GS2023+338: a new class of X-ray transient source?

TRANSIENT X-ray sources are generally considered to comprise accreting binary systems containing either a black hole or a neutron star^1-5. 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 10^21.7-10^22.8 H atoms cm^-2. This unusual behaviour remains to be explained.