arXiv subjects

Vahé Petrosian

Publications and source records attributed to Vahé Petrosian.

At least 19 recordsLinked to original sources

arXiv2026

Cosmological Evolution of Gamma Ray Bursts

Gamma-ray bursts (GRBs) are classified as long (LGRBs) and short (SGRBs), with collapsars and compact-object mergers (NS-NS or NS-Black Holes) as progenitors, respectively. LGRBs are expected to follow the cosmic star formation rate (SFR), while SGRBs follow a delayed version of the SFR. However, this division has come under question, most prominently by observational evidence of an excess of LGRBs at low redshifts by several investigations, summarized in \cite{Petrosian_2024}. Two recent observations of low-redshift LGRBs show associations with kilonovae. Both of these indicate compact mergers as a potential source of LGRBs as well. Most results showing this separation are based on analyses of small (less than 200) samples of LGRBs with measured redshifts. The aim of this paper is to use a larger sample of LGRBs. The number of LGRBs with measured redshifts has increased by more than a factor of 2 over the last decade. To this data set we add a sample of LGRBs whose redshifts are estimated using a machine learning (ML) method (\cite{Narendra_2025}). To account for the observational selection bias due to redshift measurements, we use the non-parametric, non-binning Efron-Petrosian method to establish the degree of correlation between luminosity and redshift, \textit{the luminosity evolution}, and then use the Lynden-Bell $C^-$ method to obtain the luminosity function. We find a low redshift excess for the larger sample with measured redshifts. Adding the sources with ML-estimated redshifts, which shows overabundance of the mid-range redshifts, the excess is reduced.

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arXiv2025

Cosmological Evolution of Fast Radio Bursts and The Star Formation Rate

We investigate the cosmological evolution of the luminosity and redshift of FRBs. As is the case for all extragalactic sources, we are dealing with data that are truncated by observational selection effects, the most important being the flux limit, which introduces the so-called Eddington-Malmquist bias. In addition, for FRBs, there is a significant uncertainty in the redshifts obtained from the observed dispersion measures (DMs). To correct for the truncation we use the non-parametric, non-binning Efron-Petrosian and Lynden-Bell methods, which give unbiased distributions of luminosities and redshifts and their cosmological evolution. To quantify the redshift uncertainty, we use a data set which accounts for uncertainties of contribution to the DM of the host galaxy, and DM uncertainties due to fluctuation in the intergalactic medium. This data, in addition to a mean redshift, gives the one-sigma errors. We construct three samples with lower, mean, and upper redshifts and apply the above methods to each. For the three samples, we find similar (1) $\sim 3σ$ evidence for luminosity evolution, (2) a luminosity function that can be fit by a simple broken power law, and (3) a comoving density formation rate that decreases rapidly with redshift unlike the cosmic star formation rate (SFR). This rate is similar to that of short gamma-ray bursts, which are believed to result from compact star mergers with a formation rate delayed relative to the SFR. This may further support the hypothesis that magnetars are the progenitors of FRBs.

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arXiv2021

Cosmological Evolution of Fermi-LAT Gamma-ray Blazars Using Novel Nonparametric Methods

Multi-wavelength analyses of spectra of active galactic nuclei (AGNs) provide useful information on the physical processes in the accretion disk and jets of black holes. This, however, is limited to bright sources and may not represent the population as a whole. Another approach is through the investigation of the cosmological evolution of the luminosity function (LF) that shows varied evolution (luminosity and density) at different wavelengths. These differences and the correlations between luminosities can shed light on the Jet-accretion disk connection. Most such studies use forward fitting parametric methods that involve several functions and many parameters. We use non-parametric, non-binning methods developed by Efron and Petrosian, and Lynden-Bell, for obtaining unbiased description of the evolution of the LF, from data truncated by observational selection effects. We present analysis of the evolution of gamma-ray LF of blazars with main focus on flat spectrum radio quasars (FSRQs). This requires analysis of both gamma-ray and optical data, essential for redshift measurements, and a description of the joint LF. We use a new approach which divides the sample into two sub-samples, each with its own flux limit. We use the Fermi-LAT and GAIA observations, and present results on the gamma-ray LF and its evolution, and determine the intrinsic correlation between the gamma-ray and optical luminosities corrected for the well known false correlation induced by their similar redshift dependence and evolution of the two luminosities. We also present a direct estimation of the contribution of blazars to the spectrum of the extragalactic gamma-ray background.

