arXiv subjects

Vahe' Petrosian

Publications and source records attributed to Vahe' Petrosian.

33 records · Page 2Linked to original sources

arXiv2004

Nonthermal Activity and Particle Acceleration in Clusters of Galaxies

Evidence for nonthermal activity in clusters of galaxies is well established from radio observations of synchrotron emission by relativistic electrons, and new windows (in EUV and Hard X-ray ranges) have provided more powerful tools for its investigation. The hard X-ray observations, notably from Coma, are summarized and results of a new RXTE observations of a high redshift cluster are presented. It is shown that the most likely emission mechanisms for these radiations is the inverse Compton scattering of the cosmic microwave background photons by the same electrons responsible for the radio radiation. Various scenarios for acceleration of the electrons are considered and it is shown that the most likely model is episodic acceleration by shocks or turbulence, presumably induced by merger activity, of high energy electrons injected into the intercluster medium by galaxies or active galactic nuclei.

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arXiv2004

RHESSI Observations of a Simple Large X-ray Flare on 11-03-2003

We present data analysis and interpretation of a simple X-class flare observed with RHESSI on November 3, 2003. In contrast to other X-class flares observed previously, this flare shows a very simple morphology with well defined looptop (LT) and footpoint (FP) sources. The almost monotonic upward motion of the LT source and increase in separation of the two FP sources are consistent with magnetic reconnection models proposed for solar flares. In addition, we find that the source motions are relatively slower during the more active phases of hard X-ray emission; the emission centroid of the LT source shifts toward higher altitudes with the increase of energy; the separation between the LT emission centroids at two different photon energies is anti-correlated with the FP flux. Non-uniformity of the reconnecting magnetic fields could be a possible explanation of these features.

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arXiv2004

Electron Acceleration around the Supermassive Black Hole at the Galactic Center

The recent detection of variable infrared emission from Sagittarius A*, combined with its previously observed flare activity in X-rays, provides compelling evidence that at least a portion of this object's emission is produced by nonthermal electrons. We show here that acceleration of electrons by plasma wave turbulence in hot gases near the black hole's event horizon can account both for Sagittarius A*'s mm and shorter wavelengths emission in the quiescent state, and for the infrared and X-ray flares, induced either via an enhancement of the mass accretion rate onto the black hole or by a reorganization of the magnetic field coupled to the accretion gas. The acceleration model proposed here produces distinct flare spectra that may be compared with future coordinated multi-wavelength observations. We further suggest that the diffusion of high energy electrons away from the acceleration site toward larger radii might be able to account for the observed characteristics of Sagittarius A*'s emission at cm and longer wavelengths.

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arXiv2001

New Statistical Methods for Analysis of Large Surveys: Distributions and Correlations

The aim of this paper is to describe new statistical methods for determination of the correlations among and distributions of physical parameters from a multivariate data with general and arbitrary truncations and selection biases. These methods, developed in collaboration with B. Efron of Department of Statistics at Stanford, can be used for analysis of combined data from many surveys with different and varied observational selection criteria. For clarity we will use the luminosity function of AGNs and its evolution to demonstrate the methods. We will first describe the general features of data truncation and present a brief review of past methods of analysis. Then we will describe the new methods and results from simulations testing their accuracy. Finally we will present the results from application of the methods to a sample of quasars.

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arXiv2001

Looptop Hard X-Ray Emission in Solar Flares: Images and Statistics

The discovery of hard X-ray sources near the top of a flaring loop by the HXT instrument on board the YOHKOH satellite represents a significant progress towards the understanding of the basic processes driving solar flares. In this paper we extend the previous study of limb flares by Masuda (1994) by including all YOHKOH observations up through August 1998. We report that from October 1991 to August 1998, YOHKOH observed 20 X-ray bright limb flares (where we use the same selection criteria as Masuda), of which we have sufficient data to analyze 18 events, including 8 previously unanalyzed flares. Of these 18 events, 15 show detectable impulsive looptop emission. Considering that the finite dynamic range (about a decade) of the detection introduces a strong bias against observing comparatively weak looptop sources, we conclude that looptop emission is a common feature of all flares. We summarize the observations of the footpoint to looptop flux ratio and the spectral indices. We present light curves and images of all the important newly analyzed limb flares. Whenever possible we present results for individual pulses in multipeak flares and for different loops for multiloop flares. We then discuss the statistics of the fluxes and spectral indices of the looptop and footpoint sources taking into account observational selection biases. The importance of these observations (and those expected from the scheduled HESSI satellite with its superior angular spectral and temporal resolution) in constraining acceleration models and parameters is discussed briefly.

