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Abstract

Recent studies of quasi-stellar objects (QSOs) with ROSAT suggest the existence of a significant population of soft X-ray weak QSOs (SXW QSOs) where the soft X-ray flux is ∼10-30 times smaller than in typical QSOs. Why are these QSOs soft X-ray weak, and what is their relationship to broad absorption line QSOs (BAL QSOs) and X-ray warm absorber QSOs? As a first step in a systematic study of these objects, we establish a well-defined sample of SXW QSOs that includes all αox ≤ -2 QSOs from the Boroson & Green sample of 87 Bright Quasar Survey QSOs. SXW QSOs comprise ≈11% of this optically selected QSO sample, and we find soft X-ray weakness in both radio-quiet and radio-loud QSOs. From an analysis of C IV absorption in the 55 Boroson & Green QSOs with available C IV data, we find a remarkably strong correlation between αox and the C IV absorption equivalent width. This correlation suggests that absorption is the primary cause of soft X-ray weakness in QSOs, and it reveals a continuum of absorption properties connecting unabsorbed QSOs, X-ray warm absorber QSOs, SXW QSOs, and BAL QSOs. Many of our SXW QSOs have ultraviolet absorption that is intermediate in strength between that of X-ray warm absorber QSOs and that of BAL QSOs, and their X-ray absorption is also likely to be of intermediate strength. From a practical point of view, our correlation demonstrates that selection by soft X-ray weakness is an effective (≳80% successful) and observationally inexpensive way to find low-redshift QSOs with strong and interesting ultraviolet absorption. We have also identified several notable differences between the optical emission-line properties of SXW QSOs and those of the other Boroson & Green QSOs. SXW QSOs show systematically low [O III] luminosities and equivalent widths as well as distinctive Hβ line profiles. They tend to lie toward the weak [O III] end of Boroson & Green eigenvector 1, as do many low-ionization BAL QSOs. Unabsorbed Seyfert galaxies and QSOs with similar values of eigenvector 1 have been suggested to have extreme values of a primary physical parameter, perhaps mass accretion rate relative to the Eddington rate (Ṁ/ṀEdd). If these suggestions are correct, it is likely that SXW QSOs also tend to have generally high values of Ṁ/ṀEdd. Finally, we present and discuss correlations between αox and other QSO observables after removal of the SXW QSOs.

Original languageEnglish (US)
Pages (from-to)637-649
Number of pages13
JournalAstrophysical Journal
Volume528
Issue number2 PART 1
StatePublished - Jan 10 2000

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quasars
x rays
absorbers
ultraviolet absorption
eigenvectors
radio
H lines
Seyfert galaxies
suggestion
light emission
ionization
accretion
luminosity
continuums
causes

All Science Journal Classification (ASJC) codes

  • Astronomy and Astrophysics
  • Space and Planetary Science

Cite this

Brandt, W. N., Laor, A., & Wills, B. J. (2000). On the nature of soft X-ray weak quasi-stellar objects. Astrophysical Journal, 528(2 PART 1), 637-649.
Brandt, W. N. ; Laor, A. ; Wills, Beverley J. / On the nature of soft X-ray weak quasi-stellar objects. In: Astrophysical Journal. 2000 ; Vol. 528, No. 2 PART 1. pp. 637-649.
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title = "On the nature of soft X-ray weak quasi-stellar objects",
abstract = "Recent studies of quasi-stellar objects (QSOs) with ROSAT suggest the existence of a significant population of soft X-ray weak QSOs (SXW QSOs) where the soft X-ray flux is ∼10-30 times smaller than in typical QSOs. Why are these QSOs soft X-ray weak, and what is their relationship to broad absorption line QSOs (BAL QSOs) and X-ray warm absorber QSOs? As a first step in a systematic study of these objects, we establish a well-defined sample of SXW QSOs that includes all αox ≤ -2 QSOs from the Boroson & Green sample of 87 Bright Quasar Survey QSOs. SXW QSOs comprise ≈11{\%} of this optically selected QSO sample, and we find soft X-ray weakness in both radio-quiet and radio-loud QSOs. From an analysis of C IV absorption in the 55 Boroson & Green QSOs with available C IV data, we find a remarkably strong correlation between αox and the C IV absorption equivalent width. This correlation suggests that absorption is the primary cause of soft X-ray weakness in QSOs, and it reveals a continuum of absorption properties connecting unabsorbed QSOs, X-ray warm absorber QSOs, SXW QSOs, and BAL QSOs. Many of our SXW QSOs have ultraviolet absorption that is intermediate in strength between that of X-ray warm absorber QSOs and that of BAL QSOs, and their X-ray absorption is also likely to be of intermediate strength. From a practical point of view, our correlation demonstrates that selection by soft X-ray weakness is an effective (≳80{\%} successful) and observationally inexpensive way to find low-redshift QSOs with strong and interesting ultraviolet absorption. We have also identified several notable differences between the optical emission-line properties of SXW QSOs and those of the other Boroson & Green QSOs. SXW QSOs show systematically low [O III] luminosities and equivalent widths as well as distinctive Hβ line profiles. They tend to lie toward the weak [O III] end of Boroson & Green eigenvector 1, as do many low-ionization BAL QSOs. Unabsorbed Seyfert galaxies and QSOs with similar values of eigenvector 1 have been suggested to have extreme values of a primary physical parameter, perhaps mass accretion rate relative to the Eddington rate (Ṁ/ṀEdd). If these suggestions are correct, it is likely that SXW QSOs also tend to have generally high values of Ṁ/ṀEdd. Finally, we present and discuss correlations between αox and other QSO observables after removal of the SXW QSOs.",
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Brandt, WN, Laor, A & Wills, BJ 2000, 'On the nature of soft X-ray weak quasi-stellar objects', Astrophysical Journal, vol. 528, no. 2 PART 1, pp. 637-649.

