arXiv subjects

Juri Poutanen

Publications and source records attributed to Juri Poutanen.

At least 271 records · Page 15Linked to original sources

arXiv2001

Relativistic Kinetic Equation for Induced Compton Scattering of Polarized Radiation

The relativistic kinetic equations describing time evolution and space dependence of the density matrices of polarized photons and electrons interacting via Compton scattering are deduced from the quantum Liouville equation. The induced scattering and exclusion principle are taken into account. The Bogoliubov method is used in the frame of quantum electrodynamics. The equation for polarized radiation scattered by unpolarized electrons is considered as a particular case and is reformulated in terms of the Stokes parameters. The expressions for the scattering amplitudes and cross-sections are derived simultaneously.

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arXiv2001

OSSE and RXTE Observations of GRS 1915+105: Evidence for Non-thermal Comptonization

GRS 1915+105 was observed by the CGRO/OSSE 9 times in 1995-2000, and 8 of those observations were simultaneous with those by RXTE. We present an analysis of all of the OSSE data and of two RXTE-OSSE spectra with the lowest and highest X-ray fluxes. The OSSE data show a power-law--like spectrum extending up to ~600 keV without any break. We interpret this emission as strong evidence for the presence of non-thermal electrons in the source. The broad-band spectra cannot be described by either thermal or bulk-motion Comptonization, whereas they are well described by Comptonization in hybrid thermal/ non-thermal plasmas.

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arXiv2001

An Off-line Scan of the BATSE Daily Records and a Large Uniform Sample of Gamma-Ray Bursts

During a scan of the archival BATSE daily records covering the entire 9.1 years (TJD 8369-11690) of the BATSE operation, 3906 gamma-ray bursts (GRBs) have been detected. 2068 of these GRBs are previously known BATSE triggers while 1838 of them are new non-triggered bursts. It is important that all events were detected in the same type of data and were processed with the same procedure. Therefore these 3906 GRBs constitute a uniform sample. We have created a publically available electronic data base containing this sample. We describe the procedures of the data reduction, the selection of the GRB candidates, and the statistical tests for possible non-GRB contaminations. We also describe a novel test burst method used to measure the scan efficiency and the information obtained using the test bursts. Our scan decreases the BATSE detection threshold to ~0.1 photons/sec/cm2. As a first result, we show that the differential log N - log P distribution corrected for the detection efficiency extends to low brightnesses without any indication of a turn-over. Any reasonable extrapolation of the new log N - log P to lower brightnesses imply a rate of several thousands of GRBs in the Universe per year.

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arXiv2001

Thermal Comptonization in GRS 1915+105

The Rossi X-ray Timing Explorer data of GRS 1915+105 from several observing periods are modeled with a thermal Comptonization model. Best-fit models indicate that there is a strong correlation between the inner disk temperature and the disk luminosity. The hard Comptonized luminosity does not depend significantly on the total luminosity. The spectral hardness of the Comptonized radiation, the fraction of seed soft photons scattered by the Comptonizing cloud, its Thomson optical depth, and the fraction of the total power dissipated in the optically thin hot plasma, all strongly anticorrelate with the luminosity. We find that the inner disk radius is almost constant and that the hot Comptonizing corona shrinks at high luminosities. We note that the fits using xspec diskbb + power law model underestimate the amplitude of the blackbody component (and therefore the corresponding size of the emitting region) and overestimate the absorption column density and the total, corrected for absorption, luminosity.

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arXiv2001

Bizarre Hard X-ray Outbursts of Cygnus X-1

A very high activity of Cygnus X-1 on 1999 April 19-21 was recorded by BATSE Large Area Detectors onboard the Compton Gamma-Ray Observatory. The peak luminosity was one order of magnitude higher than the normal luminosity of Cyg X-1. This fact can be critical for models of the hard state of Cyg X-1. The longest outburst lasted ~1000 s and demonstrated very unusual temporal and spectral behavior which indicates the presence of two emission components. One component is relatively soft (with a cutoff below 100 keV) and highly variable, and the other one is hard (extending above 100 keV), with much slower variability.

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arXiv2001

X-ray spectra of accretion discs with dynamic coronae

We compute the X-ray spectra produced by non-static coronae atop accretion discs around black holes and neutron stars. The hot corona is radiatively coupled to the underlying disc (the reflector) and generates an X-ray spectrum which is sensitive to the bulk velocity of the coronal plasma, beta=v/c. We show that an outflowing corona reproduces the hard-state spectrum of Cyg X-1 and similar objects. The dynamic model predicts a correlation between the observed amplitude of reflection R and the X-ray spectrum slope Gamma since both strongly depend on beta. A similar correlation was observed and its shape is well fitted by the dynamic model. The scattering of soft radiation in an outflowing corona can also account for the observed optical-UV polarization pattern in active galactic nuclei.

