Nicolas Scepi

Context . Radiation magnetohydrodynamic (RMHD) simulations are essential for comparisons with observations, particularly in the regime where fluids and radiation are dynamically coupled. Although computationally expensive, RMHD is becoming increasingly accessible with the advent of exascale computing. However, only a few public RMHD codes are currently able to fully exploit the diversity of modern accelerated architectures. Aims . We present a nonrelativistic radiative transfer module for the public magnetohydrodynamic code I DEFIX ; it is built on the Kokkos library to ensure performance portability. Our goal is to provide a user-friendly RMHD code capable of running efficiently on current and future exascale supercomputers. Methods . The radiative transfer module is based on the M1 approximation and implemented using a split explicit–implicit scheme. A reduced speed of light approximation is employed to alleviate the timestep constraint imposed by radiation. The module supports several radiation Riemann solvers and Cartesian, cylindrical, and spherical geometries in one, two, and three dimensions. The implicit step relies on a simple matrix inversion, ensuring both robustness and high performance. Users can choose between built-in opacity models or supply tabulated opacities and custom user-defined functions. Results . The radiative module of I DEFIX demonstrates excellent performance on accelerated architectures, including the AMD MI250X and MI300 partitions of the AdAstra supercomputer. On MI250X nodes, it achieves up to 7.6 × 10° cell updates per second per node, at a computational cost only 1.6 times higher than a pure magnetohydrodynamic simulation. These results establish I DEFIX as a fast, robust, and portable RMHD code suitable for the wider community.

Magnetically arrested disks (MADs) are a compelling model for explaining variability in low-luminosity active galactic nuclei (AGN), including horizon-scale outbursts. MADs experience powerful flux eruptions—episodic ejections of magnetic flux from the black hole horizon—that may drive the observed luminosity variations. In this work, we develop and validate a new formalism describing large-scale magnetic field transport in general relativistic magnetohydrodynamic simulations of MADs with geometrical thicknesses of h / R = 0.1 and h / R = 0.3. We introduce a net flux transport velocity, v _Φ , which accounts for both advective and diffusive processes. We show that MADs maintain a statistical quasi-steady state where advection and diffusion nearly balance. Flux eruptions appear as small deviations from this equilibrium, with v _Φ / V _k ≪ 1, where V _k is the local Keplerian velocity. Using this framework, we analytically derive a recurrence timescale for flux eruptions, t _rec ≃ 1500 r _g / c . This timescale closely matches simulation results. The smallness of v _Φ explains the long recurrence times of flux eruptions compared to other system timescales. We also take a closer look at the diffusion of the magnetic field by performing the first measurement of turbulent resistivity in MADs. We then estimate the turbulent magnetic Prandtl number, P m , defined as the ratio of turbulent viscosity to turbulent resistivity. We find P m ≃ 3 , consistent with shearing-box simulations of magnetorotational instability-driven turbulence. While flux eruptions excite large-scale nonaxisymmetric modes and locally enhance turbulent resistivity, magnetic field diffusion is dominated by smaller-scale turbulent motions. These results provide new insight into the nature of AGN variability and the fundamental physics of magnetic field transport.

Disc winds play a crucial role in many accreting astrophysical systems across all scales. In accreting white dwarfs (AWDs) and active galactic nuclei (AGNs), radiation pressure on spectral lines is a promising wind-driving mechanism. However, the efficiency of line driving is extremely sensitive to the ionization state of the flow, making it difficult to construct a reliable physical picture of these winds. Recently, we presented the first radiation-hydrodynamics simulations for AWDs that incorporated detailed, multidimensional ionization calculations via fully frequency-dependent radiative transfer, using the sirocco code coupled to pluto. These simulations produced much weaker line-driven winds ($\dot{M}_{\rm wind}/\dot{M}_{\rm acc} < 10^{-5}$ for our adopted parameters) than earlier studies using more approximate treatments of ionization and radiative transfer (which yielded $\dot{M}_{\rm wind}/\dot{M}_{\rm acc} \simeq 10^{-4}$). One remaining limitation of our work was the assumption of an isothermal outflow. Here, we relax this by adopting an ideal gas equation of state and explicitly solving for the multidimensional temperature structure of the flow. In the AWD setting, accounting for the thermal state of the wind does not change the overall conclusions drawn from the isothermal approximation. Our new simulations confirm the line-driving efficiency problem: the predicted outflows are too highly ionized, meaning they neither create optimal driving conditions nor reproduce the observed ultraviolet wind signatures. Possible solutions include wind clumping on subgrid scales, a softer-than-expected spectral energy distribution or additional driving mechanisms. With the physics now built into our simulations, we are well equipped to also explore line-driven disc winds in AGN.

