Research Note

The near-infrared counterpart of 4U 1636–53^⋆

¹ Astronomical Institute “Anton Pannekoek”, University of Amsterdam, PO Box 94249, 1090 GE Amsterdam, The Netherlands
² Instituto de Astrofísica de Canarias (IAC), vía Láctea s/n, La Laguna 38205, S/C de Tenerife, Spain
e-mail: russell@iac.es
³ Department of Physics, University of California, Santa Barbara, California 93106, USA
e-mail: kobrien@physics.ucsb.edu
⁴ School of Physics and Astronomy, University of Southampton, Southampton, Hampshire SO17 1BJ, UK
e-mail: t.munoz-darias@soton.ac.uk
⁵ INAF – Osservatorio Astronomico di Roma, via Frascati 33, 00040 Monteporzio Catone (Roma), Italy
e-mail: piergiorgio.casella@oa-roma.inaf.it
⁶ ISAS, Japan Aerospace Exploration Agency, 3-1-1 Yoshinodai, chuo-ku, Sagamihara, 229-8510 Kanagawa, Japan
e-mail: pgandhi@astro.isas.jaxa.jp
⁷ Space Research Institute, Russian Academy of Sciences, Profsoyuznaya 84/32, 117997 Moscow, Russia
e-mail: revnivtsev@iki.rssi.ru

Received: 6 September 2011
Accepted: 6 January 2012

Abstract

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, K_S-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 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 0.4−2.2   μm 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 0.2−0.4   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 K_S-band star as the NIR counterpart.