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arXiv2001

Understanding Spectral Variability and Time Lags in Accreting Black Holes

I review the temporal/spectral data of accreting black hole sources paying most attention to the properties of the temporal variability such as photon energy dependent auto- and cross-correlation functions, average shot profiles and hardness ratios, and the Fourier frequency dependent time/phase lags. These statistics characterize spectral changes at short time scales that are otherwise impossible to study by direct spectral analysis. The data provide strong constraints on the theoretical models for X-ray production in accreting black holes. Models for the spectral variability of the Comptonized component are briefly reviewed. It is also shown that Compton reflection can have significant impact on the observed temporal characteristics.

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arXiv2000

Observations of Seyferts by OSSE and parameters of their X-ray/gamma-ray sources

We present a summary of spectra of Seyfert galaxies observed by the OSSE detector aboard Compton Gamma Ray Observatory. We obtain average spectra of Seyferts of type 1 and 2, and find they are well fitted by thermal Comptonization. We present detailed parameter ranges for the plasma temperature and the Compton parameter in the case of spherical and slab geometries. We find both the average and individual OSSE spectra of Seyfert 2s are significantly harder than those of Seyfert 1s, which difference can be due to anisotropy of Compton reflection and/or Thomson-thick absorption.

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arXiv2000

Time Domain Analysis of Variability in Cygnus X-1: Constraints on the Emission Models

We use time domain analysis techniques to investigate the rapid variability of Cygnus X-1. We show that the cross-correlation functions between hard and soft energy bands reach values very close to unity and peak at a lag of less than 2 millisecond for energies separated by a factor of 10. This confirms that the process that produces X-ray photons at different energies is extremely coherent on short time scales and strongly constrains emission models proposed to explain Fourier-frequency-dependent time lags. We present autocorrelation functions at different energies, and note their widths decrease with increasing energy. We show that the extended Compton corona model produces auto-correlation functions whose widths increase with increasing energy, that the model of cylindrical waves moving inward through a transition disk has too large a peak lag in the cross-correlation function. Models of magnetic flaring and of drifting blobs in a hot corona can qualitatively fit the observations.

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arXiv2000

Time Lags in Compact Objects: Constraints on the Emission Models

Accreting black holes and neutron stars in their hard (low) state show not only very similar X/gamma-ray spectra but also that the behaviour of their light curves is quite similar which can be quantified as having similar power-density spectra and Fourier-frequency-dependent time/phase lags. Taken together this argues for a common mechanism of the X/gamma-ray production in these objects. This mechanism is probably a property of the accretion flow only since it does not depend on the nature of the compact object. In this paper, I review the observational data paying most attention to the properties of the temporal variability such as the time/phase lags that hopefully can help us to discriminate between different theoretical models. I also discuss the models developed to account for the basic observational facts. Particularly, I show that the commonly used Compton cloud models with constant temperature cannot explain variable sources without violating the energy conservation law. Alternative models where time lags are related to the spectral evolution during X-ray flares are discussed and compared with observations. Compton reflection from the outer edge of the accretion disc is shown to markedly affect the time lag Fourier spectrum.

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arXiv1999

Spectral evolution of magnetic flares and time lags in accreting black hole sources

We present a model for the short timescale spectral variability of accreting black holes. It describes well the time-averaged spectra as well as temporal characteristics such as power-density spectrum, time/phase lags, and coherence function of Cygnus X-1. We assume that X/gamma-rays are produced in compact magnetic flares at radii < 100GM/c^2 from the central black hole. The tendency for magnetic loops to inflate and detach from the underlying accretion disc causes the spectrum of a flare to evolve from soft to hard due to the decrease of the feedback from the cold disc, so causing time delays between hard and soft photons. We identify the observed time lags with the evolution timescales of the flares, which are of the order of the Keplerian timescale. We model the overall temporal variability using a pulse avalanche model in which each flare has a certain probability to trigger a neighbouring flare, thus occasionally producing long avalanches. The duration of the avalanches determines the Fourier frequencies where most of the power emerges.

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arXiv1998

Broad-band X-ray/gamma-ray spectra and binary parameters of GX 339-4 and their astrophysical implications

We present X-ray/gamma-ray spectra of the binary GX 339-4 observed in the hard state simultaneously by Ginga and CGRO OSSE during an outburst in 1991 September. The Ginga spectra are well represented by a power law with a photon spectral index of 1.75 and a moderately-strong Compton reflection component with a fluorescent Fe K alpha line. The OSSE data require a sharp high-energy cutoff in the power-law spectrum. The broad-band spectra are very well modelled by repeated Compton scattering in a thermal plasma with tau=1 and kT=50 keV. We also find the distance to the system to be > 3 kpc, ruling out earlier determinations of 1.3 kpc. Using this limit, the observed reddening and the orbital period, we find the allowed range of the mass of the primary is consistent with it being a black hole. The data are inconsistent with models of either homogenous or patchy coronae above the surface of an accretion disc. Rather, they are consistent with the presence of a hot inner hot disc accreting at a rate close to the maximum set by advection and surrounded by a cold outer disc. The seed photons for Comptonization are supplied by the outer cold disc and/or cold clouds within the hot disc. Pair production is negligible if electrons are thermal. The hot disc model, which scaled parameters are independent of the black-hole mass, is supported by the similarity of the spectrum of GX 339-4 to those of other black-hole binaries and Seyfert 1s. On the other hand, their spectra in the soft gamma-ray regime are significantly harder than those of weakly-magnetized neutron stars. Based on this difference, we propose that the presence of broad-band spectra corresponding to thermal Comptonization with kT of 50 keV or more represents a black-hole signature.

