Astronomy and Astrophysics – Astronomy
Scientific paper
Aug 1994
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1994pasj...46..375e&link_type=abstract
PASJ: Publications of the Astronomical Society of Japan (ISSN 0004-6264), vol. 46, no. 4, p. 375-394
Astronomy and Astrophysics
Astronomy
209
Accretion Disks, Black Holes (Astronomy), Energy Spectra, Novae, Optical Thickness, Stellar Luminosity, X Ray Astronomy, X Ray Binaries, X Ray Spectra, Astronomical Photometry, Astronomical Spectroscopy, Counters, Ginga Satellite
Scientific paper
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 (LAC) onboard Ginga from its discovery in 1991 January to its quiescent state in 1991 September. The X-ray intensity reached a maximum (approximately 8 Crab) eight days after its discovery, then decreased exponentially with an e-folding time of about approximately 30 d. Temporary increases in the luminosity were found approximately 80 d and approximately 200 d after the outburst. The source was last detected at approximately 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 (approximately 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 approximately 8 keV, and a power-law like hard-tail component, which was significant above that energy. After the transistion, the energy spectrum became much harder. The spectrum was approximately represented by a single power-law function with a photon index of approximately 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 an approximately 6 solar mass black-hole at a distance of approximately 2.5 kpc.
Aoki Takashi
Dotani Tadayasu
Ebisawa Ken
Hayashida Kiyoshi
Iga Sayuri
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