Abstract
Context. Dwarf novæ (DNe) and low mass X-ray binaries (LMXBs) show eruptions that are thought to be due to a thermal-viscous instability in their accretion disk. These eruptions provide constraints on angular momentum transport mechanisms. Aims. We explore the idea that angular momentum transport could be controlled by the dynamical evolution of the large-scale magnetic field. We study the impact of different prescriptions for the magnetic field evolution on the dynamics of the disk. This is a first step in confronting the theory of magnetic field transport with observations. Methods. We developed a version of the disk instability model that evolves the density, the temperature, and the large-scale vertical magnetic flux simultaneously. We took into account the accretion driven by turbulence or by a magnetized outflow with prescriptions taken, respectively, from shearing box simulations or self-similar solutions of magnetized outflows. To evolve the magnetic flux, we used a toy model with physically motivated prescriptions that depend mainly on the local magnetization β, where β is the ratio of thermal pressure to magnetic pressure. Results. We find that allowing magnetic flux to be advected inwards provides the best agreement with DNe light curves. This leads to a hybrid configuration with an inner magnetized disk, driven by angular momentum losses to an MHD outflow, sharply transiting to an outer weakly-magnetized turbulent disk where the eruptions are triggered. The dynamical impact is equivalent to truncating a viscous disk so that it does not extend down to the compact object, with the truncation radius dependent on the magnetic flux and evolving as Ṁ−2/3. Conclusions. Models of DNe and LMXB light curves typically require the outer, viscous disk to be truncated in order to match the observations. There is no generic explanation for this truncation. We propose that it is a natural outcome of the presence of large-scale magnetic fields in both DNe and LMXBs, with the magnetic flux accumulating towards the center to produce a magnetized disk with a fast accretion timescale.
Highlights
Magnetic fields in accretion disks are largely unconstrained with regard to observations
We studied the evolution of a non-steady, axisymmetric, DNe accretion disk undergoing eruptions due to the thermal-viscous instability
The novelty of this work is that we evolve the largescale poloidal magnetic configuration during the outburst cycle using a local toy model for magnetic flux transport
Summary
Magnetic fields in accretion disks are largely unconstrained with regard to observations. It was recently proposed that jets may be present in dwarf novæ (DNe; Körding et al 2008; Russell et al 2016; Coppejans et al 2016; Coppejans & Knigge 2020). The source of these jets may be a strong (near equipartition) large-scale poloidal magnetic field threading the disk that magneto-centrifugally accelerates and collimates the flow (Blandford & Payne 1982; Pudritz & Norman 1983; Königl 1989; Ferreira & Pelletier 1995). Slower ejections, called winds, have been observed in all the accretion disks cited above (Cordova & Mason 1982; Mauche & Raymond 1987; Crenshaw et al 2003; Miller et al 2004; Ponti et al 2012; Louvet et al 2018)
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