Astronomy and Astrophysics – Astrophysics – High Energy Astrophysical Phenomena
Scientific paper
2009-06-15
Astronomy and Astrophysics
Astrophysics
High Energy Astrophysical Phenomena
13 pages, 4 figures, MNRAS accepted; added reference
Scientific paper
We constrain the distance of the Gamma-Ray Burst (GRB) prompt emission site from the explosion centre, R, by determining the location of the electron's self absorption frequency in the GRB prompt optical-to-X/gamma-ray spectral energy distribution, assuming that the optical and the gamma-ray emissions are among the same synchrotron radiation continuum of a group of hot electrons. All possible spectral regimes are considered in our analysis. The method has only two assumed parameters, namely, the bulk Lorentz factor of the emitting source Gamma, and the magnetic field strength B in the emission region (with a weak dependence). We identify a small sample of 4 bursts that satisfy the following three criteria: (1) they all have simultaneous optical and gamma-ray detections in multiple observational time intervals; (2) they all show temporal correlations between the optical and gamma-ray light curves; and (3) the optical emission is consistent with belonging to the same spectral component as the gamma-ray emission. For all the time intervals of these 4 bursts, it is inferred that R \geq 10^{14} (Gamma/300)^{3/4} (B/10^5 Gauss)^{1/4} cm. For a small fraction of the sample, the constraint can be pinned down to R \approx 10^{14} - 10^{15} cm for Gamma ~ 300. For a second sample of bursts with prompt optical non-detections, only upper limits on R can be obtained. We find no inconsistency between the R-constraints for this non-detection sample and those for the detection sample.
Shen Rong-Feng
Zhang Bing
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