Astron observations of the Rapid Burster MXB 1730-335 and constraints on burster parameters from spectra of trailing bursts

We report on eight X-ray bursts detected by ASTRON from the Rapid Burster (RB) on 13 and 28 April and 16 August, 1983. Six of them (trailing bursts), with durations of 1.5-2 min, rise times of 5-10 s and intervals of 1-1.5 hours, exhibit spectral softening during the burst decay and may be related to the type I bursts. Two of the bursts (triangle bursts) observed on 28 April at interval of ∼28 min with much longer rise times (30-50 s) and longer durations (≃3 min), do not show distinct spectral softening. Persistent flux from RB on 16 August was estimated as F_p≃(2.0-2.4)×10^-9 erg cm^-2 s^-1. Spectral evolution of two trailing bursts was investigated by fitting their spectra in consecutive time intervals with the blackbody (BB), isothermal scattering photosphere (SP) and thermal bremsstrahlung (TB) models. Around the burst maxima the SP model fits the data best whereas in the burst tails the TB model is generally better. The BB model is worse than at least one of the two others. Interpretation of the burst spectra in terms of the BB radiation leads to improbably small neutron star mass and radius (M<0.86 M_⊙, R_NS<5 km) if the peak luminosity does not exceed the Eddington limit. Interpretation of the spectra around the burst maxima (3-15 s from the burst onset) in terms of an isothermal SP yields reasonable constraints on M, R_NS, and distance D. For instance, for the hydrogen photosphere we obtained M=(1.0-2.1)M_⊙R_NS=(7.1-16.4) km if D=11 kpc. If one postulates M=1.4 M_⊙, then D=(8.5-13) kpc for hydrogen photosphere; if, besides, D=11 kpc, then R_NS=(8.1-13.3) km. It follows also from the SP-interpretation that the photosphere radius may increase up to 20-30 km in maxima of the trailing bursts when the luminosity becomes close to the Eddington luminosity.