2000A&A...356..676S

The Wolf-Rayet star WR 146 (HM19-3, WC6+O) is the brightest WR star at radio wavelengths. We have been monitoring this system with the Westerbork Synthesis Radio Telescope (WSRT) at 1.4 and 5GHz (21 and 6cm) since 1989. The time-averaged spectral index α_5–1.4GHz≃-0.62 clearly points to a domination by non-thermal radiation, which we associate with colliding winds in this binary system. The non-thermal radio flux distribution shows a turn-over at low frequency, which we suggest to be due to free-free absorption of the synchrotron emission from the colliding wind region by plasma around the system. In the period 1989-1997 the average 1.4-GHz flux density increased from ∼61 to ∼73mJy; in the the period 1989-1999 the average 5-GHz flux density increased from ∼29 to ∼37mJy. The light-curves show three different kinds of variations: (i) a slow linear rise in a time-span of a decade; (ii) a 3.38yr periodic variation; and, (iii) rapid non-periodic variations on a time-scale of weeks. We examine whether the slow rise of the flux density could be explained by decreasing free-free absorption in the line-of-sight through the radiophotosphere of the O component, while moving in an eccentric orbit around the WR component. However, the similarity of the amplitudes (∼22% in 10yr) of the rises at 1.4 and 5GHz argues against a change in free-free absorption, expected to be strongly wavelength dependent. This points to an intrinsic flux-density variation, possibly due to modulation of the magnetic field strength resulting from orbital motion in a very-long-period eccentric binary system. The relation between the flux-density increase and orbital motion is supported by positional measurements of the 5-GHz data. We detect a possible motion of the shock zone relative to one of the control sources (ControlA) of ∼0.05'' in the 10yr observing span. At a distance of 1250pc this motion corresponds to a projected tangential velocity of about 30km/s, which is a plausible orbital velocity for a system like WR 146. Superimposed on the 1.4-GHz slow rise, we find a sinusoidal variation with a period P=3.38±0.02yr and a semi-amplitude of 4.3±0.2mJy. Adopting a distance of 1250pc to the system and a 162mas WR+O separation, we consider the observed 3.38yr period too short to be the WR+O binary period by at least two orders of magnitude. We suggest that the periodic variability is caused by a third, low-mass object, modulating the mass flow and/or the magnetic-field of the O component. Unfortunately, our 5-GHz data are far too few and not adequately spread over the whole phase to confirm that they consistently follow the 3.38yr period found in the 1.4-GHz data. The erratic `micro'-variation in the 1.4-GHz light-curve is about 4σ of the typical 0.5mJy observational uncertainty, on a time-scale of weeks to months. When irregularities in the mass flow (clumps, inhomogeneities and/or turbulence in the O and/or WR star winds) reach the wind collision region, variation in the non-thermal emission can be expected. Such irregularities can also affect the free-free line-of-sight absorption at the lowest observing frequencies.