Temporal Variability From the Two-Component Advective Flow Solution and Its Observational Evidence

Abstract

In the propagating oscillatory shock model, the oscillation of the post-shock region, i.e., the Compton cloud, causes the observed low-frequency quasi-periodic oscillations (QPOs). The evolution of QPO frequency is explained by the systematic variation of the Compton cloud size, i.e., the steady radial movement of the shock front, which is triggered by the cooling of the post-shock region. Thus, analysis of the energy-dependent temporal properties in different variability timescales can diagnose the dynamics and geometry of accretion flows around black holes. We study these properties for the high-inclination black hole source XTE J1550-564 during its 1998 outburst and the low-inclination black hole source GX 339-4 during its 2006-07 outburst using RXTE/PCA data, and we find that they can satisfactorily explain the time lags associated with the QPOs from these systems. We find a smooth decrease of the time lag as a function of time in the rising phase of both sources. In the declining phase, the time lag increases with time. We find a systematic evolution of QPO frequency and hard lags in these outbursts. In XTE J1550-564, the lag changes from hard to soft (i.e., from a positive to a negative value) at a crossing frequency (ν c ) of ∼3.4 Hz. We present possible mechanisms to explain the lag behavior of high and low-inclination sources within the framework of a single two-component advective flow model.

Evolution of accretion disc geometry of GRS 1915+105 during its χ state as revealed by TCAF solution

“…The spectrum of the lags at different QPO frequencies shows that for χ set-1 the hard lags increase monotonically with energy up to QPO frequency of ∼ 2.3 Hz and for χ set-2, up to a QPO frequency of ∼ 2.62 Hz. This is similar to what we found in the transient source XTE J1550-564 (Dutta & Chakrabarti 2016). This behavior suggests that net observed time lag of the high-energy photons depends on the relative contribution of the different physical processes responsible for the high energy emission.…”

“…Similarly, time lag monotonically increases with energy for QPO frequency ∼ 1.48 Hz, but monotonically decreases from a frequency of ∼ 2.62 Hz onward for χ set-2. A similar type of behavior was also observed for the transient source XTE J1550-564 (Dutta & Chakrabarti 2016). Physical processes producing high-energy photons im- print energy-dependent time lags on the output radiation, therefore an energy dependence in the measured lags of high energy photons is naturally expected.…”

“…Thus, analysis of the energy-dependent temporal properties in different variability timescales can diagnose the dynamics and geometry of accretion flows around black holes. This has been shown for the high inclination transient black hole source XTE J1550-564 during its 1998 outburst and the low-inclination black hole transient source GX 339-4 during its 2006-07 outburst using RXTE/PCA data (Dutta & Chakrabarti 2016). However, the dependence of lag on photon energy is more intriguing.…”

“…Repeated scatterings and Comptonization always produce longer time lags for higher energy photons. The gravitational bending of photons (i.e., focusing of emitted photons) is an energy dependent process since higher-energy photons are expected to come from regions closer to a black hole (Chatterjee et al 2017;Dutta & Chakrabarti 2016). For instance, high energy photons coming from the inner CEN-BOL in the opposite side would be focused towards the observer, but lower energy photons from farther out will not be focused.…”

Modelling of Comptonisation in low-mass X-ray binaries in the presence of fast variability

“…Recent work from Dutta & Chakrabarti (2016) and Chatterjee, Chakrabarti & Ghosh (2017), implementing the TCAF model (Chakrabarti et al 2008), linked the QPO frequency to the size of the Centrifugal pressure supported Boundary Layer (CENBOL), which serves as the corona in the TCAF model. These studies suggested that changes in the size of the CENBOL can explain the change in sign of the time lag.…”