arXiv subjects

Zachary L. Andalman

Publications and source records attributed to Zachary L. Andalman.

6 recordsLinked to original sources

arXiv2026

Beta-Particle Transport and Thermalization in Kilonova Ejecta with Detailed Atomic Microphysics

When two neutron stars collide, they eject material containing heavy nuclei formed by the rapid neutron capture process ($r$-process). As these nuclei decay, they power a bright optical/near-infrared transient known as a kilonova (KN). Modeling KN emission is a complex problem involving atomic opacities, radiation transport, and heating powered by the thermalization of radioactive decay products like $γ$-rays, $α$-particles, and $β$-particles. For heating by $γ$-rays, many KN modeling codes do full radiation transport calculations. However, heating by $α$- and $β$-particles relies on simplified descriptions of collisions and transport, and remains an important source of uncertainty in KN models. In this paper, we study the thermalization and transport of $β$-particles. To study thermalization, we use evaluated atomic physics data to estimate per-species contributions to energy deposition, scattering, and electron impact ionization, which we make available online. To include non-local effects, we develop a fully relativistic framework for charged particle transport in a spherically symmetric, homologously expanding ejecta, considering two limiting magnetic-field geometries. Non-local energy deposition and escape reduce thermalization efficiency, especially in the innermost and outermost ejecta, lowering the ejecta temperature and ionization state compared to local deposition models. Coulomb scattering partially offsets these effects by trapping particles at intermediate times $\sim 3-30$ days. Ionization by secondary electrons significantly enhances the overall ionization rate. We provide analytic prescriptions for the spatially dependent thermalization efficiency for use in future light-curve calculations. Our results demonstrate that evaluated atomic data and charged-particle transport should be incorporated into the next generation of KN models.

astro-ph.HE↗

arXiv2026

Supermassive Black Hole Growth in Massive Galaxies at Cosmic Dawn

Among the emerging excess of massive, bright galaxies at Cosmic Dawn $z \gtrsim 9$ seen by the James Webb Space Telescope, several exhibit spectral features associated with active galactic nuclei (AGN). These AGN candidates suggest that supermassive black holes (SMBHs) grow rapidly in the early Universe. In a series of numerical experiments, we investigate how SMBHs grow within and influence the most massive galaxies at Cosmic Dawn using cosmological hydrodynamic zoom-in simulations run with the adaptive mesh refinement code \textsc{ramses}. Our suite of simulations explore how super-Eddington accretion, seed mass, and the strength of feedback influence SMBH-galaxy co-evolution in the most massive galaxies ($M_\star \gtrsim 10^8 M_\odot$) of the early Universe ($z \sim 15 - 9$). The environment which our numerical experiments reside in is an overdensity that collapses into a $\sim 10^{11} M_\odot$ halo by $z \sim 9$. Within this type of environment we find that SMBH growth is sensitive to stellar feedback which generates a turbulent-multiphase interstellar medium (ISM) that stochastically starves the SMBH. In the absence of AGN feedback, we find that the SMBH is starved $\sim 50\%$ of the time after the onset of star formation in the galaxy. SMBH growth can become self-regulated by AGN feedback if the SMBH becomes massive enough, either by accretion or seeding, for its feedback to dominate the surrounding nuclear region. We find no evidence of galaxy-scale, AGN-driven quenching in the star formation rate (SFR) across all simulations in our suite.

astro-ph.GA↗

arXiv2026

Resolving the (Debate About) Nozzle Shocks in Tidal Disruption Events

When a star passes within the Roche limit of a supermassive black hole (SMBH), it is pulled apart by the BH's tidal field in a tidal disruption event (TDE). The resulting flare is powered by the circularization and accretion of bound stellar debris, which initially returns to the BH on eccentric orbits in a thin debris stream. The returning fluid elements follow inclined orbits that converge near pericenter, resulting in extreme vertical compression to scales $10^{-4}~R_\odot$ and the formation of a nozzle shock. Dissipation at the nozzle shock may affect circularization by altering the properties of the debris stream, but its role is the subject of ongoing debate. We develop an idealized model for the debris stream evolution combining 3D smoothed-particle hydrodynamics simulations, the semi-analytic affine model, and 1D finite-volume hydrodynamic simulations. Because our model is computationally cheap, we can unambiguously resolve the nozzle shock, use a realistic equation of state, and follow the debris stream evolution at many different times. Near peak fallback, Hydrogen recombination and molecular Hydrogen formation broaden the stream by a factor $\sim 5$, enhancing dissipation at the nozzle. However, the dissipation is still insufficient to directly circularize the debris by in-plane pressure gradients. Instead, the thicker stream substantially increases the likelihood that the stream self-intersects on the second orbit, despite relativistic nodal precession. The stream properties at self-intersection are sensitive to dissipation at the nozzle and the timing of focal points where the ballistic trajectories of the debris converge. Our results clarify the nozzle shock's role in circularization in TDEs, providing a foundation for more realistic circularization and emission models.

