arXiv subjects

Roland Svensson

Publications and source records attributed to Roland Svensson.

At least 19 recordsLinked to original sources

arXiv2011

A Complexity-Brightness Correlation in Gamma Ray Bursts

We observe strong correlations between the temporal properties of gamma ray bursts (GRBs) and their apparent peak brightness. The strongest effect (with a significance level of 10^{-6}) is the difference between the brightness distributions of simple bursts (dominated by a single smooth pulse) and complex bursts (consisting of overlapping pulses). The latter has a break at a peak flux of 1.5 ph/cm^2/s, while the distribution of simple bursts is smooth down to the BATSE threshold. We also observe brightness dependent variations in the shape of the average peak aligned time profile (ATP) of GRBs. The decaying slope of the ATP shows time dilation when comparing bright and dim bursts while the rising slope hardly changes. Both slopes of the ATP are deformed for weak bursts as compared to strong bursts. The interpretation of these effects is simple: a complex burst where a number of independent pulses overlap in time appears intrinsically stronger than a simple burst. Then the BATSE sample of complex bursts covers larger redshifts where some cosmological factor causes the break in the peak brightness distribution. This break could correspond to the peak in the star formation rate that was recently shown to occur at a redshift of z~1.5.

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arXiv2002

Determining Bolometric Corrections for BATSE Burst Observations

We compare the energy and count fluxes obtained by integrating over the finite bandwidth of BATSE with a measure proportional to the bolometric energy flux, the phi-measure, introduced by Borgonovo & Ryde. We do this on a sample of 74 bright, long, and smooth pulses from 55 GRBs. The correction factors show a fairly constant behavior over the whole sample, when the signal-to-noise-ratio is high enough. We present the averaged spectral bolometric correction for the sample, which can be used to correct flux data.

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arXiv2001

On the Variety of the Spectral and Temporal Behaviors of Long Gamma-Ray Burst Pulses

We find and study a variety of the spectral-temporal behavior during the decay phase of long and bright GRB pulses. Even though only a small fraction of observed bursts exhibit such pulses, these are of interest to study as they reflect individual emission episodes during the burst. We have previously found that for about half of these decays, the instantaneous photon flux ~ 1/time, which is a consequence of the validity of both a power-law hardness-intensity correlation (HIC) and an exponential hardness-fluence correlation (HFC). Here, we study a complete sample of 25 BATSE pulses (having a peak flux in 1 s resolution of > 5 photons/s/cm^2 and a S/N of 30 in at least 8 time bins) and, specifically, search for other types of decay behaviors. First, we find that a power law gives a better description of the pulse decays than a stretched exponential, the most commonly assumed pulse shape so far. Then we find that about half of the decays behave approximately as 1/time, and the other half approximately as 1/time^3. For a few of the latter decays, the two correlations, the HIC and the HFC, are constrained and found to be consistent with the light curve behavior. For these cases, the HIC is still a power law while the HFC is described by a generalized function. Finally, we briefly discuss our results in a physical context.

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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

On the "failure" of the standard model for Soft X-ray Transients in quiescence

It is currently believed that the ``standard'' accretion disk theory under-predicts the observed X-ray luminosity from Soft X-ray Transients (SXT) in quiescence by as much as 4 to 6 orders of magnitude. This failure of the standard model is considered to be an important argument for the existence of the alternative mode of accretion -- Advection Dominated Accretion Flows (ADAF) in astrophysics, since these flows allow a much higher level of X-ray emission in quiescence, in agreement with the observations. Here we point out that, in stark contrast to steady-state standard disks, such disks in quiescence (being non-steady) produce most of the X-ray emission very far from the last stable orbit. Taking this into account, these disks can accommodate the observed X-ray luminosities of SXTs rather naturally. Our theory predicts that Fe K-alpha lines from standard accretion disks in quiescence should be narrow even though the cold disk goes all the way down to the last stable orbit.

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arXiv2001

A Variety of Decays of Gamma-Ray Burst Pulses

We find and study a variety of the spectral-temporal behavior during the decay phase of the light curve of long and bright pulse structures in gamma-ray bursts. It was earlier found that for about half of these decays, the instantaneous photon flux is consistent with a power law in time, where the photon flux $\propto$ 1/time. This decay behavior is a consequence of the validity of both a power law hardness-intensity correlation (HIC) and an exponential hardness-fluence correlation (HFC). Beside this behavior, we find that the other cases approximately behave as 1/(time)$^3$. For a few of these latter decays, the two correlations (HIC and HFC) are constrained and found to be consistent with the light curve behavior. For these cases, the HIC is still a power law while the HFC is described by a generalized function. We study and describe these behaviors analytically and examine actual burst data from the complete catalog of the BATSE on the CGRO.

