The behaviour of a short gamma-ray burst (sGRB) afterglow light curve can reveal the angular structure of the relativistic jet and constrain the observer’s viewing angle θobs . The observed deceleration time of the jet, and, therefore, the time of the afterglow peak, depends on the observer’s viewing angle. A larger viewing angle leads to a later peak of the afterglow and a lower flux at peak. We utilize the earliest afterglow detections of 58 sGRBs detected with the Neil Gehrels Swift Observatory X-ray Telescope to constrain the ratio of the viewing angle θobs to the jet’s core θc . We adopt a power-law angular jet structure in both energy E( θ) ∝ θ −a and Lorentz factor ( θ) ∝ θ −b beyond the core. We find that either sGRBs are viewed within θobs /θc < 1 or the initial Lorentz factor of material in their jet’s core is extremely high ( 0 > 500). If we consider tophat jets, we constrain 90 per cent of our sample to be viewed within θobs /θc < 1 . 06 and 1.15 for our canonical and conserv ati ve afterglo w scenarios. For a subset of events with measurements of the jet break, we can constrain 0 θc 30. This confirmation that cosmological sGRBs are viewed either on-axis or very close to their jet’s core has significant implications for the nature of the prompt gamma-ray production mechanism and for the rate of future sGRB detections coincident with gravitational waves, implying that they are extremely rare.
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