A variable corona for GRS 1915+105

TL;DR

This paper models the low-frequency QPOs in GRS 1915+105 using Comptonisation, revealing how changes in corona size and feedback influence phase lag transitions from hard to soft.

Contribution

It applies a Comptonisation model to explain the frequency-dependent phase lags of type-C QPOs in GRS 1915+105, linking corona geometry to lag behavior.

Findings

Large corona (~100-150 R_g) causes soft lags at high QPO frequencies.

Corona shrinks to 3-17 R_g as QPO frequency decreases, leading to hard lags.

Feedback efficiency onto the disc diminishes at lower QPO frequencies.

Abstract

Most models of the low frequency quasi periodic oscillations (QPOs) in low-mass X-ray binaries (LMXBs) explain the dynamical properties of those QPOs. On the other hand, in recent years reverberation models that assume a lamp-post geometry have been successfull in explaining the energy-dependent time lags of the broad-band noise component in stellar mass black-holes and active galactic nuclei. We have recently shown that Comptonisation can explain the spectral-timing properties of the kilo-hertz (kHz) QPOs observed in neutron star (NS) LMXBs. It is therefore worth exploring whether the same family of models would be as successful in explaining the low-frequency QPOs. In this work, we use a Comptonisation model to study the frequency dependence of the phase lags of the type-C QPO in the BH LMXB GRS 1915+105. The phase lags of the QPO in GRS 1915+105 make a transition from hard to soft at a QPO frequency of around 1.8 Hz. Our model shows that at high QPO frequencies a large corona of ~ 100-150 R_g covers most of the accretion disc and makes it 100% feedback dominated, thus producing soft lags. As the observed QPO frequency decreases, the corona gradually shrinks down to around 3-17 R_g, and at 1.8 Hz feedback onto the disc becomes inefficient leading to hard lags. We discuss how changes in the accretion geometry affect the timing properties of the type-C QPO.