SAX J1808.4−3658
The first accreting millisecond pulsar discovered in 1998 by the Italian-Dutch BeppoSAX satellite, SAX J1808.4−3658 revealed X-ray pulsations at the 401 Hz neutron star spin frequency when it was observed during a subsequent outburst in 1998 by NASA's RXTE satellite. The neutron star is orbited by a brown dwarf binary companion with a likely mass of 0.05 solar masses, every 2.01 hours. X-ray burst oscillations and quasi-periodic oscillations in addition to coherent X-ray pulsations have been seen from SAX J1808.4-3658, making it a Rosetta stone for interpretation of the timing behavior of low-mass X-ray binaries. Paul Roche et al. discovered an optical counterpart to the X-ray source, in 1998. Its brightness varies, so in the year 2000 it was given its variable star designation, V4580 Sagittarii.
These accreting millisecond X-ray pulsars are thought to be the evolutionary progenitors of recycled radio millisecond pulsars. A total of thirteen accreting millisecond X-ray pulsars have been discovered as of January 2011. Three of them are Intermittent millisecond X-ray pulsars, i.e. they emit pulsations sporadically during the outburst.
On 21 August 2019, Neutron Star Interior Composition Explorer spotted the brightest X-ray burst so far observed. It came from SAX J1808.4−3658.