arXiv subjects

Lorenzo Sironi

Publications and source records attributed to Lorenzo Sironi.

At least 91 records · Page 5Linked to original sources

arXiv2021

Fast particle acceleration in three-dimensional relativistic reconnection

Magnetic reconnection is invoked as one of the primary mechanisms to produce energetic particles. We employ large-scale three-dimensional (3D) particle-in-cell simulations of reconnection in magnetically-dominated ($σ=10$) pair plasmas to study the energization physics of high-energy particles. We identify a novel acceleration mechanism that only operates in 3D. For weak guide fields, 3D plasmoids / flux ropes extend along the $z$ direction of the electric current for a length comparable to their cross-sectional radius. Unlike in 2D simulations, where particles are buried in plasmoids, in 3D we find that a fraction of particles with $γ\gtrsim 3σ$ can escape from plasmoids by moving along $z$, and so they can experience the large-scale fields in the upstream region. These "free" particles preferentially move in $z$ along Speiser-like orbits sampling both sides of the layer, and are accelerated linearly in time -- their Lorentz factor scales as $γ\propto t$, in contrast to $γ\propto \sqrt{t}$ in 2D. The energy gain rate approaches $\sim eE_{\rm rec}c$, where $E_{\rm rec}\simeq 0.1 B_0$ is the reconnection electric field and $B_0$ the upstream magnetic field. The spectrum of free particles is hard, $dN_{\rm free}/dγ\propto γ^{-1.5}$, contains $\sim 20\%$ of the dissipated magnetic energy independently of domain size, and extends up to a cutoff energy scaling linearly with box size. Our results demonstrate that relativistic reconnection in GRB and AGN jets may be a promising mechanism for generating ultra-high-energy cosmic rays.

astro-ph.HE↗

arXiv2021

Fast dissipation of Colliding Alfvén Waves in a Magnetically Dominated Plasma

Magnetic energy around compact objects often dominates over plasma rest mass, and its dissipation can power the object luminosity. We describe a dissipation mechanism which works faster than magnetic reconnection. The mechanism involves two strong Alfvén waves with anti-aligned magnetic fields $\boldsymbol{B}_1$ and $\boldsymbol{B}_2$ that propagate in opposite directions along background magnetic field $\boldsymbol{B}_0$ and collide. The collision forms a thin current sheet perpendicular to $\boldsymbol{B}_0$, which absorbs the incoming waves. The current sheet is sustained by electric field $\boldsymbol{E}$ breaking the magnetohydrodynamic condition $E 1/2$. It dissipates a large fraction of the wave energy, $f=(2A-1)/A^2$, reaching $100\%$ when $A=1$. The plane geometry allows one to see the dissipation process in a one-dimensional simulation. We also perform two-dimensional simulations, enabling spontaneous breaking of the plane symmetry by the tearing instability of the current sheet. At moderate $A$ of main interest the tearing instability is suppressed. Dissipation transitions to normal, slower, magnetic reconnection at $A\gg 1$. The fast dissipation described in this paper may occur in various objects with perturbed magnetic fields, including magnetars, jets from accreting black holes, and pulsar wind nebulae.

astro-ph.HE↗

arXiv2021

A fully-kinetic model for orphan gamma-ray flares in blazars

Blazars emit a highly-variable non-thermal spectrum. It is usually assumed that the same non-thermal electrons are responsible for the IR-optical-UV emission (via synchrotron) and the gamma-ray emission (via inverse Compton). Hence, the light curves in the two bands should be correlated. Orphan gamma-ray flares (i.e., lacking a luminous low-frequency counterpart) challenge our theoretical understanding of blazars. By means of large-scale two-dimensional radiative particle-in-cell simulations, we show that orphan gamma-ray flares may be a self-consistent by-product of particle energization in turbulent magnetically-dominated pair plasmas. The energized particles produce the gamma-ray flare by inverse Compton scattering an external radiation field, while the synchrotron luminosity is heavily suppressed since the particles are accelerated nearly along the direction of the local magnetic field. The ratio of inverse Compton to synchrotron luminosity is sensitive to the initial strength of turbulent fluctuations (a larger degree of turbulent fluctuations weakens the anisotropy of the energized particles, thus increasing the synchrotron luminosity). Our results show that the anisotropy of the non-thermal particle population is key to modeling the blazar emission.

astro-ph.HE↗

arXiv2021

Shock-powered radio precursors of neutron star mergers from accelerating relativistic binary winds

