z > 4 radio-quiet AGN.

[1]  M. Birkinshaw,et al.  New Visions of the X-ray Universe in the XMM-Newton and Chandra Era. Noordwijk, Nov 2001 , 2006 .

[2]  D. Schneider,et al.  X-Ray Lighthouses of the High-Redshift Universe. II. Further Snapshot Observations of the Most Luminous z≳4 Quasars with Chandra , 2005, astro-ph/0503301.

[3]  D. Berk,et al.  Soft X-Ray and Ultraviolet Emission Relations in Optically Selected AGN Samples , 2005, astro-ph/0503009.

[4]  K. Nandra,et al.  The Relationship between x-ray variability amplitude and black hole mass in active galactic nuclei , 2005, astro-ph/0501471.

[5]  The XMM-Newton view of PG quasars - I. X-ray continuum and absorption , 2004, astro-ph/0411051.

[6]  P. Capak,et al.  The Cosmic Evolution of Hard X-Ray-selected Active Galactic Nuclei , 2004, astro-ph/0410527.

[7]  D. A. Schwartz,et al.  Chandra Measurement of the X-Ray Spectrum of a Quasar at z = 5.99 , 2004, astro-ph/0410124.

[8]  A. M. Read,et al.  XMM-Newton (cross)-calibration , 2004, SPIE Astronomical Telescopes + Instrumentation.

[9]  R. McMahon,et al.  The X-Ray Spectrum of the z = 6.30 QSO SDSS J1030+0524 , 2004, astro-ph/0406561.

[10]  P. O’Brien,et al.  XMM–Newton observations of high-luminosity radio-quiet quasi-stellar objects , 2004, astro-ph/0405458.

[11]  Baltimore,et al.  Prevalence of X-Ray Variability in the Chandra Deep Field-South , 2004, astro-ph/0404418.

[12]  M. Worsley,et al.  XMM–Newton observations of GB B1428+4217: confirmation of intrinsic soft X-ray absorption , 2004, astro-ph/0403601.

[13]  Oxford,et al.  The 2dF QSO Redshift Survey – XII. The spectroscopic catalogue and luminosity function , 2004, astro-ph/0403040.

[14]  F. Mannucci,et al.  Extreme gas properties in the most distant quasars , 2003, astro-ph/0312402.

[15]  M. Worsley,et al.  XMM–Newton confirmation of a warm absorber in the z= 4.4 blazar PMN J0525−3343 , 2003, astro-ph/0310366.

[16]  D. Grupe,et al.  XMM-Newton Observations of Two High-Redshift Quasars: RX J1028-0844 and BR 0351-1034 , 2003, astro-ph/0310188.

[17]  P. O’Brien,et al.  An X‐ray Baldwin effect for the narrow Fe Kα lines observed in active galactic nuclei , 2003, astro-ph/0309394.

[18]  L. Cowie,et al.  The Changing Active Galactic Nucleus Population , 2003 .

[19]  J. Brinkmann,et al.  The near-IR properties and continuum shapes of high redshift quasars from the Sloan Digital Sky Survey , 2003, astro-ph/0308178.

[20]  M. A. Strauss,et al.  Molecular gas in the host galaxy of a quasar at redshift z = 6.42 , 2003, Nature.

[21]  S. Djorgovski,et al.  Discovery of a Clustered Quasar Pair at z ≈ 5: Biased Peaks in Early Structure Formation , 2003, astro-ph/0306423.

[22]  Xiaohui Fan,et al.  Dust emission from the most distant quasars , 2003, astro-ph/0305116.

[23]  National Radio Astronomy Observatory,et al.  Sensitive Observations at 1.4 and 250 GHz of z > 5 QSOs , 2003, astro-ph/0304097.

[24]  C. Reynolds,et al.  Fluorescent iron lines as a probe of astrophysical black hole systems , 2003 .

[25]  Tucson,et al.  Chandra and XMM-Newton Observations of the First Quasars: X-Rays from the Age of Cosmic Enlightenment , 2003, astro-ph/0302558.

[26]  J. Shields,et al.  Quasar Elemental Abundances at High Redshifts , 2003, astro-ph/0302494.

[27]  M. F. Physik,et al.  XMM-Newton observations of four high-z quasars , 2003, astro-ph/0302421.

