Spectral and timing properties of the black hole candidate X1755−338 observed in 1989–1995

We report results of an analysis of the black hole (BH) candidate source X1755-338 in 1989, 1990, and 1991 with Ginga, and in 1995 with ASCA. The spectra were well represented by a model consisting of a soft thermal emission from an accretion disk and a hard X-ray tail. The normalization of the multi-color disk model, relating to the inner disk radius, was similar to each other. The unabsorbed X-ray fluxes from the disk component in the 0.01-10 keV band were estimated to be 1.3x10^{-9}, 3.0x10^{-9}, 9.8x10^{-10}, and 2.4x10^{-9} erg s^{-1} cm^{-2} in 1989, 1990, 1991, and 1995, respectively, and are proportional to kT_{in}^4, where kT_{in} is a temperature at the inner disk radius. Based on the standard accretion disk model for a non-rotating BH, our results suggest either a small BH mass or a large inclination angle. Otherwise, X1755-338 is a rotating BH. The hard X-ray intensity was found to be variable, while the soft X-ray intensity was stable. Although the previous work showed the existence of an iron line at 6.7 keV, no clear iron line feature was found in all the spectra. We infer that most of the iron line flux reported in the previous work was due to contamination of the Galactic diffuse X-ray emission.