Person:

Sironi, Lorenzo

A dense ionized cloud of gas has been recently discovered to be moving directly toward the supermassive black hole, Sgr A*, at the Galactic center. In 2013 June, at the pericenter of its highly eccentric orbit, the cloud will be approximately 3100 Schwarzschild radii from the black hole and will move supersonically through the ambient hot gas with a velocity of vp ≈ 5400 km s–1. A bow shock is likely to form in front of the cloud and could accelerate electrons to relativistic energies. We estimate via particle-in-cell simulations the energy distribution of the accelerated electrons and show that the non-thermal synchrotron emission from these electrons might exceed the quiescent radio emission from Sgr A* by a factor of several. The enhanced radio emission should be detectable at GHz and higher frequencies around the time of pericentric passage and in the following months. The bow shock emission is expected to be displaced from the quiescent radio emission of Sgr A* by ~33 mas. Interferometric observations could resolve potential changes in the radio image of Sgr A* at wavelengths lsim 6 cm.

In systems accreting well below the Eddington rate, such as the central black hole in the Milky Way (Sgr A∗), the plasma in the innermost regions of the disk is believed to be collisionless and have two temperatures, with the ions substantially hotter than the electrons. However, whether a collisionless faster-than-Coulomb energy transfer mechanism exists in two-temperature accretion flows is still an open question. We study the physics of electron heating during the growth of ion velocity-space instabilities by means of multidimensional, fully kinetic, particle-in-cell (PIC) simulations. A background large-scale compression—embedded in a novel form of the PIC equations—continuously amplifies the field. This constantly drives a pressure anisotropy P⊥ > P because of the adiabatic invariance of the particle magnetic moments. We find that, for ion plasma beta values β0i ∼ 5–30 appropriate for the midplane of low-luminosity accretion flows (here, β0i is the ratio of ion thermal pressure to magnetic pressure), mirror modes dominate if the electron-to-proton temperature ratio is T0e/T0i 0.2, whereas for T0e/T0i 0.2 the ion cyclotron instability triggers the growth of strong Alfven-like waves, which pitch-angle ´ scatter the ions to maintain marginal stability. We develop an analytical model of electron heating during the growth of the ion cyclotron instability, which we validate with PIC simulations. We find that for cold electrons (β0e 2 me/mi, where β0e is the ratio of electron thermal pressure to magnetic pressure), the electron energy gain is controlled by the magnitude of the E-cross-B velocity induced by the ion cyclotron waves. This term is independent of the initial electron temperature, so it provides a solid energy floor even for electrons starting with extremely low temperatures. On the other hand, the electron energy gain for β0e 2 me/mi—governed by the conservation of the particle magnetic moment in the growing fields of the instability—is proportional to the initial electron temperature, and it scales with the magnetic energy of ion cyclotron waves. Our results have implications for two-temperature accretion flows as well as for solar wind and intracluster plasmas.