THRESHOLD FOR EXTENDED EMISSION IN SHORT GAMMA-RAY BURSTS

The initial pulse complex (IPC) in short gamma-ray bursts is sometimes accompanied by a softer, low-intensity extended emission (EE) component. In cases where such a component is not observed, it is not clear if it is present but below the detection threshold. Using Bayesian Block (BB) methods, we measure the EE component and show that it is present in one-quarter of a Swift/BAT sample of 51 short bursts, as was found for the Compton/BATSE sample. We simulate bursts with EE to calibrate the BAT threshold for EE detection and show that this component would have been detected in nearly half of BAT short bursts if it were present, to intensities similar to 10(-2) counts cm(-2) s(-1), a factor of 5 lower than actually observed in short bursts. In the BAT sample, the ratio of average EE intensity to IPC peak intensity, R-int, ranges over a factor of 25, R-int similar to 3 x 10(-3) to 8 x 10(-2). In comparison, for the average of the 39 bursts without an EE component, the 2 sigma upper limit is R-int < 8 x 10(-4). These results suggest that a physical threshold effect operates near R-int similar to few x 10(-3) below which the EE component is not manifest.