Study of the Prompt Emission of Short Gamma-Ray Bursts Using a Multicolor Blackbody: A Clue to the Viewing Angle

The prompt emission of short gamma-ray bursts (sGRBs) with known redshifts is analyzed using the model of a multicolor blackbody, which is interpreted as the emission from a nondissipative photosphere taking into account a power-law jet structure and the viewing geometry of the jet. We find nearly 69% and 26% of the sample is consistent with a multicolor blackbody and a pure blackbody model, respectively. Using this interpretation, we infer that nearly 57% (18%) of the sGRBs in our sample are observed within (or along the edge of) the jet core. The sGRB jets are deduced to possess a narrow core with a median θ c ∼ 3°. This suggests the rate of sGRBs that would be viewed within the jet core to be 1.8–26 Gpc −3 yr −1 . The power-law index of the decreasing Lorentz factor profile of the jet structure is deduced to be 1.3–2.2. The intrinsic luminosity is found to range between 10 48 –10 53 erg s −1 . The average values of the Lorentz factor and nozzle radius of the sGRB jets are inferred to be 210 (85) and 10 7.7 (10 9.6 ) cm for the cases when the photosphere forms in the coasting (accelerating) phase, respectively. The viability of the inferred values of the different parameters of the GRB outflow and viewing geometry within this physical interpretation enhances the prospect of the photospheric emission model explaining the observed GRB spectrum.