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arXiv1999

On the Time Evolution of Gamma-Ray Burst Pulses: A Self-Consistent Description

For the first time, the consequences of combining two well-established empirical relations, describing different aspects of the spectral evolution of observed gamma-ray burst (GRB) pulses, are explored. These empirical relations are: i) the hardness-intensity correlation, and ii) the hardness-photon fluence correlation. From these we find a self-consistent, quantitative, and compact description for the temporal evolution of pulse decay phases within a GRB light curve. In particular, we show that in the case of the two empirical relations both being valid, the instantaneous photon flux (intensity) must behave as 1/(1+ t/τ) where τis a time constant that can be expressed in terms of the parameters of the two empirical relations. The time evolution is fully defined by two initial constants, and two parameters. We study a complete sample of 83 bright GRB pulses observed by the Compton Gamma-Ray Observatory and identify a major subgroup of GRB pulses (~45 %), which satisfy the spectral-temporal behavior described above. In particular, the decay phase follows a reciprocal law in time. It is unclear what physics causes such a decay phase.

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arXiv1999

Thermalization Mechanisms in Compact Sources

There is strong observational evidence that a quasi-thermal population of electrons (or pairs) exists in compact X-ray sources. It is, however, unclear what mechanism thermalizes the particles. Here, two processes, Coulomb scattering and synchrotron self-absorption, that may be responsible for the thermalization, are reviewed. The parameter spaces in which respective process dominates are given. While the Coulomb thermalization mechanism is well-known, this is not the case for the synchrotron self-absorption thermalization. We give the arguments that synchrotron self-absorption must act as a thermalizing mechanism in sufficiently compact sources. The emitting and absorbing electrons then exchange energy efficiently with the self-absorbed synchrotron radiation field and are driven towards a relativistic or mildly relativistic thermal distribution in a few synchrotron cooling times (the "synchrotron boiler").

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arXiv1999

Old and New Advances in Black Hole Accretion Disk Theory

A summary is given of the high-lights during the Reykjavik Midsummer Symposium on Non-Linear Phenomena in Accretion Discs around Black Holes. Such high-lights include the recent advances on understanding: 1) the accretion disc solution branch dominated by advection (i.e., advection dominated accretion flows, ADAFs), 2) the importance of magnetic fields in many different respects, most importantly being responsible for the self-sustained MHD-turbulence giving rise to the disc viscosity, and 3) the details of the radiation processes giving rise to the X/gamma-ray continuum originating close to the black hole. Some old advances are unfortunately also necessary to discuss here. It is pointed out to the accretion disc research community, that many of the research papers published on ADAFs 1994-1997 do not accurately present the history of ADAF research. Some of the results that were presented as new and original actually appeared in a paper by Ichimaru already in 1977. Also not quoted are the papers from the 1970's and 1980's calculating the temperature structure of and the spectra from quasi-spherical accretion flows onto black holes. As the ADAFs are close to being quasi-spherical, the resulting spectra of ADAFs and quasi-spherical flows are almost identical. Further of the recent ADAF results are therefore not new results as is sometimes claimed. In spite of all the recent progress of various aspects of accretion flows around black holes, many of the research lines have still not been merged providing potential for further dramatic progress in coming years.

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arXiv1998

On the Shape of Pulse Spectra in Gamma-Ray Bursts

The discovery (Liang & Kargatis 1996), that the peak energy of time-resolved spectra of gamma-ray burst (GRB) pulses decays exponentially with fluence, is analytically shown to imply that the time-integrated photon number spectrum of a pulse should have a unique shape, given by an underlying E^-1 behavior. We also show that the asymptotic low energy normalization of the time-integrated spectrum is equal to the exponential decay constant. We study analytically how this general behavior is modified in more realistic situations and show that diversity is then introduced in the properties of time-integrated GRB pulse spectra. We argue that further diversity will occur in time-integrated multi-pulse (complex) GRB spectra. The total energy received per cm^2 is approximately the decay constant times the maximum peak energy of the pulse. Our analytical results connect the properties of the time-integrated pulse spectrum with those of the time-resolved spectra, and can thus be used when studying observed GRB pulse spectra. We illustrate with the bright burst GRB 910807 and comment on GRB 910525 and GRB 921207.

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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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arXiv1997

Brightness Dependent Properties of Gamma Ray Bursts

Brightness dependent correlations have been conclusively detected in the average time profiles of gamma-ray bursts (GRBs). Determining the time constants of the stretched exponential slopes of the average peak-aligned time profile of GRBs as a function of the peak brightness, we find that the post-peak slope shows time dilation when comparing bright and dim bursts, while the pre-peak slope hardly changes. Stronger bursts are thus more symmetric than weaker bursts at a high confidence level. The very weakest bursts have a different shape (i.e., a different stretched exponential index) of their average time profile as compared to brighter bursts. This difference is too large to be explained by trigger effects and Poisson noise. We interpret these correlations as being the result of an intrinsic positive correlation between brightness and complexity of GRBs. This interpretation is directly confirmed by simulations as well as by a morphological classification of bursts. The fact that such a correlation can be observed should impose new constraints on the distribution of GRBs over luminosity distance.

