2012A&A...539A..53R

The optical counterpart of the neutron star X-ray binary and well known X-ray burster, 4U 1636-53 (=4U 1636-536 = V801 Ara) has been studied for three decades. However, no infrared studies have been reported to date. Our aims are to identify and investigate the near-infrared (NIR) counterpart of 4U 1636-53. We present deep, K_S-band (2.2µm) imaging of the region of 4U 1636-53 taken with the Infrared Spectrometer And Array Camera (ISAAC) on the Very Large Telescope. Archival optical and UV data were used to infer the 0.2-2.2µm spectral energy distribution (SED). One star is located at coordinates α=16:40:55.57, δ=-53:45:05.2 (J2000; 1σ positional uncertainty of ∼0.3arcsec), which is consistent with the known optical position of 4U 1636-53; its magnitude is K_S=16.14±0.12. This star is also detected in the 2MASS survey in the J-band and has a magnitude of J=16.65 ±0.22. Assuming that the persistent emission is quite steady, the 0.4-2.2µm de-reddened SED can be described by a power law F_ν∝ν^1.5±0.3, with some possible curvature (F_ν∝ν^<∼1.5) at 0.2-0.4µm. The SED can be approximated by a blackbody of temperature ∼27000K. This is typical for an active low-mass X-ray binary, and the emission can be explained by the outer regions of a (likely irradiated) accretion disc. We therefore interpret this K_S-band star as the NIR counterpart.