arXiv subjects

V. Karas

Publications and source records attributed to V. Karas.

At least 55 records · Page 3Linked to original sources

arXiv2018

Light rays and wave fronts in strong gravity

Accretion onto black holes often proceeds via an accretion disk or a temporary disk-like pattern. Variability features observed in light curves as well as theoretical models of accretion flows suggest that accretion disks tend to be inhomogeneous -- variety of substructures (clumps) emerge within the flow. Rapid orbital motion of individual clumps then modulates the observed signal in X-rays. Furthermore, changes of spectral lines and polarization properties of the observed signal (or the absence of changes) constrain the models and reveal information about general relativity (GR) effects. In this write-up we summarize the basic equations that have been employed to study light propagation near black holes and to derive the radiation signal that can be expected at a detector within the framework of geometrical optics approximation.

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arXiv2017

Experimental Indicators of Accretion Processes in Active Galactic Nuclei

Bright Active Galactic Nuclei are powered by accretion of mass onto the super massive black holes at the centers of the host galaxies. For fainter objects star formation may significantly contribute to the luminosity. We summarize experimental indicators of the accretion processes in Active Galactic Nuclei (AGN), i.e., observable activity indicators that allow us to conclude on the nature of accretion. The Galactic Center is the closest galactic nucleus that can be studied with unprecedented angular resolution and sensitivity. Therefore, here we also include the presentation of recent observational results on Sagittarius A* and the conditions for star formation in the central stellar cluster. We cover results across the electromagnetic spectrum and find that the Sagittarius A* (SgrA*) system is well ordered with respect to its geometrical orientation and its emission processes of which we assume to reflect the accretion process onto the super massive black hole.

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arXiv2017

Bright X-ray flares from Sgr A*

We address a question whether the observed light curves of X-ray flares originating deep in galactic cores can give us independent constraints on the mass of the central supermassive black hole. To this end we study four brightest flares that have been recorded from Sagittarius A*. They all exhibit an asymmetric shape consistent with a combination of two intrinsically separate peaks that occur at a certain time-delay with respect to each other, and are characterized by their mutual flux ratio and the profile of raising/declining parts. Such asymmetric shapes arise naturally in the scenario of a temporary flash from a source orbiting near a super- massive black hole, at radius of only 10-20 gravitational radii. An interplay of relativistic effects is responsible for the modulation of the observed light curves: Doppler boosting, gravitational redshift, light focusing, and light-travel time delays. We find the flare properties to be in agreement with the simulations (our ray-tracing code sim5lib). The inferred mass for each of the flares comes out in agreement with previous estimates based on orbits of stars; the latter have been observed at radii and over time-scales two orders of magnitude larger than those typical for the X-ray flares, so the two methods are genuinely different. We test the reliability of the method by applying it to another object, namely, the Seyfert I galaxy RE J1034+396.

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arXiv2017

Nuclear Activity and the Conditions of Star-formation at the Galactic Center

The Galactic Center is the closest galactic nucleus that can be studied with unprecedented angular resolution and sensitivity. We summarize recent basic observational results on Sagittarius A* and the conditions for star formation in the central stellar cluster. We cover results from the radio, infrared, and X-ray domain and include results from simulation as well. From (sub-)mm and near-infrared variability and near-infrared polarization data we find that the SgrA* system (supermassive black hole spin, a potential temporary accretion disk and/or outflow) is well ordered in its geometrical orientation and in its emission process that we assume to reflect the accretion process onto the supermassive black hole (SMBH).

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arXiv2017

Failed radiatively Accelerated Dusty Outflow model of the Broad Line Region in Active Galactic Nuclei. I. Analytical solution

