Flaring and quiescent infrared behaviour of Cygnus X-3

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Abstract
We present new infrared observations of the exotic X-ray binary system Cygnus X-3, including high-time-resolution simultaneous H- and K-band photometry over ∼1.5 orbital periods and 1–5 µm photometry during both periods of apparent infrared quiescence and flaring activity. The simultaneous H- and K-band observations reveal the 4.8-h orbital modulation of the source, superimposed upon which are a number of extremely rapid flare events. We find rise times for these events of ≤ 15 s, and find that the source becomes redder in (H − K) during the flares. This latter effect is confirmed by 1–5 µm photometry during a period of prolonged quasi-stable flaring. We model this reddening in colour during flaring in terms of optically thin free—free emission from a hot dense plasma, possibly associated with a thermal gas in the high-velocity radio jets revealed by radio VLBI. Size, luminosity and lack of self-absorption considerations allow us to place strong constraints on possible values of Ne and T for this plasma, and we find that the dominant cooling mechanism is likely to be bremsstrahlung cooling. Dereddening of new 1–5 µm data combined with 0.7–1.0 µm results from the literature is consistent with an estimated infrared extinction to the source of 4.5 ≤ AJ ≤ 7.5 mag, and an infrared excess above a blackbody stellar continuum which is inherent to the source.

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