Y. Kaneko

The Complete Spectral Catalog of Bright BATSE Gamma-Ray Bursts

We present a systematic spectral analysis of 350 bright gamma-ray bursts (GRBs) observed with the Burst and Transient Source Experiment (BATSE; ~30 keV-2 MeV) with high temporal and spectral resolution. Our sample was selected from the complete set of 2704 BATSE GRBs based on their energy fluence or peak photon flux values to assure good statistics and included 17 short GRBs. To obtain well-constrained spectral parameters, several photon models were used to fit each spectrum. We compared spectral parameters resulting from the fits using different models, and the spectral parameters that best represent each spectrum were statistically determined, taking into account the parameterization differences among the models. A thorough analysis was performed on 350 time-integrated and 8459 time-resolved burst spectra, and the effects of integration times in determining the spectral parameters were explored. Using the results, we studied correlations among spectral parameters and their evolution pattern within each burst. The resulting spectral catalog is the most comprehensive study of spectral properties of GRB prompt emission to date and is available electronically from the High-Energy Astrophysics Science Archive Research Center (HEASARC). The catalog provides reliable constraints on particle acceleration and emission mechanisms in GRBs.

A |[gamma]|-ray burst with a high-energy spectral component inconsistent with the synchrotron shock model

Gamma-ray bursts are among the most powerful events in nature. These events release most of their energy as photons with energies in the range from 30u2009keV to a few MeV, with a smaller fraction of the energy radiated in radio, optical, and soft X-ray afterglows. The data are in general agreement with a relativistic shock model, where the prompt and afterglow emissions correspond to synchrotron radiation from shock-accelerated electrons. Here we report an observation of a high-energy (multi-MeV) spectral component in the burst of 17 October 1994 that is distinct from the previously observed lower-energy γ-ray component. The flux of the high-energy component decays more slowly and its fluence is greater than the lower-energy component; it is described by a power law of differential photon number index approximately -1 up to about 200u2009MeV. This observation is difficult to explain with the standard synchrotron shock model, suggesting the presence of new phenomena such as a different non-thermal electron process, or the interaction of relativistic protons with photons at the source.

Prompt and Afterglow Emission Properties of Gamma-Ray Bursts with Spectroscopically Identified Supernovae

We present a detailed spectral analysis of the prompt and afterglow emission of four nearby long-soft gamma-ray bursts (GRBs 980425, 030329, 031203, and 060218) that were spectroscopically found to be associated with Type Ic supernovae and compare them to the general GRB population. For each event, we investigate the spectral and luminosity evolution and estimate the total energy budget based on broadband observations. The observational inventory for these events has become rich enough to allow estimates of their energy content in relativistic and subrelativistic form. The result is a global portrait of the effects of the physical processes responsible for producing long-soft GRBs. In particular, we find that the values of the energy released in mildly relativistic outflows appears to have a significantly smaller scatter than those found in highly relativistic ejecta. This is consistent with a picture in which the energy released inside the progenitor star is roughly standard, while the fraction of that energy that ends up in highly relativistic ejecta outside the star can vary dramatically between different events.

HOW SPECIAL ARE DARK GAMMA-RAY BURSTS: A DIAGNOSTIC TOOL

We present here a comprehensive study of the optical/near-infrared (IR) upper limits for gamma-ray bursts that have an X-ray afterglow. We have extrapolated the X-ray afterglows to optical wavelengths based on the physics of the fireball blast wave model and compared these results with optical upper limits for a large sample of bursts. We find a small set of only three bursts out of a sample of 20 for which the upper limits are not compatible with their X-ray afterglow properties within the context of any blast wave model. This sparse sample does not allow us to conclusively determine the cause of this optical/near-IR deficit. Extinction in the host galaxy is a likely cause, but high redshifts and different afterglow mechanisms might also explain the deficit in some cases. We note that the three bursts appear to have higher than average gamma-ray peak fluxes. In a magnitude versus time diagram the bursts are separated from the majority of bursts with a detected optical/near-IR afterglow. However, two gamma-ray bursts with an optical afterglow (one of which is highly reddened) also fall in this region with dark bursts, making it likely that dark bursts are at the faint end of the set of optically detected bursts, and therefore the dark bursts likely form a continuum with the bursts with a detected optical afterglow. Our work provides a useful diagnostic tool for follow-up observations for potentially dark bursts; applied to the events detected with the Swift satellite, it will significantly increase our sample of truly dark bursts and shed light upon their nature.

