[12] The "4U" designation refers to the fourth (and final) Uhuru catalog.

The X-ray source is associated with a bright (6.53 V magnitude) blue supergiant 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.

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;[13] if verified it would be among the most massive known, and near the boundary of the theoretical maximum.

It was kicked out of the cluster about 2.2 million years ago by a supernova explosion.