The physical origin of the Broad Line Region in Active Galactic Nuclei is still unclear despite many years of observational studies. The reason is that the region is unresolved and the reverberation mapping results imply complex velocity field. We adopt a theory-motivated approach to identify the principal mechanism responsible for this complex phenomenon. We consider the possibility that the role of dust is essential. We assume that the local radiation pressure acting on the dust in the accretion disk atmosphere launches the outflow of material, but higher above the disk the irradiation from the central parts cause the dust evaporation and a subsequent fall back. This failed radiatively accelerated dusty outflow (FRADO) is expected to represent the material forming low ionization lines. In this paper we formulate simple analytical equations describing the cloud motion, including the evaporation phase. The model is fully described just by the basic parameters: black hole mass, accretion rate, black hole spin and the viewing angle. We study how the spectral line generic profiles correspond to this dynamics. We show that the virial factor calculated from our model strongly depends on the black hole mass in case of enhanced dust opacity, and thus it then correlates with the line width. This could explain why the virial factor measured in galaxies with pseudo-bulges differs from that obtained from objects with classical bulges although the trend predicted by the current version of the model is opposite to the observed trend.

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arXiv2017

Investigating the Relativistic Motion of the Stars Near the Supermassive Black Hole in the Galactic Center

The S-star cluster in the Galactic center allows us to study the physics close to a supermassive black hole, including distinctive dynamical tests of general relativity. Our best estimates for the mass of and the distance to Sgr A* using the three stars with the shortest period (S2, S38, and S55/S0-102) and Newtonian models are M_BH = (4.15+- 0.13 +- 0.57) x 10^6 M_sun and R_0 = 8.19 +- 0.11 +- 0.34 kpc. Additionally, we aim at a new and practical method to investigate the relativistic orbits of stars in the gravitational field near Sgr A*. We use a first-order post- Newtonian approximation to calculate the stellar orbits with a broad range of periapse distance r_p. We present a method that employs the changes in orbital elements derived from elliptical fits to different sections of the orbit. These changes are correlated with the relativistic parameter defined as Υ = r_s/r_p (with r_s being the Schwarzschild radius) and can be used to derive Υ from observational data. For S2 we find a value of Υ = 0.00088 +- 0.00080, which is consistent, within the uncertainty, with the expected value of Υ = 0.00065 derived from MBH and the orbit of S2. We argue that the derived quantity is unlikely to be dominated by perturbing influences such as noise on the derived stellar positions, field rotation, and drifts in black hole mass.

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arXiv2017

Plunging neutron stars as origin of organised magnetic field in galactic nuclei

Black holes cannot support their own internal magnetic field like, for example, compact stars can. Despite this fact observations indicate that event horizons of supermassive black holes (SMBH) are threaded by field lines along which plasma streams flow. Various magnetohydrodynamical mechanisms have been suggested to generate turbulent magnetic fields on small scales, however, the origin of the large-scale component is unclear. In this write-up we describe our progress in an on-going work and discuss the possibility of dipole-type magnetic fields being brought onto SMBH by magnetized neutron stars, which are expected to drift inward from a hidden population in the Nuclear Star Cluster. This can contribute to an organised component of the magnetic field on the characteristic length-scale of the stellar size, which thread the horizon during the final stages of the magnetized star plunge into or its close flyby around SMBH. Because of mass--size scaling relations for black holes, the effect is more important for lower-mass SMBH.

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arXiv2017

First results from the use of the relativistic and slim disc model SLIMULX in XSPEC

Ultra-Luminous X-ray sources (ULXs) are accreting black holes for which their X-ray properties have been seen to be different to the case of stellar-mass black hole binaries. For most of the cases their intrinsic energy spectra are well described by a cold accretion disc (thermal) plus a curved high-energy emission components. The mass of the black hole (BH) derived from the thermal disc component is usually in the range of 100-1000 solar masses, which have led to the idea that this might represent strong evidence of the Intermediate Mass Black Holes (IMBH), proposed to exist by theoretical studies but with no firm detection (as a class) so far. Recent theoretical and observational developments are leading towards the idea that these sources are instead stellar-mass BHs accreting at an unusual super-Eddington regime. In this paper we briefly describe the model SLIMULX that can be used in XSPEC for the fit of thermal spectra of slim discs around stellar mass black holes in the super-Eddington regime. This model consistently takes all relativistic effects into account. We present the obtained results from the fit of the X-ray spectra from NGC 5408 X-1.

