arXiv subjects

K. A. Pounds

Publications and source records attributed to K. A. Pounds.

40 records · Page 3Linked to original sources

arXiv1999

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

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 (Gamma~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 ionised 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 Ly-alpha 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% in 1.6 hours, 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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arXiv1996

Steep Spectrum Seyfert Galaxies

Seyfert-type AGN that exhibit unusually strong and steep soft X-ray spectra are reviewed, building on observations made during the ROSAT all-sky survey. Two individual cases are described in brief detail, both being identified initially in the small sample of Seyferts detected in the Wide Field Camera (WFC) survey. In one case the similarity in X-ray appearance to a high state Galactic Black Hole Candidate (GBHC) is noted, while the flare-like nature of the second example could represent a detection of the stellar capture process or a major accretion disc instability. Extending the review to include the larger (and growing) sample of steep spectrum Seyfert-type galaxies being found with the ROSAT PSPC, we draw attention to the more common properties of the group, namely steep spectra, rapid variability, strong Fe II emission and identification with narrow-line Seyfert 1 galaxies.

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arXiv1995

ROSAT PSPC observations of the infrared quasar IRAS 13349+2438: evidence for a warm absorber with internal dust

We present spatial, temporal and spectral analyses of ROSAT Position Sensitive Proportional Counter (PSPC) observations of the infrared loud quasar IRAS 13349+2438. IRAS 13349+2438 is the archetypal highly-polarized radio-quiet QSO and has an optical/infrared luminosity of about 2E46 erg/s. We detect variability in the ROSAT count rate by a factor of 4.1 in about one year, and there is also evidence for about 25 per cent variability within one week. We find no evidence for large intrinsic cold absorption of soft X-rays. These two facts have important consequences for the scattering-plus-transmission model of this object which was developed to explain its high wavelength-dependent polarization and other properties. The soft X-ray variability makes electron scattering of most of the soft X-rays difficult without a very peculiar scattering mirror. The lack of significant intrinsic cold X-ray absorption together with the large observed E(B-V) suggests either a very peculiar system geometry or, more probably, absorption by warm ionized gas with internal dust. There is evidence for an ionized oxygen edge in the X-ray spectrum. IRAS 13349+2438 has many properties that are similar to those of the Osterbrockian `narrow-line' Seyfert 1s.

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arXiv1995

THE UNUSUAL X-RAY AND OPTICAL PROPERTIES OF THE ULTRASOFT AGN ZWICKY 159.034 (RE J 1237+264)

Zwicky 159.034, one of the Seyfert galaxies identified with EUV sources detected during the ROSAT Wide Field Camera (WFC) all-sky survey, has unusual properties. The ROSAT Position Sensitive Proportional Counter (PSPC) 0.1--2.5 keV X-ray spectrum, obtained simultaneously with the WFC survey, appears extremely steep. Subsequent deeper pointed observations with ROSAT revealed that its 0.1--2.5 keV count rate had decreased by an extremely large amount (a factor of about 70). This is comparable to the variability amplitude seen in another ultrasoft Seyfert, E1615+061. There appears to be about 10 times as much flux in the soft component as in any expected hard tail suggesting that, in the absence of partial covering of the hard flux, the soft flux cannot arise via reprocessing of the hard tail. Its peculiar optical spectrum has permitted lines with widths about 1200--1500 km s^{-1}, and forbidden lines are weak or absent. Its optical spectrum shows evidence for variability, and might be used to study the role reprocessed X-rays play in the formation of optical lines.

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