Chandra and XMM-Newton observations of the first quasars: X-rays from the age of cosmic enlightenment

C. Vignali, W. N. Brandt, D. P. Schneider, S. F. Anderson, X. Fan, J. E. Gunn, S. Kaspi, G. T. Richards, Michael A. Strauss

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Abstract

We report on Chandra and XMM-Newton observations of a sample of 13 quasars at z ≈ 4.7-5.4 mostly taken from the Sloan Digital Sky Survey (SDSS). The present sample complements previous X-ray studies of z ≥ 4 quasars, in which the majority of the objects are optically more luminous and at lower redshifts. All but two of our quasars have been detected in the X-ray band, thus doubling the number of z ≥ 4.8 X-ray-detected quasars. The two nondetections are likely to be due to a short exposure time (SDSSp J033829.31+002156.3) and to the presence of intrinsic absorption (SDSSp J173744.87+582829.5). We confirm and extend to the highest redshifts the presence of a correlation between AB1450(1+z) magnitude and soft X-ray flux for z ≥ 4 quasars and the presence of a steeper optical-to-X-ray spectral energy distribution (parameterized by αox) for high-luminosity, high-redshift quasars than for lower luminosity, lower redshift quasars. The second effect is likely due to the known anticorrelation between αox and rest-frame 2500 Å luminosity, whose significance is confirmed via partial correlation analysis. The joint ≈2.5-36 keV rest-frame spectrum of the z > 4.8 SDSS quasars observed thus far by Chandra is well parameterized by a power law with photon index Γ = 1.84-0.30+0.31; this photon index is consistent with those of z ≈ 0-3 quasars and that obtained from joint spectral fitting of z ≈ 4.1-4.5 optically luminous Palomar Digital Sky Survey quasars. No evidence for widespread intrinsic X-ray absorption has been found (NH ≲ 4.0 × 1022 cm-2 on average at 90% confidence). We also obtained Hobby-Eberly Telescope (HET) photometric observations for eight of our target quasars. None of these shows significant (greater than 30%) optical variability over the time interval of a few years (in the observed frame) between the SDSS and HET observations.

Original languageEnglish (US)
Pages (from-to)2876-2890
Number of pages15
JournalAstronomical Journal
Volume125
Issue number6 1770
DOIs
StatePublished - Jun 1 2003

All Science Journal Classification (ASJC) codes

  • Astronomy and Astrophysics
  • Space and Planetary Science

Keywords

  • Early universe
  • Galaxies: active
  • Galaxies: high-redshift
  • Quasars: general
  • X-rays

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    Vignali, C., Brandt, W. N., Schneider, D. P., Anderson, S. F., Fan, X., Gunn, J. E., Kaspi, S., Richards, G. T., & Strauss, M. A. (2003). Chandra and XMM-Newton observations of the first quasars: X-rays from the age of cosmic enlightenment. Astronomical Journal, 125(6 1770), 2876-2890. https://doi.org/10.1086/374998