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

Abstract

The evolution of the C-type low frequency quasi-periodic oscillations (LFQPOs) and associated time lag in transient black hole sources as a function of time can be explained by variation of the Compton cloud size in a Two Component Advective Flow solution (TCAF). A similar study of a persistent source, GRS 1915+105, has not been attempted. We fit the evolution of QPOs with propagatory oscillating shock (POS) solution for two sets of so-called χ-state observations and find that the shock steadily recedes with almost constant velocity when QPO frequency is decreasing and the spectrum is hardening. The shock moves inward with a constant velocity v 0 = 473.0 cm s −1 and v 0 = 400.0 cm s −1 respectively in these two cases, when the QPO frequency is increasing and the spectrum softens. This behavior is similar to what was observed in XTE J1550-564 during the 1998 outburst. The time lag measured at the QPO frequency varies in a similar way as the size of the Compton cloud. Most interestingly, in both the cases, the lag switches sign (hard lag to soft lag) at a QPO frequency of ∼ 2.3 − 2.5 Hz irrespective of the energy of photons. We find, at very low frequencies < 1 Hz, the Comptonizing Efficiency (CE) increases with QPO frequency and at higher QPO frequencies the trend is opposite. The time lags become mostly positive at all energies when CE is larger than ∼ 0.85% for both the sources.

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

“…Our model, however, explains not only the lag-frequency spectrum, but also both the rms-and lag-energy spectra at different QPO frequencies. Dutta, Pal & Chakrabarti (2018) based on the TCAF model, provide an explanation opposite to ours when it comes to the evolution of the Comptonising cloud at different type-C QPO frequencies. More specifically, although the size of our corona is of the same order of magnitude as the size of the CENBOL in Dutta, Pal & Chakrabarti (2018), in their interpretation there is no systematic or rapid decrease in the CENBOL size at around 1.8 Hz.…”

“…Dutta, Pal & Chakrabarti (2018) based on the TCAF model, provide an explanation opposite to ours when it comes to the evolution of the Comptonising cloud at different type-C QPO frequencies. More specifically, although the size of our corona is of the same order of magnitude as the size of the CENBOL in Dutta, Pal & Chakrabarti (2018), in their interpretation there is no systematic or rapid decrease in the CENBOL size at around 1.8 Hz. Since a transition of a type-C QPO frequency around 1.8 Hz typically happens in matter of a few days (Dutta, Pal & Chakrabarti 2018), the predicted decrease in the size of the corona should happen on the same time scale.…”

“…More specifically, although the size of our corona is of the same order of magnitude as the size of the CENBOL in Dutta, Pal & Chakrabarti (2018), in their interpretation there is no systematic or rapid decrease in the CENBOL size at around 1.8 Hz. Since a transition of a type-C QPO frequency around 1.8 Hz typically happens in matter of a few days (Dutta, Pal & Chakrabarti 2018), the predicted decrease in the size of the corona should happen on the same time scale. Regardless of whether the decrease in corona size is systematic or very rapid, our fits to both the lag-energy and lag-frequency spectra agree that the size should be significantly lower around 1.8 Hz.…”

“…More recently, Dutta, Pal & Chakrabarti (2018) applied the TCAF model to data of GRS 1915+105, and showed how a gradual change in the size of the CENBOL can explain the time evolution of the type-C QPO frequency, above and below the 1.8 Hz limit. Dutta, Pal & Chakrabarti (2018) concluded that, as the QPO frequency decreases from over 5 Hz to below 1.8 Hz, the CENBOL increases in size from around 135 R g to over 500 R g , and then it decreases again to about 100 R g , when the QPO Here, we use the Comptonisation model of Karpouzas et al (2020) to explain the dependence of both the rms amplitude and phase lags upon QPO frequency and energy. In sections 2 and 3 we, respectively, introduce the model and explain its application to the data of Zhang et al (2020).…”

Long term X-Ray Observations of Seyfert 1 Galaxy Ark 120: On the origin of soft-excess

“…However, as discussed previously, could dominate if reflection becomes dominating (here = 0.25). Also, the size of the Compton cloud is much bigger than the what should be the 'transition radius' (see, Dutta & Chakrabarti (2016); Dutta, Pal & Chakrabarti (2018) for details) of an AGN having mass 1.5 × 10 8 ⊙ . Being an intermediate inclination angle source (Nardini et al 2011;Marinucci et al 2019), Comptonization dominates the time delay when the size of the Compton cloud is bigger.…”