Papaloizou-Pringle Instability of Magnetized Accretion Tori

TL;DR

This paper investigates how magnetic fields influence the Papaloizou-Pringle instability in accretion tori, revealing that strong magnetic fields can both suppress or induce the instability depending on the torus thickness.

Contribution

It provides a detailed analysis of magnetic effects on the PP instability in incompressible tori, highlighting conditions under which magnetic fields stabilize or destabilize the flow.

Findings

Toroidal magnetic fields suppress the PP instability in thin tori at high field strengths.

Thicker, hydrodynamically stable tori can become unstable under strong magnetic fields.

Highly magnetized tori may experience global instability even when stable against magneto-rotational instability.

Abstract

Hot accretion tori around a compact object are known to be susceptible to a global hydrodynamical instability, the so-called Papaloizou-Pringle (PP) instability, arising from the interaction of non-axisymmetric waves across the corotation radius, where the wave pattern speed matches the fluid rotation rate. However, accretion tori produced in various astrophysical situations (e.g., collapsars and neutron star binary mergers) are likely to be highly magnetized. We study the effect of magnetic fields on the PP instability in incompressible tori with various magnetic strengths and structures. In general, toroidal magnetic fields have significant effects on the PP instability: For thin tori (with the fractional width relative to the outer torus radius much less than unity), the instability is suppressed at large field strengths with the corresponding toroidal Alfven speed $v_{A\phi}\go 0.2r\Omega$ (where $\Omega$ is the flow rotation rate). For thicker tori (with the fractional width of order 0.4 or larger), which are hydrodynamically stable, the instability sets in for sufficiently strong magnetic fields (with $v_{A\phi}\go 0.2 r\Omega$). Our results suggest that highly magnetized accretion tori may be subjected to global instability even when it is stable against the usual magneto-rotational instability.