arXiv subjects

M. Dovciak

Publications and source records attributed to M. Dovciak.

At least 73 records · Page 4Linked to original sources

arXiv2007

Variation of the primary and reprocessed radiation in the flare-spot model

We study light curves and spectra (equivalent widths of the iron line and some other spectral characteristics) which arise by reprocessing on the surface of an accretion disc, following its illumination by a primary off-axis source - an X-ray 'flare', assumed to be a point-like source just above the accretion disc. We consider all general relativity effects (energy shifts, light bending, time delays, delay amplification due to the spot motion) near a rotating black hole. For some sets of parameters the reflected flux exceeds the flux from the primary component. We show that the orbit-induced variations of the equivalent width with respect to its mean value can be as high as 30% for an observer's inclination of 30 degrees, and much more at higher inclinations. We calculate the ratio of the reflected flux to the primary flux and the hardness ratio which we find to vary significantly with the spot phase mainly for small orbital radii. This offers the chance to estimate the lower limit of the black hole spin if the flare arises close to the black hole. We show the results for different values of the flare orbital radius.

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arXiv2007

Variation in the primary and reprocessed radiation from an orbiting spot around a black hole

We study light curves and spectra (equivalent widths of the iron line and some other spectral characteristics) which arise by reflection on the surface of an accretion disc, following its illumination by a primary off-axis source - an X-ray 'flare', assumed to be a point-like source just above the accretion disc resulting in a spot with radius dr/r<1. We consider General Relativity effects (energy shifts, light bending, time delays) near a rotating black hole, and we find them all important, including the light bending and delay amplification due to the spot motion. For some sets of parameters the reflected flux exceeds the flux from the primary component. We show that the orbit-induced variations of the equivalent width with respect to its mean value can be as high as 30% for the observer's inclination of 30 degrees, and much more at higher inclinations. We calculate the ratio of the reflected flux to the primary flux and the hardness ratio which we find to vary significantly with the spot phase mainly for small orbital radii. This offers the chance to estimate the lower limit of the black hole spin if the flare arises close to the black hole.

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arXiv2007

FERO (Finding Extreme Relativistic Objects): statistics of relativistic broad Fe K alpha lines in AGN

The properties of the relativistically broadened Fe K alpha line emitted in Active Galactic Nuclei (AGN) are still debated among the AGN community. Recent works seem to exclude that the broad Fe line is a common feature of AGN. The analysis of a large sample composed by 157 XMM-Newton archival observations of radio quiet AGN is presented here. This ongoing project is a development of the work reported in Guainazzi et al. 2006.

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arXiv2007

Flares from spiral waves by lensing and time-delay amplification?

Episodically accreting black holes are thought to produce flares when a chunk of particles is accelerated to high velocity near the black hole horizon. This also seems to be the case of Sagittarius A* in the Galactic Center, where the broad-band radiation is produced, likely via the synchrotron self-Compton mechanism. It has been proposed that strong-field gravitational lensing magnifies the flares. The effect of lensing is generally weak and requires a fine-tuned geometrical arrangement, which occurs with only a low probability. However, there are several aspects that make Sagittarius A* a promising target to reveal strong gravity effects. Unlike type II (obscured) active galaxies, chances are that a flare is detected at high inclination, which would be favourable for lensing. Time delays can then significantly influence the observed flare duration and the form of light-curve profiles. Here we discuss an idea that the impact of lensing amplification should be considerably enhanced when the shape of the flaring clump is appropriately elongated in the form of a spiral wave or a narrow filament, rather than a simple (circular) spot which we employed previously within the phenomenological `orbiting spot model'. By parameterizing the emission region in terms of the spiral shape and contrast, we are able to extend the spot model to more complicated sources. In the case of spirals, we notice a possibility that more photons reach a distant observer at the same moment because of interplay between lensing and light-travel time. The effect is not symmetrical with respect to leading versus trailing spirals, so in principle the source geometry can be constrained. In spite of this, the spot model seems to provide entirely adequate framework to study the currently available data.

