X-ray Afterglow limits on the viewing angles of short gamma-ray bursts

Abstract

Abstract The behavior of a short gamma-ray burst (sGRB) afterglow lightcurve 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 Γ(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 (Γ0 > 500). If we consider tophat jets, we constrain 90% of our sample to be viewed within θobs/θc < 1.06 and 1.15 for our canonical and conservative afterglow scenarios. For a subset of events with measurements of the jet break, we can constrain Γ0θc ≳ 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 (GWs), implying that they are extremely rare.