The soft X-ray properties of quasars

Laor, Ari; Fiore, F.; Elvis, M.; Wilkes, Belinda; McDowell, Jonathan

doi:10.1063/1.46725

Cited by 13 publications

(29 citation statements)

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“…The slope obtained for the intermediate redshift quasars, assuming galactic A H (case A), is steeper than the IPC and Ginga slopes and is similar to those found in the PSPC spectra of low-redshift radio-quiet quasars (Walter & Fink 1993;Laor et al 1994;Fiore et al 1994), despite the significantly lower energy range covered by the PSPC. In this case then the major evolution in x-ray slope occurs between z~0 and z~2, with little change thereafter up to z~3.…”

Section: X-ray Spectral Evolutionsupporting

confidence: 74%

X-ray spectral evolution of high redshift quasars

Bechtold¹,

Elvis²,

Fiore³

et al. 1994

The Astronomical Journal

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At z^3, the x-ray spectra of radio-loud and radio-quiet quasars are different. High-redshift radio-quiet quasars either have large absorbing columns, N u , and steeper power law spectral indices, a E , than low redshift quasars, or no absorption and similar a E 's. In contrast, the radio-loud quasars at high redshift have substantial absorption and similar a E 's to low redshift quasars. Implications for the interpretation of the evolution of the luminosity function of quasars are discussed. If the absorption arises outside the central engine for both radio-loud and radio-quiet quasars, then radio-quiet quasars differ from the radio-loud quasars in that their emitted power law spectrum has evolved with redshift. We argue that this favors models where quasars are numerous and short-lived, rather than rare and long-lived.

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Section: X-ray Spectral Evolutionsupporting

confidence: 74%

X-ray spectral evolution of high redshift quasars

Bechtold¹,

Elvis²,

Fiore³

et al. 1994

The Astronomical Journal

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“…They identified the lines as mostly L-a n d M-shell transitions of Mg, Ne, and Fe. T h e same model also implies a pronounced O VII K a X-ray absorption feature at roughly the same energy as the feature detected in 1990 by BBXRT (Madejski et al 1994), which provides strong support to the apparent association of this object with a galaxy at z = 0.116. It was suggested t h a t the higher-energy X-ray absorption feature detected in 1980 by the Einstein O G S (Canizares & Kruper 1984) a n d identified with a broadened 0 VIII L y a line might have originated in outflowing clouds t h a t had a higher abundance of O VIII t h a n of 0 VII, possibly because they crossed our line of sight closer to the continuum source.…”

Section: Spectral Observations Of Pks 2155-304supporting

confidence: 65%

“…Furthermore, in these sources there is evidence for a distinct soft X-ray component (representing a "soft excess" over an extrapolation of the ISMabsorbed hard X-ray component), whose properties are at present controversial. In particular, the first results of ROSAT PSPC observations of a complete sample of optically selected QSOs (Laor et al 1994) have pointed to a spectral index «x = 1.50 ± 0.40 in the 0.2 -2 keV range that flattens by A a x « 0.5 above ~ 2 keV. This study has concluded that steep-ax sources are characterized by a week hard component.…”

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confidence: 68%

Active Galactic Nuclei in the Extreme Ultraviolet

Königl

1995

Highlights astron.

