Discovery of absorption features of the accretion disc corona 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 low mass X-ray binary XB 1323-619 made with XMM-Newton in 2003 January. In spectral analysis of the non-dip, non-burst EPIC PN spectrum, a number of absorption lines were discovered, notably at 6.70 and 6.98 keV, which we identify with scattering by high ionization state ions Fe XXV and Fe XXVI. Such features have been seen in other dipping sources, but their origin was not understood. Curve of growth analysis provided a consistent solution in which the line ratio was reproduced assuming collisional ionization with kT = 31 keV, close to the electron temperature we previously determined for the accretion disc corona (ADC) in this source. We thus propose that the absorption lines in the dipping low mass X-ray binaries (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), based on analysis of the present observation, that spectral evolution in all of 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 BeppoSAX analysis of several sources firmly rules out. In XB 1323-619, any decrease was less than 10 ± 10 per cent and so the ionized absorber proposal can be ruled out. We find a remarkable linear increase in the rate of X-ray bursts with time over the 14-yr 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 2008 January 11. In reality, we expect the source to undergo a transition from X-ray bursting to X-ray flaring, which would confirm the suggestion we previously made that flaring is unstable nuclear burning effectively consisting of a superposition of X-ray bursts.

Neutral absorber dips in the periodic burster LMXB XB 1323-619 from Suzaku

“…Our spectral fitting results show that dipping consists of a large increase in photoelectric absorption for the Comptonized emission of the ADC, the intrinsic column density increasing from zero to 22 × 10 22 atom cm −2 in deep dipping, this being consistent with covering of an extended ADC source by an extended absorbing bulge in the outer disk as previously found for the dipping sources in general, and for XB 1323-619 in the observations with BeppoSAX (Bałucińska- Church et al 1999), with RXTE (Barnard et al (2001) and XMM (Church et al 2005). The neutron star blackbody is subject to a high column density which quickly removes this component in dipping as expected for a point-source and consistent with the source being covered by the highest column density part of the bulge in the outer disk, rather than an integration across the whole bulge as for the extended Comptonized emission.…”

“…Indeed, it would not be expected that the bulge in the outer disk would cause increased absorption in high ionization states. The line at 6.94 keV, also seen in the observation with XMM (Church et al 2005), can be identified as Fe XXVI K α , and the line at 7.6 keV can be Ni XXVI.…”

“…In the analysis of the XMM observation, absorption at 6.70 ± 0.03 keV was seen and 6.98 ± 0.04 keV, and broad emission at about 6.8 keV (Church et al 2005). Barnard et al (2001) detected an emission line at 6.43±0.21 keV in a RXTE observation of XB 1323-619.…”

Disk emission and atmospheric absorption lines in black hole candidate 4U 1630-472

“…In XB 1323-619 a number of absorption lines (6.7 and 6.9 keV) were discovered in non-dip, non-burst spectra (Church et al 2005). Curve-of-growth analysis has provided a consistent solution in which a line ratio was reproduced by assuming collisional ionization with kT = 31 keV, close to the accretion disk corona (ADC) electron temperature in this source.…”

“…Curve-of-growth analysis has provided a consistent solution in which a line ratio was reproduced by assuming collisional ionization with kT = 31 keV, close to the accretion disk corona (ADC) electron temperature in this source. Thus, Church et al (2005) proposed that those absorption lines in the dipping low-mass X-ray binaries (LMXB) are produced in the ADC.…”