On the radio emitting high mass X-ray binary LS 5039

¹ Astronomy Centre, CPES, University of Sussex, Brighton, BN1 9QH, UK
² Department of Physics and Astronomy, University College London, Gower Street, London, WC1E 6BT, England, UK
³ Physics Department, University of Crete, PO Box 2208, 71003, Heraklion, Greece
⁴ Foundation for Research and Technology-Hellas, 71110, Heraklion, Greece
⁵ Department of Physics and Astronomy, University of Wales, Cardiff, CF24 3YB, Wales, UK
⁶ Astronomical Institute of St. Petersburg University, St. Petersburg 198904, Russia
⁷ Isaac Newton Institute of Chile, St. Petersburg Branch
⁸ Departmento de Astronomia, Universidad de Valencia, 46100 Burjassot, Valencia, Spain
⁹ Department of Physics & Astronomy, University of Southampton, Highfield, Southampton, SO17 1BJ, UK
¹⁰ Observatoire de Strasbourg, 11 rue de l'Université, 67000 Strasbourg, France
¹¹ Astrophysics Research Institute, Liverpool John Moores University, Liverpool, L41 1LD, UK

Corresponding author: J. S. Clark, jsc@star.ucl.ac.uk

Received: 24 April 2001
Accepted: 26 June 2001

Abstract

We present new optical -near-IR spectroscopic and photometric observations of the newly discovered galactic microquasar LS 5039, which indicate a classification for the mass donor in the system of O6.5V((f)). Optical spectroscopy and photometry shows no variability over a timescale of years, and we find no evidence of modulation by, or emission from the compact companion in these data. However significant photometric variability ( mag) is present in the H and K bands between 1995-2000. Such variability has been observed in other radio bright X-ray binaries where it has been attributed to synchrotron emission from the jet. However, given the non-thermal spectral index of the radio emission in LS 5039 this explanation appears unlikely, predicting a near-IR flux ~3 orders of magnitude too small to contribute significantly at such wavelengths. Nightly optical photometry over a 21 day period between 2000 May-June reveals variability at a level of a few hundredths of a magnitude, with no periodicity or long term trend visible. Likewise, while the radio lightcurves show moderate variability ( per cent of the mean flux density) we find no evidence of periodic modulation -Monte Carlo simulations constrain any such periodic variability to <4 per cent modulation at 2.25 GHz. The differences in behaviour between LS 5039 and Cygnus X-1 -the most closely related radio emitting High Mass X-ray Binary -are likely to be a result of the weaker stellar wind and probable greater orbital separation of LS 5039 compared to Cyg X-1.