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ABSTRACT GW170817/GRB170817A has offered unprecedented insight into binary neutron star post-merger systems. Its Prompt and afterglow emission imply the presence of a tightly collimated relativistic jet with a smooth transverse structure. However, it remains unclear whether and how the central engine can produce such structured jets. Here, we utilize 3D general relativistic magnetohydrodynamic simulations starting with a black hole surrounded by a magnetized torus with properties typically expected of a post-merger system. We follow the jet, as it is self-consistently launched, from the scale of the compact object out to more than three orders of magnitude in distance. We find that this naturally results in a structured jet, which is collimated by the disc wind into a half-opening angle of roughly 10°; its emission can explain features of both the prompt and afterglow emission of GRB170817A for a 30° observing angle. Our work is the first to compute the afterglow, in the context of a binary merger, from a relativistic magnetized jet self-consistently generated by an accreting black hole, with the jet’s transverse structure determined by the accretion physics and not prescribed at any point. 

Abstract Magnetic reconnection is invoked as one of the primary mechanisms to produce energetic particles. We employ large-scale 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 an 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 ≳ 3 σ 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 ∝ t , in contrast to γ ∝ t in 2D. The energy gain rate approaches ∼ eE rec c , where E rec ≃ 0.1 B 0 is the reconnection electric field and B 0 the upstream magnetic field. The spectrum of free particles is hard, dN free / d γ ∝ γ − 1.5 , contains ∼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. 

ABSTRACT Relativistic jets from supermassive black holes are among the most powerful and luminous astrophysical systems in Universe. We propose that the open magnetic field lines through the black hole, which drive a strongly magnetized jet, may have their polarity reversing over time scales related to the growth of the magnetorotational dynamo in the disc, resulting in dissipative structures in the jet characterized by reversing toroidal field polarities, referred to as ‘stripes’. The magnetic reconnection between the stripes dissipates the magnetic energy and powers jet acceleration. The striped jet model can explain the jet acceleration, large-scale jet emission, and blazar emission signatures consistently in a unified physical picture. Specifically, we find that the jet accelerates to the bulk Lorentz factor Γ ≳ 10 within 1-parsec distance from the central engine. The acceleration slows down but continues at larger distances, with intrinsic acceleration rate $$\dot{\Gamma }/\Gamma$$ between $0.0005$ and $$0.005~\rm {yr^{-1}}$$ at tens of parsecs, which is in very good agreement with recent radio observations. Magnetic reconnection continuously accelerates non-thermal particles over large distances from the central engine, resulting in the core-shift effect and overall flat-to-inverted synchrotron spectrum. The large-scale spectral luminosity peak νpeak is antiproportional to the location of the peak of the dissipation, which is set by the minimal stripe width lmin. The blazar zone is approximately at the same location. At this distance, the jet is moderately magnetized, with the comoving magnetic field strength and dissipation power consistent with typical leptonic blazar model parameters. 

ABSTRACT The most extreme active galactic nuclei are the radio active ones whose relativistic jet propagates close to our line of sight. These objects were first classified according to their emission-line features into flat-spectrum radio quasars (FSRQs) and BL Lacertae objects (BL Lacs). More recently, observations revealed a trend between these objects known as the blazar sequence, along with an anticorrelation between the observed power and the frequency of the synchrotron peak. In this work, we propose a fairly simple idea that could account for the whole blazar population: all jets are launched with similar energy per baryon, independently of their power. In the case of FSRQs, the most powerful jets manage to accelerate to high-bulk Lorentz factors, as observed in the radio. As a result, they have a rather modest magnetization in the emission region, resulting in magnetic reconnection injecting a steep particle–energy distribution and, consequently, steep emission spectra in the γ-rays. For the weaker jets, namely BL Lacs, the opposite holds true; i.e. the jet does not achieve a very high bulk Lorentz factor, leading to more magnetic energy available for non-thermal particle acceleration, and harder emission spectra at frequencies ≳ GeV. In this scenario, we recover all observable properties of blazars with our simulations, including the blazar sequence for models with mild baryon loading (50 ≲ μ ≲ 80). This interpretation of the blazar population therefore tightly constrains the energy per baryon of blazar jets regardless of their accretion rate. 

ABSTRACT 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 ∼1 Jy·ms at ∼GHz frequencies lasts ∼1–500 ms following the merger for a source at ∼3 Gpc (Bd/1012 G)8/9, where Bd is the dipole field strength of the more strongly magnetized star. Given an outflow geometry concentrated along the binary equatorial plane, the signal may be preferentially observable for high-inclination systems, that is, those least likely to produce a detectable gamma-ray burst. 

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.