X-ray/optical bursts from GS1826-24

We report results from the first simultaneous X-ray (RXTE) and optical (SAAO) observations of the low-mass X-ray binary GS 1826-24 in 1998 June. A type I burst was detected in both X-ray and optical wavelengths. Its energy-dependent profile, energetics and spectral evolution provide evidence for an increase in the X-ray burning area but not for photospheric radius expansion. However, we may still derive an upper limit for its distance of 7.5 ± 0.5kpc, assuming a peak flux of ∼2.8 × 10-8 erg cm-2 s-1. A ∼3-s optical delay with respect to the X-ray burst is also observed, and we infer that this is related to the X-ray reprocessing in the accretion disc. The delay provides additional support for the recently proposed orbital period of ∼2h. We also present an ASCA observation from 1998 March, during which two X-ray bursts were detected.