During the final stages of a compact object merger, if at least one of the binary components is a magnetized neutron star (NS), then its orbital motion substantially expands the NS's open magnetic flux -- and hence increases its wind luminosity -- relative to that of an isolated pulsar. As the binary orbit shrinks due to gravitational radiation, the power and speed of this binary-induced inspiral wind may (depending on pair loading) secularly increase, leading to self-interaction and internal shocks in the outflow beyond the binary orbit. The magnetized forward shock can generate coherent radio emission via the synchrotron maser process, resulting in an observable radio precursor to binary NS merger. We perform 1D relativistic hydrodynamical simulations of shock interaction in the accelerating binary NS wind, assuming that the inspiral wind efficiently converts its Poynting flux into bulk kinetic energy prior to the shock radius. This is combined with the shock maser spectrum from particle-in-cell simulations, to generate synthetic radio light curves. The precursor burst with a fluence of $\sim1$ Jy$\cdot$ms at $\sim$GHz frequencies lasts $\sim 1-500$ ms following the merger for a source at $\sim3$ Gpc ($B_{\rm d}/10^{12}$ G)$^{8/9}$, where $B_{\rm d}$ is the dipole field strength of the more strongly-magnetized star. Given an outflow geometry concentrated along the binary equatorial, the signal may be preferentially observable for high-inclination systems, i.e. those least likely to produce a detectable gamma-ray burst.

astro-ph.HE↗

arXiv2020

Kink-driven magnetic reconnection in relativistic jets: consequences for X-ray polarimetry of BL Lacs

We investigate with relativistic MHD simulations the dissipation physics of BL Lac jets, by studying the synchrotron polarization signatures of particles accelerated by the kink instability in a magnetically-dominated plasma column. The nonlinear stage of the kink instability generates current sheets, where particles can be efficiently accelerated via magnetic reconnection. We identify current sheets as regions where s = J d/B is above some predefined threshold (where B is the field strength, J the current density and d the grid scale), and assume that the particle injection efficiency scales as proportional to the square of the current. X-ray emitting particles have short cooling times, so they only probe the field geometry of their injection sites. In contrast, particles emitting in the optical band, which we follow self-consistently as they propagate away from their injection sites while cooling, sample a larger volume, and so they may be expected to produce different polarimetric signatures. We find that the degree of polarization is roughly the same between X-ray and optical bands, because even the optical-emitting particles do not travel far from the current sheet where they were injected, due to lack of sufficient kink-generated turbulence. The polarization angle shows a different temporal evolution between the two bands, due to the different regions probed by X-ray and optical emitting particles. In view of the upcoming IXPE satellite, our results can help constrain whether kink-induced reconnection (as opposed to shocks) can be the source of multi-wavelength emission from BL Lacs.

astro-ph.HE↗

arXiv2020

Self-modulation of Fast Radio Bursts

Fast Radio Bursts (FRBs) are extreme astrophysical phenomena entering the realm of non-linear optics, a field developed in laser physics. A classical non-linear effect is self-modulation. We examine the propagation of FRBs through the circumburst environment using the idealised setup of a monochromatic linearly-polarised GHz wave propagating through a uniform plasma slab of density $N$ at distance $R$ from the source. We find that self-modulation occurs if the slab is located within a critical radius $R_{\rm crit}\sim 10^{17}(N/10^2{\rm\; cm}^{-3})(L/10^{42}{\rm\; erg\; s}^{-1}){\rm\; cm}$, where $L$ is the isotropic equivalent of the FRB luminosity. Self-modulation breaks the burst into pancakes transverse to the radial direction. When $R\lesssim R_{\rm crit}$, the transverse size of the pancakes is smaller than the Fresnel scale. The pancakes are strongly diffracted as the burst exits the slab, and interference between the pancakes produces a frequency modulation of the observed intensity with a sub-GHz bandwidth. When $R\sim R_{\rm crit}$, the transverse size of the pancakes becomes comparable with the Fresnel scale, and the effect of diffraction is weaker. The observed intensity is modulated on a timescale of ten microseconds, which corresponds to the radial width of the pancakes. Our results suggest that self-modulation may cause the temporal and frequency structure observed in FRBs.

