Revisit to the ASCA Data for GRO J1655-40: Hardening Factor in the Disk

A practical process of determining the hardening (color correction) factor from observational data is proposed and applied to four ASCA observations of GRO J1655-40. The last observation covered a whole orbit period of the binary system, which enables us to study the phase-resolved hardening factor. We found that the color correction factor does not change with orbital phase, whereas it may vary significantly (1.3 -- 2.6) between different observations and the finally corrected physical inner disk radius remains reasonably stable over a large range of luminosity. We applied GR corrections and estimated the angular momentum of the source. We constrained the specific angular momentum in the range between 0.83 and 1, which is consistent with that of an extremal Kerr black hole.