arXiv subjects

Vahé Petrosian

Publications and source records attributed to Vahé Petrosian.

23 records · Page 2Linked to original sources

arXiv2004

Stochastic Acceleration of 3He and 4He by Parallel Propagating Plasma Waves

Stochastic acceleration of $^3$He and $^4$He from a thermal background by parallel propagating turbulent plasma waves with a single power-law spectrum of the wavenumber is studied. In the model, both ions interact with several resonant modes. When one of these modes dominates, the acceleration rate is reduced considerably. At low energies, this happens for $^4$He, but not for $^3$He where contributions from the two stronger modes are comparable so that acceleration of $^3$He is very efficient. As a result, the acceleration of $^4$He is suppressed by a barrier below $\sim 100$ keV nucleon$^{-1}$ and there is a prominent quasi-thermal component in the $^4$He spectra, while almost all the injected $^3$He ions are accelerated to high energies. This accounts for the large enrichment of $^3$He at high energies observed in impulsive solar energetic particle events. With reasonable plasma parameters this also provides a good fit to the spectra of both ions. Beyond $\sim 1$ MeV nucleon$^{-1}$, the spectrum of $^3$He is softer than that of $^4$He, which is consistent with the observed decrease of the $^3$He to $^4$He ratio with energy. This study also indicates that the acceleration, Coulomb losses and diffusive escape of the particles from the acceleration site {\it all} play important roles in shaping the ion spectra. This can explain the varied spectral shapes observed recently by the {\it Advanced Composition Explorer}.

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arXiv2002

Nonthermal Radiation and Acceleration of Electrons in Clusters of Galaxies

Recent observations of excess radiation at extreme ultraviolet and hard X-ray energies straddling the well known thermal soft X-ray emission have provided new tools and puzzles for investigation of the acceleration of nonthermal particles in the intercluster medium of clusters of galaxies. It is shown that these radiations can be produced by the inverse Compton upscattering of the cosmic microwave background photons by the same population of relativistic electrons that produce the well known diffuse radio radiation via the synchrotron mechanism. It is shown that the commonly discussed discrepancy between the value of the magnetic field required for the production of these radiation with that obtained from Faraday rotation measures could be resolved by more realistic models and by considerations of observational selection effects. In a brief discussion of the acceleration process it is argued that the most likely scenario is reacceleration of injected relativistic electrons involving shocks and turbulence. The seed electrons cannot be the background thermal electrons because of energetic considerations, and a steady state situation may not agree with the details of the observed spectra. Episodic generation of shocks and turbulence or episodic injection of relativistic electrons is a more likely scenario for acceleration.

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arXiv2002

Looptop and Footpoint Impulsive Hard X-Rays and Stochastic Electron Acceleration in Solar Flares

The discovery of hard X-rays from tops of flaring loops by the HXT of YOHKOH represents a significant progress in the understanding of solar flares. This report describes the properties of 20 limb flares observed by YOHKOH from October 1991 to August 1998, 15 of which show detectable impulsive looptop emission. Considering the finite dynamic range (about a decade) of the detection it can be concluded that looptop emission is a common feature of all flares. The light curves and images of a representative flare are presented and the statistical properties of the footpoint and looptop fluxes and spectral indexes are summarized. The importance of these observations, and those expected from RHESSI with its superior angular, spectral and temporal resolution, in constraining the acceleration models and parameters is discussed. briefly.

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arXiv2001

On The Non Thermal Emission and Acceleration of Electrons in Coma and Other Clusters of Galaxies

Some clusters of galaxies in addition to thermal bremsstrahlung (TB), emit diffuse radiation from the intercluster medium (ICM) at radio, EUV and hard x-ray (HXR) ranges. The radio radiation is due to synchrotron by relativistic electrons, and the inverse Compton (IC) scattering by the cosmic microwave background radiation of the same electrons is the most natural source for the HXR and perhaps the EUV emissions. However, simple estimates give a weaker magnetic field than that suggested by Faraday rotation measurements. Consequently, non-thermal bremsstrahlung (NTB) and TB have also been suggested as sources of these emissions. We show that NTB cannot be the source of the HXRs and that the difficulty with the low magnetic field in the IC model is alleviated if we take into account the effects of observational bias, nonisotropic pitch angle distribution and spectral breaks. We derive a spectrum for the radiating electrons and discuss acceleration scenarios. We show that continuous and in situ acceleration in the ICM of the background thermal electrons requires unreasonably high energy input and acceleration of injected relativistic electrons gives rise to a much flatter spectrum than desired, unless a large fraction of electrons escape the ICM, in which case one obtains EUV and HXR emissions extending well beyond the boundaries of the cluster. A continuous emission by a cooling spectrum resulting from interaction with ICM of electrons accelerated elsewhere also suffers from similar shortcomings. The most likely scenario appears to be an episodic injection-acceleration model, whereby one obtains a time dependent spectrum that for certain phases of its evolution satisfies all the requirements.

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arXiv1998

Nonparametric Methods for Doubly Truncated Data

Truncated data plays an important role in the statistical analysis of astronomical observations as well as in survival analysis. The motivating example for this paper concerns a set of measurements on quasars in which there is double truncation. That is, the quasars are only observed if their luminosity occurs within a certain finite interval, bounded at both ends, with the interval varying for different observations. Nonparametric methods for the testing and estimation of doubly truncated data are developed. These methods extend some known techniques for data that is only truncated on one side, in particular Lynden-Bell's estimator and the truncated version of Kendall's tau statistic. However the kind of hazard function arguments that underlie the one-sided methods fail for two-sided truncation. Bootstrap and Markov Chain Monte Carlo techniques are used here in their place. Finally, we apply these techniques to the quasar data, answering a question about their long-term luminosity evolution.

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