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arXiv2007

Iron lines from transient and persisting hot spots on AGN accretion disks

[abridged] We model the X-ray reprocessing from a strong co-rotating flare above an accretion disk in active galactic nuclei. We explore the horizontal structure and evolution of the underlying hot spot. To obtain the spectral evolution seen by a distant observer, we apply a general relativity ray-tracing technique. We concentrate on the energy band around the iron K-line, where the relativistic effects are most pronounced. Persistent flares lasting for a significant fraction of the orbital time scale and short, transient flares are considered. In our time-resolved analysis, the spectra recorded by a distant observer depend on the position of the flare/spot with respect to the central black hole. If the flare duration significantly exceeds the light travel time across the spot, then the spot horizontal stratification is unimportant. On the other hand, if the flare duration is comparable to the light travel time across the spot radius, the lightcurves exhibit a typical asymmetry in their time profiles. The sequence of dynamical spectra proceeds from more strongly to less strongly ionized re-emission. At all locations within the spot the spectral intensity increases towards edge-on emission angles, revealing the limb brightening effect. Future X-ray observatories with significantly larger effective collecting areas will enable to spectroscopically map out the azimuthal irradiation structure of the accretion disk and to localize persistent flares. If the hot spot is not located too close to the marginally stable orbit of the black hole, it will be possible to probe the reflecting medium via the sub-structure of the iron K-line. Indications for transient flares will only be obtained from analyzing the observed lightcurves on the gravitational time scale of the accreting supermassive black hole.

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arXiv2007

On the orientation of the Sagittarius A* system

The near-infrared emission from the black hole at the Galactic center (Sgr A*) has unique properties. The most striking feature is a suggestive periodic sub-structure that has been observed in a couple of flares so far. Using near-infrared polarimetric observations and modelling the quasi-periodicity in terms of an orbiting blob, we try to constrain the three dimensional orientation of the Sgr A* system. We report on so far unpublished polarimetric data from 2003. They support the observations of a roughly constant mean polarization angle of 60 degr \pm 20 degr from 2004-2006. Prior investigations of the 2006 data are deepened. In particular, the blob model fits are evaluated such that constraints on the position angle of Sgr A* can be derived. Confidence contours in the position-inclination angle plane are derived. On a 3sigma level the position angle of the equatorial plane normal is in the range 60 degr - 108 degr (east of north) in combination with a large inclination angle. This agrees well with recent independent work in which radio spectral/morphological properties of Sgr A* and X-ray observations, respectively, have been used. However, the quality of the presently available data and the uncertainties in our model bring some ambiguity to our conclusions.

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arXiv2007

Constraining global parameters of accreting black holes by modeling magnetic flares

We present modeling results for the reprocessed radiation expected from magnetic flares above AGN accretion disks. Relativistic corrections for the orbital motion of the flare and for the curved space-time in the vicinity of the black hole are taken into account. We investigate the local emission spectra, as seen in a frame co-orbiting with the disk, and the observed spectra at infinity. We investigate long-term flares at different orbital phases and short-term flares for various global parameters of the accreting black hole. Particular emphasis is put on the relation between the iron Kalpha line and the Compton hump as these two features can be simultaneously observed by the Suzaku satellite and later by Simbol-X.

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arXiv2007

Constraints on a strong X-ray flare in the Seyfert galaxy MCG-6-30-15

We discuss implications of a strong flare event observed in the Seyfert galaxy MCG-6-30-15 assuming that the emission is due to localized magnetic reconnection. We conduct detailed radiative transfer modeling of the reprocessed radiation for a primary source that is elevated above the disk. The model includes relativistic effects and Keplerian motion around the black hole. We show that for such a model setup the observed time-modulation must be intrinsic to the primary source. Using a simple analytical model we then investigate time delays between hard and soft X-rays during the flare. The model considers an intrinsic delay between primary and reprocessed radiation, which measures the geometrical distance of the flare source to the reprocessing sites. The observed time delays are well reproduced if one assumes that the reprocessing happens in magnetically confined, cold clouds.

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arXiv2007

Modeling the X-ray fractional variability spectrum of Active Galactic Nuclei using multiple flares

Using Monte-Carlo simulations of X-ray flare distributions across the accretion disk of active galactic nuclei (AGN), we obtain modeling results for the energy-dependent fractional variability amplitude. Referring to previous results of this model, we illustrate the relation between the shape of the point-to-point fractional variability spectrum, F_pp, and the time-integrated spectral energy distribution, F_E. The results confirm that the spectral shape and variability of the iron Kalpha line are dominated by the flares closest to the disk center.

