Inertial-acoustic oscillations of black hole accretion discs with large-scale poloidal magnetic fields

Abstract

We study the effect of large-scale magnetic fields on the non-axisymmetric inertialacoustic modes (also called p-modes) trapped in the innermost regions of accretion discs around black holes (BHs). These global modes could provide an explanation for the high-frequency quasi-periodic oscillations (HFQPOs) observed in BH X-ray binaries. There may be observational evidence for the presence of such large-scale magnetic fields in the disks since episodic jets are observed in the same spectral state when HFQPOs are detected. We find that a large-scale poloidal magnetic field can enhance the corotational instability and increase the growth rate of the purely hydrodynamic overstable p-modes. In addition, we show that the frequencies of these overstable p-modes could be further reduced by such magnetic fields, making them agree better with observations.

What Can We Learn about GRB from the Variability Timescale Related Correlations?

“…Cao et al (2014) investigated the capability of the NDAF to drag the large-scaled magnetic field inward to the BH, and the result shows that the competition between the inherent diffusion and the accretion driven advection of the magnetic field leads to a oscillating accretion as well as an episodic jet, the oscillation timescale can be about one second which is comparable with the observed variation in the soft extended emission of short GRBs. An alternative mechanism involving the temporal variation is referred to the disk's inertial-acoustic oscillation which has been extensively discussed (e.g., Kato 1978;Wagoner et al 2001;Yu & Lai 2015). For aperiodic variation, some authors thought it reflects the possible propagating fluctuations in the disk (e.g., Lyubarskii 1997; Lin et al 2016).…”

“…An alternative mechanism involving the temporal variation is referred to the disk's inertial-acoustic oscillation which has been extensively discussed (e.g. Kato 1978;Wagoner et al 2001;Yu & Lai 2015). For aperiodic variation, some authors thought it reflects the possible propagating fluctuations in the disk (e.g., Lyubarskii 1997;Lin et al 2016).…”

Quasi-periodic oscillations and the global modes of relativistic, MHD accretion discs

“…Modes pushed to the inner edge of the disc would need to rely on excitation sufficient to overcome the damping effects of radial inflow (Ferreira 2010), and might require some reflection mechanism for their confinement. Such a reflection is required for explanations of HFQPOs that involve 2D, non-axisymmetric inertialacoustic modes, which local analyses suggest are less susceptible to the effects of large-scale magnetic fields and subject to amplification at co-rotation (Lai & Tsang 2009;Fu & Lai 2011;Lai et al 2012;Yu & Lai 2015).…”