David L. Band

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.

BATSE observations of gamma-ray burst spectra. 2: Peak energy evolution in bright, long bursts

We investigate spectral evolution in 37 bright, long gamma-ray bursts observed with the Burst and Transient Source Experiment (BATSE) spectroscopy detectors. High-resolution spectra are chracterized by the energy of the peak of nu F(sub nu), and the evolution of this quantity is examined relative to the emission intensity. In most cases it is found that this peak energy either rises with or slightly precedes major intensity increases and softens for the remainder of the pulse. Interpulse emission is generally harder early in the burst. For bursts with multiple intensity pulses, later spikes tend to be softer than earlier ones, indicating that the energy of the peak of nu F(sub nu) is bounded by an envelope which decays with time. Evidence is found that bursts in which the bulk of the flux comes well after the event which triggers the instrument tend to show less peak energy variability and are not as hard as several bursts in which the emission occurs promptly after the trigger. Several recently proposed burst models are examined in light of these results and no qualitative conflicts with the observations presented here are found.

Testing the Gamma-Ray Burst Energy Relationships

Building on Nakar & Pirans analysis of the Amati relation relating gamma-ray burst peak energies, Ep, and isotropic energies, Eiso, we test the consistency of a large sample of BATSE bursts with the Amati and Ghirlanda (which relate peak energies and actual gamma-ray energies, Eγ) relations. Each of these relations can be expressed as a ratio of the different energies that is a function of redshift (for both the Amati and Ghirlanda relations) and beaming fraction fB (for the Ghirlanda relation). The most rigorous test, which allows bursts to be at any redshift, corroborates Nakar & Pirans result—88% of the BATSE bursts are inconsistent with the Amati relation—while only 1.6% of the bursts are inconsistent with the Ghirlanda relation if fB = 1. Even when we allow for a real dispersion in the Amati relation, we find an inconsistency. Modeling the redshift distribution results in an energy ratio distribution for the Amati relation that is shifted by an order of magnitude relative to the observed distribution; any subpopulation satisfying the Amati relation can comprise at most ~18% of our burst sample. A similar analysis of the Ghirlanda relation depends sensitively on the beaming fraction distribution for small values of fB; for reasonable estimates of this distribution about a third of the burst sample is inconsistent with the Ghirlanda relation. Our results indicate that these relations are an artifact of the selection effects of the burst sample in which they were found; these selection effects may favor subpopulations for which these relations are valid.

Postlaunch Analysis of Swift’s Gamma-Ray Burst Detection Sensitivity

The dependence of Swifts detection sensitivity on a bursts temporal and spectral properties shapes the detected burst population. Using simplified models of the detector hardware and the burst trigger system, I find that Swift is more sensitive to long, soft bursts than CGROs BATSE, a reference detector because of the large burst database it has accumulated. Swift has increased sensitivity in the parameter space region into which time dilation and spectral redshifting move high-redshift bursts.

On the Consistency of Gamma-Ray Burst Spectral Indices with the Synchrotron Shock Model

The current scenario for gamma-ray bursts (GRBs) involves internal shocks for the prompt GRB emission phase and external shocks for the afterglow phase. Assuming optically thin synchrotron emission from isotropically distributed energetic shocked electrons, GRB spectra observed with a low-energy power-law spectral index greater than � 2 (for positive photon number indices E � ) indicate a problem with this model. For spectra that do not violate this condition, additional tests of the shock model can be made by comparing the low- and high-energy spectral indices, on the basis of the model’s assertion that synchrotron emission from a single power-law distribution of electrons is responsible for both the low-energy and the high-energy powerlaw portions of the spectra. We find in most cases that the inferred relationship between the two spectral indices of observed GRB spectra is inconsistent with the constraints from the simple optically thin synchrotron shock emission model. In this sense, the prompt burst phase is different from the afterglow phase, and this difference may be related to anisotropic distributions of particles or to their continual acceleration in shocks during the prompt phase. Subject headings: gamma rays: bursts — radiation mechanisms: nonthermal — shock waves

Prospects for GRB Science with the Fermi Large Area Telescope

The Large Area Telescope (LAT) instrument on the Fermi mission will reveal the rich spectral and temporal gamma-ray burst (GRB) phenomena in the >100 MeV band. The synergy with Fermis Gamma-ray Burst Monitor detectors will link these observations to those in the well explored 10-1000 keV range; the addition of the >100 MeV band observations will resolve theoretical uncertainties about burst emission in both the prompt and afterglow phases. Trigger algorithms will be applied to the LAT data both onboard the spacecraft and on the ground. The sensitivity of these triggers will differ because of the available computing resources onboard and on the ground. Here we present the LATs burst detection methodologies and the instruments GRB capabilities.

