X-ray/Optical Bursts from GS 1826-24

Abstract

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 but not fpr photospheric radius expansion. However, we may still derive an upper limit for its distance of $7.5\pm0.5$ kpc, assuming a peak flux of 2.8\times 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.