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arXiv2019

Gamma-ray Bursts as distance indicators through a machine learning approach

Gamma-ray bursts (GRBs) are spectacularly energetic events, with the potential to inform on the early universe and its evolution, once their redshifts are known. Unfortunately, determining redshifts is a painstaking procedure requiring detailed follow-up multi-wavelength observations often involving various astronomical facilities, which have to be rapidly pointed at these serendipitous events. Here we use Machine Learning algorithms to infer redshifts from a collection of observed temporal and spectral features of GRBs. We obtained a very high correlation coefficient ($0.96$) between the inferred and the observed redshifts, and a small dispersion (with a mean square error of $0.003$) in the test set. The addition of plateau afterglow parameters improves the predictions by $61.4\%$ compared to previous results. The GRB luminosity function and cumulative density rate evolutions, obtained from predicted and observed redshift are in excellent agreement indicating that GRBs are effective distance indicators and a reliable step for the cosmic distance ladder.

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arXiv2018

The Relation Between Escape and Scattering Times of Energetic Particles in a Turbulent Magnetized Plasma: Application to Solar Flares

A knowledge of the particle escape time from the acceleration regions of many space and astrophysical sources is of critical importance in the analysis of emission signatures produced by these particles and in the determination of the acceleration and transport mechanisms at work. This paper addresses this general problem, in particular in solar flares, where in addition to scattering by turbulence, the magnetic field convergence from the acceleration region towards its boundaries also influences the particle escape. We test an (approximate) analytic relation between escape and scattering times, and the field convergence rate, based on the work of Malyshkin and Kulsrud (2001), valid for both strong and weak diffusion limits and isotropic pitch angle distribution of the injected particles, with a numerical model of particle transport. To this end, a kinetic Fokker-Planck transport model of particles is solved with a stochastic differential equation scheme assuming different initial pitch angle distributions. This approach enables further insights into the phase-space dynamics of the transport process, which would otherwise not be accessible. We find that in general the numerical results agree well with the analytic equation for the isotropic case, however, there are significant differences weak diffusion regime for non-isotopic cases, especially for distributions beamed along the magnetic field lines. The results are important in the interpretation of observations of energetic particles in solar flares, and other similar space and astrophysical acceleration sites and for the determination of acceleration-transport coefficients, commonly used in Fokker-Planck type kinetic equations.

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arXiv2018

Implications of loop-top origin for microwave, hard X-ray, and low-energy gamma-ray emissions from behind the limb flares

The Fermi gamma-ray Space Telescope (Fermi) has detected hard X-ray (HXR) and gamma-ray photons from three flares, which according to \stereo occurred in active regions behind the limb of the Sun as delineated by near Earth instruments. For two of these flares \r has provided HXR images with sources located just above the limb, presumably from the loop top (LT) region of a relatively large loop. Fermi-Gamma-ray Burst Monitor has detected HXRs and gamma-rays, and RSTN has detected microwaves emissions with similar light curves. This paper presents a quantitative analysis of these multi-wavelength observations assuming that HXRs and microwaves are produced by electrons accelerated at the LT source, with emphasize on the importance of the proper treatment of escape of the particles from the acceleration-source region and the trans-relativistic nature of the analysis. The observed spectra are used to determine the magnetic field and relativistic electron spectra. It is found that a simple power-law in momentum (with cut off above a few 100 MeV) agrees with all observations, but in energy space a broken power law spectrum (steepening at rest mass energy) may be required. It is also shown that the production of the $>100$ MeV photons detected by The Fermi-Large Area Telescope at the LT source would require more energy compared to photospheric emission. These energies are smaller than that required for electrons, so that the possibility that all the emissions originate in the LT cannot be ruled out on energetic grounds. However, the differences in the light curves and emission centroids of HXRs and $>100$ MeV gamma-rays favour a different source for the latter.

