Speaker
Dr
Hirokazu Odaka
(ISAS/JAXA)
Description
An astrophysical system harboring a strong gravity star such as a black hole or a neutron star accretes gas from the circumstellar environment, releasing enormous gravitational energy via accretion onto the deep potential. Since a large fraction of the accretion power is released in a form of X-ray radiation, X-ray spectral and temporal information is an important probe to study the energetic phenomena in the accreting system. Recent improvement in observational instruments allows us to obtain high-quality data (i.e. high statistics and high energy/timing/spatial resolutions) which contains information on the central engine and the circumstellar environment of black holes and neutron stars in great detail. However, precise comparison between the high-quality data and theoretical models requires careful treatment of X-ray generation in the accreted plasma and radiative transfer. In order to treat the radiative transfer precisely, we have developed a general-purpose calculation framework of radiative transfer based on Monte Carlo methods called MONACO (Odaka et al. 2011). MONACO depends upon the Geant4 toolkit library for particle tracking since the library has sophisticated treatment of the tracking in a complicated geometry. In addition, the universality of the Geant4 physical process handling enables us to introduce into MONACO our original code of physical processes that are optimized for astrophysical modeling. The framework currently provides three physics lists: (1) Cold matter—photoelectric absorption followed by fluorescence and scattering by electrons bound to neutral atoms or molecules are responsible for generating spectral features of X-ray reflection, (2) Photoionized plasma—photoionization and photoexcitation of various ion species result in emission lines and absorption lines, and (3) Comptonization—this process plays an important role in cooling a hot accretion flow through X-ray radiation. All of these processes have to be considered in a moving frame since bulk or random motion of the matter interacting with a photon produces a Doppler shift or broadening, which provides us with crucial information on physical conditions of the matter including its dynamics. We have applied this Monte Carlo framework to various astrophysical objects from galactic X-ray binaries (neutron stars or black holes) to extragalactic supermassive black holes. In this talk, we present the code design and how we apply the framework to real astrophysical problems. We then discuss issues arising when utilizing the Geant4 library for the astrophysical applications.
Primary author
Dr
Hirokazu Odaka
(ISAS/JAXA)
