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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., $sigma_wsim 0.1-1$, where the magnetization $sigma_w$ is the ratio of magnetic energy density to enthalpy density). For both electrons and protons, we find that heating at high $beta_{rm i}$ (here, $beta_{rm i}$ is the ratio of proton thermal pressure to magnetic pressure) is dominated by adiabatic compression (adiabatic heating), while at low $beta_{rm i}$ it is accompanied by a genuine increase in entropy (irreversible heating). For our fiducial $sigma_w=0.1$, the irreversible heating efficiency at $beta_{rm i}lesssim 1$ is nearly independent of the electron-to-proton temperature ratio $T_{rm e}/T_{rm i}$ (which we vary from $0.1$ up to $1$), and it asymptotes to $sim 2%$ of the inflowing magnetic energy in the low-$beta_{rm i}$ limit. Protons are heated more efficiently than electrons at low and moderate $beta_{rm i}$ (by a factor of $sim7$), whereas the electron and proton heating efficiencies become comparable at $beta_{rm i}sim 2$ if $T_{rm e}/T_{rm i}=1$, when both species start already relativistically hot. We find comparable heating efficiencies between the two species also in the limit of relativistic reconnection ($sigma_wgtrsim 1$). 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.
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
Using fully kinetic simulations, we study the scaling of the inflow speed of collisionless magnetic reconnection from the non-relativistic to ultra-relativistic limit. In the anti-parallel configuration, the inflow speed increases with the upstream m
Cosmic sources of gamma-ray radiation in the GeV range are often characterized by violent variability, in particular this concerns blazars, gamma-ray bursts, and the pulsar wind nebula Crab. Such gamma-ray emission requires a very efficient particle
Magnetic reconnection, especially in the relativistic regime, provides an efficient mechanism for accelerating relativistic particles and thus offers an attractive physical explanation for nonthermal high-energy emission from various astrophysical so
Particle acceleration and heating at mildly relativistic magnetized shocks in electron-ion plasma are investigated with unprecedentedly high-resolution two-dimensional particle-in-cell simulations that include ion-scale shock rippling. Electrons are