SAX J1808.4−3658

{{short description|Neutron star in the constellation Sagittarius}}

{{refimprove|date=January 2017}}

{{Starbox short

|name=SAX J1808.4−3658

|constell=Sagittarius

|ra={{RA|18|08|27.54}}{{cite simbad|title=SAX J1808.4-3658|accessdate=15 January 2017}}

|dec={{DEC|−36|58|44.3}}

|class=Neutron star

|appmag_v=16.51

|names=V4580 Sgr, PSR J1808−3658, SWIFT J1808.5−3655, INTREF 881, XTE J1808−369

|Simbad=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.{{cite journal|doi=10.1038/28557|title=A millisecond pulsar in an X-ray binary system|year=1998|last1=Wijnands|first1=Rudy|last2=van der Klis|first2=Michiel|journal=Nature|bibcode = 1998Natur.394..344W|volume=394|issue=6691|pages=344–346|s2cid=4428048}} 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.

File:V4580SgrLightCurve.png light curve for V4580 Sagittarii, adapted from Elebert et al. (2009)]]

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 (HETE J1900.1-2455, Aql X-1 and SAX J1748.9-2021), i.e. they emit pulsations sporadically during the outburst.

On 21 August 2019 (UTC; 20 August in the US), Neutron Star Interior Composition Explorer (NICER) spotted the brightest X-ray burst so far observed.[https://www.space.com/nicer-telescope-brightest-x-ray-burst.html NICER Telescope Spots Brightest X-Ray Burst Ever Observed] It came from SAX J1808.4−3658.