arXiv subjects

C. Done

Publications and source records attributed to C. Done.

88 records · Page 5Linked to original sources

arXiv2000

Structure of the Circumnuclear Region of Seyfert 2 Galaxies Revealed by RXTE Hard X-ray Observations of NGC 4945

We report the Rossi X-ray Timing Explorer observations of the heavily absorbed Seyfert 2 galaxy NGC 4945, which confirm the large column density of the absorber, corresponding to the optical depth to Thomson scattering of about 2.4. The hard X-ray flux (8-30 keV) measured by us is rapidly variable with a large amplitude, on time scales of a day or less. This implies that the bulk of the extreme absorption does not originate in a parsec-size, geometrically thick molecular torus: limits on the amount of scattered flux require that the optically thick material on parsec scales must be rather geometrically thin, subtending a half-angle of less than 10 degrees as seen from the nucleus. This presents a problem in accounting for the Cosmic X-ray Background primarily with AGN possessing such geometry.

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arXiv2000

The relativistic iron line profile in the Seyfert 1 Galaxy IC4329a

We present simultaneous ASCA and RXTE data on the bright Seyfert 1 galaxy IC4329a. The iron line is significantly broadened, but not to the extent expected from an accretion disk which extends down to the last stable orbit around a black hole. We marginally detect a narrow line component, presumably from the molecular torus, but even including this gives a line profile from the accretion disk which is significantly narrower that that seen in MCG-6-30-15, and is much more like that seen from the low/hard state galactic black hole candidates. This is consistent with the inner disk being truncated before the last stable orbit, forming a hot flow at small radii as in the ADAF models. However, we cannot rule out the presence of an inner disk which does not contribute to the reflected spectrum, either because of extreme ionisation suppressing the characteristic atomic features of the reflected spectrum or because the X-ray source is intrinsically anisotropic, so it does not illuminate the inner disk. The source was monitored by RXTE every 2 days for 2 months, and these snapshot spectra show that there is intrinsic spectral variability. The data are good enough to disentangle the power law from the reflected continuum and we see that the power law softens as the source brightens. The lack of a corresponding increase in the observed reflected spectrum implies that either the changes in disk inner radial extent/ionisation structure are small, or that the variability is actually driven by changes in the seed photons which are decoupled from the hard X-ray mechanism.

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arXiv1999

On the Origin of the MeV Gamma-Ray Background

In this paper, we suggest a new hypothesis for explaining the spectrum of the extragalactic MeV gamma-ray background as observed by COMPTEL and SMM. We propose that both the flux level and spectrum can be accounted for as a superposition of non-thermal MeV tails in the spectra of Seyfert galaxies and other AGN. Although present detectors are not sensitive enough to obtain MeV data from individual extragalactic sources, indirect evidence in support of our hypothesis is found in OSSE and COMPTEL observations of the galactic black hole candidate Cygnus X-1.

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arXiv1999

Hard X-ray Emission From Low Mass X-ray Binaries

We report on Rossi X-ray Timing Explorer observations of four type I X-ray bursters; namely 1E1724-3045, GS1826-238, SLX1735-269 and KS1731-260. The first three were in a low state (LS) whereas KS1731-260 was in a high state. The LS sources have very similar power spectra, displaying high frequency noise up to \~200 Hz. For KS1731-260, its power spectrum is dominated by noise at frequencies below 20 Hz. In addition a quasi-periodic oscillation at 1200 Hz is detected. The 1-200 keV LS spectra are all consistent with resulting from thermal Comptonization with an electron temperature (kTe) around 25-30 keV. For KS1731-260, the spectrum is also dominated by Comptonization, but with kTe around 3 keV and no significant hard X-ray emission. With the exception of GS1826-238, there is an underlying soft component. For all sources, we have detected an iron line at 6.4 keV. A reflection component is present in the spectra of GS1826-238 and SLX1735-269. We suggest a model in which the region of main energy release, where hard X-rays are produced would be an optically thin boundary layer merged with an Advection Dominated Accretion Flow (ADAF). The soft component observed probably represents the unscattered emission from the accretion disk of variable inner radius. When the accretion rate increases, the inner disk radius shrinks, the strength of the reflected component and associated iron line increase, and the Comptonization region cools off in response to an increased cooling flux from the accretion disk and from the reprocessed/reflected component. Finally, in the light of these observations, we discuss extensively the various criteria recently proposed to distinguish between non-quiescent accreting black holes and neutron stars.

