Speaker
Description
The Swift Supergiant Fast X-ray Transients Project was born 18 years ago as one of the initiatives to boost secondary science. It exploited Swift's fast automatic slewing, multi-wavelength capability, its flexible observing schedule and very low overheads to detect, characterise, and understand the physics underlying Supergiant Fast X- ray Transients (SFXT), high-mass X-ray binaries with an O-B supergiant displaying X-ray outbursts reaching luminosities of $10^{38}$ erg s$^{-1}$ and surprising large dynamic ranges. The strategy combined follow-ups of outbursts, when detailed broad-band spectroscopy is possible, with long-term monitoring campaigns, when the out-of-outburst fainter states can be observed. Several fundamental discoveries can be attributed to this simple but effective plan: the record luminosity in an SFXT outburst (up to the Eddington luminosity for a 1.4M$_{\rm sun}$ neutron star) bringing the dynamic range up up to a factor of 1 million, a quantification of an intermediate flux state where SFXTs spend most of their time, and a variability detected at all timescales that have been probed.
The lasting legacy of this project now includes two catalogues based on the first $\sim100$ months of the Swift mission. The first is a collection of over a thousand Swift/BAT flares from 11 SFXTs, with immediate applications of an estimate of the expected number of SFXTs in the Milky Way, and the number of flares expected from other future missions. The second includes all the SFXT Swift onboard (GRB-like) triggers and provides tools to predict whether a transient with no X-ray counterpart may be an SFXT candidate, tools can be exploited to develop future X-ray missions with large FoV instruments.
The year 2025 marks the 20th anniversary of the recognition of SFXTs as a class of astrophysical sources, and we highlight the Swift future initiatives to continue their investigation.