astro-ph.HE↗

arXiv2020

Reconnection-driven particle acceleration in relativistic shear flows

Particle energization in shear flows is invoked to explain non-thermal emission from the boundaries of relativistic astrophysical jets. Yet, the physics of particle injection, i.e., the mechanism that allows thermal particles to participate in shear-driven acceleration, remains unknown. With particle-in-cell simulations, we study the development of Kelvin-Helmholtz (KH) instabilities seeded by the velocity shear between a relativistic magnetically-dominated electron-positron jet and a weakly magnetized electron-ion ambient plasma. We show that, in their nonlinear stages, KH vortices generate kinetic-scale reconnection layers, which efficiently energize the jet particles, thus providing a first-principles mechanism for particle injection into shear-driven acceleration. Our work lends support to spine-sheath models of jet emission - with a fast core/spine surrounded by a slower sheath - and can explain the origin of radio-emitting electrons at the boundaries of relativistic jets.

astro-ph.HE↗

arXiv2020

Electron Heating in Perpendicular Low-Beta Shocks

Collisionless shocks heat electrons in the solar wind, interstellar blast waves, and hot gas permeating galaxy clusters. How much shock heating goes to electrons instead of ions, and what plasma physics controls electron heating? We simulate 2-D perpendicular shocks with a fully kinetic particle-in-cell code. For magnetosonic Mach number $\mathcal{M}_\mathrm{ms} \sim 1$-$10$ and plasma beta $β_\mathrm{p} \lesssim 4$, the post-shock electron/ion temperature ratio $T_\mathrm{e}/T_\mathrm{i}$ decreases from $1$ to $0.1$ with increasing $\mathcal{M}_\mathrm{ms}$. In a representative $\mathcal{M}_\mathrm{ms}=3.1$, $β_\mathrm{p}=0.25$ shock, electrons heat above adiabatic compression in two steps: ion-scale $E_\parallel = \vec{E} \cdot \hat{b}$ accelerates electrons into streams along $\vec{B}$, which then relax via two-stream-like instability. The $\vec{B}$-parallel heating is mostly induced by waves; $\vec{B}$-perpendicular heating is mostly adiabatic compression by quasi-static fields.

physics.space-ph↗

arXiv2020

A First-Principle Model for Polarization Swings during Reconnection-Powered Flares

We show that magnetic reconnection in a magnetically-dominated fast-cooling plasma can naturally produce bright flares accompanied by rotations in the synchrotron polarization vector. With particle-in-cell simulations of reconnection, we find that flares are powered by efficient particle acceleration at the interface of merging magnetic flux ropes, or "plasmoids". The accelerated particles stream through the post-merger plasmoid towards the observer, thus progressively illuminating regions with varying plane-of-sky field direction, and so leading to a rotation in the observed polarization vector. Our results provide evidence for magnetic reconnection as the physical cause of high-energy flares from the relativistic jets of blazars (which recent observations have shown to be frequently associated with polarization rotations), and provide a first-principle physical mechanism for such flares.

astro-ph.HE↗

arXiv2020

Kinetic Simulations of Radiative Magnetic Reconnection in the Coronae of Accreting Black Holes

We perform two- and three-dimensional particle-in-cell simulations of reconnection in magnetically-dominated pair plasmas subject to strong Compton cooling. Reconnection under such conditions operates in accretion disk coronae around black holes, which produce hard X-rays through Comptonization. Our simulations show that most of the plasma in the reconnection layer is kept cold by Compton losses and locked in magnetically-dominated plasmoids with small thermal pressure. Compton drag clears cavities inside plasmoids and affects their bulk motions. These effects, however, weakly change the reconnection rate and the plasmoid size distribution from those in non-radiative reconnection. This demonstrates that the reconnection dynamics is governed by similar magnetic stresses in both cases and weakly affected by thermal pressure. We examine the energy distribution of particles energized by radiative reconnection and observe two components. (1) A mildly-relativistic peak, which results from bulk motions of cooled plasmoids. This component receives most of the dissipated reconnection power and dominates the output X-ray emission. The peak has a quasi-Maxwellian shape with an effective temperature of 100 keV. Thus, it mimics thermal Comptonization used previously to fit hard-state spectra of accreting black holes. (2) A high-energy tail, which receives 20% of dissipated reconnection power. It is populated by particles accelerated impulsively at X-points or "picked up" by fast outflows from X-points. The high-energy particles immediately cool, and their inverse Compton emission explains the MeV spectral tail detected in the hard state of Cyg X-1. Our first-principle simulations support reconnection as a mechanism powering hard X-ray emission from accreting black holes.

