Low-Energy Line Emission in Cygnus X--2: a Study with ASCA, BBXRT and the Einstein SSS

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Session 102 -- X-Ray Variables, Binaries and Transients
Display presentation, Thursday, 12, 1995, 9:20am - 6:30pm

[102.04] Low-Energy Line Emission in Cygnus X--2: a Study with ASCA, BBXRT and the Einstein SSS

A.~P.~Smale, L.~Angelini (USRA, NASA-GSFC), N.~E.~White (NASA-GSFC), K.~Mitsuda, T.~Dotani (ISAS, Japan)

Cygnus X--2 was observed on 1993 June 18--19 for 29 hours using the ASCA GIS and SIS detectors. The observation covered orbital phases $\phi$=0.99--0.12 of the 9.84-day binary cycle (where $\phi$=0.0 is the inferior conjunction of the neutron star). The deadtime-corrected 2--10 keV luminosity of the source is 9$\times$10$^{37}$ ergs~s$^{-1}$, and irregular dipping activity is observed with dip durations of order 1000s and depths of 15--20\%, superimposed on a smooth longer-term variation of $\sim$8\%. The dips are not associated with an increase in absorption, and we see no overall correlation between hardness and intensity. This temporal and spectral behavior implies that the source was observed in a interval of relatively stable accretion, probably on the Horizontal Branch of its Z-diagram.

The model that best fits the continuum emission consists of a Comptonized component with $kT_{ee}$=1.7 keV and $\tau$=24, plus a blackbody with $kT_{bb}$=0.6 keV, with the blackbody contributing 12\% of the total flux. We observe a strong, low-energy emission feature in the SIS spectrum, which can be modeled using a Gaussian with energy E=1.02$\pm$0.2 keV, FWHM 325$\pm$50 eV, and equivalent width EW=60$\pm$15 eV, but is more likely due to a complex of unresolved Fe L-shell (XVIII-XXIV) lines.

Reanalysis of Cyg X--2 data from BBXRT (December 1990; Smale {\sl et al.}~1993, 410, 796) and the Einstein SSS (June 1979) show that this line emission varies strongly. The BBXRT data show the source on the lower portion of the Normal Branch, with only marginal evidence for E=1 keV emission (EW$\sim$10 eV). The SSS data reveal emission at E=1.0 keV with equivalent width 55 eV, plus an additional feature at E=0.78 keV with EW=20 eV. Collating these findings with other archival (Einstein OGS, EXOSAT) results, we find no clear pattern relating the line emission parameters to source spectrum, phase or intensity.

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