We present mid-infrared spectrophotometric results obtained with
ISO on the peculiar X-ray binary Cygnus X-3 in quiescence, at
orbital phases 0.83 to 1.04. The 2.4-12 microns continuum radiation observed with ISOPHOT-S can be explained by thermal free-free emission in an expanding wind with, above 6.5 microns, a possible additional black-body component with temperature
T ~ 250 K and radius R ~ 5000 Rsun at 10 kpc, likely due to
thermal emission by circumstellar dust.
The observed brightness and continuum spectrum closely
match that of the Wolf-Rayet star WR 147, a WN8+BO5 binary system, when rescaled at the same 10 kpc distance as Cygnus X-3. A rough mass loss estimate assuming a WN wind gives
~ 8.10^-5 Msun/yr. A line at ~ 4.3 micron with a more than 4.3 sigma detection level, and with a dereddened flux of 126 mJy, is interpreted as the expected He I 3p-3s line at 4.295 microns, a prominent line in the WR 147 spectrum.
These results are consistent with a Wolf-Rayet like companion of
WN8 type to the compact object in Cyg X-3, a later type than
suggested by earlier works. |