BROADBAND SPECTRAL PROPERTIES OF BRIGHT HIGH-ENERGY GAMMA-RAY BURSTS OBSERVED WITH BATSE AND EGRET

We present the spectral analysis of duration-integrated broadband spectra (in ~30 keV–200 MeV) of 15 bright BATSE gamma-ray bursts (GRBs). Some GRB spectra are very hard, with their spectral peak energies being above the BATSE LAD passband limit of ~2 MeV. In such cases, their high-energy spectral parameters (peak energy and high-energy power-law indices) cannot be adequately constrained by BATSE LAD data alone. A few dozen bright BATSE GRBs were also observed with EGRETs calorimeter, TASC, in multi-MeV energy band, with a large effective area and fine energy resolution. Combining the BATSE and TASC data, therefore, affords spectra that span four decades of energy (30-200 MeV), allowing for a broadband spectral analysis with good statistics. Studying such broadband high-energy spectra of GRB prompt emission is crucial, as they provide key clues to understanding its gamma-ray emission mechanism. Among the 15 GRB spectra, we found two cases with a significant high-energy excess, and another case with a extremely high peak energy (Epeak 170 MeV). There have been very limited number of GRBs observed at MeV energies and above, and only a few instruments have been capable of observing GRBs in this energy band with such high sensitivity. Thus, our analysis results presented here should also help predict GRB observations with current and future high-energy instruments such as AGILE and GLAST, as well as with ground-based very-high-energy telescopes.

Discovery of a Distinct Higher Energy Spectral Component in GRB941017

We report an observation of a multi‐MeV spectral component in the burst of 17 October 1994 that is distinct from the previously observed lower energy gamma‐ray component. This higher‐energy component is described by a power law of differential photon number index ∼ −1 up to 200 MeV. Its flux decays more slowly and its fluence is more than 3 times the fluence of the lower‐energy component. Despite the unique behavior of this higher‐energy component, the lower‐energy component behaves similarly to most GRBs, presenting a hard‐to‐soft temporal evolution.

COMPTEL Observation of GRB941017 with Distinct High‐Energy Component

The joint spectral analysis of GRB941017 with BATSE and EGRET data revealed the existence of a distinct MeV spectral component that decayed slower than the lower energy component. The event was also observed with COMPTEL burst modules, which provides burst spectra in the energy range of 300 keV to 10 MeV. Due to the limited energy overlap between the BATSE Large Area Detector and the EGRET Total Absorption Shower Counter spectra, the relative normalization between the two instruments is poorly constrained. The COMPTEL spectra complement the energy ranges of the BATSE and EGRET data and are used herein to confirm and improve upon the previous analysis. Using the data from all three instruments, we present the result of joint spectral analysis for GRB941017. Including the COMPTEL data improved the statistics for the time interval in which the high energy component is more apparent.

BATSE‐EGRET Combined Spectral Fits

Combining BATSE and EGRET data yields spectral fits over the broadest energy range for the prompt phase of gamma‐ray bursts. The spectra from the BATSE data have previously been reported; however, the addition of higher energy EGRET data further constrains the values of the peak energy and high‐energy spectral index. The EGRET data from the TASC (Total Absorption Shower Counter) begin at 1 MeV and for brighter bursts extend beyond 10 MeV. These data are combined using the separate response matrices of each detector and a common spectral fitting algorithm, RMFIT, which has been extensively used with BATSE data. The spectrum from GRB910503 is presented.

Spectral Time Evolution for GRBs Observed by BATSE and EGRET‐TASC

Analysis of the time evolution of GRB spectra yields important information about the radiation mechanisms taking place in Gamma‐Ray Bursts. Spectroscopy of BATSE data using the Band function showed a generalized hard‐to‐soft temporal evolution of the GRB spectra. This analysis was limited to bursts whose spectrum peaks below 1 MeV. The EGRET’s calorimeter TASC (Total Absorption Shower Counter) was sensitive to energies above 1 MeV and beyond 10 MeV for brighter bursts. In order to give better spectral fits over a broader energy range, we have combined BATSE and TASC data for 8 bursts that showed significant detection in more than one time interval in TASC data. We observed that the spectrum of a single peak evolves from hard‐to‐soft, except for one burst where the high‐energy spectral index β stayed constant with time. For bursts with multiple peaks, no evident time evolution was observed.

Spectral Evolution of Two High-Energy Gamma-Ray Bursts

The prompt emission of the gamma-ray bursts is found to be very energetic, releasing ~10^51 ergs in a flash. However, their emission mechanism remains unclear and understanding their spectra is a key to determining the emission mechanism. Many GRB spectra have been analyzed in the sub-MeV energy band, and are usually well described with a smoothly broken power-law model. We present a spectral analysis of two bright bursts (GRB910503 and GRB930506), using BATSE and EGRET spectra that cover more than four decades of energy (30 keV - 200 MeV). Our results show time evolutions of spectral parameters (low-energy & high-energy photon indices and break energy) that are difficult to reconcile with a simple shock-acceleration model.