The X-ray spectral properties and variability of luminous high-redshift active galactic nuclei

O. Shemmer, W. N. Brandt, C. Vignali, D. P. Schneider, Xiaohui Fan, G. T. Richards, Michael A. Strauss

Research output: Contribution to journalArticle

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Abstract

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 Chandrd). 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 F = 1.97-0.04 +0.06, with no detectable intrinsic dispersion from source to source. We also obtain a strong constraint on the mean intrinsic column density, NH ≲ 10 22 - 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 radioquiet AGN.

Original languageEnglish (US)
Pages (from-to)729-739
Number of pages11
JournalAstrophysical Journal
Volume630
Issue number2 I
DOIs
StatePublished - Sep 10 2005

Fingerprint

active galactic nuclei
radio
x rays
XMM-Newton telescope
newton
timescale
photons
K lines
power law
hydrogen

Keywords

  • Galaxies: active
  • Galaxies: nuclei
  • Quasars: individual (SDSS 0231-0728)
  • X-rays: galaxies

ASJC Scopus subject areas

  • Space and Planetary Science

Cite this

Shemmer, O., Brandt, W. N., Vignali, C., Schneider, D. P., Fan, X., Richards, G. T., & Strauss, M. A. (2005). The X-ray spectral properties and variability of luminous high-redshift active galactic nuclei. Astrophysical Journal, 630(2 I), 729-739. https://doi.org/10.1086/432050

The X-ray spectral properties and variability of luminous high-redshift active galactic nuclei. / Shemmer, O.; Brandt, W. N.; Vignali, C.; Schneider, D. P.; Fan, Xiaohui; Richards, G. T.; Strauss, Michael A.

In: Astrophysical Journal, Vol. 630, No. 2 I, 10.09.2005, p. 729-739.

Research output: Contribution to journalArticle

Shemmer, O, Brandt, WN, Vignali, C, Schneider, DP, Fan, X, Richards, GT & Strauss, MA 2005, 'The X-ray spectral properties and variability of luminous high-redshift active galactic nuclei', Astrophysical Journal, vol. 630, no. 2 I, pp. 729-739. https://doi.org/10.1086/432050
Shemmer, O. ; Brandt, W. N. ; Vignali, C. ; Schneider, D. P. ; Fan, Xiaohui ; Richards, G. T. ; Strauss, Michael A. / The X-ray spectral properties and variability of luminous high-redshift active galactic nuclei. In: Astrophysical Journal. 2005 ; Vol. 630, No. 2 I. pp. 729-739.
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AU - Fan, Xiaohui

AU - Richards, G. T.

AU - Strauss, Michael A.

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