GRB Spectral Hardness and Afterglow Properties

A possible relationship between the presence of a radio afterglow and gamma-ray burst spectral hardness is discussed. The correlation is marginally significant; the spectral hardness of the bursts with radio afterglows apparently results from a combination of the break energy Ebreak and the high-energy spectral index beta. If valid, this relationship would indicate that the afterglow does carry information pertaining to the GRB central engine.Comment: 5 pages, 3 figures, presented at the 5th Huntsville Gamma-Ray Burst Symposiu

A Simple BATSE Measure of GRB Duty Cycle

We introduce a definition of gamma-ray burst (GRB) duty cycle that describes the GRB's efficiency as an emitter; it is the GRB's average flux relative to the peak flux. This GRB duty cycle is easily described in terms of measured BATSE parameters; it is essentially fluence divided by the quantity peak flux times duration. Since fluence and duration are two of the three defining characteristics of the GRB classes identified by statistical clustering techniques (the other is spectral hardness), duty cycle is a potentially valuable probe for studying properties of these classes.Comment: 4 pages, 1 figure, presented at the 5th Huntsville Gamma-Ray Burst Symposiu

Synchrotron Emission as the Source of GRB Spectra, Part II: Observations

We test the models of synchrotron emission presented in Part I of this series (Lloyd & Petrosian, these proceedings) against the distributions and evolution of GRB spectral parameters (particularly the low energy index, $\alpha$). With knowledge of the $E_{p}$ distribution and the correlation between $\alpha$ and $E_{p}$ presented in Part I, we show how to derive the expected distribution of $\alpha$ from fits to optically thin synchrotron spectra, and compare this with the observed distribution. We show that there is no difficulty explaining bursts below the ``line of death'', $\alpha < -2/3$, and that these bursts indicate that the spectrum of accelerated electrons must flatten or decline at low energies. Bursts with low energy spectral indices that fall above this limit are explained by the synchrotron self-absorption frequency entering the lower end of the BATSE window. Finally, we discuss a variety of spectral evolution behavior seen in GRBs and explain this behavior in the context of synchrotron emission from internal shocks.Comment: To appear in the proceedings of the 5th Huntsville Symposium on Gamma Ray Burst

Improved Limits on Sterile Neutrino Dark Matter using Full-Sky Fermi Gamma-Ray Burst Monitor Data

A sterile neutrino of ~keV mass is a well motivated dark matter candidate. Its decay generates an X-ray line that offers a unique target for X-ray telescopes. For the first time, we use the Gamma-ray Burst Monitor (GBM) onboard the Fermi Gamma-Ray Space Telescope to search for sterile neutrino decay lines; our analysis covers the energy range 10-25 keV (sterile neutrino mass 20-50 keV), which is inaccessible to X-ray and gamma-ray satellites such as Chandra, Suzaku, XMM-Newton, and INTEGRAL. The extremely wide field of view of the GBM enables a large fraction of the Milky Way dark matter halo to be probed. After implementing careful data cuts, we obtain ~53 days of full sky observational data. We observe an excess of photons towards the Galactic Center, as expected from astrophysical emission. We search for sterile neutrino decay lines in the energy spectrum, and find no significant signal. From this, we obtain upper limits on the sterile neutrino mixing angle as a function of mass. In the sterile neutrino mass range 25-40 keV, we improve upon previous upper limits by approximately an order of magnitude. Better understanding of detector and astrophysical backgrounds, as well as detector response, will further improve the sensitivity of a search with the GBM.Comment: 16 pages, 11 figures, references added, discussion expanded, some typos fixed, matches the published versio

Testing the Gamma-Ray Burst Energy Relationships

Building on Nakar & Piran's analysis of the Amati relation relating gamma-ray burst peak energies E_p and isotropic energies E_iso, we test the consistency of a large sample of BATSE bursts with the Amati and Ghirlanda (which relates peak energies and actual gamma-ray energies E_gamma) 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 f_B (for the Ghirlanda relation). The most rigorous test, which allows bursts to be at any redshift, corroborates Nakar & Piran's 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 f_B=1. Even when we allow for a real dispersion in the Amati relation we find an inconsistency. Modelling 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 sub-population 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 f_B; 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 sub-populations for which these relations are valid.Comment: 17 pages, 4 figures. To appear in ApJ, 627, #2 (10 July 2005

Exploring Physically-Motivated Models to Fit Gamma-Ray Burst Spectra

We explore fitting gamma-ray burst spectra with three physically-motivated models, and thus revisit the viability of synchrotron radiation as the primary source of GRB prompt emission. We pick a sample of 100 bright GRBs observed by the Fermi Gamma-ray Burst Monitor (GBM), based on their energy flux values. In addition to the standard empirical spectral models used in previous GBM spectroscopy catalogs, we also consider three physically-motivated models; (a) a Thermal Synchrotron model, (b) a Band model with a High-energy Cutoff, and (c) a Smoothly Broken Power Law (SBPL) model with a Multiplicative Broken Power Law (MBPL). We then adopt the Bayesian information criterion (BIC) to compare the fits obtained and choose the best model. We find that 42% of the GRBs from the fluence spectra and 23% of GRBs from the peak-flux spectra have one of the three physically-motivated models as their preferred one. From the peak-flux spectral fits, we find that the low-energy index distributions from the empirical model fits for long GRBs peak around the synchrotron value of -2/3, while the two low-energy indices from the SBPL+MBPL fits of long GRBs peak close to the -2/3 and -3/2 values expected for a synchrotron spectrum below and above the cooling frequency.Comment: arXiv admin note: text overlap with arXiv:2103.1352