Discovery of absorption features of the ADC and systematic acceleration of the X-ray burst rate in XB 1323-619

Abstract

We present results from analysis of the observation of the dipping, quasi-periodic bursting LMXB XB 1323-619 made with XMM-Newton in Jan., 2003. A number of absorption lines were discovered in the spectrum, notably at 6.70 and 6.98 keV which we identify with scattering by ions Fe XXV and Fe XXVI. Such features have been seen in other dipping sources, but their origin has not been understood. Curve of growth analysis provided a consistent solution assuming collisional ionization with kT=45 keV, close to the electron temperature we previously determined for the ADC in this source. We thus propose that the absorption lines in the dipping LMXB are produced in the ADC. Spectral evolution in dipping was well-described by the progressive covering model which we have previously shown to give very good explanations of many dipping sources. We discuss the proposal of Boirin et al. (2004b), that spectral evolution in the dipping LMXB may be explained by subjecting the continuum to a highly ionized absorber. This would require a decrease in X-ray intensity by a factor of ~3 in dipping at energies where photoelectric absorption is not effective (40-100 keV) which previous analysis of BeppoSAX data on several sources rules out, e.g. in XB 1323-619, any decrease was less than 10+/-10%. We find a remarkable linear increase in the rate of X-ray bursts with time over the 14-year period since 1989, and a systematic non-linear increase in source luminosity (L). The linear variation of burst rate with L shows that the burst rate is proportional to mass accretion rate, and if continued implies that the gap between bursts will become zero on January 11, 2008. In reality we expect the source to undergo a transition from X-ray bursting to X-ray flaring confirming that flaring is unstable nuclear burning.