The Neil Gehrels Swift Observatory is a NASA medium explorer satellite developed in collaboration with Italy and the United Kingdom. The name is not an acronym but refers to the English name of the swift, one of the birds with the most agile and fast flight, and was chosen to recall the speed of pointing of the satellite, guaranteed by a gyroscopic system controlled by electric engines.

The satellite is dedicated to the study and interpretation of gamma-ray bursts (GRBs). Launched in 2004, Swift has been observing the high-energy universe for nearly 20 years.
Swift has three instruments (BAT, UVOT, XRT). The Burst Alter Telescope (BAT) is a wide-field instrument that works in hard X-rays. BAT is responsible for revealing GRBs. Once a new GRB is discovered, Swift autonomously repoints the small-field instruments, the X-Ray Telescope (XRT) and the UltraViolet Optical Telescope (UVOT), within 60 to 120 seconds, to observe the GRB in UV. Swift observes approximately 70-100 GRBs per year and, thanks to its unique ability of fast re-pointing - to date, no satellite rotates, moves and re-points faster than Swift - it manages to observe the entire temporal evolution of the GRBs, from the outbreak until their slow fading, weeks or months later.

The initial mission objectives for Swift were:

• determine the origin of GRBs;
• classify GRBs and search for new types;
• determine how the blastwave evolves and interacts with the surrunding environment;
• use GRBs to study the early universe;
• carry out a sensitive survey of the sky in the hard X-ray band.

The Swift mission patch depicts both the spacecraft and the bird for which it was named. The observatory is named after a small, nimble bird that can grab up insects as it flies through the sky. Similarly, the observatory can swiftly turn and point its instruments to catch a gamma-ray burst "on the fly" to study both the burst and its afterglow. This afterglow phenomenon follows the initial gamma-ray flash in most bursts and it can linger in X-ray light, visible light and radio waves for hours or weeks, providing great detail for observations. Credits: NASA E/PO, Sonoma State University/Aurore Simonnet.

All these scientific objectives have been achieved and Swift still manages to reserve new surprises for us. Furthermore, thanks to the speed and flexibility of repointing, Swift over the years has been and is used to study all types of high energy sources: from comets to stars, to galactic compact objects, to active galactic nuclei, to galaxies and clusters of galaxies, often making new discoveries in every field, and becoming a point of reference for time domain astronomy.

Swift also made a fundamental contribution to the study of the optical counterparts of gravitational wave events, having been the only satellite to have observed their ultraviolet emission. The new observation run of gravitational interferometers (the so-called O4b) started on 3 April 2024, to which Swift will make a unique contribution by searching in real time for the electromagnetic counterpart of gravitational events in X-rays and in the bluest band of the optical spectrum.

Artist’s impression of the Swift spacecraft. Credits NASA/Swift.

Swift’s Ultraviolet/Optical Telescope imaged the kilonova produced by merging neutron stars in the galaxy NGC 4993 (box) on Aug. 18, 2017, about 15 hours after gravitational waves and the gamma-ray burst were detected. The source was unexpectedly bright in ultraviolet light. It faded rapidly and was undetectable in UV when Swift looked again on Aug. 29. This false-color composite combines images taken through three ultraviolet filters. Inset: Magnified views of the galaxy. Credits: NASA/Swift

The INAF-Brera Astronomical Observatory provided the optics for the X-ray telescope (XRT) and built a small ground-based robotic optical-infrared telescope (REM) dedicated to monitoring the optical-infrared emission of GRBs. Several OAB researchers are active members of the Swift-Italia group and participate in the scientific management of the satellite as Burst Advocate (BA) and XRT Burst Specialist (XBS), for the rapid and follow-up observations of GRBs detected by Swift, 24 hours a day, 7 days a week, because GRBs are unpredictable.
The analysis and interpretation of these observations, i.e. data from the Swift satellite coupled with simultaneous observations from ground-based telescopes, allows us to study the properties of GRBs in detail to understand their nature. Like Swift, in OAB we have also broadened our scientific interests by also studying transient sources in the X-band sky.