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Register a new userX-ray emission from the double-binary OB-star system QZ Car (HD 93206)
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X-ray observations of the double-binary OB-star system QZ Car (HD 93206) obtained with the Chandra X-ray Observatory over a period of roughly 2 years are presented. The orbit of systems A (O9.7 I+b2 v, PA = 21 d) and B (O8 III+o9 v, PB = 6 d) are reasonably well sampled by the observations, allowing the origin of the X-ray emission to be examined in detail. The X-ray spectra can be well fitted by an attenuated three temperature thermal plasma model, characterised by cool, moderate, and hot plasma components at kT ~ 0.2, 0.7, and 2 keV, respectively, and a circumstellar absorption of ~ 0.2 x 10^22 cm-2. Although the hot plasma component could be indicating the presence of wind-wind collision shocks in the system, the model fluxes calculated from spectral fits, with an average value of ~ 7 x 10^-13 erg s-1 cm-2, do not show a clear correlation with the orbits of the two constituent binaries. A semi-analytical model of QZ Car reveals that a stable momentum balance may not be established in either system A or B. Yet, despite this, system B is expected to produce an observed X-ray flux well in excess of the observations. If one considers the wind of the O8 III star to be disrupted by mass transfer the model and observations are in far better agreement, which lends support to the previous suggestion of mass-transfer in the O8 III + o9 v binary. We conclude that the X-ray emission from QZ Car can be reasonably well accounted for by a combination of contributions mainly from the single stars and the mutual wind-wind collision between systems A and B.
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Aims. Colliding wind binary systems have long been suspected to be high-energy (HE; 100 MeV < E < 100 GeV) {gamma}-ray emitters. {eta} Car is the most prominent member of this object class and is confirmed to emit phase-locked HE {gamma} rays from hundreds of MeV to ~100 GeV energies. This work aims to search for and characterise the very-high-energy (VHE; E >100 GeV) {gamma}-ray emission from {eta} Car around the last periastron passage in 2014 with the ground-based High Energy Stereoscopic System (H.E.S.S.). Methods. The region around {eta} Car was observed with H.E.S.S. between orbital phase p = 0.78 - 1.10, with a closer sampling at p {approx} 0.95 and p {approx} 1.10 (assuming a period of 2023 days). Optimised hardware settings as well as adjustments to the data reduction, reconstruction, and signal selection were needed to suppress and take into account the strong, extended, and inhomogeneous night sky background (NSB) in the {eta} Car field of view. Tailored run-wise Monte-Carlo simulations (RWS) were required to accurately treat the additional noise from NSB photons in the instrument response functions. Results. H.E.S.S. detected VHE {gamma}-ray emission from the direction of {eta} Car shortly before and after the minimum in the X-ray light-curve close to periastron. Using the point spread function provided by RWS, the reconstructed signal is point-like and the spectrum is best described by a power law. The overall flux and spectral index in VHE {gamma} rays agree within statistical and systematic errors before and after periastron. The {gamma}-ray spectrum extends up to at least ~400 GeV. This implies a maximum magnetic field in a leptonic scenario in the emission region of 0.5 Gauss. No indication for phase-locked flux variations is detected in the H.E.S.S. data.
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