Magnetic field transport in compact binaries. (arXiv:2007.07277v1 [astro-ph.HE])
<a href="http://arxiv.org/find/astro-ph/1/au:+Scepi_N/0/1/0/all/0/1">Nicolas Scepi</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Lesur_G/0/1/0/all/0/1">Geoffroy Lesur</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Dubus_G/0/1/0/all/0/1">Guillaume Dubus</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Jacquemin_Ide_J/0/1/0/all/0/1">Jonatan Jacquemin-Ide</a>
Dwarf novae (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.
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. We develop a version of the disk instability model
that evolves the density, the temperature and the large scale vertical magnetic
flux together. We take into account the accretion driven by turbulence or by a
magnetized outflow. To evolve the magnetic flux, we use a toy model with
physically motivated prescriptions depending mainly on the local magnetization.
We find that allowing magnetic flux to be advected inwards provides the best
agreement with DNe lightcurves. 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
$dot{M}^{-2/3}$. Models of DNe and LMXBs lightcurves typically require the
outer, viscous disk to be truncated in order to match 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,
the magnetic flux accumulating towards the center to produce a magnetized disk
with a fast accretion timescale.
Dwarf novae (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.
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. We develop a version of the disk instability model
that evolves the density, the temperature and the large scale vertical magnetic
flux together. We take into account the accretion driven by turbulence or by a
magnetized outflow. To evolve the magnetic flux, we use a toy model with
physically motivated prescriptions depending mainly on the local magnetization.
We find that allowing magnetic flux to be advected inwards provides the best
agreement with DNe lightcurves. 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
$dot{M}^{-2/3}$. Models of DNe and LMXBs lightcurves typically require the
outer, viscous disk to be truncated in order to match 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,
the magnetic flux accumulating towards the center to produce a magnetized disk
with a fast accretion timescale.
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