WSRT 1.4 and 5-GHz light curves for WR 147 (AS 431, WN8(h)+OB)

Abstract

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 5 GHz (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 +/-0.4 mJy and 26.4 +/- 0.3 mJy, respectively These give a time-averaged spectral index of alpha (5-1.4 GHz) approximate to 0.23 +/- 0.04, where S-nu proportional to nu (alpha). The departure from the value expected for thermal radiation from a spherically symmetric stellar wind, alpha = 0.6, call 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 WHz of 16 +/- mJy, 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 nonthermal 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 similar to 20 mJy and similar to 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 (similar to 10(3) yr) or the Inverse-Compton lifetime (approximate to4.5 yr), but of the order of the Row time in the colliding-wind region (similar to 80 d). 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 oil top of the stochastic variation. This variation is fitted with a sinusoid with a similar to7.9-yr period, with a reduced chi (2) 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 lon-term variation.