Abstract
TRANSIENT X-ray sources are generally considered to comprise accreting binary systems containing either a black hole or a neutron star1–5. The transient X-ray emission is thought to result from episodic mass transfer to the compact star; if this is so, transient sources should provide information on the mass-accretion mechanism. On 21 May 1989, the All-Sky Monitor6 (ASM) onboard the satellite Ginga7 discovered an unusual bright X-ray transient source, GS2023+338, in the constellation of Cygnus8. The ASM and Large-Area Counters9 (LAC), also on Ginga, have since monitored the source. Here we report the light curves and spectral evolution observed by the ASM and LAC. From 21 May to 1 June, the intensity varied by up to a factor of 500 on timescales from a few seconds to a few days10. Subsequently the rapid variations persisted, whereas slower ones disappeared. The spectral data fit a power-law model with a photon index of 0.0 to –2.5 and an absorption column density of 1021.7–1022.8 H atoms cm−2. This unusual behaviour remains to be explained.
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Kitamoto, S., Tsunemi, H., Miyamoto, S. et al. GS2023+338: a new class of X-ray transient source?. Nature 342, 518–520 (1989). https://doi.org/10.1038/342518a0
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DOI: https://doi.org/10.1038/342518a0
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