A variable corona for GRS 1915+105

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 successful 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 Comptonization 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 Comptonization 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 $100{{\ \rm per\ cent}}$ 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 on to 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.