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arXiv1998

Accretion Disc-Corona Models and X/gamma-ray Spectra of Accreting Black Holes

We discuss properties of thermal and hybrid (thermal/non-thermal) electron-positron plasmas in the pair and energy equilibria. Various accretion disc-corona models, recently proposed to explain properties of galactic as well as extragalactic accreting black holes, are confronted with the observed broad-band X-ray and gamma-ray spectra.

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arXiv1998

Modeling X/gamma-ray Spectra of Galactic Black Holes and Seyferts

Recent high quality broad-band X-ray and gamma-ray observations of Galactic black holes by ASCA, Ginga, RXTE, and CGRO show that the spectra cannot be described by simple thermal Comptonization models (even including a fully relativistic treatment). Multi-component models accounting for Comptonization by thermal and non-thermal electrons, Compton reflection of X/gamma-rays from cold matter, and emission of the optically thick accretion disk are required. In this review, I discuss recent advances in the spectral modeling of Galactic black holes and Seyferts and the codes available for such an analysis.

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arXiv1997

Thermal/non-thermal model of Cyg X-1 in the soft state

We analyze X-ray/gamma-ray observations of Cyg X-1 in the soft state. We find the hybrid thermal/non-thermal model fits the data very well. The significant contribution from the non-thermal electrons is required to account for the observed power law extending without a break to at least 800 keV. The presence of electron-positron pairs in the hot source is unlikely. We propose the geometry in which a cold accretion disk, surrounded by a hot corona, extends down to the last stable orbit. At the observed accretion rate the corona ensures the stability of the cold disk.

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arXiv1997

Unification of Spectral States of Accreting Black Holes

Several galactic black holes show transitions between spectral states. In this paper we present a unifying radiation transfer model that can fit the spectral data in both states. We fit individual broad-band (from 1 keV up to 4 MeV) spectral data for the "hard" and "soft" states of Cyg X-1 using an emission model where a central Comptonizing corona/cloud is illuminated by the soft photon emission from a cold disk. We assume that the energy is injected to the corona by two channels: a non-thermal one that injects energetic electrons into the coronal region, and a thermal one that heats injected and ambient electrons once they cool sufficiently to form a Maxwellian distribution, i.e., we consider a hybrid thermal/non-thermal model. Using simple scaling laws for the luminosity of the cold disk, the thermal dissipation/heating rate in the corona, and the rate of energy injection from a non-thermal source, all as functions of radius of the corona, we explain the hard-to-soft transition as the result of a decrease in the size of the corona and the inner radius of the cold disk by a factor ~ 5. For the case of Cyg X-1, we show that the bolometric luminosity of the source (mass accretion rate) does not change significantly during the transition, and thus the transition is probably the result of a disk instability.

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arXiv1997

The Nature of Spectral Transitions in Accreting Black Holes: The Case of Cyg X-1

Accreting black holes radiate in one of several spectral states, switching from one to another for reasons that are as yet not understood. Using the best studied example, Cyg X-1, we identify the geometry and physical conditions characterizing these states. In particular, we show that in the hard state most of the accretion energy is dissipated in a corona-like structure which fills the inner few tens of gravitational radii around the black hole and has Compton optical depth of order unity. In this state, an optically thick accretion disc extends out to greater distance, but penetrates only a short way into the coronal region. In the soft state, the optically thick disc moves inward and receives the majority of the dissipated energy, while the "corona" becomes optically thin and extends around much of the inner disc. The mass accretion rate in both states is $~10^{-8} M_{\odot}$ yr$^{-1}$.

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arXiv1997

The variable X/gamma-ray spectrum of the Seyfert 2 galaxy NGC 7172

A broad band X-, gamma-ray spectral study of the Seyfert 2 galaxy NGC 7172 is presented. We use our ASCA observations from May 1995 and combine these with the CGRO OSSE data from March 1995. The only Seyfert 2 galaxy previously to have been modelled over such a broad spectral range is NGC 4945. We find that the most probable model for the data is an absorbed power law, being affected by a high energy exponential cut-off. The power law is flat with Gamma = 1.54 \pm 0.13, while N_H = (8.1 \pm 0.6) 10^{22} cm$^{-2}$. An Fe K$α$ emission line is not required by the fits. The observed flux in the 2-10 keV range is F_{2-10} = (4.75\pm 0.09) 10^{-11} erg cm$^{-2}$ s$^{-1}$, which corresponds to a small increase since the Ginga measurement in October 1989. The spectral index of the underlying power law of NGC 7172 appears actually to have varied from 1.85 to 1.5 since the Ginga observations in 1989. The e-folding energy is relatively well constrained and lies at 140 ^{+310} _{-70} keV. We note, however, that the CGRO OSSE spectral shape appears to be variable on a time scale of weeks.

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