DISCOVERY AND X-RAY-PROPERTIES OF GS-1124-683 (=NOVA MUSCAE)

A bright X-ray nova: GS 1124-683 was discovered by the All Sky Monitor (ASM) on board Ginga on 1991 January 8. X-ray properties observed with the ASM from 1991 January 5 to the end of August are presented. The observed spectra were roughly described by a two-component model consisting of a power-law (a hard X-ray) and a disk-blackbody or a blackbody (a soft X-ray) component. In its rising phase, the hard X-ray component appeared first and the soft X-ray component followed with increase of the temperature and the emission area. After the maximum, the intensity of the energy range 1-6 keV decayed exponentially with a time constant of 31.2 +/- 0.1 days, whereas the hard component fell off more rapidly with a time constant of approximately 13 days. The state where the hard component has an appreciable fraction of the energy flux can be recognized as a very high state, which is observed in GX 339-4. With the decay of the hard X-ray component, GS 1124-683 would transfer to the so-called high state. The decay of the soft component can be interpreted to be due to the decrease of temperature of the soft X-ray component. On 70 days after the maximum, a hump of the intensity was observed. About 140 days after the maximum the spectrum becomes hard and would change into the so-called low state. Similar behaviors are observed from two ultrasoft transient sources, A0620-00 and GS 2000+25, therefore such behaviors are considered to be common for bright ultrasoft transient sources.