More Like this

1. Ultra-High-Energy Cosmic Rays Accelerated by Magnetically Dominated Turbulence

   https://doi.org/10.3847/2041-8213/ad955f  

   Comisso, Luca; Farrar, Glennys_R; Muzio, Marco_S (December 2024, The Astrophysical Journal Letters)

   Abstract Ultra-high-energy cosmic rays (UHECRs), particles characterized by energies exceeding 10¹⁸eV, are generally believed to be accelerated electromagnetically in high-energy astrophysical sources. One promising mechanism of UHECR acceleration is magnetized turbulence. We demonstrate from first principles, using fully kinetic particle-in-cell simulations, that magnetically dominated turbulence accelerates particles on a short timescale, producing a power-law energy distribution with a rigidity-dependent, sharply defined cutoff well approximated by the form $f_{\mathrm{cut}}\left(E,E_{\mathrm{cut}}\right)=\mathrm{sech}\left({\left(E/E_{\mathrm{cut}}\right)}^2\right)$. Particle escape from the turbulent accelerating region is energy dependent, witht_esc∝E^−δandδ∼ 1/3. The resulting particle flux from the accelerator follows $\mathit{dN}/\mathit{dEdt}\propto E^{-s}\mathrm{sech}\left({\left(E/E_{\mathrm{cut}}\right)}^2\right)$, withs∼ 2.1. We fit the Pierre Auger Observatory’s spectrum and composition measurements, taking into account particle interactions between acceleration and detection, and show that the turbulence-associated energy cutoff is well supported by the data, with the best-fitting spectral index being $s={2.1}_{-0.13}^{+0.06}$. Our first-principles results indicate that particle acceleration by magnetically dominated turbulence may constitute the physical mechanism responsible for UHECR acceleration. 

   more » « less
2. Spatial Intermittency of Particle Distribution in Relativistic Plasma Turbulence

   https://doi.org/10.3847/1538-4357/accd73  

   Vega, Cristian; Boldyrev, Stanislav; Roytershteyn, Vadim (June 2023, The Astrophysical Journal)

   Abstract Relativistic magnetically dominated turbulence is an efficient engine for particle acceleration in a collisionless plasma. Ultrarelativistic particles accelerated by interactions with turbulent fluctuations form nonthermal power-law distribution functions in the momentum (or energy) space,f(γ)dγ∝γ^−αdγ, whereγis the Lorenz factor. We argue that in addition to exhibiting non-Gaussian distributions over energies, particles energized by relativistic turbulence also become highly intermittent in space. Based on particle-in-cell numerical simulations and phenomenological modeling, we propose that the bulk plasma density has lognormal statistics, while the density of the accelerated particles,n, has a power-law distribution function, $P\left(n\right)\mathit{dn}\propto n^{-\beta }\mathit{dn}$. We argue that the scaling exponents are related asβ≈α+ 1, which is broadly consistent with numerical simulations. Non-space-filling, intermittent distributions of plasma density and energy fluctuations may have implications for plasma heating and for radiation produced by relativistic turbulence. 

   more » « less
3. 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. 

   more » « less
4. Intermittency and Lower Dimensional Dissipation in Incompressible Fluids

   https://doi.org/10.1007/s00205-023-01954-w  

   De_Rosa, Luigi; Isett, Philip (February 2024, Archive for Rational Mechanics and Analysis)

   Abstract In the context of incompressible fluids, the observation that turbulent singular structures fail to be space filling is known as “intermittency”, and it has strong experimental foundations. Consequently, as first pointed out by Landau, real turbulent flows do not satisfy the central assumptions of homogeneity and self-similarity in the K41 theory, and the K41 prediction of structure function exponents$$\zeta _p={p}/{3}$$ ${\zeta }_p=p/3$might be inaccurate. In this work we prove that, in the inviscid case, energy dissipation that is lower-dimensional in an appropriate sense implies deviations from the K41 prediction in everyp-th order structure function for$$p>3$$ $p>3$. By exploiting a Lagrangian-type Minkowski dimension that is very reminiscent of the Taylor’sfrozen turbulencehypothesis, our strongest upper bound on$$\zeta _p$$ ${\zeta }_p$coincides with the$$\beta $$ $\beta$-model proposed by Frisch, Sulem and Nelkin in the late 70s, adding some rigorous analytical foundations to the model. More generally, we explore the relationship between dimensionality assumptions on the dissipation support and restrictions on thep-th order absolute structure functions. This approach differs from the current mathematical works on intermittency by its focus on geometrical rather than purely analytical assumptions. The proof is based on a new local variant of the celebrated Constantin-E-Titi argument that features the use of a third order commutator estimate, the special double regularity of the pressure, and mollification along the flow of a vector field. 

   more » « less
5. 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. 

   more » « less