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1. The Origin of Power-law Spectra in Relativistic Magnetic Reconnection

   https://doi.org/10.3847/2041-8213/acfe7c  

   Zhang, Hao; Sironi, Lorenzo; Giannios, Dimitrios; Petropoulou, Maria (October 2023, The Astrophysical Journal Letters)

   Abstract Magnetic reconnection is often invoked as a source of high-energy particles, and in relativistic astrophysical systems it is regarded as a prime candidate for powering fast and bright flares. We present a novel analytical model—supported and benchmarked with large-scale three-dimensional kinetic particle-in-cell simulations in electron–positron plasmas—that elucidates the physics governing the generation of power-law energy spectra in relativistic reconnection. Particles with Lorentz factorγ≳ 3σ(here,σis the magnetization) gain most of their energy in the inflow region, while meandering between the two sides of the reconnection layer. Their acceleration time is $t_{\mathrm{acc}}\sim \gamma {\eta }_{\mathrm{rec}}^{-1}{\omega }_{\mathrm{c}}^{-1}\simeq 20\gamma {\omega }_{\mathrm{c}}^{-1}$, whereη_rec≃ 0.06 is the inflow speed in units of the speed of light andω_c=eB₀/mcis the gyrofrequency in the upstream magnetic field. They leave the region of active energization aftert_esc, when they get captured by one of the outflowing flux ropes of reconnected plasma. We directly measuret_escin our simulations and find thatt_esc∼t_accforσ≳ few. This leads to a universal (i.e.,σ-independent) power-law spectrum ${\mathit{dN}}_{\mathrm{free}}/d\gamma \propto {\gamma }^{-1}$for the particles undergoing active acceleration, and $\mathit{dN}/d\gamma \propto {\gamma }^{-2}$for the overall particle population. Our results help to shed light on the ubiquitous presence of power-law particle and photon spectra in astrophysical nonthermal sources. 

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2. Compression Acceleration of Protons and Heavier Ions at the Heliospheric Current Sheet

   https://doi.org/10.3847/1538-4357/ad6e80  

   Murtas, Giulia; Li, Xiaocan; Guo, Fan (October 2024, The Astrophysical Journal)

   Abstract Recent observations by the Parker Solar Probe (PSP) suggest that protons and heavier ions are accelerated to high energies by magnetic reconnection at the heliospheric current sheet (HCS). By solving the energetic particle transport equation in large-scale MHD simulations, we study the compression acceleration of protons and heavier ions in the reconnecting HCS. We find that the acceleration of multispecies ions results in nonthermal power-law distributions with a spectral index consistent with the PSP observations. Our study shows that the high-energy cutoff of protons can reach $E_{\max }\sim 0.1$–1 MeV depending on the particle diffusion coefficients. We also study how the high-energy cutoff of different ion species scales with the charge-to-mass ratio $E_{\max }\propto {\left(Q/M\right)}^{\alpha }$. When determining the diffusion coefficients from the quasi-linear theory with a Kolmogorov magnetic power spectrum, we find thatα∼ 0.4, which is somewhat smaller thanα∼ 0.7 observed by PSP. 

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3. 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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4. The Ionization and Dynamics of the Makani Galactic Wind

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

   Rupke, David S.; Coil, Alison L.; Perrotta, Serena; Davis, Julie D.; Diamond-Stanic, Aleksandar M.; Geach, James E.; Hickox, Ryan C.; Moustakas, John; Petter, Grayson C.; Rudnick, Gregory H.; et al (April 2023, The Astrophysical Journal)

   Abstract The Makani galaxy hosts the poster child of a galactic wind on scales of the circumgalactic medium. It consists of a two-episode wind in which the slow, outer wind originated 400 Myr ago (Episode I;R_I= 20 − 50 kpc) and the fast, inner wind is 7 Myr old (Episode II;R_II= 0 − 20 kpc). While this wind contains ionized, neutral, and molecular gas, the physical state and mass of the most extended phase—the warm, ionized gas—are unknown. Here we present Keck optical spectra of the Makani outflow. These allow us to detect hydrogen lines out tor= 30–40 kpc and thus constrain the mass, momentum, and energy in the wind. Many collisionally excited lines are detected throughout the wind, and their line ratios are consistent with 200–400 km s⁻¹shocks that power the ionized gas, withv_shock=σ_wind. Combining shock models, density-sensitive line ratios, and mass and velocity measurements, we estimate that the ionized mass and outflow rate in the Episode II wind could be as high as those of the molecular gas: $M_{\mathrm{II}}^{\mathrm{H}\mathrm{II}}\sim M_{\mathrm{II}}^{{\mathrm{H}}_2}=(1-2)\times {10}^9{M}_{\odot }$and $\mathit{dM}/{\mathit{dt}}_{\mathrm{II}}^{\mathrm{H}\mathrm{II}}\sim \mathit{dM}/{\mathit{dt}}_{\mathrm{II}}^{{\mathrm{H}}_2}=170-250M_{\odot }$yr⁻¹. The outer wind has slowed, so that $\mathit{dM}/{\mathit{dt}}_{\mathrm{I}}^{\mathrm{H}\mathrm{II}}\sim 10M_{\odot }$yr⁻¹, but it contains more ionized gas, $M_{\mathrm{I}}^{\mathrm{H}\mathrm{II}}=5\times {10}^9$M_⊙. The momentum and energy in the recent Episode II wind imply a momentum-driven flow (p“boost” ∼7) driven by the hot ejecta and radiation pressure from the Eddington-limited, compact starburst. Much of the energy and momentum in the older Episode I wind may reside in a hotter phase, or lie further into the circumgalactic medium. 

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5. Microphysics of Relativistic Collisionless Electron-ion-positron Shocks

   https://doi.org/10.3847/1538-4357/ac713e  

   Grošelj, Daniel; Sironi, Lorenzo; Beloborodov, Andrei M. (July 2022, The Astrophysical Journal)

   Abstract We perform particle-in-cell simulations to elucidate the microphysics of relativistic weakly magnetized shocks loaded with electron-positron pairs. Various external magnetizationsσ≲ 10⁻⁴and pair-loading factorsZ_±≲ 10 are studied, whereZ_±is the number of loaded electrons and positrons per ion. We find the following: (1) The shock becomes mediated by the ion Larmor gyration in the mean field whenσexceeds a critical valueσ_Lthat decreases withZ_±. Atσ≲σ_Lthe shock is mediated by particle scattering in the self-generated microturbulent fields, the strength and scale of which decrease withZ_±, leading to lowerσ_L. (2) The energy fraction carried by the post-shock pairs is robustly in the range between 20% and 50% of the upstream ion energy. The mean energy per post-shock electron scales as ${\overline{E}}_{\mathrm{e}}\propto {\left(Z_{\pm }+1\right)}^{-1}$. (3) Pair loading suppresses nonthermal ion acceleration at magnetizations as low asσ≈ 5 × 10⁻⁶. The ions then become essentially thermal with mean energy ${\overline{E}}_{\mathrm{i}}$, while electrons form a nonthermal tail, extending from $E\sim {\left(Z_{\pm }+1\right)}^{-1}{\overline{E}}_{\mathrm{i}}$to ${\overline{E}}_{\mathrm{i}}$. Whenσ= 0, particle acceleration is enhanced by the formation of intense magnetic cavities that populate the precursor during the late stages of shock evolution. Here, the maximum energy of the nonthermal ions and electrons keeps growing over the duration of the simulation. Alongside the simulations, we develop theoretical estimates consistent with the numerical results. Our findings have important implications for models of early gamma-ray burst afterglows. 

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