Swift gives a new BAT-GLIMPSE: Gamma-ray Localization using Imaging and Mosaic techniques for Pointing and Slew Epochs
The Burst Alert Telescope (BAT) onboard the Neil Gehrels Swift Observatory is capable of localizing gamma-ray transients with arcminute precision, enabling rapid multi-wavelength follow-up. However, onboard triggering is disabled during spacecraft slews, preventing the autonomous detection and localization of transients occurring during these intervals. We present here BAT-GLIMPSE, a fully autonomous, open-source pipeline for the low-latency localization of transient gamma-ray sources in Swift-BAT data. Making use of the BatAnalysis package, the pipeline combines coded-mask imaging and mosaic techniques, automatically selecting the appropriate analysis according to the spacecraft attitude and enabling localization searches during both pointing observations and spacecraft slews. We validate the performance of BAT-GLIMPSE on a sample of 66 GRBs reported in GUANO circulars. The pipeline successfully recovers arcminute positions for 43 events, consistent with published localizations, with typical offsets of $\lesssim5$ arcminutes. Approximately $88\%$ of the GRBs occurring during spacecraft slews are recovered through imaging or mosaic analyses. During the fourth LIGO-Virgo-KAGRA observing run, the role of BAT-GLIMPSE was crucial in the search for gamma-ray counterparts of gravitational waves, particularly in response to pre-merger alerts which triggered the slew of the Swift spacecraft with extremely low latency. Operating synergistically with NITRATES, BAT-GLIMPSE fills the critical gap left by slew intervals and, together, the two pipelines are estimated to double the onboard arcminute-localization rate of Swift-BAT, unlocking the full potential of the Swift mission for time-domain and multi-messenger astrophysics.