arXiv subjects

P. Magdziarz

Publications and source records attributed to P. Magdziarz.

13 recordsLinked to original sources

arXiv1999

Simultaneous EUVE/ASCA/RXTE Observations of NGC 5548

We present simultaneous observations by EUVE, ASCA, and RXTE of the type~1 Seyfert galaxy NGC 5548. These data indicate that variations in the EUV emission (at $\sim 0.2 $keV) appear to lead similar modulations in higher energy ($\ga 1 $keV) X-rays by $\sim$10--30 ks. This is contrary to popular models which attribute the correlated variability of the EUV, UV and optical emission in type~1 Seyferts to reprocessing of higher energy radiation. This behavior instead suggests that the variability of the optical through EUV emission is an important driver for the variability of the harder X-rays which are likely produced by thermal Comptonization. We also investigate the spectral characteristics of the fluorescent iron K$α$ line and Compton reflection emission. In contrast to prior measurements of these spectral features, we find that the iron K$α$ line has a relatively small equivalent width ($W_{Kα} \sim 100 $eV) and that the reflection component is consistent with a covering factor which is significantly less than unity ($Ω/2π\sim 0.4$--0.5). Notably, although the 2--10 keV X-ray flux varies by $\sim \pm 25$% and the derived reflection fraction appears to be constant throughout our observations, the flux in the Fe~K$α$ line is also constant. This behavior is difficult to reconcile in the context of standard Compton reflection models.

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arXiv1998

On Seyfert spectra and variability

We discuss the observational phenomenology of Seyferts from the point of view of the disk model, and argue that the spectral variability may be related to geometrical changes of the cold matter which provides a source of seed photons for Comptonization in a hot central disk. One possible configuration is a model with quasi-spherical accretion in the central part of the disk, with variability determined by the dynamics of the transition zone between the cold and hot disk. The soft excess component appears as a driver of spectral variability and contributes significantly to the source energetics.

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arXiv1998

Strong gravity and X-ray spectroscopy

This paper reviews the effects of general relativity in an X-ray spectrum reflected from a cold matter accreting onto a black hole. The spectrum consists of the iron K$α$ line and the Compton reflection. We sketch the overall picture of radiative processes in the central parts of the accretion flow with relation to the relativistic effects derived from the discrete features in the X-ray spectrum. We discuss implications for detection of relativistic effects and computational tools of spectral analysis.

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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

A model of the broad-band continuum of NGC 5548

We discuss a model of the central source in Seyfert 1 galaxy NGC 5548. The model assumes a three phase disk structure consisting of a cold outer disk, a hot central disk constituting a Comptonizing X/gamma source, and an intermediate unstable and complex phase emitting a soft excess component. The model qualitatively explains broad-band spectrum and variability behavior assuming that the soft excess contributes significantly to the continuum emission and drives variability by geometrical changes of the intermediate disk zone.

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arXiv1997

Gamma-ray flux variability in the sample of EGRET blazars

We analyze average gamma-ray variability statistics for the sample of blazars detected by CGRO/EGRET. We re-reduce all the available EGRET observations and analyze light curves by Monte Carlo modeling of the variability statistics including observational artifacts. We show that the observed variability behavior is dominated by the distribution of measurement errors which leads to strong systematical effects in all of the these statistics. General variability behavior detected by us is consistent with non-linear models and shows marginal correlation at long time scales in the structure function. We determine limits on distributions of the variability parameters with synthesis of flare population. We conclude that this method shows that all blazar light curves are consistent with a superposition of multiple flares.

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arXiv1997

Broad-band continuum and variability of NGC 5548

We analyze a composite broad-band optical/UV/Xgamma-ray spectrum of the Seyfert 1 galaxy NGC 5548. The spectrum consists of an average of simultaneous optical/IUE/Ginga observations accompanied by ROSAT and GRO/OSSE data from non-simultaneous observations. We show that the broad-band continuum is inconsistent with simple disk models extending to the soft X-rays. Instead, the soft-excess is well described by optically thick, low temperature, thermal Comptonization which may dominate the entire big blue bump. This might explain the observed tight UV/soft X-ray variability correlation and absence of a Lyman edge in this object. However, the plasma parameters inferred by the spectrum need stratification in optical depth and/or temperature to prevent physical inconsistency. The optical/UV/soft X-ray component contributes about half of the total source flux. The spectral variations of the soft-excess are consistent with that of the UV and argue that the components are closely related. The overall pattern of spectral variability suggests variations of the source geometry, and shows the optical/UV/soft X-ray component to be harder when brighter, while the hard X-ray component is softer when brighter.

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arXiv1996

Green's functions for transmission of X-rays and gamma-rays through cold media

Using a Monte Carlo method, we study Compton scattering and absorption of X-rays and gamma-rays in cold media. We consider transmission of X/gamma-rays through a shell of an arbitrary optical depth, for which we derive energy-dependent Green's functions. Fitting the Green functions with simple analytical formulae is in progress. We also present a simple treatment of the effect of absorption on Green's functions for Compton scattering, which allow to treat media with an arbitrary ionization state and chemical composition. Our transmission Green's functions allow to treat Thomson-thick absorbers, e.g. molecular tori of Seyfert 2s.