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arXiv2017

KYNREFREV - implementation of an X-ray reverberation model in XSPEC

We present briefly the first results obtained by the application of the KYNREFREV-reverberation model, which is ready for its use in XSPEC. This model computes the time dependent reflection spectra of the disc as a response to a flash of primary power-law radiation from a point source corona located on the axis of the black-hole accretion disc. The assumptions of the model are: central Kerr black hole, surrounded by a Keplerian, geometrically thin, optically thick, ionised disc with the possibility of defining the radial density profile and a stationary hot point-like patch of plasma located on the system rotation axis and emitting isotropic power-law radiation (lamp-post geometry). Full relativistic ray-tracing code in vacuum is used for photon paths from the corona to the disc and to the observer and from the disc to the observer. The ionisation of the disc is set for each radius according to the amount of the incident primary flux and the density of the accretion disc. In this paper we comment on some preliminary results obtained through the analysis of X-ray reverberation time-lags from 1H 0707-495 and IRAS 13224-3809.

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arXiv2016

Multi-phase environment of compact galactic nuclei: the role of the Nuclear Star Cluster

We study the conditions for the onset of Thermal Instability in the innermost regions of compact galactic nuclei, where the properties of the interstellar environment are governed by the interplay of quasi-spherical accretion onto a supermassive black hole (SMBH) and the heating/cooling processes of gas in a dense nuclear star cluster. Stellar winds are the source of material for radiatively inefficient (quasi-spherical, non-magnetised) inflow/outflow onto the central SMBH, where a stagnation point develops within the Bondi type accretion. We study the local thermal equilibrium to determine the parameter space which allows cold and hot phases in mutual contact to co-exist. We include the effects of mechanical heating by stellar winds and radiative cooling/heating by the ambient field of the dense star cluster. We consider two examples: the Nuclear Star Cluster (NSC) in the Milky Way central region (including the gaseous Mini-spiral of Sgr~A*), and the Ultra-Compact Dwarf galaxy M60-UCD1. We find that the two systems behave in different ways because they are placed in different areas of parameter space in the instability diagram: gas temperature vs. dynamical ionization parameter. In the case of Sgr~A*, stellar heating prevents the spontaneous formation of cold clouds. The plasma from stellar winds joins the hot X-ray emitting phase and forms an outflow. In M60-UCD1 our model predicts spontaneous formation of cold clouds in the inner part of the galaxy. These cold clouds may survive since the cooling timescale is shorter than the inflow/outflow timescale.

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arXiv2016

Electrically charged matter in permanent rotation around magnetized black hole: A toy model for self-gravitating fluid tori

We present an analytical approach for the equilibrium of a self-gravitating charged fluid embedded in a spherical gravitational and dipolar magnetic fields produced by a central mass. Our scheme is proposed, as a toy-model, in the context of gaseous/dusty tori surrounding supermassive black holes in galactic nuclei. While the central black hole dominates the gravitational field and it remains electrically neutral, the surrounding material has a non-negligible self-gravitational effect on the torus structure. By charging mechanisms it also acquires non-zero electric charge density, so the two influences need to be taken into account to achieve a self-consistent picture. With our approach we discuss the impact of self-gravity, represented by the term dt (ratio of the torus total mass to the mass of the central body), on the conditions for existence of the equilibrium and the morphology and typology of the tori. By comparison with a previous work without self-gravity, we show that the conditions can be different. Although the main aim of the present paper is to discuss a framework for the classification of electrically charged, magnetized, self-gravitating tori, we also mention potential astrophysical applications to vertically stratified fluid configurations.

astro-ph.HE↗

arXiv2016

Theoretical modelling of the AGN iron-line vs continuum time-lags in the lamp-post geometry

