3D MHD Simulations of Accretion onto Stars with Tilted Magnetic and Rotational Axes

Abstract

We present results of global three-dimensional (3D) magnetohydrodynamic (MHD) simulations of the accretion onto a magnetized star where both the magnetic and rotational axes of the star are tilted about the rotational axis of the disc. We observed that initially, the inner parts of the disc are warped due to the magnetic interaction between the external regions of the magnetosphere and the inner disc. Subsequently, larger parts of the disc become tilted and precess about the rotational axis of the star. After a few precessional periods, the disc becomes almost aligned with the equatorial plane of the star. This alignment may result from the winding of the magnetic field lines about the rotational axis of the star and the formation of a new magnetic configuration, where the magnetic pressure force acts to align the disc. Another possible explanation of this alignment is the magnetic Bardeen-Petterson effect where a part of the disc, precessing about the rotational axis of the star, settles in the equatorial plane due to the precessional and viscous torques in the disc.

Magnetic angle evolution in accreting neutron stars

“…Though in general accreting NS should be inclined, and accretion upon inclined dipoles was studied both analytically (Wang 1981a;Bozzo et al 2018) and numerically (Romanova et al 2009), the problem of mass accretion upon a misaligned oblique rotating dipole (where both α and χ are non-zero) is, to our knowledge, relatively unstudied. Notable exceptions are Lai et al (2011) who considered the aligning torques analytically, and Romanova et al (2020), where magnetospheric accretion upon a misaligned oblique magnetic dipole is considered by means of 3D MHD simulations. One of the interesting findings of Romanova et al (2020) is that the mass accretion rate upon the simulated star varies with time at about spin period.…”

“…Notable exceptions are Lai et al (2011) who considered the aligning torques analytically, and Romanova et al (2020), where magnetospheric accretion upon a misaligned oblique magnetic dipole is considered by means of 3D MHD simulations. One of the interesting findings of Romanova et al (2020) is that the mass accretion rate upon the simulated star varies with time at about spin period.…”

Modelling spin-up episodes in accreting millisecond X-ray pulsars

“…Taking the accretion torque modelling to the next level will probably require a shift from the analyticalphenomenological models discussed here to the full armoury of 3D numerical simulations [see, e.g., Kulkarni & Romanova (2013)]. The existing MHD codes, although still limited in terms of simulation time, have now reached a point where they can evolve an accretion flow without any symmetry imposed between the spin, disc and magnetic field axes (Romanova et al 2020). The numerical results could serve as a test of the key ingredients of the phenomenological models such as the magnetospheric radius (Kulkarni & Romanova 2013) and the functional form of the generated azimuthal magnetic field [cf.…”

Magnetic angle evolution in accreting neutron stars

“…Though in general accreting NSs should be inclined, and accretion upon inclined dipoles was studied both analytically (Wang 1981a;Bozzo et al 2018) and numerically (Romanova et al 2009), the problem of mass accretion upon a misaligned oblique rotating dipole (where both 𝛼 and 𝜒 are non-zero) is, to our knowledge, relatively unstudied. Notable exceptions are Lai et al (2011) who considered the aligning torques analytically, and Romanova et al (2020), where magnetospheric accretion upon a misaligned oblique magnetic dipole is considered by means of 3D magnetohydrodynamic simulations. One of the interesting findings of Romanova et al (2020) is that the mass accretion rate upon the simulated star varies with time at about spin period.…”

“…Notable exceptions are Lai et al (2011) who considered the aligning torques analytically, and Romanova et al (2020), where magnetospheric accretion upon a misaligned oblique magnetic dipole is considered by means of 3D magnetohydrodynamic simulations. One of the interesting findings of Romanova et al (2020) is that the mass accretion rate upon the simulated star varies with time at about spin period. As we show below, the only case when magnetic angle changes due to accretion torque is this misaligned oblique case (𝛼 ≠ 0 and 𝜒 ≠ 0), and the accretion torque variations within the spin period plays crucial role in the evolution of 𝜒.…”