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arXiv2001

Interpreting the Behavior of Time Resolved Gamma-Ray Burst Spectra

In this paper, we explore time resolved Gamma-Ray Burst (GRB) spectra in the context of the synchrotron emission model presented in Lloyd and Petrosian (2000; LP00). First, we show that our model - which involves three distinct emission regimes - can provide excellent fits to the time resolved spectra of GRBs, and we present these results for a few bursts. We then describe how the phenomenological Band spectrum (Band et al., 1993) can be interpreted in the context of our models based on the value of the low energy photon index $α$. We discuss the types of correlations one would expect to observe among the Band parameters if these models are correct. We then compare these predictions to the existing data, combining a sample of 2,026 time resolved spectra (from approximately 80 bursts). We show that the correlations found in the data are consistent with the models, and discuss the constraints they place on the emission physics. In particular, we find a 4-sigma negative correlation between the peak of the nu-Fnu spectrum, Ep, and the low energy photon index $α$ for bursts with $-2/3 < α< 0$, in contrast to what is predicted by the instrumental effect discussed in LP00. We suggest that this correlation is simply due to the mechanism responsible for producing $α$'s above the value of -2/3 - namely, a decreasing mean pitch angle of the electrons. We also show that Ep is correlated with the photon flux, and interpret this as a result of changing magnetic field or characteristic electron energy between emission episodes. Finally, we discuss the implications our results have on particle acceleration in GRBs, and prospects for further testing these models with the anticipated data from HETE-2, Swift and GLAST.

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arXiv2000

Synchrotron Radiation as the Source of Gamma-Ray Burst Spectra

We investigate synchrotron emission models as the source of gamma-ray burst spectra. We show that including the possibility for synchrotron self-absorption, a ``smooth cutoff'' to the electron energy distribution, and an anisotropic distribution for the electron pitch angles produces a whole range of low energy spectral behavior. In addition, we show that the procedure of spectral fitting to GRB data over a finite bandwidth can introduce a spurious correlation between spectral parameters - in particular, the value of the peak of the nu F_nu spectrum, E_p, and the low energy photon spectral index alpha (the lower E_p is, the lower (softer) the fitted value of alpha will be). From this correlation and knowledge of the E_p distribution, we show how to derive the expected distribution of alpha. We show that optically thin synchrotron models with an isotropic electron pitch angle distribution can explain the distribution of alpha below alpha=-2/3. This agreement is achieved if we relax the unrealistic assumption of the presence of a sharp low energy cutoff in the spectrum of accelerated electrons, and allow for a more gradual break. We show that this low energy portion of the electron spectrum can be at most flat. We also show that optically thin synchrotron models with an anisotropic electron pitch angle distribution can explain all bursts with -2/3 < alpha <= 0$. The very few bursts with low energy spectral indices that fall above alpha=0 may be due the presence of a the synchrotron self-absorption frequency entering the lower end of the BATSE window. Our results also predict a particular relationship between alpha and E_p during the temporal evolution of a GRB. We give examples of spectral evolution in GRBs and discuss how the behavior are consistent with the above models.

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arXiv1999

Synchrotron Emission as the Source of GRB Spectra, Part II: Observations

We test the models of synchrotron emission presented in Part I of this series (Lloyd & Petrosian, these proceedings) against the distributions and evolution of GRB spectral parameters (particularly the low energy index, $α$). With knowledge of the $E_{p}$ distribution and the correlation between $α$ and $E_{p}$ presented in Part I, we show how to derive the expected distribution of $α$ from fits to optically thin synchrotron spectra, and compare this with the observed distribution. We show that there is no difficulty explaining bursts below the ``line of death'', $α< -2/3$, and that these bursts indicate that the spectrum of accelerated electrons must flatten or decline at low energies. Bursts with low energy spectral indices that fall above this limit are explained by the synchrotron self-absorption frequency entering the lower end of the BATSE window. Finally, we discuss a variety of spectral evolution behavior seen in GRBs and explain this behavior in the context of synchrotron emission from internal shocks.

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arXiv1999

Synchrotron Radiation as the Source of GRB Spectra, Part I: Theory

We investigate synchrotron emission models as the source of gamma ray burst spectra. We show that allowing for synchrotron self absorption and a ``smooth cutoff'' to the electron energy distribution produces a wide range of low energy spectral behavior. We show that there exists a correlation between the value of the peak of the $νF_ν$ spectrum, $E_{p}$, and the low energy spectral index $α$ as determined by spectral fits over a finite bandwidth. Finally, we discuss the implications of synchrotron emission from internal shocks for GRB spectral evolution.

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arXiv1999

Cosmological versus Intrinsic: The Correlation between Intensity and the Peak of the nu F_nu Spectrum of Gamma Ray Bursts

We present results of correlation studies, examining the association between the peak of the nu F_nu spectrum of gamma ray bursts, E_p, with the burst's energy fluence and photon peak flux. We discuss methods to account for data truncation in E_p and fluence or flux when performing the correlation analyses. However, because bursts near the detector threshold are not usually able to provide reliable spectral parameters, we focus on results for the brightest bursts in which we can better understand the selection effects relevant to E_p and burst strength. We find that there is a strong correlation between total fluence and E_p. We discuss these results in terms of both cosmological and intrinsic effects. In particular, we show that for realistic distributions of the burst parameters, cosmological expansion alone cannot account for the correlation between E_p and total fluence; the observed correlation is likely a result of an intrinsic relation between the burst rest-frame peak energy and the total radiated energy. We investigate this latter scenario in the context of synchrotron radiation from external and internal shock models of GRBs. We find that the internal shock model is consistent with our interpretation of the correlation, while the external shock model cannot easily explain this intrinsic relation between peak energy and burst radiated energy.

