Maxim L. Litvak

OBSERVATIONS OF THE PROMPT GAMMA-RAY EMISSION OF GRB 070125

The long, bright gamma-ray burst GRB 070125 was localized by the Interplanetary Network. We present light curves of the prompt gamma-ray emission as observed by Konus-Wind, RHESSI, Suzaku WAM, and Swift BAT. We detail the results of joint spectral fits with Konus and RHESSI data. The burst shows moderate hard-to-soft evolution in its multipeaked emission over a period of about 1 minute. The total burst fluence as observed by Konus is -->1.79 ? 10?4 ergs cm?2 (20 keV-10 MeV). Using the spectroscopic redshift -->z = 1.548, we find that the burst is consistent with the Amati -->Epeak,i ? Eiso correlation. Assuming a jet opening angle derived from broadband modeling of the burst afterglow, GRB 070125 is a significant outlier to the Ghirlanda -->Epeak,i ? E? correlation. Its collimation-corrected energy release, -->E? = 2.5 ? 1052 ergs, is the largest yet observed.

The Emission Time of Gamma-Ray Bursts

The concept of emission time τN is suggested as a temporal parameter which is complementary to the classical parameters of duration times T50 and T90. The emission time is defined as the time of emission of N% of the total fluence. The definition adds the time bins of high fluence in decreasing fluence rank until N% of the fluence has been reached. The emission time interval excludes low-emission intervals of bursts, and so the emission time characterizes the state of high-power emission. The distribution of this new parameter is found to be bimodal for bright bursts. The distributions of emission time τ30 and τ50, for groups based on burst intensity, are also compared.

Average Emissivity Curve of Batse Gamma-Ray Bursts with Different Intensities

Six intensity groups with ~150 BATSE gamma-ray bursts each are compared using average emissivity curves. Time stretch factors for each of the dimmer groups are estimated with respect to the brightest group, which serves as the reference, taking into account the systematics of counts-produced noise effects and choice statistics. A stretching/intensity anticorrelation is found with good statistical significance during the average back slopes of bursts. A stretch factor ~2 is found between the 150 dimmest bursts, with peak flux less than 0.45 photons cm-2 s-1, and the 147 brightest bursts, with peak flux greater than 4.1 photons cm-2 s-1. On the other hand, while a trend of increasing stretching factor may exist for rise fronts for bursts with decreasing peak flux from greater than 4.1 photons cm-2 s-1 down to 0.7 photons cm-2 s-1, the magnitude of the stretching factor is less than ~1.4 and is therefore inconsistent with stretching factor of back slope.

Comparison of Redshift-known Gamma-Ray Bursts with the Main Groups of Bright BATSE Events

The small reference sample of six BATSE gamma-ray bursts with known redshifts from optical afterglows is compared with a comparison group of the 218 brightest BATSE bursts. These two groups are shown to be consistent both with respect to the distributions of the spectral peak parameter in the observers frame and also with respect to the distributions of the frame-independent cosmological invariant parameter (CIP). Using the known values of the redshifts (z) for the reference sample, the rest-frame distribution of spectral parameters is built. The deredshifted distribution of the spectral parameters of the reference sample is compared with distribution of these parameters for the comparison group after deredshifting by the factor 1/(1 + z), with z a free parameter. Requiring consistency between these two distributions produces a collective estimation of the best fitting redshifts for the comparison group z = 1.8-3.6. These values can be considered as the average cosmological redshift of the sources of the brightest BATSE bursts. The most probable value of the peak energy of the spectrum in the rest frame is 920 keV.

Rest frame properties of gamma-ray bursts

The small reference group of BATSE gamma-ray bursts with measured red-factors of optical afterglows is compared with the large comparison group of brightest BATSE bursts. These two groups are shown to be similar both in respect with distributions of measured Ep parameters in the Observer frame and also in respect with frame-independent distributions of newly implemented cosmological invariant parameters (CIP). Using the known values of red-factors Z for the reference group, the distribution of Ep(RF) is built for the rest frames of their sources. De-redshifted statistics of Ep(RF) are compared with observed distributions of Ep for the comparison group of bursts. From this comparison the collective estimation of red-shifts Z=2.5–3.5 has been performed for the group of brightest BATSE bursts.

Average spectral parameters of high-power emission in BATSE gamma-ray bursts

Photon spectra are investigated that were accumulated during the times of high emission in gamma-ray bursts. The corresponding spectral parameters are obtained for the sample of bright bursts in the BATSE 4B catalog with peak flux more than 2 phot/(cm2 s) on the 64 ms time scale. The difference in spectra corresponding to the short and long classes of bursts is investigated. The evolution of spectral parameters is studied along successive time intervals of high-power emission and around the moment of primary peaks.

Effect of Hardness/Intensity Correlation of Cosmic Gamma-Ray Bursts, as Explained by Models of the Extended Galactic Halo

The effect of hardness/intensity correlation has been determined, as the result of statistical study of cosmic gamma-ray bursts (GRBs) by Mitrofanov, Paciesas, and their coworkers. For five brightness groups of GRBs selected by Mallozzi and coworkers, the cosmological origin of this effect was suggested, due to the cosmological redshift. On the other hand, this effect is shown to be explained by models of the extended Galactic halo, provided that the broad distribution of radiated power is postulated for emitters of GRBs, together with intrinsic correlation between emissivity and spectral hardness of radiated photons.

Generic Difference Between Early and Late Stages of BATSE Gamma-Ray Bursts

The early and late stages of gamma-ray bursts are studied in a statistical analysis of the large sample of long BATSE events. The primary peak is used as the boundary between the early and late stages of emission. Significant differences are found between the stages: the early stage is shorter, it has harder emission, and it becomes a smaller fraction of the total burst duration for burst groups of decreasing intensity.

The technique of emission time estimation for BATSE GRBs

The technique of estimation of the emission time τ50, introduced as a new temporal parameter [1,2], is described.

The pure physical parameters of BATSE gamma ray bursts

The concept of Pure Physical Parameters (PPP, or P3) is presented, as the instrument-independent parameterization for the statistical search for basic physical signatures of gamma-ray bursts. The emission time of photon light curves and the peak of photon spectra of high power emission are proposed for each burst characterization, as P3 parameters for time variability and for spectral energy of gamma-rays. The P3 parameters are presented for 6 gamma-ray bursts in their rest frames with known red-shifts of optical afterglows.