Searcharxiv⌕ Search

arXiv subjects

S. Vaughan

Publications and source records attributed to S. Vaughan.

81 records · Page 5Linked to original sources
arXiv2002

How the X-ray spectrum of a Narrow-Line Seyfert 1 galaxy may be reflection dominated

A model for the inner regions of accretion flows is presented where, due to disc instabilities, cold and dense material is clumped into deep sheets or rings. Surrounding these density enhancements is hot, tenuous gas where coronal dissipation processes occur. We expect this situation to be most relevant when the accretion rate is close to Eddington and the disc is radiation-pressure dominated, and so may apply to Narrow-Line Seyfert~1 (NLS1) galaxies. In this scenario, the hard X-ray source is obscured for most observers, and so the detected X-ray emission would be dominated by reflection off the walls of the sheets. A simple Comptonization calculation shows that the large photon-indices characteristic of NLS1s would be a natural outcome of two reprocessors closely surrounding the hard X-ray source. We test this model by fitting the XMM-Newton spectrum of the NLS1 1H 0707-495 between 0.5 and 11 keV with reflection dominated ionized disc models. A very good fit is found with three different reflectors each subject to the same Γ=2.35 power-law. An iron overabundance is still required to fit the sharp drop in the spectrum at around 7 keV. We note that even a small corrugation of the accretion disc may result in Γ> 2 and a strong reflection component in the observed spectrum. Therefore, this model may also explain the strength and the variability characteristics of the MCG-6-30-15 Fe K line. The idea needs to be tested with further broadband XMM-Newton observations of NLS1s.

astro-ph↗
arXiv2001

XMM-Newton discovery of a sharp spectral feature at ~7 keV in the Narrow-Line Seyfert 1 galaxy 1H 0707-495

We report the first detection of a sharp spectral feature in a Narrow-Line Seyfert 1 galaxy. Using XMM-Newton we have observed 1H0707-495 and find a drop in flux by a factor of more than 2 at a rest-frame energy of ~7 keV without any detectable narrow Fe K alpha line emission. The energy of this feature suggests a connection with the neutral iron K photoelectric edge, but the lack of any obvious absorption in the spectrum at lower energies makes the interpretation challenging. We explore two alternative explanations for this unusual spectral feature: (i) partial covering absorption by clouds of neutral material and (ii) ionised disc reflection with lines and edges from different ionisation stages of iron blurred together by relativistic effects. We note that both models require an iron overabundance to explain the depth of the feature. The X-ray light curve shows strong and rapid variability, changing by a factor of four during the observation. The source displays modest spectral variability which is uncorrelated with flux.

astro-ph↗
arXiv2001

A complete sample of Seyfert galaxies selected at 1/4 keV

We have used the ROSAT Bright Source Catalogue to extract a complete sample of sources selected in the band from 0.1-0.4 keV. This 1/4 keV-selected sample is comprised of 54 Seyfert galaxies, 25 BL Lacertae objects, 4 clusters and 27 Galactic stars or binaries. Seyfert-type galaxies with ``ultrasoft'' X-ray spectra can very often be classed optically as Narrow-line Seyfert 1s (NLS1s). Such objects are readily detected in 1/4 keV surveys; the sample reported here contains 20 NLS1s, corresponding to a 40% fraction of the Seyferts. Optical spectra of the Seyfert galaxies were gathered for correlative analysis, which confirmed the well-known relations between X-ray slope and optical spectral properties (e.g., [O III]/H-beta ratio; Fe II strength, H-beta width). The various intercorrelations are most likely driven, fundamentally, by the shape of the photoionising continuum in Seyfert nuclei. We argue that a steep X-ray spectrum is a better indicator of an ``extreme'' set of physical properties in Seyfert galaxies than is the narrowness of the optical H-beta line. (Abridged)

astro-ph↗
arXiv2000

Studies of the high luminosity quasar, PDS 456

X-ray and multi-wavelength observations of the most luminous known local (z<0.3) AGN, the recently discovered radio-quiet quasar PDS 456, are presented. The spectral energy distribution shows that PDS 456 has a bolometric luminosity of 1e47 erg/s, peaking in the UV. The X-ray spectrum obtained by ASCA and RXTE shows considerable complexity. The most striking feature observed is a deep, highly-ionised, iron K edge (8.7 keV, rest-frame), originating via reprocessing from highly ionised material, possibly the inner accretion disk. PDS 456 was found to be remarkably variable for its luminosity; in one flare the X-ray flux doubled in just about 15 ksec. If confirmed this would be an unprecedented event in a high-luminosity source, with a light-crossing time corresponding to about 2RS. The implications are that either flaring occurs within the very central regions, or else that PDS 456 is a super-Eddington or relativistically beamed system.

astro-ph↗
arXiv1999

PDS 456: an Extreme Accretion Rate Quasar?

