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1. Connecting the early afterglow to the prompt GRB and the central engine in the striped jet model

   https://doi.org/10.1093/mnras/stad1865  

   Damoulakis, Michail; Duran, Rodolfo_Barniol; Giannios, Dimitrios (June 2023, Monthly Notices of the Royal Astronomical Society)

   ABSTRACT Despite a generally accepted framework for describing the gamma-ray burst (GRB) afterglows, the nature of the compact object at the central engine and the mechanism behind the prompt emission remain debated. The striped jet model is a promising venue to connect the various GRB stages since it gives a robust prediction for the relation of jet bulk acceleration, magnetization, and dissipation profile as a function of distance. Here, we use the constraints of the magnetization and bulk Lorentz of the jet flow at the large scales, where the jet starts interacting with the ambient gas in a large sample of bursts to (i) test the striped jet model for the GRB flow and (ii) study its predictions for the prompt emission and the constraints on the nature of the central engine. We find that the peak of the photospheric component of the emission predicted by the model is in agreement with the observed prompt emission spectra in the majority of the bursts in our sample, with a radiative efficiency of about 10 per cent. Furthermore, we adopt two different approaches to correlate the peak energies of the bursts with the type of central engine to find that more bursts are compatible with a neutron star central engine compared to a black hole one. Lastly, we conclude that the model favours broader distribution of stripe length-scales which results in a more gradual dissipation profile in comparison to the case, where the jet stripes are characterized by a single length-scale. 

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2. Striped Jets in Post–Neutron Star Merger Systems

   https://doi.org/10.3847/1538-4357/ace894  

   Kaufman, Emma; Christie, I. M.; Lalakos, A.; Tchekhovskoy, A.; Giannios, D. (August 2023, The Astrophysical Journal)

   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. 

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3. Relativistic non-thermal particle acceleration in two-dimensional collisionless magnetic reconnection

   https://doi.org/10.1017/S0022377822000046  

   Uzdensky, Dmitri A. (February 2022, Journal of Plasma Physics)

   Magnetic reconnection, especially in the relativistic regime, provides an efficient mechanism for accelerating relativistic particles and thus offers an attractive physical explanation for non-thermal high-energy emission from various astrophysical sources. I present a simple analytical model that elucidates key physical processes responsible for reconnection-driven relativistic non-thermal particle acceleration in the large-system, plasmoid-dominated regime in two dimensions. The model aims to explain the numerically observed dependencies of the power-law index $$p$$ and high-energy cutoff $$\gamma _c$$ of the resulting non-thermal particle energy spectrum $$f(\gamma )$$ on the ambient plasma magnetization $$\sigma$$ , and (for $$\gamma _c$$ ) on the system size $$L$$ . In this self-similar model, energetic particles are continuously accelerated by the out-of-plane reconnection electric field $$E_{\rm rec}$$ until they become magnetized by the reconnected magnetic field and eventually trapped in plasmoids large enough to confine them. The model also includes diffusive Fermi acceleration by particle bouncing off rapidly moving plasmoids. I argue that the balance between electric acceleration and magnetization controls the power-law index, while trapping in plasmoids governs the cutoff, thus tying the particle energy spectrum to the plasmoid distribution. 

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4. On the synthesis of heavy nuclei in protomagnetar outflows and implications for ultra-high energy cosmic rays

   https://doi.org/10.1093/mnras/stac1721  

   Bhattacharya, Mukul; Horiuchi, Shunsaku; Murase, Kohta (June 2022, Monthly Notices of the Royal Astronomical Society)

   ABSTRACT It has been suggested that strongly magnetized and rapidly rotating protoneutron stars (PNSs) may produce long duration gamma-ray bursts (GRBs) originating from stellar core collapse. We explore the steady-state properties and heavy element nucleosynthesis in neutrino-driven winds from such PNSs whose magnetic axis is generally misaligned with the axis of rotation. We consider a wide variety of central engine properties such as surface dipole field strength, initial rotation period, and magnetic obliquity to show that heavy element nuclei can be synthesized in the radially expanding wind. This process is facilitated provided the outflow is Poynting-flux dominated such that its low entropy and fast expansion time-scale enables heavy nuclei to form in a more efficient manner as compared to the equivalent thermal GRB outflows. We also examine the acceleration and survival of these heavy nuclei and show that they can reach sufficiently high energies ≳ 1020 eV within the same physical regions that are also responsible for powering gamma-ray emission, primarily through magnetic dissipation processes. Although these magnetized outflows generally fail to achieve the production of elements heavier than lanthanides for our explored electron fraction range 0.4–0.6, we show that they are more than capable of synthesizing nuclei near and beyond iron peak elements. 

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5. Blazar jets launched with similar energy per baryon, independently of their power

   https://doi.org/10.1093/mnras/staa3925  

   Rueda-Becerril, Jesús M; Harrison, Amanda O; Giannios, Dimitrios (January 2021, Monthly Notices of the Royal Astronomical Society)

   null (Ed.)

   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. 

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