arXiv subjects

Zach Smith

Publications and source records attributed to Zach Smith.

3 recordsLinked to original sources

arXiv2026

The nature of tilted supercritical accretion discs

In this paper, we report on the first 3D general relativistic radiation magnetohydrodynamic simulations of large supercritical accretion discs that are tilted with respect to the black hole spin axis. We explore a range of black hole spin parameters (from $a_* = -0.9$ to 0.9), initial tilts (in the range from $\beta_0 = 0^\circ$ to $30^\circ$), and target mass accretion rates. We first confirm that, for all the untilted simulations, the Eddington accretion limit is obeyed ($\dot{M}_\mathrm{BH} \lesssim \dot{M}_\mathrm{Edd}$), consistent with our previous findings. However, for tilted discs we find that the mass accretion rate can be enhanced by up to a factor of ten and that factor depends linearly on tilt $\dot{M}_\mathrm{BH} \propto \beta_0 \ge \dot{M}_\mathrm{Edd}$. This could be an important aspect in solving the puzzle of the growth of the first supermassive black holes. We also find that for a given tilt, the mass accretion rate enhancement is proportional to the magnitude of the spin. Additionally, we find that tilted supercritical accretion discs are more advective than their untilted counterparts. We attribute all of these differences to the presence of standing shocks in the inner regions of the accretion flow, a feature unique to tilted discs.

astro-ph.HE↗

arXiv2025

Long timescale numerical simulations of large, super-critical accretion discs

In this paper, we report on three of the largest (in terms of simulation domain size) and longest (in terms of duration) 3D general relativistic radiation magnetohydrodynamic simulations of super-critical accretion onto black holes. The simulations are all set for a rapidly rotating ($a_* = 0.9$), stellar-mass ($M_\mathrm{BH} = 6.62 M_\odot$) black hole. The simulations vary in their initial target mass accretion rates (assumed measured at large radius), with values sampled in the range $\dot{m}=\dot{M}/\dot{M}_\mathrm{Edd} = 1-10$. We find in practice, though, that all of our simulations settle close to a net accretion rate of $\dot{m}_\mathrm{net} = \dot{m}_\mathrm{in}-\dot{m}_\mathrm{out} \approx 1$ (over the radii where our simulations have reached equilibrium), even though the inward mass flux (measured at large radii) $\dot{m}_\mathrm{in}$ can exceed 1,000 in some cases. This is possible because the outflowing mass flux $\dot{m}_\mathrm{out}$ adjusts itself to very nearly cancel out $\dot{m}_\mathrm{in}$, so that at all radii $\dot{M}_\mathrm{net} \approx \dot{M}_\mathrm{Edd}$. In other words, these simulated discs obey the Eddington limit. We compare our results with the predictions of the slim disc (advection-dominated) and critical disc (wind/outflow-dominated) models, finding that they agree quite well with the critical disc model both qualitatively and quantitatively. We also speculate as to why our results appear to contradict most previous numerical studies of super-critical accretion.

astro-ph.HE↗

arXiv2022

Modelling Agent-Skipping Attacks in Message Forwarding Protocols

Message forwarding protocols are protocols in which a chain of agents handles transmission of a message. Each agent forwards the received message to the next agent in the chain. For example, TLS middleboxes act as intermediary agents in TLS, adding functionality such as filtering or compressing data. In such protocols, an attacker may attempt to bypass one or more intermediary agents. Such an agent-skipping attack can the violate security requirements of the protocol. Using the multiset rewriting model in the symbolic setting, we construct a comprehensive framework of such path protocols. In particular, we introduce a set of security goals related to path integrity: the notion that a message faithfully travels through participants in the order intended by the initiating agent. We perform a security analysis of several such protocols, highlighting key attacks on modern protocols.

cs.CR↗