Two-temperature Accretion Flows Research Articles

Two-temperature accretion flows in magnetic cataclysmic variables: structures of post-shock emission regions and X-ray spectroscopy

We use a two-temperature hydrodynamical formulation to determine the temperature and density structures of the post-shock accretion flows in magnetic cataclysmic variables (mCVs) and calculate the corresponding X-ray spectra. The effects of two-temperature flows are significant for systems with a massive white dwarf and a strong white-dwarf magnetic field. Our calculations show that two-temperature flows predict harder keV spectra than one-temperature flows for the same white-dwarf mass and magnetic field. This result is insensitive to whether the electrons and ions have equal temperature at the shock, but depends on the electron–ion exchange rate, relative to the rate of radiative loss along the flow. White-dwarf masses obtained by fitting the X-ray spectra of mCVs using hydrodynamic models including the two-temperature effects will be lower than those obtained using single-temperature models. The bias is more severe for systems with a massive white dwarf.

On Collisionless Electron‐Ion Temperature Equilibration in the Fast Solar Wind

We explore a mechanism, entirely new to the fast solar wind, of electron heating by lower hybrid waves to explain the shift to higher charge states observed in various elements in the fast wind at 1 AU relative to the original coronal hole plasma. This process is a variation on that previously discussed for two-temperature accretion flows by Begelman & Chiueh. Lower hybrid waves are generated by gyrating minor ions (mainly α-particles) and become significant once strong ion cyclotron heating sets in beyond 1.5 R☉. In this way the model avoids conflict with SUMER electron temperature diagnostic measurements between 1 and 1.5 R☉. The principal requirement for such a process to work is the existence of density gradients in the fast solar wind, with scale length of similar order to the proton inertial length. Similar size structures have previously been inferred by other authors from radio scintillation observations and considerations of ion cyclotron wave generation by global resonant MHD waves.

Signatures of Energetic Protons in Hot Accretion Flows: Synchrotron Coolingof Protons in Strongly Magnetized Pulsars

The existence of hot, two-temperature accretion flows is essential to the recent discussions of the low-luminosity, hard X-ray emission from accreting neutron stars and black holes in Galactic binaries and massive black holes in low-luminosity galactic nuclei. In these flows, protons are essentially virialized and relativistic energies for nonthermal protons are likely. Observational confirmation of the energetic protons' presence could further support the two-temperature accretion flow models. We point out that synchrotron emission from nonthermal relativistic protons could provide an observational signature in strongly magnetized neutron star systems. The self-absorbed synchrotron emission from an accreting neutron star with the magnetic moment ~1030 G cm3 is expected to exhibit a spectrum νIν ~ ν2 with the luminosity about a few times 1033(LX/1036 ergs s−1)0.4 ergs s-1 at ν~1015 Hz, where LX is the X-ray luminosity from the neutron star surface. The detection of the expected synchrotron signature in optical and UV bands during the low-luminosity state of the pulsar systems such as 4U 1626-67 and GX 1+4 could prove the existence of the hot, two-temperature accretion flows during their spin-down episodes. The detected optical emission in 4U 1626-67 has a spectral shape and luminosity level very close to our predictions.

Electron‐Positron Pairs in Hot Accretion Flows and Thin Disk Coronae

We investigate equilibrium accretion flows dominated by e+e- pairs. We consider one- and two-temperature accretion disk coronae above a thin disk, as well as hot optically thin two-temperature accretion flows without an underlying thin disk; we model the latter in the framework of advection-dominated accretion flows (ADAFs). In all three cases we include equipartition magnetic fields. We confirm the previous result that the equilibrium density of pairs in two-temperature ADAFs is negligible and show that the inclusion of magnetic fields and the corresponding synchrotron cooling reduces the pair density even further. Similarly, we find that pairs are unimportant in two-temperature coronae. Even when the corona has significantly enhanced heating by direct transfer of viscous dissipation in the thin disk to the corona, the inefficient Coulomb coupling between protons and electrons acts as a bottleneck and prevents the high compactness required for pair-dominated solutions. Only in the case of a one-temperature corona model do we find pair-dominated thermal equilibria. These pair-dominated solutions occur over a limited range of optical depth and temperature.

Optically Thin, Advection-Dominated Two-Temperature Disks

We present global models of optically thin, advection-dominated two-temperature accretion flows onto black holes, paying careful attention to transonic properties of the flows. We treat the physical quantities integrated over the vertical direction, and adopt the standard α-viscosity and the pseudo-Newtonian potential. Bremsstrahlung and synchrotron cooling amplified by Comptonization is considered as to be cooling processes of electrons. It is found that when moderate synchrotron cooling is included, the electron temperature distribution in the inner part of the disk is roughly flat with temperatures slightly lower than the electron rest mass energy. This high electron temperature is appropriate to explain the hard X-rays from X-ray sources and AGNs. It is also found that in the inner part of the disks the energy input to electrons from ions by the ion-electron Coulomb collision is negligible as the electron heating, i.e., the electron temperature is determined by the balance between advective heating and synchrotron cooling.

Thermal coupling of ions and electrons by collective effects in two-temperature accretion flows

A plasma instability which may lead to the thermal coupling of ions and electrons in two-temperature accretion flows on a time scale shorter than the Coulomb coupling time has been found. The processes involved with the instability are described. It is shown that the instability has time to grow only in regions of large-amplitude, small-scale MHD turbulence.

Thermal and dynamical effects of pair production on two-temperature accretion flows

The two-temperature criterion for quasi-spherical (ion-pressure supported) accretion onto a black hole, including the effects of electron-positron pair production is studied. For an interesting range of accretion rates, Coulomb interactions between protons and pairs can cool the innermost regions of the accretion flow. The cooled plasma, assumed to possess some angular momentum, will collapse onto the equatorial plane, forming an optically thick annulus. Hysteresis effects involved in the cooling criterion are expected to lead to bistability or time dependence, which may be associated with variability in certain classes of active galactic nuclei (AGNs). Radiation from the annulus may be responsible for the EUV/soft X-ray excess observed in some AGNs.