Magnetic field transport in compact binaries

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.

QPOs in compact binaries from small-scale eruptions in an inner magnetized disc

“…In the inner disk angular momentum is mostly transported by a magnetized outflow, while in the outer disk it is transported by turbulence as in an -disk. While Scepi et al (2020) focused on the long term dynamics of DNe, we focused here on the small scale behavior of the inner magnetized disk. Indeed, the modeled light curves show ≈ 0.2 mag quasi-periodic variations with a period of a few thousand seconds.…”

“…Magnetic field transport is modeled using very simple power laws to explore different behaviors. Scepi et al (2020) find that when the disk is able to advect magnetic flux inwards, it divides in two zones: an inner zone that accumulates magnetic flux, until it reaches an equilibrium magnetization, and an outer zone that empties of magnetic flux, if not replenished from the outer boundary. The hybrid disk structure is illustrated on Figure 3.…”

“…However, as we will see here, the inner zone exhibits rapid fluctuations that might be related to the observed rapid fluctuations of DNe. Figure 1 shows a light curve from Scepi et al (2020) where we can see the secular eruptions as well as small-scale, rapid fluctuations during quiescence. The shape of the light curve as well as the secular dynamics of the disk is discussed in Scepi et al (2020).…”

“…However, those eruptions are smaller than the eruptions in the outer viscous disk due to the difference of accretion time scales as we discuss in the following section. Scepi et al (2020). Bottom panel: Temperature as a function of the surface density during instant (b) at a radius of = 1.5 × 10 9 cm.…”

“…These are the subjects of this article. We first present light curves obtained from the model of Scepi et al (2020), in which rapid fluctuations have been identified ( §2). We propose that these rapid fluctuations might correspond to this subclass of QPOs.…”