Vahe Petrosian

Surface brightness and evolution of galaxies

It is well known that before the redshift-magnitude diagram of galaxies could be used for determination of the cosmological parameters one must know the evolution of the galaxies. We propose use of apparent surface brightness: which depends only on the redshift and is independent of the cosmological model and the inhomogeneities in the universe: for observational determination of the evolution of galaxies. The needed observations are isophotal angular diameters and apparent magnitudes within this or any other reasonable angular diameter. The application of the results for determination of q/sub 0/ is discussed briefly. (AIP)

On the Nonthermal Emission and Acceleration of Electrons in Coma and Other Clusters of Galaxies

Some clusters of galaxies, in addition to thermal bremsstrahlung (TB), emit detectable diffuse radiation from the intracluster medium (ICM) at radio, EUV, and hard X-ray (HXR) ranges. The radio radiation must be due to synchrotron by relativistic electrons, and the inverse Compton 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, nonthermal bremsstrahlung (NTB) and TB have also been suggested as sources of these emissions. We show that NTB cannot be the source of the HXRs (except for a short period) and that the difficulty with the low magnetic field in the IC model is alleviated if the effects of observational selection bias, nonisotropic pitch angle distribution, and spectral breaks in the energy distribution of the relativistic electrons are taken into account. From these considerations and the strength of the EUV emission, we derive a spectrum for the radiating electrons and discuss possible acceleration scenarios for its production. We show that continuous and in situ acceleration in the ICM of the background thermal electrons is difficult and requires unreasonably high energy input. Similarly, acceleration of injected relativistic electrons, say, by galaxies, seems unreasonable because it will give rise to a much flatter spectrum of electrons than required, unless a large fraction of energy input is carried away by electrons escaping the ICM, in which case one obtains EUV and HXR emissions extending well beyond the boundaries of the diffuse radio source. 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.

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 find that in a pure hydrogen plasma the dominance of the wave-proton interaction by the resonant Alfven mode reduces the acceleration rate of protons in the intermediate energy range significantly, while the electron-cyclotron and Whistler waves are very efficient in accelerating electrons from a few keV to MeV energies. The presence of such an acceleration barrier prohibits the proton acceleration under solar flare conditions. This difficulty is alleviated when we include the effects of 4He in the dispersion relation and the damping of the turbulent waves by the thermal background plasma. The additional 4He cyclotron branch of the turbulent plasma waves suppresses the proton acceleration barrier significantly, and the steep turbulence spectrum in the dissipation range makes the nonthermal component have a near power-law shape. The relative acceleration of protons and electrons is very sensitive to a plasma parameter α = ωpe/Ωe, where ωpe and Ωe are the electron plasma frequency and gyrofrequency, respectively. Protons are preferentially accelerated in weakly magnetized plasmas (large α). The formalism developed here is applicable to the acceleration of other ion species and to other astrophysical systems.

IDENTIFICATION AND PROPERTIES OF THE PHOTOSPHERIC EMISSION IN GRB090902B

The Fermi Gamma-ray Space Telescope observed the bright and long GRB090902B, lying at a redshift of z = 1.822. Together the Large Area Telescope (LAT) and the Gamma-ray Burst Monitor (GBM) cover th ...

Loop Top 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 Hard X-Ray Telescope on board the Yohkoh satellite represents a significant progress toward the understanding of the basic processes driving solar flares. In this paper we extend the previous study of limb flares by Masuda by including all Yohkoh observations up through 1998 August. We report that from 1991 October to 1998 August, 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 eight previously unanalyzed flares. Of these 18 events, 15 show detectable impulsive loop top emission. Considering that the finite dynamic range (about a decade) of the detection introduces a strong bias against observing comparatively weak loop top sources, we conclude that loop top emission is a common feature of all flares. We summarize the observations of the footpoint-to-loop top flux ratio and the spectral indexes. 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 indexes of the loop top 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.

Total Galaxy Magnitudes and Effective Radii from Petrosian Magnitudes and Radii

Petrosian magnitudes were designed to help with the difficult task of determining a galaxys total light. Although these magnitudes [taken here as the flux within 2RP, with the inverted Petrosian index 1/?(RP) = 0.2] can represent most of an objects flux, they do of course miss the light outside the Petrosian aperture (2RP). The size of this flux deficit varies monotonically with the shape of a galaxys light profile, i.e., its concentration. In the case of a de Vaucouleurs R1/4 profile, the deficit is 0.20 mag; for an R1/8 profile this figure rises to 0.50 mag. Here we provide a simple method for recovering total (S?rsic) magnitudes from Petrosian magnitudes using only the galaxy concentration (R90/R50 or R80/R20) within the Petrosian aperture. The corrections hold to the extent that S?rsics model provides a good description of a galaxys luminosity profile. We show how the concentration can also be used to convert Petrosian radii into effective half-light radii, enabling a robust measure of the mean effective surface brightness. Our technique is applied to the Sloan Digital Sky Survey Data Release 2 (SDSS DR2) Petrosian parameters, yielding good agreement with the total magnitudes, effective radii, and mean effective surface brightnesses obtained from the New York University Value-Added Galaxy Catalog S?rsic R1/n fits by Blanton and coworkers. Although the corrective procedure described here is specifically applicable to the SDSS DR2 and DR3, it is generally applicable to all imaging data where any Petrosian index and concentration can be constructed.

Recent Advances in Understanding Particle Acceleration Processes in Solar Flares

We review basic theoretical concepts in particle acceleration, with particular emphasis on processes likely to occur in regions of magnetic reconnection. Several new developments are discussed, including detailed studies of reconnection in three-dimensional magnetic field configurations (e.g., current sheets, collapsing traps, separatrix regions) and stochastic acceleration in a turbulent environment. Fluid, test-particle, and particle-in-cell approaches are used and results compared. While these studies show considerable promise in accounting for the various observational manifestations of solar flares, they are limited by a number of factors, mostly relating to available computational power. Not the least of these issues is the need to explicitly incorporate the electrodynamic feedback of the accelerated particles themselves on the environment in which they are accelerated. A brief prognosis for future advancement is offered.

A simple test of independence for truncated data with applications to redshift surveys

This paper presents an easily applied permutation test for H0, closely related to Lyden-Bells (1971) estimate of the marginal distribution of truncated data. The test is applied to two redshift-magnitude surveys, one of galaxies and one of quasars. Assuming statistical independence, testing H0 amounts to testing validity of the cosmological model. Segals (1986) chronomatic cosmological model is rejected under H0. On the other hand, for the quasar sample H0 is rejected strongly in a conventional cosmological model (and in a chronomatic model as well) indicating either incorrectness of the models or, as is more commonly assumed, indicating strong luminosity evolution.

Nonparametric Methods for Doubly Truncated Data

Abstract Truncated data play an important role in the statistical analysis of astronomical observations as well as in survival analysis. The motivating example for this article concerns a set of measurements on quasars in which there is double truncation. That is, the quasars are observed only 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 are truncated only on one side, in particular Lynden-Bells estimator and the truncated version of Kendalls 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.

Cosmological versus Intrinsic: The Correlation between Intensity and the Peak of the νFν 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 bursts 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.