ROSAT observations of γ Velorum (WC8+O9I). I. The discovery of colliding-wind X-ray emission.

Details ROSAT observations of γ Velorum (WC8+O9I). I. The discovery of colliding-wind X-ray emission. ROSAT observations of γ Velorum (WC8+O9I). I. The discovery of colliding-wind X-ray emission.

Jun 1995

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1995a%26a...298..549w&link_type=abstract

Astronomy and Astrophysics, v.298, p.549

Astronomy and Astrophysics

Astrophysics

62

Stars: Wolf-Rayet, X-Rays: Stars, Stars: Mass Loss, Binaries: Close, Stars: Individual: {Gamma} Vel

Scientific paper

We present an analysis of 13 X-ray (0.1-2.5keV) observations of γ Velorum obtained with the XRT+PSPC on the ROSAT satellite. The first set of 10 observations were obtained in our Guest Observer AO2 programme, with 9 at roughly 8-day intervals in the 78^d^.5 binary period, providing nearly complete phase coverage. Of these, the last two (secured at phases .48 and .53) show respectively, a factor of 2 and 4 increase in X-ray emission, compared to the (relatively) constant flux at other phases. The increase in flux at phase .5 in our AO2 data is due to an additional, harder component superimposed on the (largely unvarying) soft X-ray flux reflecting `low-state' emission. Rough spectral fitting suggests that this additional harder component has kT>=2keV and L_x_>=10^32^erg/s, but these cannot be fully constrained within the limited energy range of the PSPC. Three further PSPC observations, secured in a Target of Opportunity programme during AO3, confirm that the increase in X-ray emission at φ~.5 is cycle/phase repeatable. We conclude that this is due to emission produced in the wind collision, observed through a cavity in the WC8 wind around the O-star. The derived, well-defined light curve of this colliding-wind X-ray emission is asymmetric, due to orbital motion effects, with a half-opening angle of ~25^o^. Spectral fitting to the `low-state' PSPC data yields: log N_H_=20.2+/-0.2 and kT=0.19keV (=T=2.210^6^K). The derived N_H_ is consistent with purely ISM absorption, for which log N_H_=19.9+/-0.1 is derived from IUE Lyman-α data. Adopting a distance of 0.5kpc, yields a mean `low-state` X-ray luminosity of 2.5x10^31^erg/s. We consider this low-state X-ray emission arises in shocked material in the WC8 wind caused by radiatively-driven instabilities. The ROSAT observations show that γ Velorum is located in a cluster of (at least) 6 resolved sources within the central 3x5'of the FOV. The total PSPC flux of these other sources is comparable to the `low-state' PSPC flux for γ Velorum alone. These cluster sources were not resolved in previous Einstein IPC X-ray data, and they clearly contributed to previous estimates of the luminosity and spectrum assigned to γ Velorum in the 0.4-4.5keV range. We have calculated state-of-the-art hydrodynamical models of colliding winds in the γ Velorum system. These models predict copious X-ray emission with kT~3-5keV, consistent with, but not constrained by the soft ROSAT data available. Our modelling shows: (a) a pronounced cavity and high temperatures shocked region does form around the O-star, (b) the predicted half-opening angle is ~20-30^o^, (c) at ROSAT energies, wind-collision X-ray emission should only be seen around phase .5, with a luminosity ~10^33^erg/s (strongly dependent on the presumed O-star wind velocity at the interface region). Within current uncertainties in the binary system parameters we conclude that our model results are fully compatible with the observed X-ray light curve. Further, harder X-ray observations of the colliding wind emission we have discovered in γ Velorum are needed to quantify its spectrum, luminosity and absorption, and provide important tests and constraints on the complex theory of the colliding wind phenomenon. We have recently been awarded time on the ASCA satellite to secure these data.

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