Magnetically dominated discs in tidal disruption events and quasi-periodic eruptions

Abstract

ABSTRACT The classical radiation pressure instability has been a persistent theoretical feature of thin, radiatively efficient accretion discs with accretion rates $\sim $ 1 per cent–100 per cent of the Eddington rate. But there is only limited evidence of its occurrence in nature: rapid heartbeat oscillations of a few X-ray binaries and now, perhaps, the new class of hourly X-ray transients called quasi-periodic eruptions (QPEs). The accretion discs formed in tidal disruption events (TDEs) have been observed to peacefully trespass through the range of unstable accretion rates without exhibiting any clear sign of the instability. We try to explain the occurrence or otherwise of this instability in these systems, by constructing steady state 1D models of thin magnetic accretion discs. The local magnetic pressure in the disc is assumed to be dominated by toroidal fields arising from a dynamo sourced by magneto-rotational instability (MRI). We choose a physically motivated criterion of MRI saturation, validated by recent magnetohydrodynamic simulations, to determine the disc magnetic pressure. The resulting magnetic pressure support efficiently shrinks: (1) the parameter space of unstable mass accretion rates, explaining the absence of instability in TDEs and (2) the range of unstable radii in the inner accretion disc, which can shorten the quasi-periods of instability limit-cycles by more than three orders of magnitude, explaining the short periods of QPEs. In addition to examining stability of strongly magnetized discs, we predict other observational signatures such as spectral hardening and jet luminosities to test the compatibility of our disc models with observations of TDE discs.