Context. X-ray binaries (XRBs) exhibit a spectral hysteresis for luminosities in the range 10⁻² ≲ L/L_Edd ≲ 0.3, with a hard X-ray spectral state that persists from quiescent luminosities up to ≳0.3L_Edd, transitioning to a soft spectral state that survives with decreasing luminosities down to ∼10⁻²L_Edd. Aims. We present a possible approach to explain this behavior based on the thermal properties of a magnetically arrested disk simulation. Methods. By post-processing the simulation to include radiative effects, we solved for all the thermal equilibrium solutions as the accretion rate, Ṁ, varies during the XRB outburst. Results. For an assumed scaling of the disk scale height and accretion speed with temperature, we find that two solutions exist in the range of 10⁻³ ≲ Ṁ/Ṁ_Eddington ≲ 0.1 at r = 8 r_g (4 × 10−2 ≲ Ṁ/Ṁ_Eddington ≲ 0.5 at r = 3 r_g): a cold, optically thick solution, and a hot, optically thin one. This opens the possibility of a natural thermal hysteresis in the right range of luminosities for XRBs. We stress that our scenario for the hysteresis does not require us to invoke the strong advection-dominated accretion flow principle, nor does it require the magnetization of the disk to change during the XRB outburst. In fact, our scenario requires a highly magnetized disk in the cold soft state to reproduce the transition from soft to hard state at the right luminosities. Our scenario therefore predicts a jet, although possibly very weakly dissipative, in the soft state of XRBs. We also predict that if active galactic nuclei have similar hysteresis cycles and are strongly magnetized, they undergo a transition from soft to hard state at much lower L/L_Edd than XRBs.

Outflows are critical components of many astrophysical systems, including accreting compact binaries and active galactic nuclei (AGN). These outflows can significantly affect a system’s evolution and alter its observational appearance by reprocessing the radiation produced by the central engine. sirocco (Simulating Ionization and Radiation in Outflows Created by Compact Objects – or ‘the code formerly known as python’) is a Sobolev-based Monte Carlo ionization and radiative transfer code. It is designed to simulate the spectra produced by any system with an azimuthally symmetric outflow, from spherical stellar winds to rotating, biconical accretion disc winds. Wind models can either be parametrized or imported, e.g. from hydrodynamical simulations. The radiation sources include an optically thick accretion disc and various central sources with flexible spectra and geometries. The code tracks the ‘photon packets’ produced by the sources in any given simulation as they traverse and interact with the wind. The code assumes radiative near-equilibrium, so the thermal and ionization state can be determined iteratively from these interactions. Once the physical properties in the wind have converged, sirocco can be used to generate synthetic spectra at a series of observer sightlines. Here, we describe the physical assumptions, operation, performance and limitations of the code. We validate it against tardis, cmfgen, and cloudy, finding good agreement, and present illustrative synthetic spectra from disc winds in cataclysmic variables, tidal disruption events, AGN, and X-ray binaries. sirocco is publicly available on GitHub, alongside its associated data, documentation and sample input files covering a wide range of astrophysical applications.

Context. Relativistic magnetic reconnection studies have so far focused on symmetric configurations, where the upstream plasma has identical properties on the two sides of the layer. Yet, just like nonrelativistic reconnection on the dayside of the Earth’s magnetosphere, relativistic reconnection can also operate at the interface between highly asymmetric environments. The boundary layer between a relativistic jet and an accretion flow forming around a supermassive black hole can present asymmetric configurations in terms of plasma composition, bulk velocity, temperature, and magnetization. Aims. We conducted the first study of relativistic magnetic reconnection where the upstream plasma is composed of electron-positron pairs on one side, and electrons and ions on the other. We also investigated the impact of a relativistic symmetric shear flow applied along the reconnecting field lines. Methods. We simulated magnetic reconnection using 2D particle-in-cell simulations. The initial setup was adapted from a classic Harris layer without a guide field, modified to accommodate plasma-composition and shear asymmetries in the upstream medium. Results. For a composition-asymmetric setup, we find that the reconnection dynamics is driven by the electron-ion side, which is the plasma with the lowest magnetization. The energy partition favors accelerating ions at the expense of electrons even more than in a corresponding symmetric setup. With respect to shear, a super-Alfvénic upstream decreases the laboratory-frame reconnection rate, but, unlike in nonrelativistic studies, does not shut off reconnection completely. Conclusions. The asymmetries examined in this work lower the overall efficiency of electron acceleration relative to corresponding symmetric configurations. In the context of a black hole jet-disk boundary, asymmetric reconnection alone is probably not efficient at accelerating electrons to very high energies, but it might facilitate plasma mixing and particle injection for other acceleration channels at the interface.