Volume 376, Number 2, September II 2001
|Page(s)||476 - 483|
|Section||Interstellar and circumstellar matter|
|Published online||15 September 2001|
On the radio emitting high mass X-ray binary LS 5039
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, firstname.lastname@example.org
Accepted: 26 June 2001
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.
Key words: stars: early-type / stars: individual: LS 5039 / X-rays: binaries
© ESO, 2001
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