Cygnus X-3 in an ``Ultrahigh'' X-Ray State with No Detected K alpha Line Emission

Cyg 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 in data from OSO 8, Ariel S, HEAO 1 A-2, the EXOSAT GSPC, Tenma, and Ginga, with measured physical widths (FWHM) ranging from 0.6 to 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 luminosity of ∼2 × 10³⁸ ergs s^-1 (1-10 keV; at 10 kpc), above the Eddington limit. The continuum is well described by a blackbody with temperature kT = 1.13±0.02 keV, constituting 93% of the flux, plus an additional soft component. 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. Ninety percent confidence limits are typically 20 eV lower.

We discuss these results in the context of the various physical models for Cyg X-3, and conclude that the high X-ray luminosity can be attributed to an increase in the blackbody component from the neutron star surface and inner accretion disk. We speculate that an increase in the mass-accretion rate may have caused the inner disk to become optically and geometrically thick, shielding the outer disk and much of the corona from the bulk of the central X-ray flux. Alternatively, a decrease in the extent of the azimuthal structure on the disk edge may have resulted in greater visibility of the central X-ray emitting region.