Effects of Synchrotron Cooling and Pair Production on Collisionless Relativistic Reconnection

Abstract

High energy radiation from nonthermal particles accelerated in relativistic magnetic reconnection is thought to be important in many astrophysical systems, ranging from blazar jets and black hole accretion disk coronae to pulsars and magnetar flares. The presence of a substantial density of high energy photons (>MeV) in these systems can make two-photon pair production (γγ → e − e + ) an additional source of plasma particles and can affect the radiative properties of these objects. We present the results of novel particle-in-cell simulations that track both the radiated synchrotron photons and the created pairs, with which we study the evolution of a two-dimensional reconnecting current sheet in pair plasma. Synchrotron radiation from accelerated particles in the current sheet produces hot secondary pairs in the upstream which are later advected into the current sheet where they are reaccelerated and produce more photons. In the optically thin regime, when most of the radiation is leaving the upstream unaffected, this process is self-regulating and depends only on the background magnetic field and the optical depth of photons to pair production. The extra plasma loading also affects the properties of reconnection. We study how the inflow of the secondary plasma, with multiplicities up to several hundred, reduces the effective magnetization of the plasma, suppressing the acceleration and thus decreasing the high energy photon spectrum cutoff. This offers an explanation for the weak dependence of the observed gamma-ray cutoff in pulsars on the magnetic field at the light cylinder.

Reconnection and particle acceleration in three-dimensional current sheet evolution in moderately magnetized astrophysical pair plasma

“…In such environments, reconnection in the relativistic regime has become a promising explanation for high-energy emission, especially since particle-in-cell (PIC) simulations have convincingly demonstrated that, in addition to heating plasma, reconnection can accelerate a significant fraction of particles to very high energies, yielding a non-thermal power-law energy distribution of particles (e.g. Zenitani & Hoshino 2001, 2005a,b, 2007Jaroschek et al 2004;Lyubarsky & Liverts 2008;Liu et al 2011;Sironi & Spitkovsky 2011Bessho & Bhattacharjee 2012;Cerutti et al 2012bCerutti et al , 2013Cerutti et al , 2014aKagan, Milosavljević & Spitkovsky 2013;Guo et al 2014Guo et al , 2015Guo et al , 2016bGuo et al , a, 2019Dahlin, Drake & Swisdak 2015, 2017Nalewajko et al 2015;Sironi, Petropoulou & Giannios 2015;Sironi, Giannios & Petropoulou 2016;Werner et al 2016Werner et al , 2018Werner, Philippov & Uzdensky 2019;Werner & Uzdensky 2017;Ball, Sironi & Özel 2018, 2019Hakobyan, Philippov & Spitkovsky 2019;Hakobyan et al 2021;Li et al 2019;Schoeffler et al 2019;Guo et al 2020;Kilian et al 2020;Mehlhaff et al 2020;Sironi & Beloborodov 2020;Zhang, Sironi & Giannios 2021). High-energy non-thermal particle acceleration (NTPA) followed by synchrotron or i...…”

Generation of Cold Magnetized Relativistic Plasmas at the Rear of Thin Foils Irradiated by Ultra-High-Intensity Laser Pulses

“…Additionally, we note that it can be expected that next-generation laser facilities carrying hundreds of joules in sub-picosecond pulses will be able to create conditions in which the effects of radiation cooling and generation of electron-positron pairs become observable. Recently, it has been proposed that they can play an important role in relativistic magnetic reconnection [29,30]. The study of such a possibility is an interesting possible subject of a future work.…”

A Coupled Guiding Center–Boris Particle Pusher for Magnetized Plasmas in Compact-object Magnetospheres

“…We choose to run our tests in two distinct setups, with and without the effect of strong gravity. We consider static (time-invariant) electromagnetic field configurations given by PiC simulations of an isolated current sheet (in flat spacetime; see Hakobyan et al 2019) and of a current sheet in a BH magnetosphere (in Kerr spacetime;…”

“…Our electromagnetic field configuration is a fully developed reconnecting current sheet. This state represents a snapshot from a PiC simulation of a magnetized pair plasma in flat spacetime (Hakobyan et al 2019) with TRISTAN-MP (Buneman 1993;Spitkovsky 2005). The initial configuration consists of a Harris-like profile for the magnetic field B y (x) = B 0 tanh(x/a) (Harris 1962), where B 0 is the asymptotic magnetic field strength and a is the current sheet half-width, in a two-dimensional box of size x × y = 5000d e × 5600d e (with grid spacing ∆x = 0.2d e ), where d e = c/ω p is the electron skin depth and ω p is the plasma frequency.…”

“…The Particle-in-Cell (PiC) algorithm is the most widely employed method to solve the kinetic equations, owing its success to its simplicity, reliability, and remarkable performance on parallel architectures. Special-relativistic (SR) PiC simulations are used to study the dynamics of magnetized relativistic plasma, and have illuminated the physics of fundamental kinetic processes such as particle acceleration in shocks (Spitkovsky 2008;Sironi & Spitkovsky 2009a,b), magnetic reconnection (Guo et al 2014;Sironi & Spitkovsky 2014;Werner & Uzdensky 2017;Werner et al 2018;Hakobyan et al 2019), and turbulence (Comisso & Sironi 2018Zhdankin et al 2018Zhdankin et al , 2019. More recently, PiC simulations have been applied to model the plasma magnetospheres of compact objects, resulting in first-principles models of coherent radio emission from non-stationary reconnection (Philippov et al 2019) and pair discharges (Philippov et al 2020), γ-ray emission from reconnecting current sheets (Cerutti et al 2016;Kalapotharakos et al 2018;Philippov & Spitkovsky 2018), and Xray emission in magnetar magnetospheres (Chen & Beloborodov 2017).…”