arXiv subjects

B. Czerny

Publications and source records attributed to B. Czerny.

At least 109 records · Page 6Linked to original sources

arXiv2001

The Nature of the Emission Components in the Quasar/NLS1 PG1211+143

We present the study of the emission properties of the quasar PG1211+143, which belongs to the class of Narrow Line Seyfert 1 galaxies. On the basis of observational data analyzed by us and collected from the literature, we study the temporal and spectral variability of the source in the optical/UV/X-ray bands and we propose a model that explains the spectrum emitted in this broad energy range. In this model, the intrinsic emission originating in the warm skin of the accretion disk is responsible for the spectral component that is dominant in the softest X-ray range. The shape of reflected spectrum as well as Fe K line detected in hard X-rays require the reflecting medium to be mildly ionized (xi~500). We identify this reflector with the warm skin of the disk and we show that the heating of the skin is consistent with the classical alpha P_{tot} prescription, while alpha P_{gas} option is at least two orders of magnitude too low to provide the required heating. We find that the mass of the central black hole is relatively small (M_BH~10^7- 10^8 Msun, which is consistent with the Broad Line Region mapping results and characteristic for NLS1 class.

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arXiv2001

Black hole masses from power density spectra: determinations and consequences

We analyze the scaling of the X-ray power density spectra with the mass of the black hole on the example of Cyg X-1 and Seyfert 1 galaxy NGC 5548. We show that the high frequency tail of the power density spectrum can be successfully used for determination of the black hole mass. We determine the masses of the black holes in 6 Broad Line Seyfert 1 galaxies, 5 Narrow Line Seyfert 1 galaxies and two QSOs using available power density spectra. The proposed scaling is clearly appropriate for other Seyfert galaxies and QSOs. In all but 1 normal Seyferts the resulting luminosity to the Eddington luminosity ratio is smaller than 0.15, with a source MCG -6-15-30 being an exception. The applicability of the same scaling to Narrow Line Seyfert 1 is less clear and there may be a systematic shift between the power spectra of NLS1 and S1 galaxies of the same mass, leading to underestimation of the black hole mass. However, both the method based on variability and the method based on spectral fitting show that those galaxies have relatively low masses and high luminosity to the Eddington luminosity ratio, supporting the view of those objects as analogs of galactic sources in their high/soft or very high state based on the overall spectral shape. Bulge masses of their host galaxies are similar to normal Seyfert galaxies so they do not follow the black hole mass-bulge mass relation for Seyfert galaxies, being evolutionary less advanced, as suggested by Mathur (2000). The bulge mass-black hole mass relation in our sample is consistent with being linear, with black hole to bulge ratio $\sim$ 0.03 %, similar to Wandel (1999) and Laor (1998, 2001) for low mass objects but significantly shifted from the relation of Magorrian et al. (1998) and McLure & Dunlop (2000).

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arXiv2000

What do the UV Spectra of Narrow-line Seyfert 1 Galaxies tell us about their BLR?

We study the UV spectra of narrow-line Seyfert~1 (NLSy1) galaxies and compare them with ``normal'' AGN. Similar to their optical lines, the NLSy1s show narrower UV lines. They are also characterized by weaker CIV 1549, CIII] 1909, and stronger AlIII 1857 emission. These UV line properties add to the optical and X-ray properties known to be part of the Boroson and Green eigenvector 1. We show that the steep soft-X-rays, which characterize the NLSy1s SEDs, change the equilibrium of the two phase cloud-intercloud medium resulting in somewhat higher BLR cloud densities, lower ionization parameter and larger BLR radii. These modified conditions can explain the unusual emission line properties we find in NLSy1. Using a specific model of an accretion disk with corona presented by Witt, Czerny and Zycki (1997), we also show that the steep soft and hard-X-ray continua can be explained if the $L/L_{Edd}$ ratios are larger than in ``normal'' Seyfert1/QSO strengthening earlier suggestions that the $L/L_{Edd}$ is the physical parameter driving this eigenvector.

