No detection of the iron line in Cygnus X--3 by BBXRT

Cygnus X--3 is well-known to be the brightest source of 6.7 keV iron K-line photons in the X-ray sky; this line has been unambiguously detected using OSO-8, Ariel V, HEAO-1 A-2, the EXOSAT PSPC, Tenma and Ginga, with measured physical widths (FWHM) ranging from 0.6--1.0 keV and equivalent widths typically from 0.5 to 1.7 keV. However, when the Shuttle-borne X-ray telescope BBXRT observed this source on 1990 December 5, the line was not detected. The BBXRT data show Cyg X--3 to be in an unusually high, soft state, with a 1--10 keV luminosity of ~ 2times 10(38) erg s(-1) (at 10 kpc). The continuum is well described by a blackbody with temperature kT=1.12+/-0.03 keV, constituting 93% of the flux, plus a soft component which may be associated with the halo emission. No iron line is seen in the spectrum, with formal 99% confidence upper limits on the equivalent width of 160 eV, 120 eV and 75 eV for lines with FWHM=1 keV, 600 eV and 200 eV respectively, for a line in the energy range 6.1--7.1 keV (90% confidence limits are typically 20 eV lower). We discuss these results in the context of the various physical models for the Cygnus X--3 system, and conclude that the high luminosity is attributable to an increase in the blackbody component from the surface of the neutron star or inner accretion disk, due perhaps to an increase in dot M. The blackbody component thus swamps both the usually-observed power law component from the outer accretion disk and the Fe K-line.