Late-time X-ray Behavior of Short Gamma-Ray Bursts: Implications for Energetics and Rates

The distribution of jet angles for short gamma-ray bursts (GRBs) is critical to constrain because it directly affects the true energy scale and event rate, paramenters of interest for coincident short GRB-gravitational wave detections with Advanced LIGO. Our current knowledge of the opening angle distribution comes almost exclusively from X-ray observations at >1 day after the burst. Here, we propose XMM TOO observations to monitor a short GRB afterglow and constrain its collimation, either from the detection of a jet break to determine the opening angle, or the non-detection of a break to place a lower limit of >10-15 degrees.