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arXiv1999

The May 1989 outburst of the Soft X-ray Transient GS 2023+338 (V404 Cyg)

We re-analyze archival Ginga data of the soft X-ray transient source GS 2023+338 covering the beginning of its May 1989 outburst. The source showed a number of rather unusual features: very high and apparently saturated luminosity, dramatic flux and spectral variability (often on ~1 sec time scale), generally very hard spectrum, with no obvious soft thermal component characteristic for soft/high state. We describe the spectrum obtained at the maximum of flux and we demonstrate that it is very different from spectra of other soft X-ray transients at similar luminosity. We confirm previous suggestions that the dramatic variability was due to heavy and strongly variable photo-electric absorption. We also demonstrate that for a short time the source's spectrum did look like a typical soft/high state spectrum but that this coincided with very heavy absorption.

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arXiv1998

On accretion flow in Soft X-ray Transients

We have analyzed archival Ginga data of a number of Soft X-ray Transient sources and modelled them in an attempt to constrain the geometry and physical properties of accretion. Our models include a self-consistent description of X-ray reprocessing, linking the properties of the reflected continuum and those of the iron fluorescence Kαline, and allowing for relativistic smearing of spectral features. We derive constraints on the inner extent of the disk and its ionization, assuming a generic geometry of a central X-ray source and an external accretion disk. Fitting our models to the data we have found that in some sources - GS 1124-68 (Nova Muscae 1991) and GS 2000+25 - the evolution during decline proceeded in qualitative agreement with recent theoretical models based on advection-dominated solutions of accretion flow. These models link the high/soft-low/hard transition with a change of geometry, most importantly, the inner radius of the optically thick disk. Quantitatively, our analysis requires serious revision of the model assumptions. A remarkably exceptional case is that of GS 2023+338, where the source behavior both during outburst and decline was quite different than the other, more ``standard'' ones. However, we found that the source did go into the high/soft state, contrary to claims in literature.

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arXiv1998

X-ray spectral evolution of GS 2023+338 (V404 Cyg) during decline after outburst

We have re-analyzed archival GINGA data of the soft X-ray transient source GS2023+308 (V404 Cyg) covering the decline phase of its May 1989 outburst. Our spectral modelling includes the relativistically smeared Compton reflected continuum and iron Kαfluorescent line near 6.5 keV produced by X-ray illumination of the accretion disc. This gives a powerful diagnostic of the accretion geometry, with the amplitude of the reprocessed components showing the solid angle subtended by the disc, while the detailed shape of the relativistic smearing shows how close this material is to the black hole event horizon. We find that reflection is always significantly present in the spectra, but that its fractional contribution decreases as the decline progresses. The amount of relativistic smearing is also consistent with decreasing during the decline, although this is poorly constrained except for the brightest spectra. One plausible scenario explaining this evolution is of an optically thick disc with inner radius increasing as a function of time, with the X-ray source in the form of a central corona. This is similar to the evolution inferred for other X-ray transient sources, such as Nova Muscae, except that the underlying power law spectrum of GS 2023+338 stayed constant as the disc geometry changed. This challenges the underlying assumption of almost all models for the spectra of accreting black holes, namely that the hard X-rays are formed by Comptonization of seed photons from the accretion disc.