Context: Theoretical modelling of time-lags between variations in the Fe K$α$ emission and the X-ray continuum might shed light on the physics and geometry of the X-ray emitting region in active galaxies (AGN) and X-ray binaries. We here present the results from a systematic analysis of time-lags between variations in two energy bands ($5-7$ vs $2-4\,\mathrm{keV}$) for seven X-ray bright and variable AGN. Aims: We estimate time-lags as accurately as possible and fit them with theoretical models in the context of the lamp-post geometry. We also constrain the geometry of the X-ray emitting region in AGN. Methods: We used all available archival \textit{XMM-Newton} data for the sources in our sample and extracted light curves in the $5-7$ and $2-4\,\mathrm{keV}$ energy bands. We used these light curves and applied a thoroughly tested (through extensive numerical simulations) recipe to estimate time-lags that have minimal bias, approximately follow a Gaussian distribution, and have known errors. Using traditional $χ^2$ minimisation techniques, we then fitted the observed time-lags with two different models: a phenomenological model where the time-lags have a power-law dependence on frequency, and a physical model, using the reverberation time-lags expected in the lamp-post geometry. The latter were computed assuming a point-like primary X-ray source above a black hole surrounded by a neutral and prograde accretion disc with solar iron abundance. We took all relativistic effects into account for various X-ray source heights, inclination angles, and black hole spin values.

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arXiv2016

eXTP -- enhanced X-ray Timing and Polarimetry Mission

eXTP is a science mission designed to study the state of matter under extreme conditions of density, gravity and magnetism. Primary targets include isolated and binary neutron stars, strong magnetic field systems like magnetars, and stellar-mass and supermassive black holes. The mission carries a unique and unprecedented suite of state-of-the-art scientific instruments enabling for the first time ever the simultaneous spectral-timing-polarimetry studies of cosmic sources in the energy range from 0.5-30 keV (and beyond). Key elements of the payload are: the Spectroscopic Focusing Array (SFA) - a set of 11 X-ray optics for a total effective area of about 0.9 m^2 and 0.6 m^2 at 2 keV and 6 keV respectively, equipped with Silicon Drift Detectors offering <180 eV spectral resolution; the Large Area Detector (LAD) - a deployable set of 640 Silicon Drift Detectors, for a total effective area of about 3.4 m^2, between 6 and 10 keV, and spectral resolution <250 eV; the Polarimetry Focusing Array (PFA) - a set of 2 X-ray telescope, for a total effective area of 250 cm^2 at 2 keV, equipped with imaging gas pixel photoelectric polarimeters; the Wide Field Monitor (WFM) - a set of 3 coded mask wide field units, equipped with position-sensitive Silicon Drift Detectors, each covering a 90 degrees x 90 degrees FoV. The eXTP international consortium includes mostly major institutions of the Chinese Academy of Sciences and Universities in China, as well as major institutions in several European countries and the United States. The predecessor of eXTP, the XTP mission concept, has been selected and funded as one of the so-called background missions in the Strategic Priority Space Science Program of the Chinese Academy of Sciences since 2011. The strong European participation has significantly enhanced the scientific capabilities of eXTP. The planned launch date of the mission is earlier than 2025.

astro-ph.IM↗

arXiv2016

Monitoring the Galactic Centre with Australia Telescope Compact Array

The supermassive black hole, Sagittarius A* (Sgr A*), at the centre of the Milky Way undergoes regular flaring activity which is thought to arise from the innermost region of the accretion flow. We performed the monitoring observations of the Galactic Centre to study the flux-density variations at 3mm using the Australia Telescope Compact Array (ATCA) between 2010 and 2014. We obtain the light curves of Sgr A* by subtracting the contributions from the extended emission around it, and the elevation and time dependent gains of the telescope. We perform structure function analysis and the Bayesian blocks representation to detect flare events. The observations detect six instances of significant variability in the flux density of Sgr A* in three observations, with variations between 0.5 to 1.0 Jy, which last for 1.5 $-$ 3 hours. We use the adiabatically expanding plasmon model to explain the short time-scale variations in the flux density. We derive the physical quantities of the modelled flare emission, such as the source expansion speed $v_{\mathrm{exp}}$, source sizes, spectral indices, and the turnover frequency. These parameters imply that the expanding source components are either confined to the immediate vicinity of Sgr A* by contributing to the corona or the disc, or have a bulk motion greater than $v_{\mathrm{exp}}$. No exceptional flux density variation on short flare time-scales was observed during the approach and the flyby of the dusty S-cluster object (DSO/G2). This is consistent with its compactness and the absence of a large bow shock.