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arXiv1997

X-rays and gamma rays from active galactic nuclei

Various types of active galactic nuclei (AGN) are briefly discussed, with an emphasis on the theory of recent X-ray and $γ$-ray observations of the subclass, Seyfert 1 galaxies. The large radiation power from AGN is thought to originate from gravitational power released by matter accreting onto a supermassive black hole. The physical mechanisms responsible for the energy release and the geometry of the gaseous components are still uncertain in spite of three decades of observational and theoretical studies. The recent X-ray and gamma-ray observations, however, start to provide useful constraints on the models. This kind of interpretation of the observations is possible due to theoretical developments during the last decade of radiative transfer of X-rays in both optically thick and thin media of various geometries. Particular attention is paid to various accretion disk-corona models. Recent work on exact radiative transfer in such geometries are reviewed.

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arXiv1997

Two-phase pair corona model for AGN: physical modelling and diagnostics

The predictions of the two-phase accretion disc-corona models for active galactic nuclei are compared with observations. We discuss the possibility to use X-ray spectral slopes, equivalent widths of the iron line, and the observed flux-spectral index correlation as diagnostics of the X/gamma-ray source compactness and geometry as well as of the cold disc temperature. As an example of the application of the modelling tools, we use XSPEC to fit the broad-band data of Seyfert 1 galaxy, IC4329A, with a theoretical spectrum from a hemisphere-corona.

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arXiv1997

The X/gamma-ray spectral properties of NGC 7172

We present a combined, non-simultaneous, ASCA GIS and CGRO OSSE spectrum of the Seyfert 2 galaxy, NGC 7172, and make broad band spectral fits. The only Seyfert 2 galaxy previously studied over such a broad band is NGC 4945. We find that the most probable model for the data is a power law with an exponential cut-off being affected by a neutral absorber. The best fit parameters are found to be $Γ= 1.47 \pm 0.15$ and $\NH = (7.8 \pm 0.6) 10^{22}$ cm$^{-2}$. The spectral index of the underlying power law of NGC 7172 has therefore varied from 1.8 to 1.5 since the Ginga observations in 1989. For this simple model the e-folding energy at $88^{+65}_{-28}$ keV is relatively well constrained. The observed flux in the 2-10 keV range is $F_{2-10}=4.7 10^{-11}$ erg cm$^{-2}$ s$^{-1}$, which corresponds to a small increase since the Ginga measurement in October 1989.

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arXiv1997

Average properties of the time behaviour of gamma-ray bursts

The average peak-aligned profile of all bursts detected by BATSE with sufficient data quality has a simple ``stretched'' exponential shape, F ~ exp[-(t/t_0)^{1/3}], where $t$ is the time measured from the time for the peak flux of the event, and $t_0$ is a time constant. We study the behaviour of $t_0$ of both the post-peak and the pre-peak slopes of the average time profile as a function of the peak brightness range of the burst sample. We found that the post-peak slope shows time dilation when comparing bright and dim bursts, while the pre-peak slope hardly changes. Thus dimmer bursts have a different shape -- they are more asymmetric. This shape-brightness correlation is observed at a 99.6% confidence level. Such a correlation has a natural explanation within the pulse avalanche model, which is briefly described. Complex events, consisting of many pulses are more symmetric and are intrinsically brighter. Bursts consisting of one or a few pulses are intrinsically weaker and more asymmetric. For such a correlation to be observable requires that the luminosity distance distribution of GRBs to be different from a power-law.

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arXiv1996

The Synchrotron Boiler: a Thermalizer in Seyfert Galaxies

There are difficulties in understanding what keeps the plasma thermalized in compact sources, especially during rapid variations of the emitted flux. Particle-particle collisions are too inefficient in hot rarefied plasmas, and a faster process is called for. Synchrotron absorption is such a process. We show that relativistic electrons can thermalize in a few synchrotron cooling times by emitting and absorbing cyclo-synchrotron photons. The resulting equilibrium distribution is a Maxwellian at low energies, with a high energy power law tail when Compton cooling is important. Assuming that the particles emit completely self absorbed synchrotron radiation while they at the same time Compton scatter ambient UV photons, we calculate the time dependent behavior of the distribution function, and the final high energy spectra.

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arXiv1996

A simple formula for the thermal pair annihilation line emissivity

We introduce a simple and convenient fitting formula for the thermal annihilation line from pair plasmas in cosmic sources. The fitting formula is accurate to 0.04\% and is valid at all photon energies and temperatures of interest. The commonly used Gaussian line profile is not a good approximation for broader lines.

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