Here, we invoke an extend accretion disc corona (ADC) model to explain the long-term time lags detected in NS-LMXBs, including the long-term time lags of GX 349+2 that we derive in this work. Analysing the dip and non-dip spectra of NS-LMXBs, Church & Bałucińska-Church (1993, 1995) proposed a Birmingham model which consists of a blackbody component interpreted as the emission from a point source, i.e. the NS, and a power-law component that might be resulted from the Comptonization of thermal emission in an ADC above the accretion disc. Through dip ingress time technique, Church & Bałucińska-Church (2004) measured the radius of the ADC and developed the Birmingham model into an extended ADC model. The measured radius of a thin, hot corona above the accretion disc varies in the range of ∼(2–70)× 104 km. Therefore, the corona is very extended and the disc is substantially covered by the corona. In the extended ADC model, almost all soft X-ray photons from the accretion disc are inversely Comptonized by the energetic electrons from the extended ADC, which produces the observed hard X-rays, while the observed soft X-rays are interpreted as the emission from the NS; the accretion disc is illuminated by the emission of the NS, leading to the production of the extended ADC above the disc. This model was successfully applied to Cyg-like Z sources (Church, Halai & Bałucińska-Church 2006; Jackson, Church & Bałucińska-Church 2009; Bałucińska-Church et al. 2010) and Sco-like Z sources (including GX 349+2) (Church et al. 2012), as well as atoll sources (Church et al. 2014), so it could be a universal model for NS-LMXBs. Since the extended ADC model is a unified model for NS-LMXBs, we try to interpret the long-term time lags in NS-LMXBs with the help of this model. The extended ADC and the NS are two independent emitting regions, which satisfies the request that the hard and soft X-rays for long-term time lags are emitted from two distant regions, as discussed above. In order to explain the long-term time lags detected in NS-LMXBs in terms of the extended ADC model, we introduce two time-scales. One is the Comptonization time-scale during which the disc seed photons are inversely Comptonized by the high-energy electrons in the extended ADC, and another is the viscous time-scale in the order of hundreds of seconds (Lei et al. 2008), during which the accreting matter flows from the disc on to the NS. The hard X-ray time lags will be produced if the Comptonization time-scale is less than the viscous time-scale, and, contrarily, the soft X-ray time lags will be observed if the Comptonization time-scale is larger than the viscous time-scale. It is noted that a minority of positively correlated short-term time lags (1 s) are listed in Tables 1 and 3, which are derived with the CCF method in our work. These short-term time lags cannot be explained by the models reviewed in section 4.1, because those models are used to interpret the short-term time lags produced in two adjacent energy intervals, while these short-term time lags obtained in this work are derived from two distant energy intervals, i.e. 2–5 kev and 16–30 keV energy intervals. In the frame of the extended ADC model, we propose that these short-term time lags will be observed under the circumstance that the Comptonization time-scale is comparable with the viscous time-scale.
Citation: Church et al. 2014
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