4U 1700−37

4U 1700−37 is one of the stronger binary X-ray sources in the sky, and is classified as a high-mass X-ray binary. It was discovered by the Uhuru satellite.^[13] The "4U" designation refers to the fourth (and final) Uhuru catalog.

4U 1700−37

A light curve for V884 Scorpii. The main plot, from TESS data,[1] shows the variation over several orbital periods. The inset plot, adapted from van Paradijs et al. (1984),[2] shows the average variation as a function of orbit phase.
Observation data Epoch J2000      Equinox J2000
Constellation	Scorpius
Right ascension	17h 03m 56.77256s[3]
Declination	−37° 50′ 38.9133″[3]
Apparent magnitude (V)	6.51 (- 6.57) - 6.60[4]
Characteristics
Spectral type	O6Iafcp[5]
U−B color index	−0.727[6]
B−V color index	+0.253[6]
Variable type	Ellipsoidal + HMXB[4]
Astrometry
Radial velocity (Rv)	−75.00±7.4[7] km/s
Proper motion (μ)	RA: 2.414[3] mas/yr Dec.: 5.022[3] mas/yr
Parallax (π)	0.6327±0.0259 mas[3]
Distance	5,200 ± 200 ly (1,580 ± 60 pc)
Absolute magnitude (MV)	−6.52[8]
Orbit[9]
Period (P)	3.4118 ± 0.0001 d
Semi-major axis (a)	35±1 R☉[10]″
Eccentricity (e)	0.17±0.06
Inclination (i)	62±1[10]°
Periastron epoch (T)	JD 2443702.62±0.21
Argument of periastron (ω) (primary)	54±24°
Semi-amplitude (K1) (primary)	10.06 ± 0.66 km/s
Details[11]
O star
Mass	46±5[10] M☉
Radius	21.9+1.3 −0.5 R☉
Luminosity	660,000 L☉
Temperature	35,000±1,000 K
X-ray object
Mass	1.96±0.19[10] M☉
Other designations
V884 Sco, CD−37°11206, HD 153919, HIP 83499, SAO 208356[12]
Database references
SIMBAD	data

The X-ray source is associated with a bright (6.53 V magnitude) blue supergiant star HD 153919, which is orbited by an accreting neutron star.^[10][14] The X-ray source is eclipsed by the star every 3.4 days, but no pulsations have yet been observed. The source is one of the ten brightest persistent X-ray sources in the 10-100 keV hard X-ray energy region.

Evidence of Compton cooling during an X-ray flare recorded by the Chandra X-ray telescope strongly suggests that the compact object is a neutron star;^[14] if verified it would be among the most massive known, and near the boundary of the theoretical maximum.

4U 1700−37 is a runaway system. It has a high velocity of 63±5 km/s with respect to its parent cluster, NGC 6231. It was kicked out of the cluster about 2.2 million years ago by a supernova explosion.^[15]