We present quasi-simultaneous ASCA and RXTE observations of the most luminous known AGN in the local (z<0.3) Universe, the recently discovered quasar PDS 456. Multiwavelength observations have been conducted which show that PDS 456 has a bolometric luminosity of 10^47 erg/s peaking in the UV part of the spectrum. In the X-ray band the 2-10 keV (rest-frame) luminosity is 10^45 erg/s. The broad-band X-ray spectrum obtained with ASCA and RXTE contains considerable complexity. The most striking feature observed is a very deep, ionised iron K edge, observed at 8.7 keV in the quasar rest-frame. We find that these features are consistent with reprocessing from highly ionised matter, probably the inner accretion disk. PDS 456 appeared to show a strong (factor of 2.1) outburst in just 17ksec, although non-intrinsic sources cannot be completely ruled out. If confirmed, this would be an unusual event for such a high-luminosity source, with the light-crossing-time corresponding to 2 Schwarzschild radii. The implication would be that flaring occurs within the very central regions, or else that PDS 456 is a `super-Eddington' or relativistically beamed system. Overall we conclude on the basis of the extreme blue/UV luminosity, the rapid X-ray variability and from the imprint of highly ionised material on the X-ray spectrum, that PDS 456 is a quasar with an unusually high accretion rate.

astro-ph↗
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.

astro-ph↗
arXiv1999

X-ray spectral complexity in narrow-line Seyfert 1 galaxies

We present a systematic analysis of the X-ray spectral properties of a sample of 22 ``narrow-line'' Seyfert 1 galaxies for which data are available from the ASCA public archive. Many of these sources, which were selected on the basis of their relatively narrow H-beta line width (FWHM <= 2000 km/s), show significant spectral complexity in the X-ray band. Their measured hard power-law continua have photon indices spanning the range 1.6 - 2.5 with a mean of 2.1, which is only slightly steeper than the norm for ``broad-line'' Seyfert 1s. All but four of the sources exhibit a soft excess, which can be modelled as blackbody emission (T_{bb} ~ 100 - 300 eV) superposed on the underlying power-law. This soft component is often so strong that, even in the relatively hard bandpass of ASCA, it contains a significant fraction, if not the bulk, of the X-ray luminosity, apparently ruling out models in which the soft excess is produced entirely through reprocessing of the hard continuum. Most notably, 6 of the 22 objects show evidence for a broad absorption feature centred in the energy range 1.1 - 1.4 keV, which could be the signature of resonance absorption in highly ionized material. A further 3 sources exhibit ``warm absorption'' edges in the 0.7 - 0.9 keV bandpass. Remarkably, all 9 ``absorbed'' sources have H-beta line widths below 1000 km/s, which is less than the median value for the sample taken as a whole. This tendency for very narrow line widths to correlate with the presence of ionized absorption features in the soft X-ray spectra of NLS1s, if confirmed in larger samples, may provide a further clue in the puzzle of active galactic nuclei.

astro-ph↗
arXiv1999

The ROSAT Wide Field Camera Extragalactic Survey

We report the results of a new analysis of the ROSAT Wide Field Camera (WFC) all-sky survey data, designed to detect extragalactic sources of extreme ultraviolet (EUV) radiation in regions of low Galactic N_H. We identify a total of 19 active galactic nuclei (AGN), more than double the number of confirmed AGN in the published WFC (2RE) survey. Our sample contains 8 narrow-line Seyfert 1 galaxies, making this the first reasonably-sized, complete sample of these extreme AGN, along with 6 broad-line Seyfert 1s and 5 BL Lacertae objects. These EUV-selected sources typically have steep soft X-ray spectra with a mean power-law energy index alpha_X=-2. The derived AGN luminosity function is rather flat and appears to cut off above a luminosity of 10^46 erg/s/keV, measured monochromatically at 200 eV. Narrow-line Seyfert~1s account for roughly half of the local (z~0) volume emissivity in the EUV band.

astro-ph↗