SAX J1808.4−3658

Neutron star in the constellation Sagittarius

SAX J1808.4−3658
Observation data Epoch J2000.0 Equinox J2000.0 (ICRS)
Constellation	Sagittarius
Right ascension	18^h 08^m 27.54^s^[1]
Declination	−36° 58′ 44.3″^[1]
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

Constellation of Sagittarius

Stars

Bayer	α (Rukbat) β¹ (Arkab Prior) β² (Arkab Posterior) γ¹ (W) γ² (Alnasl) δ (Kaus Media) ε (Kaus Australis) ζ (Ascella) η θ¹ θ² ι κ¹ κ² λ (Kaus Borealis) μ (Polis) ν¹ (Ainalrami) ν² ξ¹ ξ² ο π (Albaldah) ρ¹ ρ² σ (Nunki) τ υ φ χ¹ χ² χ³ ψ ω (Terebellum)
Flamsteed	1 4 5 6 7 9 12 14 15 16 17 18 21 23 24 25 26 28 29 30 31 33 43 (d) 50 51 (h¹) 52 (h²) 53 54 (e¹) 55 (e²) 56 (f) 57 59 (b) 60 (A) 61 (g) 62 (c) 63 65 63 Oph
Variable	T U X (3) Y RR RS RY RZ VX VY VZ WZ XX XZ YY YZ AQ AS AV BB FN GM GU KW MV V348 V350 V356 V440 V505 V523 V526 V617 V630 V725 V732 V774 V919 V1017 V1059 V1216 V1223 V1647 V1942 V3792 V3795 V3804 V3879 V3885 V3890 V3892 V3894 V3903 (11) V3961 V3984 V4021 V4024 V4028 V4029 V4030 V4046 V4050 V4064 V4072 V4089 V4091 V4138 V4140 V4160 V4169 V4190 V4198 V4199 V4200 V4332 V4333 V4334 V4371 V4375 V4381 V4387 V4405 V4407 V4434 V4444 V4580 V4633 V4641 V4647 (Pistol Star) V4650 V4728 V4743 V4745 V4998 V5112 V5115 V5116 V5125 V5157 V5555 V5558 V5652 V5668 V5856
HR	6617 6679 6680 6691 6692 6693 6704 6716 6739 6748 6762 6766 6769 6777 6780 6788 6798 6835 6836 6838 6842 6864 6874 6889 6907 6909 6914 6919 6933 6936 6944 6960 6972 6988 6990 6998 7026 7029 7035 7038 7072 7104 7126 7128 7151 7159 7164 7166 7168 7182 7186 7195 7203 7205 7211 7230 7239 7240 7246 7265 7276 7277 7282 7316 7317 7327 7330 7334 7350 7355 7360 7367 7378 7380 7392 7398 7410 7433 7443 7454 7473 7491 7496 7507 7532 7538 7585 7616 7627 7629 7630 7631 7639 7643 7652 7658 7659 7703 7706 7725 7765 7799 7801 7808
HD	163296 163758 164604 (Pincoya) 164816 165052 169142 170657 171238 175390 175754 180902 181342 181720 183577 187085 187150 190647 313846 316285
Other	2MASS 19281982-2640123 2MASS J18352154–3123385 2MASS J19513587-3510375 4U 1755-33 4U 1820-30 A1742-294 Ap 1-11 AS 295 BD -18 5550 Bursting Pulsar CWISEP J1935-1546 GCIRS 13E 745 GGD 27 GRO J1744−28 GRS 1758-258 GS 1826-24 GX 3-1 GX 5-1 GX 9-1 GX 13+1 HATS-36 Herschel 36 HETE J1900.1-2455 IRAS 17423-1755 IRAS 18162−2048 GCIRS 3 GCIRS 7 GCIRS 8* GCIRS 16SW Gomez's Hamburger G0.238-0.071 K2-2016-BLG-0005L LBV 1806-20 LS IV -12 111 MACHO 176.18833.411 MACHO-1997-BLG-41 MACHO-98-BLG-35 NGC 6822-WR 12 PSR B1744−24A PSR B1749−28 PSR B1757−24 PSR B1758−23 PSR B1800−21 PSR B1821−24 PSR B1857−26 PSR J1808−2024 PSR J1747−2958 PSR J1748−2446ad S2 S55 S62 S4716 SAX J1747.0−2853 SAX J1808.4−3658 SAX J1810.8−2609 SWEEPS J175853.92−291120.6 SWEEPS J175902.00-291323.7 SWEEPS J175902.67−291153.5 Swift J1745-26 SWIFT J1756.9−2508 Th 4-4 W33A WASP-67 WD 0032−317 WR 102 c ea ka WR 101-2 WR 104 WR 110 WR 111 WR 112 XTE J1748−288 XTE J1807−294 XTE J1810−197 XTE J1814−338

Exoplanets	HATS-36b HD 164604 b HD 181720 b K2-2016-BLG-0005Lb KMT-2022-BLG-0440L b MOA-2007-BLG-192Lb MOA-2007-BLG-400Lb MOA-2009-BLG-387Lb MOA-2010-BLG-477Lb OGLE-2003-BLG-235Lb OGLE-2005-BLG-169Lb OGLE-2018-BLG-1119Lb OGLE-TR-10b OGLE-TR-56b SWIFT J1756.9-2508 b

Star
clusters

NGC	NGC 6440 NGC 6520 NGC 6522 NGC 6528 NGC 6530 NGC 6540 NGC 6544 NGC 6553 NGC 6558 NGC 6569 NGC 6603 NGC 6624 NGC 6638 NGC 6642 NGC 6717 NGC 6723
Other	1806−20 cluster 2MASS-GC02 Arches Cluster CO-0.40-0.22 Messier 18 Messier 21 Messier 22 Messier 23 Messier 25 Messier 28 Messier 54 Messier 55 Messier 69 Messier 70 Messier 75 Quintuplet cluster Small Sagittarius Star Cloud

Nebulae

NGC	NGC 6822 (Bubble Nebula) NGC 6445 NGC 6559 NGC 6563 NGC 6565 NGC 6578 NGC 6589 NGC 6590 NGC 6818
Other	Barnard 92 G1.9+0.3 HCN-0.009-0.044 Lagoon Nebula Large Molecule Heimat M 1-42 M2-42 Omega Nebula Pistol Nebula Red Spider Nebula Ring Nebula Trifid Nebula

Galaxies

NGC	NGC 6822 NGC 6902
Other	PKS 2000-330 Sagittarius Dwarf Irregular Galaxy Sagittarius Dwarf Spheroidal Galaxy

Galaxy clusters	ESO 593-8

Astronomical events	GLEAM-X J162759.5−523504.3 MOA-2007-BLG-192L MOA-2007-BLG-400L MOA-2009-BLG-387L MOA-2010-BLG-477L MOA-2011-BLG-262L OGLE-2003-BLG-235L OGLE-2005-BLG-169L Nova Sagittarii 1982