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Issue A&A
Volume 420, Number 3, June IV 2004
Page(s) 1061 - 1077
Section Stellar atmospheres
DOI http://dx.doi.org/10.1051/0004-6361:20041030-1



A&A 420, 1061-1077 (2004)
DOI: 10.1051/0004-6361:20041030

Quasi-simultaneous XMM-Newton and VLA observation of the non-thermal radio emitter HD 168112 (O5.5III(f $\mathsf{^+}$))

M. De Becker1, G. Rauw1, R. Blomme2, W. L. Waldron3, H. Sana1, J. M. Pittard4, P. Eenens1, 5, I. R. Stevens6, M. C. Runacres2, S. Van Loo2 and A. M. T. Pollock7

1  Institut d'Astrophysique, Université de Liège, Allée du 6 Août, Bât. B5c, 4000 Liège (Sart Tilman), Belgium
2  Royal Observatory of Belgium, Avenue Circulaire 3, 1180 Brussels, Belgium
3  L-3 Communications Government Services, Inc., 1801 McCormick Drive, Suite 170, Largo, MD 20774, USA
4  Department of Physics & Astronomy, University of Leeds, Leeds LS2 9JT, UK
5  Departamento de Astronomia, Universidad de Guanajuato, Apartado 144, 36000 Guanajuato, GTO, Mexico
6  School of Physics & Astronomy, University of Birmingham, Edgbaston Birmingham B15 2TT, UK
7  ESA/Vilspa, Apartado 50727, 28080 Madrid, Spain

(Received 14 January 2004 / Accepted 10 March 2004 )

Abstract
We report the results of a multiwavelength study of the non-thermal radio emitter HD 168112 (O5.5III(f +)). The detailed analysis of two quasi-simultaneous XMM-Newton and VLA observations reveals strong variability of this star both in the X-ray and radio ranges. The X-ray observations separated by five months reveal a decrease of the X-ray flux of ~30%. The radio emission on the other hand increases by a factor 5-7 between the two observations obtained roughly simultaneously with the XMM-Newton pointings. The X-ray data reveal a hard emission that is most likely produced by a thermal plasma at kT ~ 2-3 keV while the VLA data confirm the non-thermal status of this star in the radio waveband. Comparison with archive X-ray and radio data confirms the variability of this source in both wavelength ranges over a yet ill defined time scale. The properties of HD 168112 in the X-ray and radio domain point towards a binary system with a significant eccentricity and an orbital period of a few years. However, our optical spectra reveal no significant changes of the star's radial velocity suggesting that if HD 168112 is indeed a binary, it must be seen under a fairly low inclination.


Key words: radiation mechanisms: non-thermal -- stars: early-type -- X-rays: stars -- radio continuum: stars -- stars: individual: HD 168112

Offprint request: M. De Becker, debecker@astro.ulg.ac.be

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