The Flared Disc Project: RXTE and ASCA observations of X 1822--371

Abstract

We present archival Rossi X-ray Timing Explorer (RXTE) and simultaneous Advanced Satellite for Cosmology and Astrophysics (ASCA) data of the eclipsing low mass X-ray binary (LMXB) X 1822-371. Our spectral analysis shows that a variety of simple models can fit the spectra relatively well. Of these models, we explore two in detail through phase-resolved fits. These two models represent the case of a very optically thick and a very optically thin corona. While systematic residuals remain at high energies, the overall spectral shape is well approximated. The same two basic models are fitted to the X-ray light curve, which shows sinusoidal modulations interpreted as absorption by an opaque disc rim of varying height. The geometry we infer from these fits is consistent with previous studies: the disc rim reaches out to the tidal truncation radius, while the radius of the corona (approximated as spherical) is very close to the circularization radius. Timing analysis of the RXTE data shows a time-lag from hard to soft consistent with the coronal size inferred from the fits. Neither the spectra nor the light curve fits allow us to rule out either model, leaving a key ingredient of the X 1822-371 puzzle unsolved. Furthermore, while previous studies were consistent with the central object being a 1.4 M⊙ neutron star, which has been adopted as the best guess scenario for this system, our light curve fits show that a white dwarf or black hole primary can work just as well. Based on previously published estimates of the orbital evolution of X 1822-371, however, we suggest that this system contains either a neutron star or a low mass (≲2.5 M⊙) black hole and is in a transitional state of duration shortward of 107 yr.