Theoretical astrophysics and plasma physics at RPC

Abstract:

Chaos is present in most stellar dynamical systems and manifests itself through the exponential growth of small perturbations. Exponential divergence drives time irreversibility and increases the entropy in the system. A numerical consequence is that integrations of the N-body problem unavoidably magnify truncation and rounding errors to macroscopic scales. Hitherto, a quantitative relation between chaos in stellar dynamical systems and the level of irreversibility remained undetermined. In this work, we study chaotic three-body systems in free fall initially using the accurate and precise N-body code Brutus, which goes beyond standard double-precision arithmetic. We demonstrate that the fraction of irreversible solutions decreases as a power law with numerical accuracy. This can be derived from the distribution of amplification factors of small initial perturbations. Applying this result to systems consisting of three massive black holes with zero total angular momentum, we conclude that up to 5 per cent of such triples would require an accuracy of smaller than the Planck length in order to produce a time-reversible solution, thus rendering them fundamentally unpredictable.

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.

Abstract:

Refer to readme.pdf in the repository.