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arXiv2000

Vertical structure of the accreting two-temperature corona and the transition to an ADAF

We investigate the model of the disc/corona accretion flow around the black hole. The model, parameterized by the total (i.e. disk plus corona) accretion rate, $\dot m$, mass of the black hole, $M$ and the viscosity parameter, $α$, uniquely determines the fraction of energy released in the corona as a function of radius and, in particular, the transition radius to the single-phase ADAF flow. Self-consistent solutions with the mass exchange between phases display radial dependence of the parameters. This model reproduces all characteristic luminosity states of accretion black hole without any additional ad hoc assumptions. In particular. the mechanism of the disk evaporation leads to a new, almost horizontal branch on the accretion flow's stability curve (i.e. the dependence of accretion rate on surface density) at the critical accretion rate.

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arXiv2000

A cloud model of active galactic nuclei: the iron K-alpha line diagnostics

The origin and the profile of the iron K-alpha line from active galactic nuclei is being studied. The model with a quasi-spherical distribution of clouds around a compact galactic nucleus is explored in which the intrinsic spectral feature around 6-7 keV is substantially affected by Comptonization of primary X-rays. The predicted spectral profiles are influenced mainly by motion of the clouds in strong gravitational field of the centre, and by orientation of the source with respect to the observer. The case of the Seyfert galaxy MCG-6-30-15 is examined in more detail and free parameters of the model are determined. A robust result is obtained: acceptable fits to the observed profile correspond to the spherical distribution of the clouds with strong self-obscuration effects and with rather high value of ionization parameter. This is a different situation from that considered in the model of a cold illuminated disc. Nevertheless, geometrically flat (disc-type) configurations are not rejected.

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arXiv2000

X-ray reprocessing in Narrow-Line Seyfert 1 Galaxies: Ton S180 and Ark 564

We present the results of spectral analysis of the ASCA data for the Narrow-Line Seyfert 1 galaxy (NLS1) Ton S180 and simultaneous ASCA and RXTE data modelling for the NLS1 Ark 564. We model both the primary and reflected continuum as well as the iron K alpha line, the energy of which depends on the ionization state of the reprocessor. We show that the reprocessing matter is mildly ionized, and we find the soft to hard luminosity ratio to be about 2.5. The accretion rate approximately corresponds to the Eddington limit value.

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arXiv2000

Disk/corona model: The transition to ADAF

We propose a model of the accretion flow onto a black hole consisting of the accretion disk with an accreting two-temperature corona. The model is based on assumptions about the radiative and conductive energy exchange between the two phases and the pressure equilibrium. The complete model is determined by the mass, the accretion rate, and the viscosity parameter. We present the radial dependencies of parameters of such a two-phase flow, with advection in the corona and the disk/corona mass exchange due to evaporation/condensation included, and we determine the transition radius from a two-phase disk/corona accretion to a single-phase optically thin flow (ADAF) in the innermost part of the disk as a function of accretion rate. We identify the NLS1 galaxies with objects accreting at a rate close to the Eddington accretion rate. The strong variability of these objects may be related to the limit cycle behaviour expected in this luminosity range, as the disk, unstable due to the dominance by the radiation pressure, oscillates between the two stable branches: the advection-dominated optically thick branch and the evaporation branch.

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arXiv2000

The UV spectra of NLS1s - Implications for their broad line regions

We study the UV spectra of NLS1 galaxies and compare them with typical Seyfert 1 galaxies and quasars. The NLS1 spectra show narrower UV lines as well as weaker CIV lambda 1549 and CIII] lambda 1909 emission. We show that these line properties are due to a lower ionization parameter and somewhat higher BLR cloud densities. These modified conditions can be explained by the hotter big blue bumps observed in NLS1s, which are in turn due to higher L/L_Edd ratios, as shown by our accretion disk and corona modeling of the NLS1 continua. We also present evidence that the Boroson & Green eigenvector 1, which is correlated with the optical and UV emission-line properties, is not driven by orientation and hence NLS1s, which have extreme eigenvector 1 values, are not viewed from an extreme viewing angle.

