Cygnus X-3 with ISO: investigating the wind

Abstract

We have observed the energetic binary Cygnus X-3 in both quiescent and flaring states between 4 and 16μm using the ISO satellite. We find that the quiescent source shows the thermal free-free spectrum typical of a hot, fast stellar wind, such as from a massive helium star. The quiescent mass-loss rate arising from a spherically symmetric, non-accelerating wind is found to be in the range (0.4-2.9)×10^-4M_solaryr^-1, consistent with other infrared and radio observations, but considerably larger than the 10^-5M_solaryr^-1 deduced from both the orbital change and the X-ray column density. There is rapid, large-amplitude flaring at 4.5 and 11.5μm at the same time as enhanced radio and X-ray activity, with the infrared spectrum apparently becoming flatter in the flaring state. We believe that non-thermal processes are operating, perhaps along with enhanced thermal emission.