[28]  Edward J. Wollack,et al.  First-Year Wilkinson Microwave Anisotropy Probe (WMAP) Observations: Determination of Cosmological Parameters , 2003, astro-ph/0302209.

[29]  V. Narayanan,et al.  A Survey of z > 5.7 Quasars in the Sloan Digital Sky Survey. II. Discovery of Three Additional Quasars at z > 6 , 2003, astro-ph/0301135.

[30]  J. Shields,et al.  Chandra Survey of Radio-quiet, High-Redshift Quasars , 2002, astro-ph/0204462.

[31]  G. Richards,et al.  X-rays from the first massive black holes , 2002, astro-ph/0411355.

[32]  Astronomy,et al.  X-Ray Lighthouses of the High-Redshift Universe: Probing the Most Luminous z > 4 Palomar Digital Sky Survey Quasars with Chandra , 2002, astro-ph/0210475.

[33]  et al,et al.  Optical and Radio Properties of Extragalactic Sources Observed by the FIRST Survey and the Sloan Digital Sky Survey , 2002, astro-ph/0202408.

[34]  M. SubbaRao,et al.  Spectroscopic Target Selection in the Sloan Digital Sky Survey: The Quasar Sample , 2002, astro-ph/0202251.

[35]  J. Manners,et al.  The X‐ray variability of high‐redshift QSOs , 2001, astro-ph/0110461.

[36]  V. Narayanan,et al.  Evolution of the Ionizing Background and the Epoch of Reionization from the Spectra of z ∼ 6 Quasars , 2001, astro-ph/0111184.

[37]  Astrophysics,et al.  Exploratory Chandra Observations of the Highest-Redshift Quasars: X-Rays from the Dawn of the Modern Universe , 2001, astro-ph/0108001.

[38]  Xiaohui Fan,et al.  A 250 GHz Survey of High-Redshift Quasars from the Sloan Digital Sky Survey , 2001, astro-ph/0103252.

[39]  P. Gondoin,et al.  XMM-Newton observatory. I. The spacecraft and operations , 2001 .

[40]  Walter A. Siegmund,et al.  The Sloan Digital Sky Survey: Technical Summary , 2000, astro-ph/0006396.

[41]  Brazil,et al.  Radio Properties of z > 4 Optically Selected Quasars , 2000, astro-ph/0001394.

[42]  W. Brandt,et al.  The X-Ray Properties of z > 4 Quasars , 2000, astro-ph/0001299.

[43]  Cambridge,et al.  Variability of the extreme z = 4.72 blazar, GB 1428+4217 , 1999, astro-ph/9907033.

[44]  R. Nichol,et al.  High-Redshift Quasars Found in Sloan Digital Sky Survey Commissioning Data , 1999, astro-ph/0103228.

[45]  G. Palumbo,et al.  ASCA and ROSAT X-Ray Spectra of High-Redshift Radio-loud Quasars , 1996, astro-ph/9610204.

[46]  Matthew A. Bershady,et al.  Linear Regression for Astronomical Data with Measurement Errors and Intrinsic Scatter , 1996, astro-ph/9605002.

[47]  M. Irwin,et al.  APM z>4 QSO Survey: Spectra and Intervening Absorption Systems , 1996, astro-ph/9604021.

[48]  A. Zdziarski,et al.  Angle-dependent Compton reflection of X-rays and gamma-rays , 1995 .

[49]  B. Wilkes,et al.  The Soft X-Ray Properties of a Complete Sample of Optically Selected Quasars. II. Final Results , 1994, astro-ph/9609164.

[50]  D. McCammon,et al.  Photoelectric absorption cross sections with variable abundances , 1992 .

[51]  J. Dickey,et al.  H I in the Galaxy , 1990 .

[52]  J. Gunn,et al.  A study of ten quasars with redshifts greater than four , 1989 .

[53]  Maarten Schmidt,et al.  VLA observations of objects in the Palomar Bright Quasar Survey , 1989 .

[54]  N. Grevesse,et al.  Abundances of the elements: Meteoritic and solar , 1989 .

[55]  G. Zamorani,et al.  The X-ray spectra of the extragalactic sources in the Einstein extended medium sensitivity survey , 1988 .

[56]  Belinda J. Wilkes,et al.  Quasar energy distributions. I - Soft X-ray spectra of quasars , 1987 .

[57]  Y. Avni,et al.  Energy spectra of X-ray clusters of galaxies , 1976 .