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1. Generation of Large-scale Magnetic Fields Upstream of Gamma-Ray Burst Afterglow Shocks

   https://doi.org/10.3847/1538-4357/add404  

   Golant, Ryan; Vanthieghem, Arno; Grošelj, Daniel; Sironi, Lorenzo (June 2025, The Astrophysical Journal)

   Abstract The origins of the magnetic fields that power gamma-ray burst (GRB) afterglow emission are not fully understood. One possible channel for generating these fields involves the pre-conditioning of the circumburst medium: in the early afterglow phase, prompt photons streaming ahead of the GRB external shock can pair produce, seeding the upstream with drifting electron–positron pairs and triggering electromagnetic microinstabilities. To study this process, we employ 2D periodic particle-in-cell simulations in which a cold electron–proton plasma is gradually enriched with warm electron–positron pairs injected at mildly relativistic speeds. We find that continuous pair injection drives the growth of large-scale magnetic fields via filamentation-like instabilities; the temporal evolution of the field is self-similar and depends on a single parameter, ${\left(\alpha /\left(t_{\mathrm{f}}{\omega }_{\mathrm{pi}}\right)\right)}^{1/2}t{\omega }_{\mathrm{pi}}$ , whereαis the ratio of final pair beam density to background plasma density,t_fis the duration of pair injection, andω_piis the plasma frequency of background protons. Extrapolating our results to parameter regimes realistic for long GRBs, we find that upstream pair enrichment generates weak magnetic fields on scales much larger than the proton skin depth; for bright bursts, the extrapolated coherence scale at a shock radius ofR ∼ 10¹⁷cm is $\left({\lambda }_y\right)\sim 100c/{\omega }_{\mathrm{pi}}$ , and the corresponding magnetization isσ ∼ 10⁻⁸for typical circumburst parameters. These results may help explain the persistence of magnetic fields at large distances behind GRB shocks. 

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2. Electron Acceleration at Quasi-parallel Nonrelativistic Shocks: A 1D Kinetic Survey

   https://doi.org/10.3847/1538-4357/ad7c4c  

   Gupta, Siddhartha; Caprioli, Damiano; Spitkovsky, Anatoly (November 2024, The Astrophysical Journal)

   Abstract We present a survey of 1D kinetic particle-in-cell simulations of quasi-parallel nonrelativistic shocks to identify the environments favorable for electron acceleration. We explore an unprecedented range of shock speedsv_sh≈ 0.067–0.267c, Alfvén Mach numbers ${\mathcal{M}}_{\mathrm{A}}=5–40$ , sonic Mach numbers ${\mathcal{M}}_{\mathrm{s}}=5–160$ , as well as the proton-to-electron mass ratiosm_i/m_e= 16–1836. We find that high Alfvén Mach number shocks can channel a large fraction of their kinetic energy into nonthermal particles, self-sustaining magnetic turbulence and acceleration to larger and larger energies. The fraction of injected particles is ≲0.5% for electrons and ≈1% for protons, and the corresponding energy efficiencies are ≲2% and ≈10%, respectively. The extent of the nonthermal tail is sensitive to the Alfvén Mach number; when ${\mathcal{M}}_{\mathrm{A}}\lesssim 10$ , the nonthermal electron distribution exhibits minimal growth beyond the average momentum of the downstream thermal protons, independently of the proton-to-electron mass ratio. Acceleration is slow for shocks with low sonic Mach numbers, yet nonthermal electrons still achieve momenta exceeding the downstream thermal proton momentum when the shock Alfvén Mach number is large enough. We provide simulation-based parameterizations of the transition from thermal to nonthermal distribution in the downstream (found at a momentum around $p_{\mathrm{i},\mathrm{e}}/m_{\mathrm{i}}{v}_{\mathrm{sh}}\approx 3\sqrt{m_{\mathrm{i},\mathrm{e}}/m_{\mathrm{i}}}$ ), as well as the ratio of nonthermal electron to proton number density. The results are applicable to many different environments and are important for modeling shock-powered nonthermal radiation. 

