Theoretical astrophysics and plasma physics at RPC

Abstract:

Galactic nuclei harbouring a central supermassive black hole (SMBH), possibly surrounded by a dense nuclear cluster (NC), represent extreme environments that house a complex interplay of many physical processes that uniquely affect stellar formation, evolution, and dynamics. The discovery of gravitational waves (GWs) emitted by merging black holes (BHs) and neutron stars (NSs), funnelled a huge amount of work focused on understanding how compact object binaries (COBs) can pair up and merge together. Here, we review from a theoretical standpoint how different mechanisms concur with the formation, evolution, and merger of COBs around quiescent SMBHs and active galactic nuclei (AGNs), summarising the main predictions for current and future (GW) detections and outlining the possible features that can clearly mark a galactic nuclei origin.

Abstract:

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.