Truncated accretion discs in black hole X-ray binaries: dynamics and variability signatures

Abstract

ABSTRACT Variable features in black hole X-ray binaries (BH-XRBs) are observed in different energy ranges and time-scales. The physical origin of different spectral states in BH-XRBs and their relations with the underlying accretion disc are still elusive. To investigate the intermediate state of BH-XRBs during outburst, we simulate a truncated accretion disc around a Kerr black hole using a general relativistic magnetohydrodynamical (GRMHD) framework under axisymmetry with adaptively refined mesh. Additionally, we have also carried out radiative transfer calculations for understanding the implications of disc dynamics on emission. Dynamically, the inner edge of the truncated accretion disc oscillates in a quasi-periodic fashion (QPO). The QPO frequency of oscillations (νQPO, max) increases as the magnetic field strength and magnetic resistivity increase. However, as the truncation radius increases, νQPO, max decreases. In our simulation models, frequency varies between $7\times (10\, {\rm M}_{\odot }/M_{\rm BH})$ Hz $\lesssim \nu _{\rm QPO, max}\lesssim 20 \times (10\, {\rm M}_{\odot }/M_{\rm BH})$ Hz, which is in the range of low-frequency QPOs. We further find evidence of transient shocks in the highly accreting stage during oscillation. Such a transient shock acts as an extended hot post-shock corona around the black hole that has an impact on its radiative properties. The radiative transfer calculations show signatures of these oscillations in the form of modulation in the edge-brightened structure of the accretion disc.