Accretion of Low Angular Momentum Material onto Black Holes: Radiation Properties of Axisymmetric MHD Flows

Context: Numerical simulations of MHD accretion flows in the vicinity of a supermasssive black hole provide important insights to the problem of why and how systems – such as the Galactic Center – are underluminous and variable. In particular, the simulations indicate that low angular momentum accretion flow is strongly variable both quantitatively and qualitatively. This variability and a relatively low mass accretion rate are caused by an interplay between a rotationally supported torus, its outflow and a nearly non-rotating inflow. Aims: To access applicability of such flows to real objects, we examine the dynamical MHD studies with computations of the time dependent radiation spectra predicted by the simulations. Methods: We calculate synthetic broadband spectra of accretion flows using Monte Carlo techniques. Our method computes the plasma electron temperature allowing for the pressure work, ion-electron coupling, radiative cooling, and advection. The radiation spectra are calculated by taking into account thermal synchrotron and bremsstrahlung radiation, self absorption, and Comptonization processes. We also explore effects of non-thermal electrons. We apply this method to calculate spectra predicted by the time-dependent model of an axisymmetic MHD flow accreting onto a black hole presented by Proga and Begelman. Results: Our calculations show that variability in an accretion flow is not always reflected in the corresponding spectra, at least not in all wavelengths. We find no one-to-one correspondence between the accretion state and the predicted spectrum. For example, we find that two states with different properties – such as the geometry and accretion rate – could have relatively similar spectra. However, we also find two very different states with very different spectra. Existence of nonthermal radiation may be necessary to explain X-rays flaring because thermal bremsstrahlung, that dominates X-ray emission, is produced at relatively large radii where the flow changes are small and slow.