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arXiv2018

The Fermi bubbles from stochastic acceleration of electrons in a Galactic outflow

The discovery of the Fermi bubbles---a huge bilobular structure seen in GeV gamma-rays above and below the Galactic center---implies the presence of a large reservoir of high energy particles at $\sim 10 \, \text{kpc}$ from the disk. The absence of evidence for a strong shock coinciding with the edge of the bubbles, and constraints from multi-wavelength observations point towards stochastic acceleration by turbulence as a likely mechanism of acceleration. We have investigated the time-dependent acceleration of electrons in a large-scale outflow from the Galactic centre. For the first time, we present a detailed numerical solution of the particle kinetic equation that includes the acceleration, transport and relevant energy loss processes. We also take into account the addition of shock acceleration of electrons at the bubble's blast wave. Fitting to the observed spectrum and surface brightness distribution of the bubbles allows determining the transport coefficients, thereby shedding light on the origin of the Fermi bubbles.

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arXiv2016

Hard X-Ray Emission from Partially Occulted Solar Flares: RHESSI Observations in Two Solar Cycles

Flares close to the solar limb, where the footpoints are occulted, can reveal the spectrum and structure of the coronal loop-top source in X-rays. We aim at studying the properties of the corresponding energetic electrons near their acceleration site, without footpoint contamination. To this end, a statistical study of partially occulted flares observed with RHESSI is presented here, covering a large part of solar cycles 23 and 24. We perform a detailed spectra, imaging and light curve analysis for 116 flares and include contextual observations from SDO and STEREO when available, providing further insights into flare emission that was previously not accessible. We find that most spectra are fitted well with a thermal component plus a broken power-law, non-thermal component. A thin-target kappa distribution model gives satisfactory fits after the addition of a thermal component. X-rays imaging reveals small spatial separation between the thermal and non-thermal components, except for a few flares with a richer coronal source structure. A comprehensive light curve analysis shows a very good correlation between the derivative of the soft X-ray flux (from GOES) and the hard X-rays for a substantial number of flares, indicative of the Neupert effect. The results confirm that non-thermal particles are accelerated in the corona and estimated timescales support the validity of a thin-target scenario with similar magnitudes of thermal and non-thermal energy fluxes.

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arXiv2016

Data-driven Radiative Hydrodynamic Modeling of the 2014 March 29 X1.0 Solar Flare

Spectroscopic observations of solar flares provide critical diagnostics of the physical conditions in the flaring atmosphere. Some key features in observed spectra have not yet been accounted for in existing flare models. Here we report a data-driven simulation of the well-observed X1.0 flare on 2014 March 29 that can reconcile some well-known spectral discrepancies. We analyzed spectra of the flaring region from the Interface Region Imaging Spectrograph (IRIS) in MgII h&k, the Interferometric BIdimensional Spectropolarimeter at the Dunn Solar Telescope (DST/IBIS) in H$α$ 6563 Å and CaII 8542 Å, and the Reuven Ramaty High Energy Solar Spectroscope Imager (RHESSI) in hard X-rays. We constructed a multi-threaded flare loop model and used the electron flux inferred from RHESSI data as the input to the radiative hydrodynamic code RADYN to simulate the atmospheric response. We then synthesized various chromospheric emission lines and compared them with the IRIS and IBIS observations. In general, the synthetic intensities agree with the observed ones, especially near the northern footpoint of the flare. The simulated MgII line profile has narrower wings than the observed one. This discrepancy can be reduced by using a higher microturbulent velocity (27 km/s) in a narrow chromospheric layer. In addition, we found that an increase of electron density in the upper chromosphere within a narrow height range of $\approx$800 km below the transition region can turn the simulated MgII line core into emission and thus reproduce the single peaked profile, which is a common feature in all IRIS flares.