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arXiv1998

Observation of Cen A by the Rossi X-ray Timing Explorer

The Rossi X-ray Timing Explorer made a short (10 ks) observation of the radio galaxy Centaurus A on 14 August 1996. Analysis of the combined 2.5-240 keV spectrum has revealed a heavily absorbed(NH=9.42+/-0.24 e22 cm-2) primary power law (index=1.86+/-0.015) and an iron line due to fluorescence of cold matter (EW=162+/-25 eV). Flux from either a jet, primary flux scattered into the line of sight, or primary flux seen through a partial absorber was not required. The iron line width is unresolved at the 95% confidence level (sigma < 0.54 keV). No significant variability in the iron line flux is seen from measurements over the last two decades, while the overall continuum flux varied by more than a factor of four, which implies that the line emission region is distant from that of the primary emission. While radio-quiet Seyfert galaxies exhibit spectral components attributable to Compton reflection from cold matter, Cen A reveals no such component (exposed solid angle ratio < 0.09). This supports unified models of active galaxies that have little difference between Seyfert 2 and low luminosity radio galaxies

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arXiv1998

The broad-band X-ray spectrum of Mrk 3

We have used non-simultaneous Ginga, ASCA and ROSAT observations to investigate the complex X-ray spectrum of the Seyfert 2 galaxy Mrk 3. We find that the composite spectrum can be well described in terms of a heavily cut-off hard X-ray continuum, iron K_alpha emission and a soft X-ray excess, with spectral variability confined to changes in the continuum normalisation and the flux in the iron line. Previous studies have suggested that the power-law continuum in Mrk 3 is unusually hard. We obtain a canonical value for the energy index of the continuum (i.e. alpha approx 0.7) when a warm absorber (responsible for an absorption edge observed near 8 keV) is included in the spectral model. Alternatively, the inclusion of a reflection component yields a comparable power-law index. The soft-excess flux cannot be modelled solely in terms of pure electron scattering of the underlying power-law continuum. However, a better fit to the spectral data is obtained if we include the effects of both emission and absorption in a partially photoionized scattering medium. In particular the spectral feature prominent at $\sim 0.9$ keV could represent O VIII recombination radiation produced in a hot photoionized medium. We discuss our results in the context of other recent studies of the soft X-ray spectra of Seyfert 2 galaxies.

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arXiv1998

The X-ray emission of the polar BL Hydri

We report on the analysis of the ASCA and BeppoSAX X-ray observations of the polar system BL Hyi. Emission from both poles is apparent in both observations; the emission from the second pole varies from cycle to cycle, indicating non-stationary accretion there. The temperature of the post-shock region is estimated to be about 10 keV. Inclusion of both complex absorption and Compton reflection significantly improves the quality of the fit. No soft X-ray component is observed; the BeppoSAX/LECS upper limit to the soft component is in agreement with theoretical expectations for this low magnetic field system.

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arXiv1998

BeppoSAX observations of the long period polar system V1309 Ori

We present BeppoSAX observations of the peculiar long period polar system V1309 Ori (RXJ0515.6+0105). The source was detected simultaneously at soft and, for the first time, at hard X-rays with the LECS and the MECS detectors. Both, the LECS and the MECS light curves are irregular with a bursting/flaring type behaviour indicating inhomogeneous accretion onto the white dwarf. This peculiar variability, together with an extreme high soft-to-hard X-ray luminosity ratio, indicates that in V1309 Ori accretion occurs predominantly in highly compressed chunks or ``blobs'' of matter. From coordinated ESO optical spectroscopy, we find indications that the magnetic field strength of the white dwarf is < 70 MG, not expected either from the 8 hr orbital period synchronism or from the strong soft-to-hard X-ray ratio suggesting alternative solutions for sustaining synchronism in this system.

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arXiv1997

Relativistic smearing of the reflection spectrum in Galactic Black Hole Candidates

We identify the reflected component in the GINGA spectra of Nova Muscae, a Black Hole transient system which has been used as the prototype for the recent advection dominated disk models. We see that the reflected spectrum is generally significantly relativistically smeared, and use this and the amount of reflection to track the innermost extent of the accretion disk. We see that the optically thick disk does retreat during the decline, but more slowly than predicted by the advective models, posing problems for this description of the accretion flow.

