Unification of Pulses in Long and Short Gamma-Ray Bursts: Evidence from Pulse Properties and their Correlations

We demonstrate that distinguishable gamma-ray burst pulses exhibit similar behaviors as evidenced by correlations among the observable pulse properties of duration, peak luminosity, fluence, spectral hardness, energy-dependent lag, and asymmetry. Long and Short burst pulses exhibit these behaviors, suggesting that a similar process is responsible for producing all GRB pulses. That these properties correlate in the observer's frame indicates that intrinsic correlations are strong enough to not be diluted into insignificance by the dispersion in distances and redshift. We show how all correlated pulse characteristics can be explained by hard-to-soft pulse evolution, and we demonstrate that "intensity tracking" pulses not having these properties are not single pulses; they instead appear to be composed of two or more overlapping hard-to-soft pulses. In order to better understand pulse characteristics, we recognize that hard-to-soft evolution provides a more accurate definition of a pulse than its intensity variation. This realization, coupled with the observation that pulses begin near-simultaneously across a wide range of energies, leads us to conclude that the observed pulse emission represents the energy decay resulting from an initial injection, and that one simple and as yet unspecified physical mechanism is likely to be responsible for all gamma-ray burst pulses regardless of the environment in which they form and, if GRBs originate from different progenitors, then of the progenitors that supply them with energy.Comment: 35 pages including 11 figures and 4 tables, accepted for publication in The Astrophysical Journa

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

Heterogeneity in Short Gamma-ray Bursts

We analyze the Swift/BAT sample of short gamma-ray bursts, using an objective Bayesian Block procedure to extract temporal descriptors of the bursts' initial pulse complexes (IPCs). The sample comprises 12 and 41 bursts with and without extended emission (EE) components, respectively. IPCs of non-EE bursts are dominated by single pulse structures, while EE bursts tend to have two or more pulse structures. The medians of characteristic timescales - durations, pulse structure widths, and peak intervals - for EE bursts are factors of ~ 2-3 longer than for non-EE bursts. A trend previously reported by Hakkila and colleagues unifying long and short bursts - the anti-correlation of pulse intensity and width - continues in the two short burst groups, with non-EE bursts extending to more intense, narrower pulses. In addition we find that preceding and succeeding pulse intensities are anti-correlated with pulse interval. We also examine the short burst X-ray afterglows as observed by the Swift/XRT. The median flux of the initial XRT detections for EE bursts (~ 6 x 10^-10 erg cm^-2 s^-1) is ~> 20 x brighter than for non-EE bursts, and the median X-ray afterglow duration for EE bursts (~ 60,000 s) is ~ 30 x longer than for non-EE bursts. The tendency for EE bursts toward longer prompt-emission timescales and higher initial X-ray afterglow fluxes implies larger energy injections powering the afterglows. The longer-lasting X-ray afterglows of EE bursts may suggest that a significant fraction explode into more dense environments than non-EE bursts, or that the sometimes-dominant EE component efficiently powers the afterglow. Combined, these results favor different progenitors for EE and non-EE short bursts.Comment: 30 pages, 11 figures, 3 tables; accepted to The Astrophysical Journa

Search for Gamma-Ray Burst Classes with the RHESSI Satellite

A sample of 427 gamma-ray bursts (GRBs), measured by the RHESSI satellite, is studied statistically with respect to duration and hardness ratio. Standard statistical tests are used, such as $\chi^2$, F-test and the maximum likelihood ratio test, in order to compare the number of GRB groups in the RHESSI database with that of the BATSE database. Previous studies based on the BATSE Catalog claim the existence of an intermediate GRB group, besides the long and short groups. Using only the GRB duration $T_{90}$ as information and $\chi^2$ or F-test, we have not found any statistically significant intermediate group in the RHESSI data. However, maximum likelihood ratio test reveals a significant intermediate group. Also using the 2-dimensional hardness / $T_{90}$ plane, the maximum likelihood analysis reveals a significant intermediate group. Contrary to the BATSE database, the intermediate group in the RHESSI data-set is harder than the long one. The existence of an intermediate group follows not only from the BATSE data-set, but also from the RHESSI one.Comment: Accepted for publication in Astronomy and Astrophysics, 9 pages, 4 figure

A New Discriminator for Gamma-Ray Burst Classification: The Epeak-Fluence Energy Ratio

Using the derived gamma-ray burst E_peak and fluences from the complete BATSE 5B Spectral Catalog, we study the ensemble characteristics of the E_peak-fluence relation for GRBs. This relation appears to be a physically meaningful and insightful fundamental discriminator between long and short bursts. We discuss the results of the lower limit test of the E_peak-E_iso relations in the E_peak-fluence plane for BATSE bursts with no observed redshift. Our results confirm the presence of two GRB classes as well as heavily suggesting two different GRB progenitor types.Comment: 8 pages, 3 figures, accepted for publication in Ap