MoCA: a Monte Carlo code for accretion in Astrophysics

Abstract

Accretion is a common mechanism in astrophysics and, due to its peerless efficiency it is the only one able to power the most luminous objects in the sky. From the point of view of energetics the question is very simple: the gravity of the accreting source attracts matter which, due to its angular momentum, forms an accretion disc. The gravitational potential energy is converted in kinetic energy and vis- cosity forces inside the disc convert this energy into radiation. The efficiency of this process is the highest in nature and, if the gravita- tional potential well is deep enough, huge luminosities can be reached. Compact objects are among the most exotic sources in the Universe and their luminosity is produced by accretion. The class of compact objects includes Neutron Stars (NSs), White Dwarfs (WDs) and Black Holes (BHs), objects which show a very high compactness, M/R, and differ from all the other stars because there are no thermonuclear fu- sion processes that produce radiation and counteract their gravity. When these objects are in a binary system, matter from the compan- ion star accretes onto the compact object forming an accretion disc which can produce a large amount of radiation in X-ray band. These systems are called X-ray binaries (XRBs). The X-ray spectrum pro- duced is complex and constituted by several components, due to the circumnuclear material which scatters, absorbs and reprocesses X-ray photons produced by the disc. Furthermore, the spectral energy dis- tribution can have a different shape depending on the state of the XRB. A common feature in all XRBs X-ray spectra is a strong Iron K emission line at 6.4 keV . The line is produced by the reprocessing of X-ray photons absorbed by the neutral disc. These lines often show a broad energy profile and the origin of this broadening is still matter of debate. The two common interpretations are that it is given either by General Relativistic effects or by Comptonization (or, of course, by a combination of the two). One way to discriminate between these explanations is given by the polarization signal. In the case of Comp- tonization, in fact, some degree of linear polarization is expected to be measured in the line flux while for GR broadening it is not predicted. Another case of astrophysics interest in which X-ray polarization plays a key role is on the determination of the corona geometry in Ac- tive Galactic Nuclei (AGN). The term AGN refers to a few percent of galaxies whose emission from the inner region is ascribed to accretion onto a super massive black hole (SMBH). As for XRBs, the emission, in all the energy bands of the electromagnetic spectrum, is complex and constituted by several components. One important difference is that in AGN the accretion disc radiates in the UV/optical band rather than in X-ray band. Therefore, X-ray emission in AGN is due only to the scattering electrons cloud which surrounds the inner part of the disc, known as ‘corona’. The characteristics of the corona, especially the geometry, are still unknown. Measuring the polarization signal in the X-ray spectra of AGN, offers the unique possibility to constrain some of the corona properties. In the framework of this scenario we developed MoCA, a code dedicated to the study of the spectrum and the polarization signal produced in accreting sources. We applied our code to the two issues described before: the spectrum and the polarization signal of the con- tinuum X-ray emission in AGN and the Comptonization of the Iron line in XRBs. The code includes all special relativistic corrections and it is modular, allowing, with minor modifications, to be applied to different accreting systems and astrophysical situations.