Prof Steven Balbus FRS, FInstP

Abstract:

The fundamental difference between incompressible ideal magnetohydrodynamics and the dynamics of a non-conducting fluid is that magnetic fields exert a tension force that opposes their bending; magnetic fields behave like elastic strings threading the fluid. It is natural, therefore, to expect that a magnetic field tangled at small length scales should resist a large-scale shear in an elastic way, much as a ball of tangled elastic strings responds elastically to an impulse. Furthermore, a tangled field should support the propagation of ‘magnetoelastic waves’, the isotropic analogue of Alfvén waves on a straight magnetic field. Here, we study magnetoelasticity in the idealised context of an equilibrium tangled field configuration. In contrast to previous treatments, we explicitly account for intermittency of the Maxwell stress, and show that this intermittency necessarily decreases the frequency of magnetoelastic waves in a stable field configuration. We develop a mean-field formalism to describe magnetoelastic behaviour, retaining leading-order corrections due to the coupling of large- and small-scale motions, and solve the initial-value problem for viscous fluids subjected to a large-scale shear, showing that the development of small-scale motions results in anomalous viscous damping of large-scale waves. Finally, we test these analytic predictions using numerical simulations of standing waves on tangled, linear force-free magnetic-field equilibria.

Abstract:

We model the light curves of the novel and extremely luminous transient ASASSN-15lh at nine different frequencies, from infrared to ultraviolet photon energies, as an evolving relativistic disc produced in the aftermath of a tidal disruption event (TDE). Good fits to all nine light curves are simultaneously obtained when Macc ≃ 0.07 M⊙ is accreted on to a black hole of mass M ≃ 109 M⊙ and near-maximal rotation a/rg = 0.99. The best-fitting black hole mass is consistent with a number of existing estimates from galactic scaling relationships. If confirmed, our results represent the detection of one of the most massive rapidly spinning black holes to date, and are strong evidence for a TDE origin for ASASSN-15lh. This would be the first TDE to be observed in the disc-dominated state at optical and infrared frequencies.

Abstract:

We perform a detailed numerical and analytical study of the properties of observed light curves from relativistic thin discs, focussing on observational bands most appropriate for comparison with tidal disruption events (TDEs). We make use of asymptotic expansion techniques applied to the spectral emission integral, using time-dependent disc temperature profiles appropriate for solutions of the relativistic thin disc equation. Rather than a power law associated with bolometric disc emission L ∼ t−n, the observed X-ray flux from disc-dominated TDEs will typically have the form of a power law multiplied by an exponential (see equation 91). While precise details are somewhat dependent on the nature of the ISCO stress and disc-observer orientational angle, the general form of the time-dependent flux is robust and insensitive to the exact disc temperature profile. We present numerical fits to the UV and X-ray light curves of ASASSN-14li, a particularly well observed TDE. This modelling incorporates strong gravity optics. The full 900 d of ASASSN-14li X-ray observations are very well fit by a simple relativistic disc model, significantly improving upon previous work. The same underlying model also fits the final 1000 d of ASASSN-14li observations in three different UV bandpasses. Finally, we demonstrate that the analytic formulae reproduce the properties of full numerical modelling at both UV and X-ray wavelengths with great fidelity.