Spectral Evolution of the Bright X-Ray Nova GS 1124-68 (Nova Muscae 1991) Observed with Ginga

Abstract

Abstract A bright X-ray nova, GS 1124-68 (Nova Muscae 1991, GRS 1124-68), which later turned out to be a promising black-hole binary with evidence of large mass, was observed with the Large Area Counters onboard Ginga from its discovery in 1991 January to its quiescent state in 1991 September. The X-ray intensity reached a maximum (~ 8 Crab) eight days after its discovery, then decreased exponentially with an e-folding time of about ~ 30 d. Temporary increases in the luminosity were found ~ 80 d and ~ 200 d after the outburst. The source was last detected at ~ 2 mCrab on the 239th day after the outburst; although subsequent observations were carried out on days 260 and 282, the source was below the detection limit (~ 0.3 mCrab). GS 1124-68 exhibited such a drastic state transition that was very reminiscent of the high-low transition of black-hole candidates Cyg X-1 and GX 339-4, between the 131st day (May 18) and 157th day (June 13) after the outburst. Before the transition, the energy spectrum was characterized by a thermal soft component which was dominant below ~ 8 keV, and a power-law like hard-tail component, which was significant above that energy. After the transition, the energy spectrum became much harder. The spectrum was approximately represented by a single power-law function with a photon index of ~ 1.7. Intense short time variations, which were quite similar to those in the low state of Cyg X-1, GX 339-4 and GS 2023+338, became prominent down to a timescale of milliseconds. The outburst mechanism, the origin of the soft component and the state transition can be favorably explained in terms of an optically thick accretion-disk model around a ~ 6M black-hole at a distance of ~ 2.5 kpc.