Absorption in X-ray spectra of high-redshift quasars

Martin Elvis, Fabrizio Fiore, Belinda Wilkes, Jonathan Mcdowell, Jill Bechtold

Research output: Contribution to journalArticle

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Abstract

We present evidence that X-ray absorption is common in high-redshift quasars. We have studied six high-redshift (z ∼ 3) quasars with the ROSAT PSPC of which four are in directions of low Galactic NH. Three out of these four show excess absorption, while only three in ∼ 50 z ≲ 0.4 quasars do, indicating that such absorption must be common, but not ubiquitous, at high redshifts, and that the absorbers must lie at z > 0.4. The six quasars were: S5 0014+81, Q0420-388, PKS 0438-436, S4 0636+680. PKS 2000-330, PKS 2126-158, which have redshifts between 2.85 and 3.78. PKS 0438-436 and PKS 2126-158 show evidence for absorption above the local Galactic value at better than 99.999% confidence level. If the absorber is at the redshift of the quasar, then values of NH = [0.86(+0.49, -0.28)] × 1022 atoms cm-2 for PKS 0438-436, and NH = [1.45(+1.20, -0.64)] × 1022 atoms cm-2 for PKS 2126-158, are implied, assuming solar abundances. The spectrum of S4 0636+680 also suggests the presence of a similarly large absorption column density at the 98% confidence level. This absorption reverses the trend for the most luminous active galactic nuclei (AGN) to have the least X-ray absorption, so a new mechanism is likely to be responsible. Intervening absorption due to damped Lyα systems is a plausible cause. We also suggest, as an intrinsic model, that intracluster material, e.g., a cooling flow, around the quasar could account for both the X-ray spectrum and other properties of these quasars. All the quasars are radio-loud and three are gigahertz peaked (two of the three showing absorption). No excess absorption above the Galactic value is seen toward Q0420-388. This quasar has two damped Lyα systems at z = 3.08. The limit on the X-ray column density implies a low ionization fraction, N(H I)/N(H) ≳ 4 × 10-3 (3 σ), for solar abundances, for these systems, and can set a weak limit on the size of the absorber. In the emitted frame these PSPC spectra cover the band ∼0.5-10 keV, which has been well observed for low-redshift quasars and AGN. Comparison of high and low-redshift spectra in this emitted band shows no change of mean spectral index greater than ΔαE > 0.3 (99% confidence) with either redshift or luminosity, for radio-loud quasars.

Original languageEnglish (US)
Pages (from-to)60-72
Number of pages13
JournalAstrophysical Journal
Volume422
Issue number1
StatePublished - Feb 10 1994

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quasars
x rays
confidence
absorbers
active galactic nuclei
radio
ionization
atoms
cooling
luminosity
trends
causes

Keywords

  • Quasars: absorption lines
  • X-rays: galaxies

ASJC Scopus subject areas

  • Space and Planetary Science

Cite this

Elvis, M., Fiore, F., Wilkes, B., Mcdowell, J., & Bechtold, J. (1994). Absorption in X-ray spectra of high-redshift quasars. Astrophysical Journal, 422(1), 60-72.

Absorption in X-ray spectra of high-redshift quasars. / Elvis, Martin; Fiore, Fabrizio; Wilkes, Belinda; Mcdowell, Jonathan; Bechtold, Jill.

In: Astrophysical Journal, Vol. 422, No. 1, 10.02.1994, p. 60-72.

