X-ray Observations of Bow Shocks around Runaway O Stars. The case of $ζ$ Oph and BD+433654

J. A. Toalá, L. M. Oskinova, A. González-Gaán, M. A. Guerrero, R. Ignace, M. Pohl

Published 2016-02-25Version 1

Non-thermal radiation has been predicted within bow shocks around runaway stars by recent theoretical works. We present X-ray observations towards the runaway stars $\zeta$ Oph (Chandra and Suzaku) and BD+433654 (XMM-Newton) to search for the presence of non-thermal X-ray emission. We found no evidence of non-thermal emission spatially coincident with the bow shocks, nonetheless, diffuse emission is detected in the vicinity of $\zeta$ Oph. After a careful analysis of its spectral characteristics we conclude that this emission has a thermal nature with a plasma temperature of $T \approx 2 \times10^{6}$ K. The cometary shape of this emission seems to be in line with recent predictions of radiation-hydrodynamic models of runaway stars. The case of BD+433654 is puzzling as non-thermal emission has been reported in a previous work for this source.