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arXiv2016

Signatures of X-ray reverberation in the power spectra of AGN

We compute fully relativistic disc response functions in the case of the "lamp-post" geometry using the full observed reflection spectrum for various X-ray source heights, disc inclination, and spin values of the central black hole. Since the observed PSD is equal to the product of the intrinsic power spectrum with the "transfer function" (i.e. the Fourier transform of the disc response function), we are able to predict the observed PSDs in the case of X-ray illumination of the inner disc. The observed PSD should show a prominent dip at high frequencies and an oscillatory behaviour, with a decreasing amplitude, at higher frequencies. The reverberation "echo" features should be more prominent in energy bands where the reflection component is more pronounced. The frequency of the dip is independent of energy, and it is mainly determined by the black hole mass and the X-ray source height. The amplitude of the dip increases with increasing black hole spin and inclination angle, as long as the height of the "lamp" is smaller than ~10 gravitational radii. The detection of the X-ray reverberation signals in the PSDs can provide further evidence for X-ray illumination of the inner disc in AGN. Our results are largely independent of the assumed geometry of the disc-corona system, as long as it does not change with time, and the disc response function is characterized by a sharp rise, a "plateau", and a decline at longer times. Irrespective of the geometry, the frequency of the main dip should decrease with increasing "mean time" of the response function, and the amplitude of the dip should increase with increasing reflection fraction.

astro-ph.HE↗

arXiv2015

Reflection nebulae in the Galactic Center: the case for soft X-ray imaging polarimetry

The origin of irradiation and fluorescence of the 6.4 keV bright giant molecular clouds surrounding Sgr A*, the central supermassive black hole of our Galaxy, remains enigmatic. Testing the theory of a past active period of Sgr A* requires X-ray polarimetry. In this paper, we show how modern imaging polarimeters could revolutionize our understanding of the Galactic Center. Through Monte Carlo modeling, we produce a 4-8 keV polarization map of the Galactic Center, focusing on the polarimetric signature produced by Sgr B1, Sgr B2, G0.11-0.11, Bridge E, Bridge D, Bridge B2, MC2, MC1, Sgr C3, Sgr C2, and Sgr C1. We estimate the resulting polarization, include polarized flux dilution by the diffuse plasma emission detected toward the GC, and simulate the polarization map that modern polarimetric detectors would obtain assuming the performances of a mission prototype. The eleven reflection nebulae investigated in this paper present a variety of polarization signatures, ranging from nearly unpolarized to highly polarized (about 77%) fluxes. A major improvement in our simulation is the addition of a diffuse, unpolarized plasma emission that strongly impacts soft X-ray polarized fluxes. The dilution factor is in the range 50% - 70%, making the observation of the Bridge structure unlikely even in the context of modern polarimetry. The best targets are the Sgr B and Sgr C complexes, and the G0.11-0.11 cloud. An exploratory observation of a few hundred kilo-seconds of the Sgr B complex would allow a significant detection of the polarization and be sufficient to derive hints on the primary source of radiation. A more ambitious program (few Ms) of mapping the giant molecular clouds could then be carried out to probe with great precision the turbulent history of Sgr A*, and place important constraints on the composition and three-dimensional position of the surrounding gas.

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arXiv2015

Accretion, ejection and reprocessing in supermassive black holes

This is a White Paper in support of the mission concept of the Large Observatory for X-ray Timing (LOFT), proposed as a medium-sized ESA mission. We discuss the potential of LOFT for the study of active galactic nuclei. For a summary, we refer to the paper.

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arXiv2015

The Galactic Center Black Hole Laboratory

The super-massive 4 million solar mass black hole Sagittarius~A* (SgrA*) shows flare emission from the millimeter to the X-ray domain. A detailed analysis of the infrared light curves allows us to address the accretion phenomenon in a statistical way. The analysis shows that the near-infrared flare amplitudes are dominated by a single state power law, with the low states in SgrA* limited by confusion through the unresolved stellar background. There are several dusty objects in the immediate vicinity of SgrA*. The source G2/DSO is one of them. Its nature is unclear. It may be comparable to similar stellar dusty sources in the region or may consist predominantly of gas and dust. In this case a particularly enhanced accretion activity onto SgrA* may be expected in the near future. Here the interpretation of recent data and ongoing observations are discussed.

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