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1. Synchrotron Firehose Instability

   https://doi.org/10.3847/1538-4357/acaf54  

   Zhdankin, Vladimir; Kunz, Matthew W.; Uzdensky, Dmitri A. (February 2023, The Astrophysical Journal)

   Abstract We demonstrate using linear theory and particle-in-cell (PIC) simulations that a synchrotron-cooling collisionless plasma acquires pressure anisotropy and, if the plasma beta is sufficiently high, becomes unstable to the firehose instability, in a process that we dub the synchrotron firehose instability (SFHI). The SFHI channels free energy from the pressure anisotropy of the radiating, relativistic electrons (and/or positrons) into small-amplitude, kinetic-scale, magnetic-field fluctuations, which pitch-angle scatter the particles and bring the plasma to a near-thermal state of marginal instability. The PIC simulations reveal a nonlinear cyclic evolution of firehose bursts interspersed by periods of stable cooling. We compare the SFHI for electron–positron and electron–ion plasmas. As a byproduct of the growing electron-firehose magnetic-field fluctuations, magnetized ions gain a pressure anisotropy opposite to that of the electrons. If these ions are relativistically hot, we find that they also experience cooling due to collisionless thermal coupling with the electrons, which we argue is mediated by a secondary ion-cyclotron instability. We suggest that the SFHI may be activated in a number of astrophysical scenarios, such as within ejecta from black hole accretion flows and relativistic jets, where the redistribution of energetic electrons from low to high pitch angles may cause transient bursts of radiation. 

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2. Hard Synchrotron Spectra from Magnetically Dominated Plasma Turbulence

   https://doi.org/10.3847/2041-8213/ab93dc  

   Comisso, Luca; Sobacchi, Emanuele; Sironi, Lorenzo (June 2020, The Astrophysical Journal)
3. Multimodal Synchrotron Approach: Research Needs and Scientific Vision

   https://doi.org/10.1080/08940886.2020.1701380  

   Chen-Wiegart, Yu-Chen Karen; Waluyo, Iradwikanari; Kiss, Andrew; Campbell, Stuart; Yang, Lin; Dooryhee, Eric; Trelewicz, Jason R.; Li, Yiyang; Gates, Bruce; Rivers, Mark; et al (February 2020, Synchrotron Radiation News)
4. The Synchrotron-based Far-infrared Spectrum of Glycolaldehyde

   https://doi.org/10.3847/1538-4365/abde40  

   Collier, Breanna; Krueger, Kristopher; Miller, Isaac; Zhao, Jianbao; Billinghurst, Brant E.; Raston, Paul L. (March 2021, The Astrophysical Journal Supplement Series)

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5. Synchrotron Pair Production Equilibrium in Relativistic Magnetic Reconnection

   https://doi.org/10.3847/1538-4357/acb68a  

   Chen, Alexander Y.; Uzdensky, Dmitri; Dexter, Jason (February 2023, The Astrophysical Journal)

   Abstract Magnetic reconnection is ubiquitous in astrophysical systems, and in many such systems the plasma suffers from significant cooling due to synchrotron radiation. We study relativistic magnetic reconnection in the presence of strong synchrotron cooling, where the ambient magnetization,σ, is high and the magnetic compactness,ℓ_B, of the system is of order unity. In this regime,e^±pair production from synchrotron photons is inevitable, and this process can regulate the magnetizationσsurrounding the current sheet. We investigate this self-regulation analytically and find a self-consistent steady state for a given magnetic compactness of the system and initial magnetization. This result helps estimate the self-consistent upstream magnetization in systems where plasma density is poorly constrained, and can be useful for a variety of astrophysical systems. As illustrative examples, we apply it to study the properties of reconnecting current sheets near the supermassive black hole of M87, as well as the equatorial current sheet outside the light cylinder of the Crab pulsar. 

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