Setting the Triggering Thresholds on Swift

The Burst Alert Telescope (BAT) on Swift has two main types of “rate” triggers: short and long. Short trigger time scales range from 4ms to 64ms, while long triggers are 64ms to ≈ 16 seconds. While both short and long trigger have criteria with one background sample (traditional “one‐sided” triggers), the long triggers can also have criteria with two background samples (“bracketed” triggers) which remove trends in the background. Both long and short triggers can select energy ranges of 15–25, 15–50, 25–100 and 50–350 KeV. There are more than 180 short triggering criteria and approximately 500 long triggering criteria used to detect gamma ray bursts. To fully utilize these criteria, the thresholds must be set correctly. The optimum thresholds are determined by a tradeoff between avoiding false triggers and capturing as many bursts as possible. We use realistic simulated orbital variations, which are the prime cause of false triggers.