2002ApJ...573..789C

We have measured the precise position of the 38 s eclipsing X-ray pulsar OAO 1657-415 with the Chandra X-Ray Observatory: α(J2000)=17^h00^m48{fs}90, δ(J2000)=-41°39'21".6, errorradius=0".5. Based on the previously measured pulsar mass function and X-ray eclipse duration, this 10.4 day 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 nondetections for this star are all consistent with a highly reddened B supergiant (A_V=20.4±1.3) at a distance of 6.4±1.5 kpc. This implies an X-ray luminosity of 3x10³⁶ ergs.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.