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arXiv2016

Acceleration of Thermal Protons By Generic Phenomenological Mechanisms

We investigate heating and acceleration of protons from a thermal gas with a generic diffusion and acceleration model, and subject to Coulomb scattering and energy loss, as was carried out in Petrosian & East (2008) for electrons. As protons gain energy their loss to electrons becomes important. Thus, we need to solve the coupled proton-electron kinetic equation. We numerically solve the coupled Fokker-Plank equations and computes the time evolution of the spectra of both particles. We show that this can lead to a quasi-thermal component plus a high energy nonthermal tail. We determine the evolution of nonthermal tail and the quasi-thermal component. The results may be used to explore the possibility of inverse bremsstrahlung radiation as a source of hard X-ray emissions from hot sources such as solar flares, accretion disk coronas and the intracluster medium of galaxy clusters. We find that emergence of nonthermal protons is accompanied by excessive heating of the entire plasma, unless the turbulence needed for scattering and acceleration is steeper than Kolmogorov and the acceleration parameters, the duration of the acceleration, and/or the initial distributions are significantly fine-tuned. These results severely constraint the feasibility of nonthermal inverse bremsstrahlung process producing hard X-ray emissions. However the nonthermal tail may be the seed particles for further re-acceleration to relativistic energies, say by a shock. In the Appendix we present some tests of the integrity of the algorithm used and present a new formula for the energy loss rate due to inelastic proton-proton interactions.

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arXiv2015

Solar Flare Chromospheric Line Emission: Comparison Between IBIS High-resolution Observations and Radiative Hydrodynamic Simulations

Solar flares involve impulsive energy release, which results in enhanced radiation over a broad spectral range and a wide range of heights. In particular, line emission from the chromosphere can provide critical diagnostics of plasma heating processes. Thus, a direct comparison between high-resolution spectroscopic observations and advanced numerical modeling results could be extremely valuable, but has not yet been attempted. In this paper, we present in this paper such a self-consistent investigation of an M3.0 flare observed by the Dunn Solar Telescope's Interferometric Bi-dimensional Spectrometer (IBIS) on 2011 September 24 which we have modeled with the radiative hydrodynamic code RADYN. We obtained images and spectra of the flaring region with IBIS in H$α$ 6563 Å and Ca II 8542 Å, and with the Reuven Ramaty High Energy Solar Spectroscope Imager (RHESSI) in X-rays. The latter observations were used to infer the non-thermal electron population, which was passed to RADYN to simulate the atmospheric response to electron collisional heating. We then synthesized spectral lines and compared their shapes and intensities to those observed by IBIS and found a general agreement. In particular, the synthetic Ca II 8542 Å profile fits well to the observed profile, while the synthetic H$α$ profile is fainter in the core than for the observation. This indicates that H$α$ emission is more responsive to the non-thermal electron flux than the Ca II 8542 Å emission. We suggest that it is necessary to refine the energy input and other processes is necessary to resolve this discrepancy.

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arXiv2014

Determination of Acceleration Mechanism Characteristics Directly and Non-Parametrically from Observations: Application to Supernova Remnants

We have developed an inversion method for determination of the characteristics of the acceleration mechanism directly and non-parametrically from observations, in contrast to the usual forward fitting of parametric model variables to observations. In two recent papers (Petrosian & Chen 2010, Chen & Petrosian 2013), we demonstrate the efficacy of this inversion method by its application to acceleration of electrons in solar flares based on stochastic acceleration by turbulence. Here we explore its application for determining the characteristics of shock acceleration in supernova remnants (SNRs) based on the electron spectra deduced from the observed nonthermal radiation from SNRs and the spectrum of the cosmic ray electrons observed near the Earth. These spectra are related by the process of escape of the electrons from SNRs and energy loss during their transport in the galaxy. Thus, these observations allow us to determine spectral characteristics of the momentum and pitch angle diffusion coefficients, which play crucial roles in both direct acceleration by turbulence and in high Mach number shocks. Assuming that the average electron spectrum deduced from a few well known SNRs is representative of those in the solar neighborhood we find interesting discrepancies between our deduced forms for these coefficients and those expected from well known wave-particle interactions. This may indicate that the standard assumptions made in treatment of shock acceleration need revision. In particular, the escape of particles from SNRs may be more complex than generally assumed.