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arXiv1996

The average X-ray/gamma-ray spectrum of radio-quiet Seyfert 1s

We have obtained the average 1--500 keV spectrum of radio-quiet Seyfert 1s using data from EXOSAT, Ginga, HEAO, and GRO/OSSE. The spectral fit to the combined average EXOSAT and OSSE data is fully consistent with that for Ginga and OSSE, confirming results from an earlier Ginga/OSSE sample. The average spectrum is well-fitted by a power-law X-ray continuum with an energy spectral index of $α\simeq 0.9$ moderately absorbed by an ionized medium and with a Compton reflection component. A high-energy cutoff (or a break) in the the power-law component at a few hundred keV or more is required by the data. We also show that the corresponding average spectrum from HEAO A1 and A4 is fully compatible with that obtained from EXOSAT, Ginga and OSSE. These results confirm that the apparent discrepancy between the results of Ginga (with $α\simeq 0.9$) and the previous results of EXOSAT and HEAO (with $α\simeq 0.7$) is indeed due to ionized absorption and Compton reflection first taken into account for Ginga but not for the previous missions. Also, our results confirm that the Seyfert-1 spectra are on average cut off in gamma-rays at energies of at least a few hundred keV, not at $\sim 40$ keV (as suggested earlier by OSSE data alone). The average spectrum is compatible with emission from either an optically-thin relativistic thermal plasma in a disk corona, or with a nonthermal plasma with a power-law injection of relativistic electrons.

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arXiv1996

Broad-band gamma-ray and X-ray spectra of NGC 4151 and their implications for physical processes and geometry

We study gamma-ray observations of NGC 4151 by GRO/OSSE contemporaneous with X-ray observations by ROSAT and Ginga in 1991 June and with ASCA in 1993 May. The spectra are well modeled by thermal Comptonization and a dual neutral absorber. We also find, for the first time for NGC 4151, a Compton-reflection spectral component in the Ginga/OSSE data. The OSSE spectra of those and other observations of NGC 4151 are statistically undistinguishable from the average OSSE spectrum of radio-quiet Seyfert 1s. NGC 4151 observed in 1991 and 1993 has the intrinsic X-ray/gamma-ray spectrum typical for Seyfert 1s, and the main property distinguishing it from other Seyfert 1s is a large absorbing column of $\sim 10^{23}$ cm$^{-2}$. We find no evidence for a strong, broad and redshifted, Fe K$α$ line component in the ASCA spectrum of 1993 May. On the other hand, we confirm previous studies in that archival X-ray data do imply strong intrinsic X-ray variability and hardness of the intrinsic spectrum in low X-ray states. An observed softening of the intrinsic X-ray spectrum with the increasing flux implies variability in gamma-rays weaker than in X-rays, which agrees with the 100 keV flux changing only within a factor of 2 in archival OSSE and Granat/SIGMA observations. The relative hardness of the intrinsic X-ray spectrum rules out the homogeneous hot corona/cold disk model for this source. Instead, the hot plasma has to subtend a small solid angle as seen from the source of UV radiation. If the hot plasma is purely thermal, it consists of electrons rather than $e^+e^-$ pairs. On the other hand, the plasma can be pair-dominated if a small fraction of the power is nonthermal.

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arXiv1996

The origin of the correlation between the UV and X-rays in NGC 4151

We propose that the UV/X-ray correlation observed in NGC 4151 by EXOSAT and IUE is due to reemission of the X-ray flux absorbed by cold clouds in the line of sight. This transmission model satisfies the energy balance, provides a good fit to the X-ray and UV data, and predicts no Compton reflection of X-rays (as observed). The correlation can alternatively be explained by reprocessing of X-rays and gamma-rays by a cold accretion disk. A specific model in which X-rays and gamma-rays are emitted by a dissipative patchy corona and have the spectrum cut off at about 100 keV (as seen by OSSE) is shown to be energetically possible even if it does not give as satisfactory a fit as the transmission model. However, the model predicts a Compton- reflection spectral component, whose presence remains to be tested by future detectors sensitive above 10 keV.

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arXiv1996

The nature of the X-ray source in NGC 4151

Analysis of broad-band X/$γ$-ray spectra of NGC~4151 from contemporaneous observations by {\it GRO}/OSSE, {\it ROSAT}, {\it Ginga} and {\it ASCA}, shows that the data are well modelled with an intrinsic spectrum due to thermal Comptonization. The X-ray spectral index changes from $α\sim$ 0.4 to 0.7, and temperature stays at about 50 keV. The X-ray spectrum varies in such a way that it is consistent with the roughly constant soft $γ$-rays, with pivots at $\sim$ 100 keV. The UV/X-ray correlation observed by {\it EXOSAT} and {\it IUE} can be explained by two specific models with reprocessing of X-rays by cold matter. The first one is based on reemision of the X-ray flux absorbed by clouds in the line of sight. The model predicts no Compton reflection which is consistent with the broad-band spectra. The second model, assumes reprocessing of X-rays and $γ$-rays by a cold accretion disk with dissipative patchy corona. The homogenous corona model is ruled out here, since the hardness of the X-ray spectrum implies that the plasma is photon starved. The accretion disk model predicts Compton reflection which is only marginally allowed by the observations. Both our models satisfy the energy balance, and provide a good fit to the X/$γ$-rays and UV data.

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