SAX J1808.4−3658
Observation data
Epoch J2000.0      Equinox J2000.0 (ICRS)
Constellation Sagittarius
Right ascension 17h 45m 39.73s[1]
Declination −29° 00′ 29.7″[1]
Apparent magnitude (V) 16.51
Spectral typeNeutron star
Other designations
V4580 Sgr, PSR J1808−3658, SWIFT J1808.5−3655, INTREF 881, XTE J1808−369
Database references
SIMBADdata

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

  1. 1 2 "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.
  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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