On the nature of soft X-ray weak quasi-stellar objects. / Brandt, W. N.; Laor, A.; Wills, Beverley J.

In: Astrophysical Journal, Vol. 528, No. 2 PART 1, 10.01.2000, p. 637-649.

Research output: Contribution to journalArticle

TY - JOUR

T1 - On the nature of soft X-ray weak quasi-stellar objects

AU - Brandt, W. N.

AU - Laor, A.

AU - Wills, Beverley J.

PY - 2000/1/10

Y1 - 2000/1/10

N2 - Recent studies of quasi-stellar objects (QSOs) with ROSAT suggest the existence of a significant population of soft X-ray weak QSOs (SXW QSOs) where the soft X-ray flux is ∼10-30 times smaller than in typical QSOs. Why are these QSOs soft X-ray weak, and what is their relationship to broad absorption line QSOs (BAL QSOs) and X-ray warm absorber QSOs? As a first step in a systematic study of these objects, we establish a well-defined sample of SXW QSOs that includes all αox ≤ -2 QSOs from the Boroson & Green sample of 87 Bright Quasar Survey QSOs. SXW QSOs comprise ≈11% of this optically selected QSO sample, and we find soft X-ray weakness in both radio-quiet and radio-loud QSOs. From an analysis of C IV absorption in the 55 Boroson & Green QSOs with available C IV data, we find a remarkably strong correlation between αox and the C IV absorption equivalent width. This correlation suggests that absorption is the primary cause of soft X-ray weakness in QSOs, and it reveals a continuum of absorption properties connecting unabsorbed QSOs, X-ray warm absorber QSOs, SXW QSOs, and BAL QSOs. Many of our SXW QSOs have ultraviolet absorption that is intermediate in strength between that of X-ray warm absorber QSOs and that of BAL QSOs, and their X-ray absorption is also likely to be of intermediate strength. From a practical point of view, our correlation demonstrates that selection by soft X-ray weakness is an effective (≳80% successful) and observationally inexpensive way to find low-redshift QSOs with strong and interesting ultraviolet absorption. We have also identified several notable differences between the optical emission-line properties of SXW QSOs and those of the other Boroson & Green QSOs. SXW QSOs show systematically low [O III] luminosities and equivalent widths as well as distinctive Hβ line profiles. They tend to lie toward the weak [O III] end of Boroson & Green eigenvector 1, as do many low-ionization BAL QSOs. Unabsorbed Seyfert galaxies and QSOs with similar values of eigenvector 1 have been suggested to have extreme values of a primary physical parameter, perhaps mass accretion rate relative to the Eddington rate (Ṁ/ṀEdd). If these suggestions are correct, it is likely that SXW QSOs also tend to have generally high values of Ṁ/ṀEdd. Finally, we present and discuss correlations between αox and other QSO observables after removal of the SXW QSOs.

AB - Recent studies of quasi-stellar objects (QSOs) with ROSAT suggest the existence of a significant population of soft X-ray weak QSOs (SXW QSOs) where the soft X-ray flux is ∼10-30 times smaller than in typical QSOs. Why are these QSOs soft X-ray weak, and what is their relationship to broad absorption line QSOs (BAL QSOs) and X-ray warm absorber QSOs? As a first step in a systematic study of these objects, we establish a well-defined sample of SXW QSOs that includes all αox ≤ -2 QSOs from the Boroson & Green sample of 87 Bright Quasar Survey QSOs. SXW QSOs comprise ≈11% of this optically selected QSO sample, and we find soft X-ray weakness in both radio-quiet and radio-loud QSOs. From an analysis of C IV absorption in the 55 Boroson & Green QSOs with available C IV data, we find a remarkably strong correlation between αox and the C IV absorption equivalent width. This correlation suggests that absorption is the primary cause of soft X-ray weakness in QSOs, and it reveals a continuum of absorption properties connecting unabsorbed QSOs, X-ray warm absorber QSOs, SXW QSOs, and BAL QSOs. Many of our SXW QSOs have ultraviolet absorption that is intermediate in strength between that of X-ray warm absorber QSOs and that of BAL QSOs, and their X-ray absorption is also likely to be of intermediate strength. From a practical point of view, our correlation demonstrates that selection by soft X-ray weakness is an effective (≳80% successful) and observationally inexpensive way to find low-redshift QSOs with strong and interesting ultraviolet absorption. We have also identified several notable differences between the optical emission-line properties of SXW QSOs and those of the other Boroson & Green QSOs. SXW QSOs show systematically low [O III] luminosities and equivalent widths as well as distinctive Hβ line profiles. They tend to lie toward the weak [O III] end of Boroson & Green eigenvector 1, as do many low-ionization BAL QSOs. Unabsorbed Seyfert galaxies and QSOs with similar values of eigenvector 1 have been suggested to have extreme values of a primary physical parameter, perhaps mass accretion rate relative to the Eddington rate (Ṁ/ṀEdd). If these suggestions are correct, it is likely that SXW QSOs also tend to have generally high values of Ṁ/ṀEdd. Finally, we present and discuss correlations between αox and other QSO observables after removal of the SXW QSOs.

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Brandt WN, Laor A, Wills BJ. On the nature of soft X-ray weak quasi-stellar objects. Astrophysical Journal. 2000 Jan 10;528(2 PART 1):637-649.