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arXiv2000

Accretion and Emission Processes in AGN: the UV-X Connection

The main characteristics of the average spectrum of radio quiet AGN in the UV and X-ray range are reviewed, and the emission mechanisms are discussed in the framework of accretion disk models, in particular the ``irradiated cold relativistic disk". It is shown that some problems arise in confronting the predictions of the model to the observations. We propose an alternative model in terms of a hot disk surrounded by a cold Compton thick medium. Finally we mention problems with remote regions of the accretion disk.

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arXiv2000

Toy model of obscurational variability in active galactic nuclei

We propose an explanation of the variability of AGN based on a cloud model of accretion onto a black hole. These clouds, present at a distance of 10-100 Schwarzschild radii, possibly come from the disrupted innermost disk and they can partially obscure the central X-ray source due to their extreme Compton optical depth. We consider the implications of this scenario using a toy model of AGN spectra and support the results with full radiative transfer computations from the codes TITAN and NOAR. We show that small random rearrangements of the cloud distribution can happen on timescales of the order of 10^2 - 10^6 s and may lead to relatively high variability amplitude in the X-ray band if the mean covering factor is large. The normalized variability amplitude in the UV band is either the same as in the X-ray band, if the contribution from the dark sides of the clouds to the UV band is negligible, or it is smaller. The X-ray spectrum should basically preserve its spectral shape even during the high amplitude variability, within the frame of our model.

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arXiv1999

Obscuration model of Variability in AGN

There are strong suggestions that the disk-like accretion flow onto massive black hole in AGN is disrupted in its innermost part (10-100 Rg), possibly due to the radiation pressure instability. It may form a hot optically thin quasi spherical (ADAF) flow surrounded by or containing denser clouds due to the disruption of the disk. Such clouds might be optically thick, with a Thompson depth of order of 10 or more. Within the frame of this cloud scenario (Collin-Souffrin et al. 1996, Czerny & Dumont 1998), obscuration events are expected and the effect would be seen as a variability. We consider expected random variability due to statistical dispersion in location of clouds along the line of sight for a constant covering factor. We discuss a simple analytical toy model which provides us with the estimates of the mean spectral properties and variability amplitude of AGN, and we support them with radiative transfer computations done with the use of TITAN code of Dumont, Abrassart & Collin (1999) and NOAR code of Abrassart (1999).

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arXiv1998

Vertical Structure of Accretion Discs with Hot Coronae in AGN

We study vertical structure of radiation pressure dominated disc with a hot corona. We include all the relevant processes like bound--free opacity and convection. We show that the presence of the corona modifies considerably the density and the opacity of the disc surface layers which are important from the point of view of spectrum formation. The surface of the disc with a corona is much denser and less ionized than the surface of a bare disc. Such a disc is likely to produce a neutral reflection and a local spectrum close to a black body. This effect will help to reconcile the predictions of accretion disc models with the observational data since a neutral reflection and a lack of Lyman edge are generally seen in AGN.

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arXiv1998

Comment on Viscous Stability of Relativistic Keplerian Accretion Disks

Recently Ghosh (1998) reported a new regime of instability in Keplerian accretion disks which is caused by relativistic effects. This instability appears in the gas pressure dominated region when all relativistic corrections to the disk structure equations are taken into account. We show that he uses the stability criterion in completely wrong way leading to inappropriate conclusions. We perform a standard stability analysis to show that no unstable region can be found when the relativistic disk is gas pressure dominated.

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arXiv1998

Power density spectrum of NGC 5548 and the nature of its variability

We derive power density spectra in the optical and X-ray band in the timescale range from several years down to $\sim $ a day. We suggest that the optical power density spectrum consists of two separate components: long timescale variations and short timescale variations, with the dividing timescale around 100 days. The shape of the short timescale component is similar to X-ray power density spectrum which is consistent with the interpretation of short timescale optical variations being caused by X-ray reprocessing. We show that the observed optical long timescale variability is consistent with thermal pulsations of the accretion disc.