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arXiv1999

Cosmological Signatures in Temporal and Spectral Characteristics of Gamma-Ray Bursts

There have been several suggestions of the existence of cosmological redshift signatures in the temporal and spectral characteristics of gamma-ray bursts. However, recent discoveries of afterglows and redshift measurements indicate the presence of broad ``luminosity functions'' which may overwhelm such weaker cosmological signatures. The primary goal of this paper is to determine if the intrinsic and cosmological dispersions can be separated. We have expanded the search for cosmological signatures to several other temporal and spectral features, have determined correlations which could arise from the cosmological redshift of the sources and have carried out tests to determine if the observed correlations can be due to cosmology. We find that the intrinsic dispersions are the dominant factors and that detection of cosmological signatures must await accumulation of a much larger number of identification with galaxies and measurements of redshifts.

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arXiv1998

Distribution of Spectral Characteristics and the Cosmological Evolution of GRBs

We investigate the cosmological evolution of GRBs, using the total gamma ray fluence as a measure of the burst strength. This involves an understanding of the distributions of the spectral parameters of GRBs as well as the total fluence distribution - both of which are subject to detector selection effects. We present new non-parametric statistical techniques to account for these effects, and use these methods to estimate the true distribution of the peak of the nu F_nu spectrum, E_p, from the raw distribution. The distributions are obtained from four channel data and therefore are rough estimates. Here, we emphasize the methods and present qualitative results. Given its spectral parameters, we then calculate the total fluence for each burst, and compute its cumulative and differential distributions. We use these distributions to estimate the cosmological rate evolution of GRBs, for three cosmological models. Our two main conclusions are the following: 1) Given our estimates of the spectral parameters, we find that there may exist a significant population of high E_p bursts that are not detected by BATSE, 2) We find a GRB co-moving rate density quite different from that of other extragalactic objects; in particular, it is different from the recently determined star formation rate.

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arXiv1998

New and Old Tests of Cosmological Models and Evolution of Galaxies

We describe the classical cosmological tests, such as the Log$N$-Log$S$, redshift-magnitude and angular diameter tests, and propose some new tests of the evolution of galaxies and the universe. Most analyses of these tests treat the problem in terms of a luminosity function and its evolution which can lead to incorrect conclusions when dealing with high redshift sources. We develop a proper treatment in three parts. In the first part we describe these tests based on the isophotal values of the quantities such as flux, size or surface brightness. We show the shortcomings of the simple point source approximation based solely on the luminosity function and consideration of the flux limit. We emphasize the multivariate nature of the problem and quantify the effects of other selection biases due to the surface brightness and angular size limitations. In these considerations the surface brightness profile plays a critical role. In the second part we show that considerable simplification over the complicated isophotal scheme is achieved if these test are carried out in some sort of metric scheme, for example that suggested by Petrosian (1976). This scheme, however, is limited to well resolved sources. Finally, we describe the new tests, which use the data to a fuller extent than the isophotal or metric based tests, and amount to simply counting the pixels or adding their intensities as a function of the pixel surface brightness, instead of dealing with surface brightness, sizes and fluxes of individual galaxies. We show that the data analysis and its comparison with the theoretical models of the distributions and evolution of galaxies has the simplicity of the metric test and utilizes the data more fully than the isophotal test.

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arXiv1997

Distribution of Fluences versus Peak Fluxes and the Cosmological Density Evolution of the Rate of GRBs

We argue that recent observations of afterglows and some theoretical considerations indicate that the gamma-ray fluence may be a better measure of strength of a burst than its peak flux. We discuss how the distribution of the fluence, or any physical quantity related to the peak counts (or flux), can be obtained with proper correction for threshhold effects. Using this method, we compare the cumulative and differential distributions of flux and fluence. We find some differences between these distributions, but more remarkable are the similarities between the distributions. The other striking feature is how different these distributions are from those of other extragalactic objects. Using the fluence distributions, we derive the expected comoving density rate evolution for different assumed total luminosity; we compare this with the GRB rate expected from the recently determined star formation rate.

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arXiv1997

The Extent of the Spectral Bias in BATSE: The True Distribution of the nu F_nu Peak Energy

The distributions of spectral characteristics and their correlations with fluence, peak flux, or duration, are essential in understanding the nature of GRBs. However, the selection effects involved in detecting GRBs can distort these distributions. Here, we discuss how to deal with selection effects involving the peak energy E_p of the GRB nu F_nu spectrum, which suffers from both an upper and lower threshold. We describe a new method to account for this double-sided truncation, and show that the true distribution of E_p is significantly different from the observed distribution.

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