The near-infrared counterpart of 4U 1636-53

Context. 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.

Aims. Our aims are to identify and investigate the near-infrared (NIR) counterpart of 4U 1636-53.

Methods. We present deep, KS-band (2.2 μm) 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 0.2−2.2 μm spectral energy distribution (SED).

Results. One star is located at coordinates α = 16:40:55.57, δ = −53:45:05.2 (J2000; 1σ positional uncertainty of ~0.3 arcsec), which is consistent with the known optical position of 4U 1636-53; its magnitude is KS = 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 0.4−2.2 μm de-reddened SED can be described by a power law Fν ∝ ν1.5 ± 0.3, with some possible curvature (Fν ∝ ν ≲ 1.5) at 0.2−0.4 μm. The SED can be approximated by a blackbody of temperature ~27 000 K. 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 KS-band star as the NIR counterpart.