astro-ph.HE↗

arXiv2020

Tilting Instability of Magnetically Confined Spheromaks

We consider the tilting instability of a magnetically confined spheromak using 3D MHD and relativistic PIC calculations with an application to astrophysical plasmas, specifically those occurring in magnetar magnetospheres. The instability is driven by the counter alignment of the spheromak's intrinsic magnetic dipole with the external magnetic field. Initially the spheromak rotates - tilts - trying to lower its magnetic potential energy. As a result a current sheet forms between the internal magnetic field of a spheromak and the confining field. Magnetic reconnection sets in; this leads to the annihilation of the newly counter-aligned magnetic flux of the spheromak. This occurs on few Alfvén time scales. In the case of higher order (second order) spheromak, the internal core is first pushed out of the envelope, resulting in formation of two nearly independent tilting spheromaks. Thus, the magnetically twisted outer shell cannot stabilize the inner core. During dissipation, helicity of the initial spheromak is carried away by torsional Alfvén waves, violating the assumptions of the Taylor relaxation theorem. In applications to magnetars' giant flares, fast development of tilting instabilities, and no stabilization of the higher order spheromaks, make it unlikely that trapped spheromaks are responsible for the tail emission lasting hundreds of seconds.

physics.plasm-ph↗

arXiv2020

The synchrotron maser emission from relativistic magnetized shocks: Dependence on the pre-shock temperature

Electromagnetic precursor waves generated by the synchrotron maser instability at relativistic magnetized shocks have been recently invoked to explain the coherent radio emission of Fast Radio Bursts. By means of two-dimensional particle-in-cell simulations, we explore the properties of the precursor waves in relativistic electron-positron perpendicular shocks as a function of the pre-shock magnetization $σ\gtrsim 1$ (i.e., the ratio of incoming Poynting flux to particle energy flux) and thermal spread $Δγ\equiv kT/mc^2=10^{-5}-10^{-1}$. We measure the fraction $f_ξ$ of total incoming energy that is converted into precursor waves, as computed in the post-shock frame. At fixed magnetization, we find that $f_ξ$ is nearly independent of temperature as long as $Δγ\lesssim 10^{-1.5}$ (with only a modest decrease of a factor of three from $Δγ=10^{-5}$ to $Δγ=10^{-1.5}$), but it drops by nearly two orders of magnitude for $Δγ\gtrsim 10^{-1}$. At fixed temperature, the scaling with magnetization $f_ξ\sim 10^{-3}\,σ^{-1}$ is consistent with our earlier one-dimensional results. For our reference $σ=1$, the power spectrum of precursor waves is relatively broad (fractional width $\sim 1-3$) for cold temperatures, whereas it shows pronounced line-like features with fractional width $\sim 0.2$ for $10^{-3} \lesssim Δγ\lesssim 10^{-1.5} $. For $σ\gtrsim 1$, the precursor waves are beamed within an angle $\simeq σ^{-1/2}$ from the shock normal (as measured in the post-shock frame), as required so they can outrun the shock. Our results can provide physically-grounded inputs for FRB emission models based on maser emission from relativistic shocks.

astro-ph.HE↗

arXiv2020

Hard Synchrotron Spectra from Magnetically Dominated Plasma Turbulence

Synchrotron emission from astrophysical nonthermal sources usually assumes that the emitting particles are isotropic. By means of large-scale two- and three-dimensional particle-in-cell simulations, we demonstrate that the dissipation of magnetically-dominated ($σ_0\gg1$) turbulence in pair plasmas leads to strongly anisotropic particle distributions. At Lorentz factors $\sim σ_0 γ_{th0}$ (here, $γ_{th0}$ is the initial Lorentz factor), the particle velocity is preferentially aligned with the local magnetic field; instead, the highest energy particles are roughly isotropic. This energy-dependent anisotropy leads to a synchrotron spectral flux $νF_ν\propto ν^s$ that is much harder than for isotropic particles. Remarkably, for $σ_0\gg1$ we find that the solid-angle-averaged spectral slope in the slow cooling regime is $s\sim 0.5-0.7$ for a wide range of turbulence fluctuations, $0.25\lesssim δB_{\rm rms0}^2/B_0^2\lesssim 4$, despite significant variations in the power-law energy spectrum of nonthermal particles. This is because weaker turbulence levels imprint a stronger degree of anisotropy, thereby counteracting the effect of the steeper particle spectrum. The synchrotron spectral slope may be even harder, $s\gtrsim 0.7$, if the observer is in the plane perpendicular to the mean magnetic field. Our results are independent of domain size and dimensionality. Our findings may help explaining the origin of hard synchrotron spectra of astrophysical nonthermal sources, most notably the radio spectrum of Pulsar Wind Nebulae.

