Observations of Cygnus X-3 in Quiescence in the $2.4 - 12 \mu$m Range with ISO

Abstract.

We present mid-infrared spectrophotometric and imaging photometric results obtained with ISO on the peculiar X-ray binary Cygnus X-3. The observations covered the orbital phases 0.83 to 1.04, during quiescence. The flux observed with ISOCAM at $11.5\mu$m is $3.51\pm 0.73$ mJy, in good agreement with that observed in the K and mm ranges, during quiescence (Fender et al, 1999; Ogley et al, 1999). Spectrophotometry with ISOPHOT-S in the range $2.4 - 12 \mu$m shows an unresolved line at about $4 \mu$m, peaking at $61\pm 10$ mJy above a mean flux in the $3.5 - 4.5 \mu$m range of $9.0 \pm 3.2$ mJy; it is interpreted as the expected HeII(10-8) line at $4.05 \mu$m.