astro-ph.HE↗

arXiv2026

On the origin of anomalous dissipation in simulations of tidal disruption events

In a tidal disruption event (TDE), a star is destroyed by the tidal field of a supermassive black hole. The stellar debris is initially placed on highly elliptical orbits, and a longstanding question in TDE theory is: How does the stellar debris circularize into a disc and accrete? The originally proposed answer to this question is self-intersection shocks, where relativistic apsidal precession results in a strong collision between the incoming and outgoing material. However, global simulations of TDEs tend to find enhanced hydrodynamical dissipation prior to any intersections of the debris orbits, with the material ``fanning out'' into a wide-angle and partially-unbound outflow upon passing through pericenter. We show that this dissipation is numerical in origin and arises from a combination of 1) the change in the kinematics of the debris as it passes through pericenter, with its velocity profile along the stream transitioning from strongly diverging pre-pericenter to strongly converging post-pericenter, and 2) the dependence of numerical algorithms (viscosity switches for particle-based methods and Riemann solvers for Godunov-based schemes) on the diverging vs. converging nature of the fluid. We support this conclusion with analytical and numerical modeling. We discuss possible resolutions to these issues as well as the implications of our findings in the context of observations.

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arXiv2024

Nozzle Shocks, Disk Tearing and Streamers Drive Rapid Accretion in 3D GRMHD Simulations of Warped Thin Disks

The angular momentum of gas feeding a black hole (BH) is typically misaligned with respect to the BH spin, resulting in a tilted accretion disk. Rotation of the BH drags the surrounding space-time, manifesting as Lense-Thirring torques that lead to disk precession and warping. We study these processes by simulating a thin ($H/r=0.02$), highly tilted ($\mathcal{T}=65^\circ$) accretion disk around a rapidly rotating ($a=0.9375$) BH at extremely high resolutions, which we performed using the general-relativistic magnetohydrodynamic (GRMHD) code H-AMR. The disk becomes significantly warped and continuously tears into two individually precessing sub-disks. We find that mass accretion rates far exceed the standard $α$-viscosity expectations. We identify two novel dissipation mechanisms specific to warped disks that are the main drivers of accretion, distinct from the local turbulent stresses that are usually thought to drive accretion. In particular, we identify extreme scale height oscillations that occur twice an orbit throughout our disk. When the scale height compresses, `nozzle' shocks form, dissipating orbital energy and driving accretion. Separate from this phenomenon, there is also extreme dissipation at the location of the tear. This leads to the formation of low-angular momentum `streamers' that rain down onto the inner sub-disk, shocking it. The addition of low angular momentum gas to the inner sub-disk causes it to rapidly accrete, even when it is transiently aligned with the BH spin and thus unwarped. These mechanisms, if general, significantly modify the standard accretion paradigm. Additionally, they may drive structural changes on much shorter timescales than expected in $α$-disks, potentially explaining some of the extreme variability observed in active galactic nuclei.

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arXiv2021

Tidal Disruption Disks Formed and Fed by Stream-Stream and Stream-Disk Interactions in Global GRHD Simulations

When a star passes close to a supermassive black hole (BH), the BH's tidal forces rip it apart into a thin stream, leading to a tidal disruption event (TDE). In this work, we study the post-disruption phase of TDEs in general relativistic hydrodynamics (GRHD) using our GPU-accelerated code H-AMR. We carry out the first grid-based simulation of a deep-penetration TDE ($β$=7) with realistic system parameters: a black-hole-to-star mass ratio of $10^6$, a parabolic stellar trajectory, and a nonzero BH spin. We also carry out a simulation of a tilted TDE whose stellar orbit is inclined relative to the BH midplane. We show that for our aligned TDE, an accretion disk forms due to the dissipation of orbital energy with $\sim$20 percent of the infalling material reaching the BH. The dissipation is initially dominated by violent self-intersections and later by stream-disk interactions near the pericenter. The self-intersections completely disrupt the incoming stream, resulting in five distinct self-intersection events separated by approximately 12 hours and a flaring in the accretion rate. We also find that the disk is eccentric with mean eccentricity e$\approx$0.88. For our tilted TDE, we find only partial self-intersections due to nodal precession near pericenter. Although these partial intersections eject gas out of the orbital plane, an accretion disk still forms with a similar accreted fraction of the material to the aligned case. These results have important implications for disk formation in realistic tidal disruptions. For instance, the periodicity in accretion rate induced by the complete stream disruption may explain the flaring events from Swift J1644+57.

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