X - ray / Optical Bursts from GS 1826 – 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 June 1998. 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 ± 0.5 kpc, 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 disk into the optical. This provides support for the recently proposed orbital period of ∼2 h. We also present an ASCA observation from March 1998, during which two X-ray bursts were detected.