4U 1700 37

4U 1700-37 is one of the stronger binary X-ray sources in the sky. It was discovered by the Uhuru satellite. The "4U" designation refers to the Fourth (and final) Uhuru catalog. It is associated with a bright (6.53 V magnitude) supergiant Of star HD 153919, which is orbited by an accreting compact object that must be either a neutron star or a black hole. 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.