Prompt optical emission and synchrotron self-absorption constraints on emission site of GRBs |
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| Authors: | Rong-Feng Shen Bing Zhang |
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| Institution: | Department of Astronomy, University of Texas at Austin, Austin, TX 78712, USA;Department of Physics and Astronomy, University of Nevada at Las Vegas, Las Vegas, NV 89154, USA |
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| Abstract: | 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/γ-ray spectral energy distribution, assuming that the optical and the γ-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 Γ and the magnetic field strength B in the emission region (with a weak dependence). We identify a small sample of four bursts that satisfy the following three criteria: (1) they all have simultaneous optical and γ-ray detections in multiple observational time intervals, (2) they all show temporal correlations between the optical and γ-ray light curves and (3) the optical emission is consistent with belonging to the same spectral component as the γ-ray emission. For all the time intervals of these four bursts, it is inferred that R ≥ 1014 (Γ/300)3/4 ( B /105 G)1/4 cm. For a small fraction of the sample, the constraint can be pinned down to R ≈ 1014–1015 cm for Γ～ 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. |
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| Keywords: | radiation mechanisms: non-thermal radiative transfer gamma-rays: bursts gamma-rays: theory |
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