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arXiv2013

Determination of Stochastic Acceleration Model Characteristics in Solar Flares

Following our recent paper (Petrosian & Chen 2010), we have developed an inversion method to determine the basic characteristics of the particle acceleration mechanism directly and non-parametrically from observations under the leaky box framework. In the above paper, we demonstrated this method for obtaining the energy dependence of the escape time. Here, by converting the Fokker-Planck equation to its integral form, we derive the energy dependences of the energy diffusion coefficient and direct acceleration rate for stochastic acceleration in terms of the accelerated and escaping particle spectra. Combining the regularized inversion method of Piana et al. 2007 and our procedure, we relate the acceleration characteristics in solar flares directly to the count visibility data from RHESSI. We determine the timescales for electron escape, pitch angle scattering, energy diffusion, and direct acceleration at the loop top acceleration region for two intense solar flares based on the regularized electron flux spectral images. The X3.9 class event shows dramatically different energy dependences for the acceleration and scattering timescales, while the M2.1 class event shows a milder difference. The M2.1 class event could be consistent with the stochastic acceleration model with a very steep turbulence spectrum. A likely explanation of the X3.9 class event could be that the escape of electrons from the acceleration region is not governed by a random walk process, but instead is affected by magnetic mirroring, in which the scattering time is proportional to the escape time and has an energy dependence similar to the energy diffusion time.

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arXiv2009

Cascade and Damping of Alfvén-Cyclotron Fluctuations: Application to Solar Wind Turbulence

It is well-recognized that the presence of magnetic fields will lead to anisotropic energy cascade and dissipation of astrophysical turbulence. With the diffusion approximation and linear dissipation rates, we study the cascade and damping of Alfvén-cyclotron fluctuations in solar plasmas numerically. For an isotropic case the steady-state turbulence spectra are nearly isotropic in the inertial range and can be fitted by a single power-law function with a spectral index of -3/2, similar to the Iroshnikov-Kraichnan phenomenology. Beyond the MHD regime the kinetic effects make the spectrum softer at higher wavenumbers. In the dissipation range the turbulence spectrum cuts off at the wavenumber, where the damping rate becomes comparable to the cascade rate, and the cutoff wavenumber changes with the wave propagation direction. The angle averaged turbulence spectrum of the isotropic model resembles a broken power-law. Taking into account the Doppler effects, the model naturally reproduces the broken power-law turbulence spectra observed in the solar wind and predicts that a higher break frequency always comes along with a softer dissipation range spectrum that may be caused by the increase of the turbulence intensity, the reciprocal of the plasma β, and/or the angle between the solar wind velocity and the mean magnetic field. These predictions can be tested by detailed comparisons with more accurate observations.

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arXiv2008

Cascade and Damping of Alfvén-Cyclotron Fluctuations: Application to Solar Wind Turbulence Spectrum

With the diffusion approximation, we study the cascade and damping of Alfvén-cyclotron fluctuations in solar plasmas numerically. Motivated by wave-wave couplings and nonlinear effects, we test several forms of the diffusion tensor. For a general locally anisotropic and inhomogeneous diffusion tensor in the wave vector space, the turbulence spectrum in the inertial range can be fitted with power-laws with the power-law index varying with the wave propagation direction. For several locally isotropic but inhomogeneous diffusion coefficients, the steady-state turbulence spectra are nearly isotropic in the absence of damping and can be fitted by a single power-law function. However, the energy flux is strongly polarized due to the inhomogeneity that leads to an anisotropic cascade. Including the anisotropic thermal damping, the turbulence spectrum cuts off at the wave numbers, where the damping rates become comparable to the cascade rates. The combined anisotropic effects of cascade and damping make this cutoff wave number dependent on the wave propagation direction, and the propagation direction integrated turbulence spectrum resembles a broken power-law, which cuts off at the maximum of the cutoff wave numbers or the $^4$He cyclotron frequency. Taking into account the Doppler effects, the model can naturally reproduce the broken power-law wave spectra observed in the solar wind and predicts that a higher break frequency is aways accompanied with a greater spectral index change that may be caused by the increase of the Alfvén Mach number, the reciprocal of the plasma beta, and/or the angle between the solar wind velocity and the mean magnetic field. These predictions can be tested by future observations.

