ASCA OBSERVATIONS OF THE IRON LINE STRUCTURE IN CYGNUS X-1

The black hole candidate Cyg X-l was observed with the ASCA satellite on multiple occasions in 1993 and 1994 for a total exposure of about 7 days. The energy resolution of the ASCA SIS (Delta E/E approximate to 2% at 6 keV) allows the spectral structure in the iron K band to be s tudied in greater detail than before. We detected an iron emission lin e at 6.4 keV in almost all the data with an intrinsic line width of le ss than or similar to 0.2 keV (1 sigma) and an equivalent width of 10- 30 eV. The line parameters show no clear orbital phase dependence. A b road edge feature was observed at E greater than or similar to 7 keV i n all the spectra. Both the narrow iron emission-line and broad edge f eatures are consistent with disk reflection from the outer parts of an ionized accretion disk where the most populous iron ion species is Fe XI.