SAX J1808.4−3658

SAX J1808.4−3658
Observation data; Epoch J2000.0 Equinox J2000.0 (ICRS)
Constellation	Sagittarius
Right ascension	18h 08m 27.54s
Declination	−36° 58′ 44.3″
Apparent magnitude (V)	16.51
Spectral type	Neutron star
Other designations
	V4580 Sgr, PSR J1808−3658, SWIFT J1808.5−3655, INTREF 881, XTE J1808−369
Database references
SIMBAD	data

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.^[2] 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.

A red band light curve for V4580 Sagittarii, adapted from Elebert *et al.* (2009)^[3]

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.^[4] It came from SAX J1808.4−3658.

References

^ ^a ^b "SAX J1808.4-3658". SIMBAD. Centre de données astronomiques de Strasbourg. Retrieved 15 January 2017.
^ Wijnands, Rudy; van der Klis, Michiel (1998). "A millisecond pulsar in an X-ray binary system". Nature. 394 (6691): 344–346. Bibcode:1998Natur.394..344W. doi:10.1038/28557. S2CID 4428048.
^ Elebert, P.; Reynolds, M. T.; Callanan, P. J.; Hurley, D. J.; Ramsay, G.; Lewis, F.; Russell, D. M.; Nord, B.; Kane, S. R.; Depoy, D. L.; Hakala, P. (May 2009). "Optical spectroscopy and photometry of SAX J1808.4-3658 in outburst". Monthly Notices of the Royal Astronomical Society. 395 (884–894). arXiv:0901.3991. Bibcode:2009MNRAS.395..884E. doi:10.1111/j.1365-2966.2009.14562.x.
^ NICER Telescope Spots Brightest X-Ray Burst Ever Observed

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[simbad-1] "SAX J1808.4-3658". SIMBAD. Centre de données astronomiques de Strasbourg. Retrieved 15 January 2017.

[2] Wijnands, Rudy; van der Klis, Michiel (1998). "A millisecond pulsar in an X-ray binary system". Nature. 394 (6691): 344–346. Bibcode:1998Natur.394..344W. doi:10.1038/28557. S2CID 4428048.

[Elebert-3] Elebert, P.; Reynolds, M. T.; Callanan, P. J.; Hurley, D. J.; Ramsay, G.; Lewis, F.; Russell, D. M.; Nord, B.; Kane, S. R.; Depoy, D. L.; Hakala, P. (May 2009). "Optical spectroscopy and photometry of SAX J1808.4-3658 in outburst". Monthly Notices of the Royal Astronomical Society. 395 (884–894). arXiv:0901.3991. Bibcode:2009MNRAS.395..884E. doi:10.1111/j.1365-2966.2009.14562.x.

[4] NICER Telescope Spots Brightest X-Ray Burst Ever Observed

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