Beginning this month, NuSTAR has removed the 20 ks minimum exposure time requirement for target-of-opportunity (ToO) requests. This change has been made to enable NuSTAR help fill the gap in NASA capabilities to observe transient high-energy phenomena during the recent suspension of science operations by the Neil Gehrels Swift Observatory, as well as the suspension of NICER operations since June 2025. For the last two decades, Swift has been the primary utility for rapid follow-up of high-energy transient events, performing multiple ToO observations every day.  Last month, Swift suspended science operations in an effort to reduce atmospheric drag and slow the spacecraft’s orbital decay.  Halting these activities will enable controllers to keep Swift in an orientation that minimizes drag effects, extending its time in orbit in anticipation of the reboost mission planned for launch this summer. The shortest exposure for NuSTAR observations had been set at a 12-hour duration to match flux depths of previous sky surveys and to avoid performing many long slews between targets. This week that limit has been removed for Directors Discretionary Time requests to perform high energy ToO observations of transient events. NuSTAR will not match the rapid slew capability nor the soft X-ray energy and UV sensitivity of the instruments on the Swift observatory, but the project expects to be able to respond to some requests that Swift would have performed, while the community waits for Swift operations to resume later this year.

Author: Karl Forster (NuSTAR Science Operations Lead, Caltech)