We have measured the precise position of the 38 s eclipsing X-ray pulsar OAO 1657 - 415 with the Chandra X-Ray Observatory : \alpha ( J2000 ) = 17 ^ { h } 00 ^ { m } 48 \fs 90 , \delta ( J2000 ) = -41 ^ { \circ } 39′21 \farcs 6 , error radius = 0 \farcs 5 .
Based on the previously measured pulsar mass function and X-ray eclipse duration , this 10.4-d high-mass X-ray binary is believed to contain a B supergiant companion .
Deep optical imaging of the field did not detect any stars at the Chandra source position , setting a limit of V > 23 .
However , near-infrared imaging revealed a relatively bright star ( J = 14.1 , H = 11.9 , K _ { s } = 10.7 ) coincident with the Chandra position , and we identify this star as the infrared counterpart of OAO 1657 - 415 .
The infrared colors and magnitudes and the optical non-detections for this star are all consistent with a highly reddened B supergiant ( A _ { V } = 20.4 \pm 1.3 ) at a distance of 6.4 \pm 1.5 kpc .
This implies an X-ray luminosity of 3 \times 10 ^ { 36 } erg s ^ { -1 } ( 2–10 keV ) .
Infrared spectroscopy can verify the spectral type of the companion and measure its radial velocity curve , yielding a neutron star mass measurement .