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Astronomy & Astrophysics

Quasi-simultaneous XMM-Newton and VLA observation of the
non-thermal radio emitter HD168112 (O5.5III(f+))#

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 et de Géophysique - Université de Liège, Allée du 6 Août, Bât B5c, B-4000 Liège (Sart Tilman), Belgium
2 Royal Observatory of Belgium, Avenue Circulaire 3, B-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, United Kingdom
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, United Kingdom
7 European Space Agency, Vilspa, Apartado 50727, 28080 Madrid, Spain

* Research Associate FNRS, Belgium
** Research Fellow FNRS, Belgium

# Based on observations with XMM-Newton, an ESA Science Mission with instruments and contributions directly funded by ESA Member states and the USA (NASA). Also based on observations collected with the VLA, an instrument of the National Radio Astronomy Observatory, which is a facility of the National Science Fondation operated by the Associated Universities Inc.. Optical data were collected at the European Southern Observatory (La Silla, Chile) and at the Observatorio Astronómico Nacional of San Pedro Mártyr (Mexico).


     We report the results of a multiwavelength study of the non-thermal radio emitter HD168112 (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 HD168112 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 HD168112 is indeed a binary, it must be seen under a fairly low inclination.

Key Words
Radiation mechanisms: non-thermal -- Stars: early-type -- Stars: individual: HD168112 -- Stars: winds, outflows -- X-rays: stars -- Radio continuum: stars

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