arXiv subjects

Fabio Buffoli

Publications and source records attributed to Fabio Buffoli.

3 recordsLinked to original sources

arXiv2026

An Operative Viability Boundary for Relaxed Single-Particle Penrose Extraction in Kerr-Vaidya Spacetimes

We present a corrected numerical investigation of the Penrose process in the Kerr-Vaidya metric for a rotating black hole losing mass at constant rate. We fix two errors from an earlier version: an incorrect metric component g_r phi (verified by transformation from Boyer-Lindquist coordinates) and the omission of the Wald/Christodoulou area theorem in the split optimization. Using the standard single-particle treatment (energy and angular momentum conserved at the split, but not the escaping fragment's radial momentum), we compute optimal trajectories and the bounded energy gain Delta E = E3 - E1 versus spin a/M and mass-loss rate alpha. For static Kerr holes, Delta E rises with spin from about 0.106 M at a/M = 0.8 to 0.245 M at a/M = 0.99. In the dynamic case, holding a fixed while m(v) decreases causes the effective spin a/m(v) to drift strongly toward extremality (from 0.81 to 0.9999), a geometric effect that makes static-dynamic comparisons at equal nominal a/M misleading. We map the critical mass-loss rate alpha_crit(a) above which no viable extraction is found (using m >= 0), obtaining alpha_crit(0.90) ~ 0.036, alpha_crit(0.95) ~ 0.019 and alpha_crit(0.99) ~ 0.0038, decreasing with spin. A check with full four-momentum conservation recovers the Bardeen-Press-Teukolsky bound near the horizon, but escaping solutions are rare away from it; thus our main results refer to the relaxed single-particle formulation common in the literature.

gr-qc↗

arXiv2026

Hamiltonian variational reconstruction of the 3D magnetic geometry of relativistic jets: accuracy across GRMHD models and the fundamental sign degeneracy

Every resolved image of a relativistic jet encodes its magnetic field, yet no image records which way the field points along the axis. Synchrotron intensity and linear polarization are blind to this: both probe the field only through even combinations of its components. We introduce H-MOG, a variational method that reconstructs the 3D field on a lattice from two projected observables, the jet width W(z) and the linear polarization p(z), regularized by a Hamiltonian prior and optimized with automatic differentiation. We apply H-MOG to ten GRMHD simulations spanning MAD and SANE states and five black-hole spins (a* = -0.94, -0.5, 0, +0.5, +0.94), and test three routes to break the intrinsic sign degeneracy of the reconstruction: a Faraday rotation-measure term, full 3D sampling, and a Blandford-Znajek spin prior. The unsigned field orientation is recovered at <|cos|> ~ 0.95-0.98, far above the random expectation of 0.5. The sense of the poloidal field, however, is not recovered, and we prove it cannot be: W and p are invariant under B -> -B. All three routes to break this degeneracy fail for distinct physical reasons; the spin-sense relation in these turbulent jets is not monotonic, differing sharply between MAD and SANE. Recovering the sense requires a parity-odd observable: Faraday tomography or circular polarization.

astro-ph.HE↗

arXiv2026

HAMCOR: A physics-driven Hamiltonian framework for inferring AGN coronal geometry from X-ray reverberation lags

We present HAMCOR (Hamiltonian-based AGN Multi-constraint CORonal inference framework), a geometry-agnostic method for inferring the X-ray coronal structure of accreting black holes using reverberation-lag measurements. Unlike conventional template-fitting approaches, HAMCOR reframes coronal geometry inference as the ground-state selection of a physical Hamiltonian. The corona is represented as a discrete emissivity distribution over a cylindrical grid, and its geometry emerges from five competing physical constraints: magnetic coherence, lag consistency, illumination consistency, pair-production stability, and energy budget feasibility. Minimisation is performed via projected gradient descent with Armijo backtracking line search on the probability simplex. We validate HAMCOR on three synthetic geometries (lamppost, column, ring) using the same grid as the real-data fits, recovering spatial correlations rho = 0.24, 0.50, 0.12 and fractional lag errors below 24 per cent. A hyperparameter sensitivity analysis confirms robustness over more than one order of magnitude in the coupling constants. We apply HAMCOR to five sources spanning seven orders of magnitude in black hole mass: four AGN observed with XMM-Newton (Mrk 335, 1H 0707-495, IRAS 13224-3809, MCG-6-30-15) and the stellar-mass black hole binary Cyg X-1 (M_bh = 14.8 M_sun), recovering consistent extended disc-corona geometries across the full mass range. We further present a multi-epoch analysis of Mrk 335 across five XMM-Newton observations (2006-2019), revealing that the coronal centroid remains stable at (R_c, z_c) ~ (6.3, 0.5) r_g across flux states spanning a factor of ~15 in reverberation lag amplitude, arguing against a collapsing or expanding lamppost. Schwarzschild-Shapiro delay corrections amount to ~79 per cent of the flat-spacetime lag on average; the recovered spatial morphology is robust to this correction.

astro-ph.HE↗