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arXiv1997

The X-ray spectrum of the polar BY Cam

ASCA and GINGA X-ray data from the magnetic Cataclysmic Variable BY Cam show that the spectrum is strongly affected by complex absorption, probably from the pre-shock accretion column. The intrinsic emission from the shock is significantly better described by the theoretically expected multi-temperature structure rather than a single temperature plasma, but with cyclotron cooling probably suppressing the highest temperature bremsstrahlung components. Reflection of this multi-temperature emission from the white dwarf surface is also significantly detected. All these spectral complexities are required to gain a physically self-consistent picture of the X-ray spectrum.

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arXiv1997

Complex Absorption and Reflection of a Multi-temperature Cyclotron-Bremsstrahlung X-ray Cooling Shock in BY Cam

We re-analyse the ASCA and GINGA X-ray data from BY Cam, a slightly asynchronous magnetic accreting white dwarf. The spectra are strongly affected by complex absorption, which we model as a continuous (power law) distribution of covering fraction and column of neutral material. This absorption causes a smooth hardening of the spectrum below 3 keV, and is probably produced by material in the preshock column which overlies the X-ray emission region. The ASCA data show that the intrinsic emission from the shock is not consistent with a single temperature plasma. Significant iron L emission co-existing with iron K shell lines from H and He-like iron clearly shows that there is a wide range of temperatures present, as expected from a cooling shock structure. The GINGA data give the best constraints on the maximum temperature emission in the shocked plasma, with kT(max)=21(+18,-4) keV. Cyclotron cooling should also be important, which supresses the highest temperature bremsstrahlung components, so the X-ray data only give a lower limit on the mass of the white dwarf of M> 0.5 solar masses. Reflection of the multi-temperature bremsstrahlung emission from the white dwarf surface is also significantly detected. We stress the importance of modelling ALL these effects in order to gain a physically self-consistent picture of the X-ray spectra from polars in general and BY Cam in particular.

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arXiv1997

Geometry of accretion in Soft X-ray Transients

We present results of modelling Ginga data of a number of Soft X-ray Transient sources performed with the aim of constraining the geometry of accretion during various stages of the sources' evolution. Assuming a generic geometry of a central, extended X-ray source with an external, optically thick accretion disk, we use consistent models of X-ray reprocessing to constrain the inner radius of the accretion disk and its ionization state. We show that the evolution of GS 1124-68 (Nova Muscae 1991) and GS 2000+25 agree qualitatively with the recent ideas linking the high/low state transition with changing the inner radius of the optically thick disk, but the case of GS 2023+33 (V404 Cyg) poses a problem for any model.

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arXiv1997

The X-ray Spectrum of the Dwarf Nova SS Cyg in quiescence and outburst

We reanalyse archival GINGA and ASCA X-ray data from SS Cyg in both quiescence and outburst, using multi-temperature plasma models for the continuum and line emission, together with their reflection from the X-ray illuminated white dwarf and accretion disk. Reflection is clearly detected in all the spectra, but its contribution is larger in the softer X-ray spectra seen in outburst than in quiescence. This supports models in which the quiescent inner disk is not present or not optically thick, so that the only reflector is the white dwarf surface rather than the white dwarf plus the disk. We detect partially ionized absorption in the ASCA outburst spectrum, which is probably the X-ray signature of the outflowing wind. The ASCA data also allow a detailed measure of the elemental abundances. We find that all detectable lines from the hot plasma (except perhaps Si and S) are a factor $\sim 2.5\times$ weaker than expected from solar abundances. We examine possible deviations from coronal equilibrium, but conclude that the heavy elements are truly underabundant in SS Cyg. In quiescence the underlying intrinsic spectrum is consistent with a single temperature plasma. This conflicts with the expected cooling of the material, and requires that either the plasma is reheated or that the cooler emission is masked by absorption in an optically thin inner disk which affects only that part of the boundary layer emission below the disk. By contrast, the outburst spectrum is much softer and is dominated by the cooling components but has a total hot plasma luminosity similar to that in quiescence, showing that the optically thin plasma emission is important even in outburst.

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