XTE J1739−285

XTE J1739−285 is a low-mass X-ray binary in the constellation Ophiuchus, situated approximately 13,000 light-years from Earth. It was first observed on 19 October 1999 by NASA's Rossi X-ray Timing Explorer satellite.
It had previously been claimed that XTE J1739−285 was the fastest-spinning celestial body yet known, with a frequency of 1122 Hz. However, a re-analysis of these data by other astronomers has been unable to reproduce this result.
XTE J1739−285 has been proposed as a possible quark star, as well as 3C 58.