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arXiv2008

Heating and Acceleration of Intracluster Medium Electrons by Turbulence

In this paper we investigate the feasibility of bremsstrahlung radiation from `nonthermal' electrons as a source of hard X-rays from the intracluster medium of clusters of galaxies. With an exact treatment of the Coulomb collisions in a Fokker-Planck analysis of the electron distribution we find that the severe difficulties with lifetimes of `nonthermal' particles found earlier by Petrosian (2001) using a cold target model remain problematic. We then address possible acceleration of background electrons into a nonthermal tail. We assume a simplified but generic acceleration rate and determine the expected evolution of an initially Maxwellian distribution of electrons. We find that strong nonthermal components arise only for rapid rate of acceleration which also heats up the entire plasma. These results confirm the conclusion that if the observed `nonthermal' excesses are due to some process accelerating the background thermal electrons this process must be short lived.

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arXiv2005

Stochastic Acceleration of $^3$He and $^4$He in Solar Flares by Parallel Propagating Plasma Waves: General Results

We study the acceleration in solar flares of $^3$He and $^4$He from a thermal background by parallel propagating plasma waves with a general broken power-law spectrum that takes into account the turbulence generation processes at large scales and the thermal damping effects at small scales. The exact dispersion relation for a cold plasma is used to describe the relevant wave modes. Because low-energy $α$-particles only interact with small scale waves in the $^4$He-cyclotron branch, where the wave frequencies are below the $α$-particle gyro-frequency, their pitch angle averaged acceleration time is at least one order of magnitude longer than that of $^3$He ions, which mostly resonate with relatively higher frequency waves in the proton-cyclotron (PC) branch. The $α$-particle acceleration rate starts to approach that of $^3$He beyond a few tens of keV nucleon$^{-1}$, where $α$-particles can also interact with long wavelength waves in the PC branch. However, the $^4$He acceleration rate is always smaller than that of $^3$He. Consequently, the acceleration of $^4$He is suppressed significantly at low energies, and the spectrum of the accelerated $α$-particles is always softer than that of $^3$He. The model gives reasonable account of the observed low-energy $^3$He and $^4$He fluxes and spectra in the impulsive solar energetic particle events observed with the {\it Advanced Composition Explorer}. We explore the model parameter space to show how observations may be used to constrain the model.

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arXiv2004

Stochastic Acceleration of Electrons and Protons. I. Acceleration by Parallel Propagating Waves

Stochastic acceleration of electrons and protons by waves propagating parallel to the large scale magnetic fields of magnetized plasmas is studied with emphasis on the feasibility of accelerating particles from a thermal background to relativistic energies and with the aim of determining the relative acceleration of the two species in one source. In general, the stochastic acceleration by these waves results in two distinct components in the particle distributions, a quasi-thermal and a hard nonthermal, with the nonthermal one being more prominent in hotter plasmas and/or with higher level turbulence. This can explain many of the observed features of solar flares. Regarding the proton to electron ratio, we include the effects of $^4$He in the dispersion relation and the damping of the turbulent waves by the thermal background plasma. The relative acceleration of protons and electrons is very sensitive to the ratio of the electron plasma frequency to its gyro-frequency. Protons are preferentially accelerated in weakly magnetized plasmas. The formalism developed here is applicable to the acceleration of other ion species and to other astrophysical systems.

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