Redshifts and Luminosities for 112 Gamma-Ray Bursts

Two different luminosity indicators have recently been proposed for gamma-ray bursts that use gamma-ray observations alone. They relate the burst luminosity (L) with the time lag between peaks in hard and soft energies (τlag) and the spikiness or variability of the bursts light curve (V). These relations are currently justified and calibrated with only six or seven bursts with known redshifts. We have examined BATSE data for τlag and V for 112 bursts. (1) A strong correlation between τlag and V exists, and it is exactly as predicted from the two proposed relations. This is proof that both luminosity indicators are reliable. (2) GRB 830801 is the all-time brightest burst, yet with a small V and a large τlag, and hence it is likely the closest known event, being perhaps as close as 3.2 Mpc. (3) We have combined the luminosities as derived from both indicators as a means to improve the statistical and systematic accuracy when compared with the accuracy from either method alone. The result is a list of 112 bursts with good luminosities and hence redshifts. (4) The burst-averaged hardness ratio rises strongly with the luminosity of the burst. (5) The burst luminosity function is a broken power law, with the break at L = 2 × 1052 ergs. The numbers in logarithmic bins scale as L-2.8±0.2 above the break and as L-1.7±0.1 below the break. (6) The comoving number density of GRBs varies with redshift roughly as (1 + z)3.5±0.3 between 0.2 < z < 5. This demonstrates that the burst rate follows the star formation rate at low redshifts, as expected since long bursts are generated by very massive stars. Excitingly, this result also provides a measure of the star formation rate out to z ~ 5 with no effects from reddening, and the rate is rising uniformly for redshifts above 2.

Energetics of Gamma-Ray Bursts

We determine the distribution of total energy emitted by gamma-ray bursts for bursts with fluence and distance information. Our core sample consists of eight bursts with BATSE spectra and spectroscopic redshifts. We extend this sample by adding four bursts with BATSE spectra and host galaxy R magnitudes. From these R magnitudes, we calculate a redshift probability distribution; this method requires a model of the host galaxy population. From a sample of 10 bursts with both spectroscopic redshifts and host galaxy R magnitudes (some do not have BATSE spectra), we find that the burst rate is proportional to the galaxy luminosity at the epoch of the burst. Assuming that the total energy emitted has a lognormal distribution, we find that the average emitted energy (assumed to be radiated isotropically) is Eγiso = 1.3 × 1053 ergs (for H0 = 65 km s-1 Mpc-1, Ωm = 0.3, and ΩΛ = 0.7); the distribution has a logarithmic width of σγ = 1.7. The corresponding distribution of X-ray afterglow energy (for seven bursts) has EX,iso = 3.5 × 1051 ergs and σX = 1.3. For completeness, we also provide spectral fits for all bursts with BATSE spectra for which there were afterglow searches.

BATSE spectroscopy catalog of bright gamma-ray bursts

This paper presents comprehensive results on the spectra of 30 bright gamma ray bursts (GRBs) as observed by the Spectroscopy Detectors (SDs) of the Burst And Transient Source Experiment (BATSE). The data selection was strict in including only spectra that are of high reliability for continuum shape studies. This BATSE Spectroscopy Catalog presents fluences, model fits (for five spectral models for three energy ranges), and photon spectra in a standard manner for each burst. Complete information is provided to describe the data selection and analysis procedures. The catalog results are also presented in electronic format (from the Compton Observatory Science Support Center) and CD-ROM format (AAS CD-ROM series, Vol. 2). These electronic formats also present the count spectra and detector response matrices so as to allow for independent study and fitting by researchers outside the BATSE Team. This BATSE Spectroscopy Catalog complements the catalog from BATSE Large Area Detector (LAD) data by Fishman et al. (1994).

A Search for Early Optical Emission at Gamma-Ray Burst Locations by the Solar Mass Ejection Imager (SMEI)

The Solar Mass Ejection Imager (SMEI) views nearly every point on the sky once every 102 minutes and can detect point sources as faint as R ~ 10 mag. Therefore, SMEI can detect or provide upper limits for the optical afterglow from gamma-ray bursts in the tens of minutes after the burst, when different shocked regions may emit optically. Here we provide upper limits for 58 bursts between 2003 February and 2005 April.