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arXiv2006

Magnetic flares in Active Galactic Nuclei: Modeling the iron Kalpha-line

The X-ray spectra of Active Galactic Nuclei (AGN) are complex and vary rapidly in time as seen in recent observations. Magnetic flares above the accretion disk can account for the extreme variability of AGN. They also explain the observed iron Kalpha fluorescence lines. We present radiative transfer modeling of the X-ray reflection due to emission from magnetic flares close to the marginally stable orbit. The hard X-ray primary radiation coming from the flare source illuminates the accretion disk. A Compton reflection/reprocessed component coming from the disk surface is computed for different emission directions. We assume that the density structure remains adjusted to the hydrostatic equilibrium without external illumination because the flare duration is only a quarter-orbit. The model takes into account the variations of the incident radiation across the hot spot underneath the flare source. The integrated spectrum seen by a distant observer is computed for flares at different orbital phases close to the marginally stable orbit of a Schwarzschild black hole and of a maximally rotating Kerr black hole. The calculations include relativistic and Doppler corrections of the spectra using a ray tracing technique. We explore the practical possibilities to map out the azimuthal irradiation pattern of the inner accretion disks and conclude that the next generation of X-ray satellites should reveal this structure from iron Kalpha line profiles and X-ray lightcurves.

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arXiv2006

Statistics of relativistically broadened Fe K-alpha lines in AGN

We present preliminary results on the properties of relativistically broadened Fe K-alpha lines in a collection of more then 100 Active Galactic Nuclei (AGN) observed by the XMM-Newton EPIC-pn camera. Our main conclusions can be summarized as follows: a) we detect broad lines in about 25% of the sample objects. This fraction increases to 42+/-13% if we consider only objects with more than 10000 counts in the hard (2-10 keV) band, and to 50+/-32% for the small sub-sample (6 objects) of type~1 Piccinotti AGN with optimal XMM-Newton exposure (at least 200000 counts in the hard band); b) we find no significant difference in the detection rate of broad lines between obscured and unobscured AGN; c) the strongest relativistic profiles are measured in low-luminosity (< 10^43 erg/s) AGN; d) Equivalent Widths (EWs) associated with relativistic profiles in stacked spectra are ~150 eV for all luminosity classes; e) models of relativistically broadened iron line profiles ("kyrline", Dovciak et al. 2005), which include full relativistic treatment of the accretion disk emission around a Kerr black hole in the strong gravity regime, yield an average disk inclination angle ~30 degrees, and a radial dependence of the disk emissivity profile ~-3. The distribution of EW is very broad, with =2.4. We estimate that an investment of about 1 Ms of XMM-Newton time would be required to put these results on a sound statistical basis.

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arXiv2006

K-band polarimetry of an Sgr A* flare with a clear sub-flare structure

Context: The supermassive black hole at the Galactic center, SgrA*, shows frequent radiation outbursts, often called 'flares'. In the near-infrared some of these flares were reported as showing intrinsic quasi-periodicities. The flux peaks associated with the quasi-periodic behavior were found to be highly polarized. Aims: The aim of this work is to present new evidence to support previous findings of the properties of the polarized radiation from SgrA* and to again provide strong support for the quasi-periodicity of ~18+-3 min reported earlier. Methods: Observations were carried out at the European Southern Observatory's Very Large Telescope on Paranal, Chile. We used the NAOS/CONICA adaptive optics/near-infrared camera instrument. By fitting the polarimetric lightcurves with a hot-spot model, we addressed the question of whether the data are consistent with this model. To fit the observed data we used a general relativistic ray-tracing code in combination with a simple hot-spot/ring model. Results: We report on new polarization measurements of a K-band flare from the supermassive black hole at the Galactic center. The data provide very strong support for a quasi-periodicity of 15.5+-2 min. The mean polarization of the flare is consistent with the direction of the electric field vector that was reported in previous observations. The data can be modeled successfully with a combined blob/ring model. The inclination i of the blob orbit must be i > 20 deg on a 3sigma level, and the dimensionless spin parameter of the black hole is derived to be a > 0.5.

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arXiv2006

Near-infrared polarimetry setting constraints on the orbiting spot model for Sgr A* flares

Context: Recent near-infrared polarization measurements of SgrA* show that its emission is significantly polarized during flares and consists of a non- or weakly polarized main flare with highly polarized sub-flares. The flare activity suggests a quasi-periodicity of ~20 minutes in agreement with previous observations. Aims: By simultaneous fitting of the lightcurve fluctuations and the time-variable polarization angle, we address the question of whether these changes are consistent with a simple hot spot/ring model, in which the interplay of relativistic effects plays the major role, or whether some more complex dependency of the intrinsic emissivity is required. Methods: We discuss the significance of the 20min peak in the periodogram of a flare from 2003. We consider all general relativistic effects that imprint on the polarization degree and angle and fit the recent polarimetric data, assuming that the synchrotron mechanism is responsible for the intrinsic polarization and considering two different magnetic field configurations. Results: Within the quality of the available data, we think that the model of a single spot in addition to an underlying ring is favoured. In this model the broad near-infrared flares of Sgr A* are due to a sound wave that travels around the MBH once while the sub-flares, superimposed on the broad flare, are due to transiently heated and accelerated electrons which can be modeled as a plasma blob. Within this model it turns out that a strong statement about the spin parameter is difficult to achieve, while the inclination can be constrained to values > 35 deg on a 3sigma level.

