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Abstract Models invoking magnetic reconnection as the particle acceleration mechanism within relativistic jets often adopt a gradual energy dissipation profile within the jet. However, such a profile has yet to be reproduced in first-principles simulations. Here we perform a suite of 3D general relativistic magnetohydrodynamic simulations of post–neutron star merger disks with an initially purely toroidal magnetic field. We explore the variations in both the microphysics (e.g., nuclear recombination, neutrino emission) and system parameters (e.g, disk mass). In all of our simulations, we find the formation of magnetically striped jets. The stripes result from the reversals in the poloidal magnetic flux polarity generated in the accretion disk. The simulations display large variations in the distributions of stripe duration,τ, and power, 〈P_Φ〉. We find that more massive disks produce more powerful stripes, the most powerful of which reaches 〈P_Φ〉 ∼ 10⁴⁹erg s⁻¹atτ∼ 20 ms. The power and variability that result from the magnetic reconnection of the stripes agree with those inferred in short-duration gamma-ray bursts. We find that the dissipation profile of the cumulative energy is roughly a power law in both radial distance,z, andτ, with a slope in the range of ∼1.7–3; more massive disks display larger slopes. 

ABSTRACT The afterglow emission from gamma-ray bursts (GRBs) is believed to originate from a relativistic blast wave driven into the circumburst medium. Although the afterglow emission from radio up to X-ray frequencies is thought to originate from synchrotron radiation emitted by relativistic, non-thermal electrons accelerated by the blast wave, the origin of the emission at high energies (HE; ≳GeV) remains uncertain. The recent detection of sub-TeV emission from GRB 190114C by the Major Atmospheric Gamma Imaging Cherenkov Telescopes (MAGIC) raises further debate on what powers the very high energy (VHE; ≳300 GeV) emission. Here, we explore the inverse Compton scenario as a candidate for the HE and VHE emissions, considering two sources of seed photons for scattering: synchrotron photons from the blast wave (synchrotron self-Compton or SSC) and isotropic photon fields external to the blast wave (external Compton). For each case, we compute the multiwavelength afterglow spectra and light curves. We find that SSC will dominate particle cooling and the GeV emission, unless a dense ambient infrared photon field, typical of star-forming regions, is present. Additionally, considering the extragalactic background light attenuation, we discuss the detectability of VHE afterglows by existing and future gamma-ray instruments for a wide range of model parameters. Studying GRB 190114C, we find that its afterglow emission in the Fermi-Large Area Telescope (LAT) band is synchrotron dominated. The late-time Fermi-LAT measurement (i.e. t ∼ 104 s), and the MAGIC observation also set an upper limit on the energy density of a putative external infrared photon field (i.e. $${\lesssim} 3\times 10^{-9}\, {\rm erg\, cm^{-3}}$$), making the inverse Compton dominant in the sub-TeV energies. 

ABSTRACT Blazar emission models based on magnetic reconnection succeed in reproducing many observed spectral and temporal features, including the short-duration luminous flaring events. Plasmoids, a self-consistent by-product of the tearing instability in the reconnection layer, can be the main source of blazar emission. Kinetic simulations of relativistic reconnection have demonstrated that plasmoids are characterized by rough energy equipartition between their radiating particles and magnetic fields. This is the main reason behind the apparent shortcoming of plasmoid-dominated emission models to explain the observed Compton ratios of BL Lac objects. Here, we demonstrate that the radiative interactions among plasmoids, which have been neglected so far, can assist in alleviating this contradiction. We show that photons emitted by large, slow-moving plasmoids can be a potentially important source of soft photons to be then upscattered, via inverse Compton, by small fast-moving, neighbouring plasmoids. This interplasmoid Compton scattering process can naturally occur throughout the reconnection layer, imprinting itself as an increase in the observed Compton ratios from those short and luminous plasmoid-powered flares within BL Lac sources, while maintaining energy equipartition between radiating particles and magnetic fields.