astro-ph.HE↗

arXiv2020

Constraints on the Engines of Fast Radio Bursts

We model the sample of fast radio bursts (FRB), including the newly discovered CHIME repeaters, using the synchrotron blast wave model of Metzger, Margalit & Sironi (2019). This model postulates that FRBs are precursor radiation from ultra-relativistic magnetized shocks generated as flare ejecta from a central engine collide with an effectively stationary external medium. Downward drifting of the burst frequency structure naturally arises from deceleration of the blast-wave. The data are consistent with FRBs being produced by flares of energy $E_{\rm flare} \sim 10^{43}-10^{46}(f_ξ/10^{-3})^{-4/5}$ erg, where $f_ξ$ is the maser efficiency, and minimum bulk Lorentz factors $Γ\approx 10^2-10^3$, which generate the observed FRBs at shock radii $r_{\rm sh} \sim 10^{12}-10^{13}$ cm. We infer upstream densities $n_{\rm ext}(r_{\rm sh}) \sim 10^{2}-10^{4}$ cm$^{-3}$ and radial profiles $n_{\rm ext} \propto r^{-k}$ showing a range of slopes $k \approx [-2,1]$ (which are seen to evolve between bursts), broadly consistent with the upstream medium being the inner edge of an ion-loaded shell released by a recent energetic flare. The burst timescales, energetics, rates, and external medium properties are consistent with repeating FRBs arising from young, hyper-active flaring magnetars, but the methodology presented is generally applicable to any central engine which injects energy impulsively into a dense magnetized medium. Uncertainties and variations of the model are discussed, including the effects of the strong electric field of the FRB wave (strength parameter $a \gg 1$) on the upstream medium. One-dimensional particle-in-cell simulations of magnetized shocks into a pair plasma are presented which demonstrate that high maser efficiency can be preserved, even in the limit $a \gg 1$ in which the FRB wave accelerates the upstream electrons to ultra-relativistic speeds.

astro-ph.HE↗

arXiv2019

The interplay of magnetically-dominated turbulence and magnetic reconnection in producing nonthermal particles

Magnetized turbulence and magnetic reconnection are often invoked to explain the nonthermal emission observed from a wide variety of astrophysical sources. By means of fully-kinetic 2D and 3D PIC simulations, we investigate the interplay between turbulence and reconnection in generating nonthermal particles in magnetically-dominated pair plasmas. A generic by-product of the turbulence evolution is the generation of a nonthermal particle spectrum with a power-law energy range. The power-law slope $p$ is harder for larger magnetizations and stronger turbulence fluctuations, and it can be as hard as $p < 2$. The Larmor radius of particles at the high-energy cutoff is comparable to the size $l$ of the largest eddies. Plasmoid-mediated reconnection, which self-consistently occurs in the turbulent plasma, controls the physics of particle injection. Then, particles are further accelerated by stochastic scattering off turbulent fluctuations. The work done by parallel electric fields - naturally expected in reconnection layers - is responsible for most of the initial energy increase, and is proportional to the magnetization $σ$ of the system, while the subsequent energy gain, which dominates the overall energization of high-energy particles, is powered by the perpendicular electric fields of turbulent fluctuations. The two-stage acceleration process leaves an imprint in the particle pitch-angle distribution: low-energy particles are aligned with the field, while the highest energy particles move preferentially orthogonal to it. The energy diffusion coefficient of stochastic acceleration scales as $D_γ\sim 0.1σ(c/l)γ^2$, where $γ$ is the particle Lorentz factor. This results in fast acceleration timescales $t_{acc}\sim (3/σ)\,l/c$. Our findings have important implications for understanding the generation of nonthermal particles in high-energy astrophysical sources.

astro-ph.HE↗

arXiv2019

The Mechanism of Electron Injection and Acceleration in Trans-Relativistic Reconnection