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The extreme ultraviolet (EUV) wavelength interval (commonly defined to constitute the range 68 -912 A, or 13.6 -182 eV) lies between the intensively studied FUV (912 -3000 A) and X-ray (1 -68 A) spectral regimes, but until recently has not been subjected to systematic investigations. This lack of information has been particularly acute in the case of active galactic nuclei (AGNs), where the very ability to carry out successful EUV observations has been seriously questioned. This has stemmed from the fact that, for a Galactic H I column density NQ{B. I) = 1 x 10 20 c m -2 , the interstellar transmission factor is 0.42 at 60 A, but a factor of 10 smaller at 100 A. In turn, there are no known regions of the interstellar medium (ISM) that have [JVQ(H I)/10 20 cm -2 ] < 0.6 (resp., 0.8) for positive (resp., negative) latitudes (e.g., Heiles 1991).There are, however, several reasons why EUV observations of AGNs could provide important information about the nature of these objects. In the case of radioquiet QSOs, the peak power is known to be emitted in the EUV range (several hundred A). Furthermore, in these sources there is evidence for a distinct soft X-ray component (representing a "soft excess" over an extrapolation of the ISMabsorbed hard X-ray component), whose properties are at present controversial. In particular, the first results of ROSAT PSPC observations of a complete sample of optically selected QSOs (Laor et al. 1994) have pointed to a spectral index «x = 1.50 ± 0.40 in the 0.2 -2 keV range that flattens by A a x « 0.5 above ~ 2 keV. This study has concluded that steep-ax sources are characterized by a week hard component. A similar distribution of a\ has been inferred for soft X-raybright QSOs and Seyfert 1 galaxies observed in the ROSAT all-sky survey. On the other hand, previous observations (enumerated in Laor et al. 1994) by the Einstein and EXOSAT satellites yielded different soft X-ray spectral slopes and led to an alternative explanation of the steep-ax objects. These questions could in principle be clarified by EUV measurements, which would then also help determine the soft X-ray emission mechanism.EUV observations could also provide valuable information on AGNs whose emission is dominated by a nonthermal component. In particular, the 0.5 -3.5 KeV spectrum of BL Lac objects (BLOs) appears to be a power-law extension of the UV continuum, and the entire UV-X-ray spectrum can be interpreted in terms of 517 /.

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“…It seems likely that the distinctive observational properties of this subclass of active galactic nuclei (AGN) relate to some fundamental physical parameter common to all NLS1. By analogy with the ‘high‐state’ properties of Galactic black‐hole candidates (GBHCs), it has been suggested that the fundamental ‘driver’ in NLS1 is the central black hole accreting at or above the Eddington limit (Pounds, Done & Osborne 1995; Laor et al 1997). One consequence of a high accretion rate might be that most of the power in NLS1 is liberated in a hot, photoionized accretion disc rather than a disc corona (Ross, Fabian & Mineshige 1992).…”

Section: Introductionmentioning

confidence: 99%

Evidence for an ionized disc in the narrow-line Seyfert 1 galaxy Ark 564

Vaughan

Pounds

Reeves

et al. 1999

Monthly Notices of the Royal Astronomical Society

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We present simultaneous ASCA and RXTE observations of Ark 564, the brightest known ‘narrow‐line’ Seyfert 1 in the 2–10 keV band. The measured X‐ray spectrum is dominated by a steep (Γ≈2.7) power‐law continuum extending to at least 20 keV, with imprinted Fe K‐line and edge features and an additional ‘soft excess’ below ∼1.5 keV. The energy of the iron K‐edge indicates the presence of highly ionized material, which we identify in terms of reflection from a strongly irradiated accretion disc. The high reflectivity of this putative disc, together with its strong intrinsic O viii Lyα and O viii recombination emission, can also explain much of the observed soft excess flux. Furthermore, the same spectral model also provides a reasonable match to the very steep 0.1–2 keV spectrum deduced from ROSAT data. The source is much more rapidly variable than ‘normal’ Seyfert 1s of comparable luminosity, increasing by a factor of ∼50 per cent in 1.6 h, with no measurable lag between the 0.5–2 keV and 3–12 keV bands, consistent with much of the soft excess flux arising from reprocessing of the primary power‐law component in the inner region of the accretion disc. We note, finally, that if the unusually steep power‐law component is a result of Compton cooling of a disc corona by an intense soft photon flux, then the implication is that the bulk of these soft photons lie in the unobserved extreme ultraviolet.

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The soft X-ray properties of quasars

Cited by 13 publications

References 0 publications

X-ray spectral evolution of high redshift quasars

X-ray spectral evolution of high redshift quasars

Active Galactic Nuclei in the Extreme Ultraviolet

Evidence for an ionized disc in the narrow-line Seyfert 1 galaxy Ark 564

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