GRB Population Research Articles

Low‐Luminosity Gamma‐Ray Bursts as a Unique Population: Luminosity Function, Local Rate, and Beaming Factor

Swift/BAT has detected ~ 200 long-duration GRBs, with redshift measurements for ~50 of them. We derive the luminosity function (Phi^{HL}) and the local event rate (rho_0^{HL}) of the conventional high luminosity (HL) GRBs by using the z-known Swift GRBs. Our results are generally consistent with that derived from the CGRO/BATSE data. However, the fact that Swift detected a low luminosity (LL) GRB, GRB 060218, at z=0.033 within ~ 2 year of operation, together with the previous detection of the nearby GRB 980425, suggests a much higher local rate for these LL-GRBs. We explore the possibility that LL-GRBs as a distinct GRB population from the HL-GRBs. We find that rho_0^{LL} is 325_{-177}^{+352} Gpc^{-3} yr^{-1}, which is much higher than rho_0^{HL}(1.12_{-0.20}^{+0.43} Gpc^{-3} yr^{-1}). This rate is ~ 0.7% of the local Type Ib/c SNe. Our results, together with the finding that less than 10% of Type Ib/c SNe are associated with off-beam GRBs, suggest that LL-GRBs have a beaming factor typically less than 14, or a jet angle typically wider than 31^{o}. The high local GRB rate, the small beaming factor, and low luminosity make the LL-GRBs distinct from the HL-GRBs. Although the current data could not fully rule out the possibility that both HL- and LL-GRBs are the same population, our results suggest that LL-GRBs are likely a unique GRB population and the observed low redshift GRB sample is dominated by the LL-GRBs.

Quantifying the Luminosity Evolution in Gamma‐Ray Bursts

We estimate the luminosity evolution and formation rate for more than 900 GRBs by using redshift and luminosity data calculated by Band, Norris, & Bonnell via the lag-luminosity correlation. By applying maximum likelihood techniques, we are able to infer the true distribution of the parent GRB population's luminosity function and density distributions in a way that accounts for detector selection effects. We find that after accounting for data truncation, there still exists a significant correlation between the average luminosity and redshift, indicating that distant GRBs are on average more luminous than nearby counterparts. This is consistent with previous studies showing strong source evolution and also recent observations of underluminous nearby GRBs. We find no evidence for beaming angle evolution in the current sample of GRBs with known redshift, suggesting that this increase in luminosity cannot be due to an evolution of the collimation of gamma-ray emission. The resulting luminosity function is well fit with a single power law of index L'-1.5, which is intermediate between the values predicted by the power-law and Gaussian structured jet models. We also find that the GRB comoving rate density rises steeply, with a broad peak between 1 = z = 2 followed by a steady decline above z > 3. This rate density qualitatively matches the current estimates of the cosmic star formation rate, favoring a short-lived massive star progenitor model or a binary model with a short delay between the formation of the compact object and the eventual merger.

HETE‐2Observation of Two Gamma‐Ray Bursts atz> 3

GRB 020124 and GRB 030323 constitute half the sample of gamma-ray bursts with a measured redshift greater than 3. This paper presents the temporal and spectral properties of these two gamma-ray bursts detected and localized with HETE-2. While they have nearly identical redshifts (z=3.20 for GRB 020124, and z=3.37 for GRB 030323), these two GRBs span about an order of magnitude in fluence, thus sampling distinct regions of the GRB luminosity function. The properties of these two bursts are compared with those of the bulk of the GRB population detected by HETE-2. We also discuss the energetics of GRB 020124 and GRB 030323 and show that they are compatible with the Epeak - Eiso relation discovered by Amati et al. (2002). Finally, we compute the maximum redshifts at which these bursts could have been detected by HETE-2 and we address various issues connected with the detection and localization of high-z GRBs.

An Analysis of Gamma‐Ray Burst Spectral Break Models

Typical gamma-ray burst spectra are characterized by a spectral break, Ep, which for bright BATSE bursts is found to be narrowly clustered around 300 keV. Recently identified X-ray flashes, which may account for a significant portion of the whole GRB population, seem to extend the Ep distribution to a broader range below 40 keV. Positive correlations among Ep and some other observed parameters have been noticed. On the other hand, within the cosmological fireball model, the issues concerning the dominant energy ingredient of the fireball as well as the location of the GRB emission site are still unsettled, leading to several variants of the fireball model. Here we analyze these models within a unified framework, and critically reexamine the Ep predictions in the various model variants. Attention is focused on the predictions of the narrowness of the Ep distribution in different models, and the correlations among Ep and some other measurable observables. These model properties may be tested against the current and upcoming GRB data, through which the nature of the fireball as well as the mechanism and site of the GRB emission will be identified. In view of the current data, various models are appraised through a simple Monte-Carlo simulation, and a tentative discussion about the possible nature of X-ray flashes is presented.