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arXiv2006

The structure and X-ray radiation spectra of illuminated accretion disks in AGN. III. Modeling fractional variability

[abridged] Extending the work of Czerny et al. (2004), we model the fractional variability amplitude due to distributions of hot spots co-orbiting on the accretion disk around a supermassive black hole. From defined radial distributions, our code samples random positions for the hot spots across the disk. The local spot emission is computed as reprocessed radiation coming from a compact primary source above the disk. The structure of the hot spot and the anisotropy of the re-emission are taken into account. We compute the fractional variability spectra expected from such spot ensembles and investigate dependencies on the parameters describing the radial spot distribution. We consider the fractional variability F_var with respect to the spectral mean and also the so-called point-to-point F_pp. Our method includes relativistic corrections for the curved space-time; the black hole angular momentum is a free parameter and subject to the fitting procedure. We confirm that the rms-variability spectra involve intrinsic randomness at a significant level when the number of flares appearing during the total observation time is too small. Furthermore, F_var is not always compatible with F_pp. For MCG -6-30-15, we can reproduce the short-timescale variability and model the suppressed variability in the energy range of the Kalpha line without any need to postulate reprocessing farther away from the center. An increasing rate of energy production by the flares toward the center of the disk, a fast rotation of the central black hole, and considerable suppression of the primary flare emission are required. The modeled line remains consistent with the measured equivalent width of the iron Kalpha line complex.

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arXiv2006

Variable iron-line emission near the black hole of Markarian 766

We investigate the link between ionised Fe X-ray line emission and continuum emission in the bright nearby AGN, Mrk 766. A new long (433 ks) XMM-Newton observation is analysed, together with archival data from 2000 and 2001. The contribution from ionised line emission is measured and its time variations on short (5-20 ks) timescales are correlated with the continuum emission. The ionised line flux is found to be highly variable and to be strongly correlated with the continuum flux, demonstrating an origin for the ionised line emission that is co-located with the continuum emission. Most likely the emission is ionised reflection from the accretion disc within a few A.U. of the central black hole, and its detection marks the first time that such an origin has been identified other than by fitting to spectral line profiles. Future observations may be able to measure a time lag and hence achieve reverberation mapping of AGN at X-ray energies.

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arXiv2005

The relativistic shift of narrow spectral features from black-hole accretion discs

Transient spectral features have been discovered in the X-ray spectra of Active Galactic Nuclei, mostly in the 5--7 keV energy range. Several interpretations were proposed for the origin of these features. We examined a model of Doppler boosted blue horns of the iron line originating from a spot in a black hole accretion disc, taking into account different approximations of general relativistic light rays and the resulting shift of energy of photons. We provide a practical formula for the blue horn energy of an intrinsically narrow line and assess its accuracy by comparing the approximation against an exact value, predicted under the assumption of a planar accretion disc. The most accurate approximation provides excellent agreement with the spot orbital radius down to the marginally stable orbit of a non-rotating black hole.

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arXiv2005

X-ray Variability of Active Galactic Nuclei and the Flare Model

Short-term variability of X-ray spectra has been reported for several Active Galactic Nuclei. Significant X-ray flux variations are observed within time scales down to 10^4 -- 10^5 seconds. We discuss short variability time scales in the framework of the X-ray flare model, which assumes the release of a large hard X-ray flux above a small portion of the accretion disk. The resulting observed X-ray spectrum is composed of the primary radiation as well as a reprocessed/Compton reflection component that we model performing numerical radiative transfer simulations. We conduct Monte-Carlo simulations of large flare distributions across the disk including relativistic corrections of the observed radiation and discuss general implications on AGN spectra and their variability.

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arXiv2005

Polarization signatures of strong gravity in black-hole accretion discs

We discuss the effects of strong gravity on the polarization properties of a black hole accretion disc. The intrinsic polarization is computed taking into account light scattered on the disc surface and using different approximations. The gravitational field of a black hole influences the Stokes parameters of reflected radiation propagating to a distant observer. The lamp-post model is explored as an example of a specific geometrical arrangement relevant for AGNs. The degree and the angle of polarization are computed as functions of the observer inclination angle, of the inner radius of the disc emitting region, and of other parameters of the model. The expected polarization should be detectable by new generation polarimeters. This contribution extends results reported in our recent paper (MNRAS 355, 1005; 2004).

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