THRESHOLD FOR EXTENDED EMISSION IN SHORT GAMMA-RAY BURSTS

Abstract

The initial pulse complex (IPC) in short gamma-ray bursts is sometimes\naccompanied by a softer, low-intensity extended emission (EE) component. In\ncases where such a component is not observed, it is not clear if it is present\nbut below the detection threshold. Using Bayesian Block (BB) methods, we\nmeasure the EE component and show that it is present in one quarter of a\nSwift/BAT sample of 51 short bursts, as was found for the Compton/BATSE sample.\nWe simulate bursts with EE to calibrate the BAT threshold for EE detection and\nshow that this component would have been detected in nearly half of BAT short\nbursts if it were present, to intensities ~ 10^-2 counts cm^-2 s^-1, a factor\nof five lower than actually observed in short bursts. In the BAT sample the\nratio of average EE intensity to IPC peak intensity, Rint, ranges over a factor\nof 25, Rint ~ 3 x 10^-3 to 8 x 10^-2. In comparison, for the average of the 39\nbursts without an EE component, the 2-sigma upper limit is Rint < 8 x 10^-4.\nThese results suggest that a physical threshold effect operates near Rint ~ few\nx 10^-3, below which the EE component is not manifest.\n