Electron acceleration during magnetic reconnection is thought to play a key role in time-variable high-energy emission from astrophysical systems. By means of particle-in-cell simulations of trans-relativistic reconnection, we investigate electron injection and acceleration mechanisms in low-$β$ electron-proton plasmas. We set up a diversity of density and field structures (e.g., X-points and plasmoids) by varying the guide field strength and choosing whether to trigger reconnection or let it spontaneously evolve. We show that the number of X-points and plasmoids controls the efficiency of electron acceleration, with more X-points leading to a higher efficiency. Using on-the-fly acceleration diagnostics, we also show that the non-ideal electric fields associated with X-points play a critical role in the first stages of electron acceleration. As a further diagnostic, we include two populations of test particles that selectively experience only certain components of electric fields. We find that the out-of-plane component of the parallel electric field determines the hardness of the high-energy tail of the electron energy distribution. These results further our understanding of electron acceleration in this regime of magnetic reconnection and have implications for realistic models of black hole accretion flows.

astro-ph.HE↗

arXiv2019

Interpreting Crab Nebula synchrotron spectrum: two acceleration mechanisms

We outline a model of the Crab Pulsar Wind Nebula with two different populations of synchrotron emitting particles, arising from two different acceleration mechanisms: (i) Component-I due to Fermi-I acceleration at the equatorial portion of the termination shock, with particle spectral index $p_I \approx 2.2$ above the injection break corresponding to $γ_{wind} σ_{wind} \sim 10^5$, peaking in the UV ($γ_{wind} \sim 10^2$ is the bulk Lorentz factor of the wind, $σ_{wind} \sim 10^3$ is wind magnetization); (ii) Component-II due to acceleration at reconnection layers in the bulk of the turbulent Nebula, with particle index $p_{II} \approx 1.6$. The model requires relatively slow but highly magnetized wind. For both components the overall cooling break is in the infra-red at $\sim 0.01$ eV, so that the Component-I is in the fast cooling regime (cooling frequency below the peak frequency). In the optical band Component-I produces emission with the cooling spectral index of $α_o \approx 0.5$, softening towards the edges due to radiative losses. Above the cooling break, in the optical, UV and X-rays, Component-I mostly overwhelms Component-II. We hypothesize that acceleration at large-scale current sheets in the turbulent nebula (Component-II) extends to the synchrotron burn-off limit of $ε_s \approx 100$ MeV. Thus in our model acceleration in turbulent reconnection (Component-II) can produce both hard radio spectra and occasional gamma-ray flares. This model may be applicable to a broader class of high energy astrophysical objects, like AGNe and GRB jets, where often radio electrons form a different population from the high energy electrons.

astro-ph.HE↗

arXiv2019

Relativistic Magnetic Reconnection in Electron-Positron-Proton Plasmas: Implications for Jets of Active Galactic Nuclei

Magnetic reconnection is often invoked to explain the non-thermal radiation of relativistic outflows, including jets of active galactic nuclei (AGN). Motivated by the largely unknown plasma composition of AGN jets, we study reconnection in the unexplored regime of electron-positron-proton (pair-proton) plasmas with large-scale two-dimensional particle-in-cell simulations. We cover a wide range of pair multiplicities (lepton-to-proton number ratio $κ=1-199$) for different values of the all-species plasma magnetization ($σ=1,3$ and 10) and electron temperature ($Θ_e\equiv kT_e/m_ec^2=0.1-100$). We focus on the dependence of the post-reconnection energy partition and lepton energy spectra on the hot pair plasma magnetization $σ_{e,h}$ (i.e., the ratio of magnetic to pair enthalpy densities). We find that the post-reconnection energy is shared roughly equally between magnetic fields, pairs, and protons for $σ_{e,h}\gtrsim 3$. We empirically find that the mean lepton Lorentz factor in the post-reconnection region depends on $σ, Θ_e$, and $σ_{e,h}$ as $\langle γ_e-1\rangle \approx \sqrtσ(1+4Θ_e)\left(1+σ_{e,h}/30\right)$, for $σ\ge1$. The high-energy part of the post-reconnection lepton energy distributions can be described by a power law, whose slope is mainly controlled by $σ_{e,h}$ for $κ\gtrsim 3-6$, with harder power laws obtained for higher magnetizations. We finally show that reconnection in pair-proton plasmas with multiplicities $κ\sim 1-20$, magnetizations $σ\sim 1-10$, and temperatures $Θ_e \sim 1-10$ results in particle power law slopes and average electron Lorentz factors that are consistent with those inferred in leptonic models of AGN jet emission.

astro-ph.HE↗