The optical counterpart of the binary X-ray pulsar 4U 1538 - 52

An early B supergiant star with Hα emission is identified with the binary pulsating X-ray source 4U 1538 – 52. The star is reddened with $$E(B-V)\sim2.4$$ and is at a distance of ∼ 5.5 kpc. The X-ray source has a 2−20 keV luminosity of ∼ 4 × 10³⁶ erg/s. There is evidence that the star is undergoing moderate mass loss.