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3. The Kelvin–Helmholtz Instability at the Boundary of Relativistic Magnetized Jets

   https://doi.org/10.3847/2041-8213/acdfcf  

   Chow, Anthony; Davelaar, Jordy; Rowan, Michael E; Sironi, Lorenzo (July 2023, The Astrophysical Journal Letters)

   Abstract We study the linear stability of a planar interface separating two fluids in relative motion, focusing on conditions appropriate for the boundaries of relativistic jets. The jet is magnetically dominated, whereas the ambient wind is gas-pressure-dominated. We derive the most general form of the dispersion relation and provide an analytical approximation of its solution for an ambient sound speed much smaller than the jet Alfvén speedv_A, as appropriate for realistic systems. The stability properties are chiefly determined by the angleψbetween the wavevector and the jet magnetic field. Forψ=π/2, magnetic tension plays no role, and our solution resembles the one of a gas-pressure-dominated jet. Here, only sub-Alfvénic jets are unstable ( $0<M_e\equiv \left(v/v_{\mathrm{A}}\right)\cos \theta <1$ , wherevis the shear velocity andθthe angle between the velocity and the wavevector). Forψ= 0, the free energy in the velocity shear needs to overcome the magnetic tension, and only super-Alfvénic jets are unstable ( $1<M_e<\sqrt{{(1+{\mathrm{\Gamma }}_w^2)/[1+(v_{\mathrm{A}}/c)}^2{\mathrm{\Gamma }}_w^2]}$ , with Γ_wthe wind adiabatic index). Our results have important implications for the propagation and emission of relativistic magnetized jets. 

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4. Observation of the Galactic Center PeVatron beyond 100 TeV with HAWC

   https://doi.org/10.3847/2041-8213/ad772e  

   Albert, A; Alfaro, R; Alvarez, C; Andrés, A; Arteaga-Velázquez, J C; Avila_Rojas, D; Ayala_Solares, H A; Babu, R; Belmont-Moreno, E; Bernal, A; et al (September 2024, The Astrophysical Journal Letters)

   Abstract We report an observation of ultrahigh-energy (UHE) gamma rays from the Galactic center (GC) region, using 7 yr of data collected by the High-Altitude Water Cherenkov (HAWC) Observatory. The HAWC data are best described as a point-like source (HAWC J1746-2856) with a power-law spectrum ( $\mathit{d}N/\mathit{d}E=\varphi {\left(E/26\mathrm{TeV}\right)}^{\gamma }$ ), whereγ= −2.88 ± 0.15_stat− 0.1_sysandϕ= 1.5 × 10⁻¹⁵(TeV cm²s)⁻¹ $\pm {0.3}_{\mathrm{stat}}{}_{-{0.13}_{\mathrm{sys}}}^{+{0.08}_{\mathrm{sys}}}$ extending from 6 to 114 TeV. We find no evidence of a spectral cutoff up to 100 TeV using HAWC data. Two known point-like gamma-ray sources are spatially coincident with the HAWC gamma-ray excess: Sgr A* (HESS J1745-290) and the Arc (HESS J1746-285). We subtract the known flux contribution of these point sources from the measured flux of HAWC J1746-2856 to exclude their contamination and show that the excess observed by HAWC remains significant (>5σ), with the spectrum extending to >100 TeV. Our result supports that these detected UHE gamma rays can originate via hadronic interaction of PeV cosmic-ray protons with the dense ambient gas and confirms the presence of a proton PeVatron at the GC. 

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5. Imaging nuclear shape through anisotropic and radial flow in high-energy heavy-ion collisions

   https://doi.org/10.1088/1361-6633/ae0fc3  

   Collaboration, The STAR (October 2025, Reports on Progress in Physics)

   Abstract Most atomic nuclei exhibit ellipsoidal shapes characterized by quadrupole deformationβ₂and triaxialityγ, and sometimes even a pear-like octupole deformationβ₃. The STAR experiment introduced a new ‘imaging-by-smashing’ technique ((STAR Collaboration) 2024Nature63567; Jia 2025Rep. Prog. Phys.88092301) to image the nuclear global shape by colliding nuclei at ultra-relativistic speeds and analyzing outgoing debris. Features of nuclear shape manifest in collective observables like anisotropic flowv_nand radial flow via mean transverse momentum $\left[p_{\mathrm{T}}\right]$ . We present new measurements of the variances ofv_n(n = 2, 3, and 4) and $\left[p_{\mathrm{T}}\right]$ , and the covariance of $v_n^2$ with $\left[p_{\mathrm{T}}\right]$ , in collisions of highly deformed²³⁸U and nearly spherical¹⁹⁷Au. Ratios of these observables between the two systems effectively suppress common final-state effects, isolating the strong impact of uranium’s deformation. By comparing results with state-of-the-art hydrodynamic model calculations, we extract ${\beta }_{2\mathrm{U}}$ and ${\gamma }_{\mathrm{U}}$ values consistent with those deduced from low-energy nuclear structure measurements. Measurements ofv₃and its correlation with $\left[p_{\mathrm{T}}\right]$ also provide the first experimental suggestion of a possible octupole deformation for²³⁸U. These findings provide significant support for using high-energy collisions to explore nuclear shapes on femtosecond timescales, with implications for both nuclear structure and quark-gluon plasma studies. 

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