2001A&A...368..484S

The results of more than 8-yr monitoring (1988-1997) of the Wolf-Rayet binary WR 147 (WN8(h)+OB) with the Westerbork Synthesis Radio Telescope (WSRT) are presented. When the strong winds of the Wolf-Rayet (WR) and OB binary components collide, they produce non-thermal excess radiation in the region where the two winds interact. The binary system, monitored at 1.4 and 5GHz (21 and 6cm), is not resolved by the WSRT, thus we observed the total flux density of the system. The time-averaged 5 and 1.4-GHz flux densities are 35.4±0.4mJy and 26.4±0.3mJy, respectively. These give a time-averaged spectral index of α_5–1.4GHz≃0.23±0.04, where S_ν∝ν^α. The departure from the value expected for thermal radiation from a spherically symmetric stellar wind, α=0.6, can be attributed to non-thermal emission from a bow-shaped source to the north of the thermal source associated with the WN8 star. With a possible detection at 350 MHz of 16±4mJy, in our separate study of the Cygnus region, the spectral energy distribution, after the contribution of the southern thermal source is subtracted, can be fitted by a synchrotron emission model which includes free-free absorption. The non-thermal emission originates in the region where the winds of the binary components collide. This region, therefore, contains a mixture of relativistic particles accelerated by shocks and thermal particles, responsible for the free-free absorption. We show, in a simplified model of the system, that additional free-free absorption may occur when the line of sight to the collision region passes through the radiophotosphere of the WR wind. The 1.4-GHz flux density of WR 147 varied between ∼20 mJy and ∼30 mJy. We attribute the irregular, stochastic variations with a typical timescale of about 60 days to inhomogeneities in the wind, with different mechanisms involved in the flux-density increase than in the flux-density decrease. A flux-density increase results when the inhomogeneities in the wind/clumps enter the wind collision region, fuelling the synchrotron emission. The typical timescale of the flux-density decrease is shorter than the timescale of synchrotron loss (∼10³yr) or the Inverse-Compton lifetime (≃4.5yr), but of the order of the flow time in the colliding-wind region (∼80d). Therefore, we suggest that the flux-density decrease is due to plasma outflow from the system. Furthermore, variable free-free absorption due to large clumps passing the line of sight may also cause variations in the flux density. We observe a possible long-term flux-density variation on top of the stochastic variation. This variation is fitted with a sinusoid with a ∼7.9-yr period, with a reduced χ² of 1.9. However, as the period of the sinusoid is too close to the monitoring time span, further monitoring is needed to confirm this long-term variation.