The accretion flow around the Galactic Centre black hole Sagittarius A* (Sgr A * ) is expected to have an electron temperature that is distinct from the ion temperature, due to weak Coulomb coupling in the low-density plasma. We present four two-temperature general relativistic radiative magnetohydrodynamic (GRRMHD) simulations of Sgr A * performed with the code KORAL. These simulations use different electron heating prescriptions, motivated by different models of the underlying plasma microphysics. We compare the Landau-damped turbulent cascade model used in previous work with a new prescription we introduce based on the results of particle-in-cell simulations of magnetic reconnection. With the turbulent heating model, electrons are preferentially heated in the polar outflow, whereas with the reconnection model electrons are heated by nearly the same fraction everywhere in the accretion flow. The spectra of the two models are similar around the submillimetre synchrotron peak, but the models heated by magnetic reconnection produce variability more consistent with the level observed from Sgr A * . All models produce 230 GHz images with distinct black hole shadows which are consistent with the image size measured by the Event Horizon Telescope, but only the turbulent heating produces an anisotropic 'disc-jet' structure where the image is dominated by a polar outflow or jet at frequencies below the synchrotron peak. None of our models can reproduce the observed radio spectral slope, the large near-infrared and X-ray flares, or the near-infrared spectral index, all of which suggest non-thermal electrons are needed to fully explain the emission from Sgr A * .
Hot collisionless accretion flows, such as the one in Sgr A * at our Galactic center, provide a unique setting for the investigation of magnetic reconnection. Here, protons are non-relativistic while electrons can be ultra-relativistic. By means of two-dimensional particle-in-cell simulations, we investigate electron and proton heating in the outflows of trans-relativistic reconnection (i.e., σ w ∼ 0.1 − 1, where the magnetization σ w is the ratio of magnetic energy density to enthalpy density). For both electrons and protons, we find that heating at high β i (here, β i is the ratio of proton thermal pressure to magnetic pressure) is dominated by adiabatic compression ("adiabatic heating"), while at low β i it is accompanied by a genuine increase in entropy ("irreversible heating"). For our fiducial σ w = 0.1, the irreversible heating efficiency at β i 1 is nearly independent of the electron-to-proton temperature ratio T e /T i (which we vary from 0.1 up to 1), and it asymptotes to ∼ 2% of the inflowing magnetic energy in the low-β i limit. Protons are heated more efficiently than electrons at low and moderate β i (by a factor of ∼ 7), whereas the electron and proton heating efficiencies become comparable at β i ∼ 2 if T e /T i = 1, when both species start already relativistically hot. We find comparable heating efficiencies between the two species also in the limit of relativistic reconnection (σ w 1). Our results have important implications for the two-temperature nature of collisionless accretion flows, and may provide the sub-grid physics needed in general relativistic MHD simulations.
Particle energization in shear flows is invoked to explain nonthermal emission from the boundaries of relativistic astrophysical jets. Yet the physics of particle injection, i.e., the mechanism that allows thermal particles to participate in shear-driven acceleration, remains unknown. With particle-in-cell simulations, we study the development of Kelvin-Helmholtz (KH) instabilities seeded by the velocity shear between a relativistic magnetically dominated electron-positron jet and a weakly magnetized electron-ion ambient plasma. We show that, in their nonlinear stages, KH vortices generate kinetic-scale reconnection layers, which efficiently energize the jet particles, thus providing a first-principles mechanism for particle injection into shear-driven acceleration. Our work lends support to spine-sheath models of jet emission-with a fast core/spine surrounded by a slower sheath -and can explain the origin of radio-emitting electrons at the boundaries of relativistic jets.
The plasma in low-luminosity accretion flows, such as the one around the black hole at the center of M87 or Sgr A* at our Galactic Center, is expected to be collisioness and two-temperature, with protons hotter than electrons. Here, particle heating is expected to be controlled by magnetic reconnection in the transrelativistic regime σ w ∼ 0.1-1, where the magnetization σ w is the ratio of magnetic energy density to plasma enthalpy density. By means of large-scale 2D particle-in-cell simulations, we explore for a fiducial σ w = 0.1 how the dissipated magnetic energy gets partitioned between electrons and protons, as a function of β i (the ratio of proton thermal pressure to magnetic pressure) and of the strength of a guide field B g perpendicular to the reversing field B 0 . At low β i ( 0.1), we find that the fraction of initial magnetic energy per particle converted into electron irreversible heat is nearly independent of B g /B 0 , whereas protons get heated much less with increasing B g /B 0 . As a result, for large B g /B 0 , electrons receive the overwhelming majority of irreversible particle heating (∼93% for B g /B 0 = 6). This is significantly different than the antiparallel case B g /B 0 = 0, in which electron irreversible heating accounts for only ∼18% of the total particle heating (Rowan et al. 2017). At β i ∼ 2, when both species start already relativistically hot (for our fiducial σ w = 0.1), electrons and protons each receive ∼50% of the irreversible particle heating, regardless of the guide field strength. Our results provide important insights into the plasma physics of electron and proton heating in hot accretion flows around supermassive black holes.
WarpX is a general purpose electromagnetic particle-in-cell code that was originally designed to run on many-core CPU architectures. We describe the strategy followed to allow WarpX to use the GPUaccelerated nodes on OLCF's Summit supercomputer, a strategy we believe will extend to the upcoming machines Frontier and Aurora. We summarize the challenges encountered, lessons learned, and give current performance results on a series of relevant benchmark problems.
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