Research output: Contribution to journalArticle

Elvis, M, Fiore, F, Wilkes, B, Mcdowell, J & Bechtold, J 1994, 'Absorption in X-ray spectra of high-redshift quasars', Astrophysical Journal, vol. 422, no. 1, pp. 60-72.
Elvis M, Fiore F, Wilkes B, Mcdowell J, Bechtold J. Absorption in X-ray spectra of high-redshift quasars. Astrophysical Journal. 1994 Feb 10;422(1):60-72.
Elvis, Martin ; Fiore, Fabrizio ; Wilkes, Belinda ; Mcdowell, Jonathan ; Bechtold, Jill. / Absorption in X-ray spectra of high-redshift quasars. In: Astrophysical Journal. 1994 ; Vol. 422, No. 1. pp. 60-72.
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abstract = "We present evidence that X-ray absorption is common in high-redshift quasars. We have studied six high-redshift (z ∼ 3) quasars with the ROSAT PSPC of which four are in directions of low Galactic NH. Three out of these four show excess absorption, while only three in ∼ 50 z ≲ 0.4 quasars do, indicating that such absorption must be common, but not ubiquitous, at high redshifts, and that the absorbers must lie at z > 0.4. The six quasars were: S5 0014+81, Q0420-388, PKS 0438-436, S4 0636+680. PKS 2000-330, PKS 2126-158, which have redshifts between 2.85 and 3.78. PKS 0438-436 and PKS 2126-158 show evidence for absorption above the local Galactic value at better than 99.999{\%} confidence level. If the absorber is at the redshift of the quasar, then values of NH = [0.86(+0.49, -0.28)] × 1022 atoms cm-2 for PKS 0438-436, and NH = [1.45(+1.20, -0.64)] × 1022 atoms cm-2 for PKS 2126-158, are implied, assuming solar abundances. The spectrum of S4 0636+680 also suggests the presence of a similarly large absorption column density at the 98{\%} confidence level. This absorption reverses the trend for the most luminous active galactic nuclei (AGN) to have the least X-ray absorption, so a new mechanism is likely to be responsible. Intervening absorption due to damped Lyα systems is a plausible cause. We also suggest, as an intrinsic model, that intracluster material, e.g., a cooling flow, around the quasar could account for both the X-ray spectrum and other properties of these quasars. All the quasars are radio-loud and three are gigahertz peaked (two of the three showing absorption). No excess absorption above the Galactic value is seen toward Q0420-388. This quasar has two damped Lyα systems at z = 3.08. The limit on the X-ray column density implies a low ionization fraction, N(H I)/N(H) ≳ 4 × 10-3 (3 σ), for solar abundances, for these systems, and can set a weak limit on the size of the absorber. In the emitted frame these PSPC spectra cover the band ∼0.5-10 keV, which has been well observed for low-redshift quasars and AGN. Comparison of high and low-redshift spectra in this emitted band shows no change of mean spectral index greater than ΔαE > 0.3 (99{\%} confidence) with either redshift or luminosity, for radio-loud quasars.",
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N2 - We present evidence that X-ray absorption is common in high-redshift quasars. We have studied six high-redshift (z ∼ 3) quasars with the ROSAT PSPC of which four are in directions of low Galactic NH. Three out of these four show excess absorption, while only three in ∼ 50 z ≲ 0.4 quasars do, indicating that such absorption must be common, but not ubiquitous, at high redshifts, and that the absorbers must lie at z > 0.4. The six quasars were: S5 0014+81, Q0420-388, PKS 0438-436, S4 0636+680. PKS 2000-330, PKS 2126-158, which have redshifts between 2.85 and 3.78. PKS 0438-436 and PKS 2126-158 show evidence for absorption above the local Galactic value at better than 99.999% confidence level. If the absorber is at the redshift of the quasar, then values of NH = [0.86(+0.49, -0.28)] × 1022 atoms cm-2 for PKS 0438-436, and NH = [1.45(+1.20, -0.64)] × 1022 atoms cm-2 for PKS 2126-158, are implied, assuming solar abundances. The spectrum of S4 0636+680 also suggests the presence of a similarly large absorption column density at the 98% confidence level. This absorption reverses the trend for the most luminous active galactic nuclei (AGN) to have the least X-ray absorption, so a new mechanism is likely to be responsible. Intervening absorption due to damped Lyα systems is a plausible cause. We also suggest, as an intrinsic model, that intracluster material, e.g., a cooling flow, around the quasar could account for both the X-ray spectrum and other properties of these quasars. All the quasars are radio-loud and three are gigahertz peaked (two of the three showing absorption). No excess absorption above the Galactic value is seen toward Q0420-388. This quasar has two damped Lyα systems at z = 3.08. The limit on the X-ray column density implies a low ionization fraction, N(H I)/N(H) ≳ 4 × 10-3 (3 σ), for solar abundances, for these systems, and can set a weak limit on the size of the absorber. In the emitted frame these PSPC spectra cover the band ∼0.5-10 keV, which has been well observed for low-redshift quasars and AGN. Comparison of high and low-redshift spectra in this emitted band shows no change of mean spectral index greater than ΔαE > 0.3 (99% confidence) with either redshift or luminosity, for radio-loud quasars.

AB - We present evidence that X-ray absorption is common in high-redshift quasars. We have studied six high-redshift (z ∼ 3) quasars with the ROSAT PSPC of which four are in directions of low Galactic NH. Three out of these four show excess absorption, while only three in ∼ 50 z ≲ 0.4 quasars do, indicating that such absorption must be common, but not ubiquitous, at high redshifts, and that the absorbers must lie at z > 0.4. The six quasars were: S5 0014+81, Q0420-388, PKS 0438-436, S4 0636+680. PKS 2000-330, PKS 2126-158, which have redshifts between 2.85 and 3.78. PKS 0438-436 and PKS 2126-158 show evidence for absorption above the local Galactic value at better than 99.999% confidence level. If the absorber is at the redshift of the quasar, then values of NH = [0.86(+0.49, -0.28)] × 1022 atoms cm-2 for PKS 0438-436, and NH = [1.45(+1.20, -0.64)] × 1022 atoms cm-2 for PKS 2126-158, are implied, assuming solar abundances. The spectrum of S4 0636+680 also suggests the presence of a similarly large absorption column density at the 98% confidence level. This absorption reverses the trend for the most luminous active galactic nuclei (AGN) to have the least X-ray absorption, so a new mechanism is likely to be responsible. Intervening absorption due to damped Lyα systems is a plausible cause. We also suggest, as an intrinsic model, that intracluster material, e.g., a cooling flow, around the quasar could account for both the X-ray spectrum and other properties of these quasars. All the quasars are radio-loud and three are gigahertz peaked (two of the three showing absorption). No excess absorption above the Galactic value is seen toward Q0420-388. This quasar has two damped Lyα systems at z = 3.08. The limit on the X-ray column density implies a low ionization fraction, N(H I)/N(H) ≳ 4 × 10-3 (3 σ), for solar abundances, for these systems, and can set a weak limit on the size of the absorber. In the emitted frame these PSPC spectra cover the band ∼0.5-10 keV, which has been well observed for low-redshift quasars and AGN. Comparison of high and low-redshift spectra in this emitted band shows no change of mean spectral index greater than ΔαE > 0.3 (99% confidence) with either redshift or luminosity, for radio-loud quasars.

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