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arXiv1998

Cloud model of the mean quasar spectrum

We assume a distribution of clouds optically thick for electron scattering (OTCM) which are moderately optically thin for absorption and we consider them as a model of the mean quasar spectrum of Laor et al. (1997). We show that the model is particularly sensitive to the value of the ionization parameter $ξ$ and that for $ξ\sim 500$ the model well reproduces the optical/UV/X-ray mean quasar spectrum, in agreement with the estimates of the ionization parameter based on the energy of the iron $K_α$ line. We cannot definitively reject synchrotron emission as a source of primary radiation but we favor the model in which the hard X-ray emission is produced by Compton scattering of soft photons in a central hot medium surrounded by cool clouds. In such a model clouds are located typically at the distance of $\sim 12 R_{Schw}$, with the covering factor about 0.88 and the radius of hot plasma is $\sim 9 R_{Schw}$. The model explains optical/UV emission as predominantly due to the dark sides of the clouds and the soft X-ray emission as due to the reflection by the irradiated sides of the clouds. Therefore, atomic features are expected in these bands although they are hardly present in the observational data. The kinematical effects connected with the cloud motion affect those features but do not remove them. The level of the primary emission required to model the mean quasar spectrum is too low to reproduce the equivalent width of the iron K$_α$ line correctly but more detailed computations may resolve this problem.

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arXiv1997

Spectral Components in the Optical Emission of the Seyfert Galaxy NGC 5548 and the Comparison of Intrinsic Nuclear Spectra with Accreting Corona Model

We study the extensively monitored Seyfert galaxy NGC 5548 and compare its nuclear emission with models of accretion disk with accreting corona. To obtain the intrinsic nuclear spectra from the observed spectra we had to estimate and subtract the contribution from circumnuclear components such as stars, the Balmer continuum and blended FeII lines, and the FC2 extended, featureless continuum. The true nuclear spectra were compared with a two parameter model of the accreting disk with an accreting corona, described by the mass of the central black hole and viscosity. The model that best fitted the data was for M_{BH}=1.4e8 solar masses and the viscosity parameter alpha=0.033. Such a low viscosity parameter was necessary to produce the sufficient amount of X-rays. The vertical outflow of mass from corona in the form of wind had to be neglected in our model in order to fit into high and low states that NGC 5548 underwent. The model also predicts the behavior of the overall opt/UV/X continuum of NGC 5548 during the whole five year campaign.

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arXiv1997

Implication of the discovery of the proper motion of the optical counterpart of GRB 970228 for the models of gamma-ray bursts

The accurate position determination of GRB 970228 by the Beppo/SAX Satellite led to the discovery of a fading X-ray and optical counterparts of the burst. About a month after the GRB event the proper motion of the ejecta was detected and the extended optical source has faded below the Keck detection level. We analyze these observations in the framework of the most popular gamma ray burst models. We find a lower limit on the distance to GRB 970228 d>6.6 pc. We estimate the amount of energy required to explain the motion of a point source and the variability of the extended source. We find that the cosmological models are ruled out. The Galactic corona models suffer difficulties if the distance to the burst is > 100 kpc or the variability of the extended source is directly connected with the burst No constraints are found for GRBs originating between 6.6 pc and $\sim 10$ kpc.

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arXiv1996

The role of extended corona in formation of emission lines and continuum in AGN

We study a model of an accretion disc surrounded by an extended corona of the temperature 10^7 - 10^8 K. This corona modifies the disk spectrum since it redirects a significant fraction of the emission from the central parts towards the more distant parts of the disk. The same corona is indirectly the source of the broad emission lines because we expect the formation of cool clouds at the basis of the corona due to thermal instabilities. We model the number of the clouds and their motion through the corona adopting a few different physically sound assumptions. Comparing the predicted optical/UV continua and emission line ratios and profiles with the observed values we favor a particular model of a typical quasar. It radiates at ~0.5 of the Eddington luminosity, the corona surrounding the disk is additionally heated in excess of the Inverse Compton heating, and the broad line clouds are most probably destroyed when accelerated vertically above the sound speed within the corona.

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