XTE J1739–302: An Unusual New X-Ray Transient

A new X-ray transient, designated XTE J1739-302, was discovered with the proportional counter array on the Rossi X-Ray Timing Explorer in data from 1997 August 12. Although it was the brightest source in the Galactic center region while active (about 3.0×10−9 ergs cm-2 s-1 from 2 to 25 keV), it was only observed on that one day; it was not detectable 9 days earlier or 2 days later. There is no known counterpart at other wavelengths, and its proximity to the Galactic center will make such an identification difficult due to source confusion and extinction. The X-ray spectrum and intensity suggest a giant outburst of a Be/neutron star binary, although no pulsations were observed and the outburst was shorter than is usual from these systems.