Threshold for Extended Emission in Short Gamma-Ray Bursts

TL;DR

This study investigates the presence and detectability of extended emission in short gamma-ray bursts, revealing a potential physical threshold below which EE is not observable, using Bayesian methods and simulations.

Contribution

The paper demonstrates that extended emission is present in a quarter of short GRBs and establishes a detection threshold, suggesting a physical limit for EE manifestation.

Findings

Extended emission is detected in 25% of short GRBs.

Detection threshold for EE is about 10^-2 counts cm^-2 s^-1.

A physical threshold near Rint ~ few x 10^-3 likely governs EE visibility.

Abstract

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 ~ 10^-2 counts cm^-2 s^-1, a factor of five lower than actually observed in short bursts. In the BAT sample the ratio of average EE intensity to IPC peak intensity, Rint, ranges over a factor of 25, Rint ~ 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 Rint < 8 x 10^-4. These results suggest that a physical threshold effect operates near Rint ~ few x 10^-3, below which the EE component is not manifest.