Speaker
Dr
Giovanny Bernal
(IA-UNAM)
Description
The importance of accretion as a power source was first widely recognized in the study of binary systems, especially X-ray binaries. The detailed study of interacting binary systems has revealed the importance of angular momentum in accretion. In many cases, the transferred material cannot land on the accreting star until it has rid itself of most of its angular momentum. This leads to the formation of accretion disks, which turn out to be efficient machines for extracting gravitational potential energy and converting it into radiation.
Nevertheless, although the accretion disk provides an efficient machine for the extraction of up to half the available accretion energy, our understanding of how the remaining substantial fraction of the accretion luminosity is released near the central object is rather less advanced. The existence of an inner boundary layer in the case where the disk extends down to the surface of the accreting star can be the solution. In this scenario, some part of accreting matter with large angular momentum forms a disk, while another part (participating in a sub-Keplerian rotation) undergoes practically a free-fall accretion until the centrifugal barrier becomes sufficient to halt the flow. Thus, one may expect that two distinct zones, a disk and a barrier, which are likely to be responsible for the generation of a resulting spectrum, can be formed in the vicinity of a compact object. The disk structure begins deflecting from a Keplerian one at a certain point to adjust itself to the boundary conditions at an NS surface. The transition from a Keplerian to a sub-Keplerian flow may proceed smoothly, although it is very likely that a perfect adjustment never occurs. In general, the transition should take place through the setting up of a Centrifugal Barrier (where a centrifugal force slightly exceeds the gravitational force) within the adjustment radius.
In this work we perform numerical simulations to analyze if it is possible that the transient hard x-ray tails observed in Neutron Stars LMXBS are produced by inverse Compton in a bulk comptonization scenario. Also, study the possibility that under some circumstances (namely depending on the accretion rate), matter can reach the neutron star surface with a quasi-free-fall profile but with radial component velocity never exceeding ~ 0.2 c. The numerical tool used in this work is the AMR FLASH CODE: The Flash Center for Computational Science is the home of several cross-disciplinary computational research projects, and FLASH, a publicly available multiphysics multiscale simulation code with a wide international user base. Research projects include high-energy density physics, thermonuclear-powered supernovae, exascale computing co-design, fluid-structure interactions, and development of implicit solvers for "stiff" systems.
With this powerful code we simulated an accretion disk around a neutron star. We use a 2D polar mesh (R,phi) to perform the simulations in the KanBalam cluster at UNAM. We used the PPM method supplied with FLASH distribution to solve the hydrodynamic system in the Newtonian gravity approach. The initial conditions are the standard disk values (keplerian thin disk) and customized radial boundary conditions (extended disk & rotating neutron star).
REFERENCIAS
1. Batchelor, G.K, 1967, An Introduction to Fluid Dynamics, Cambridge University Press.
2. Jeans, J., 1940. An Introduction to the Kinetic Theory of Gases, Cambridge University Press, p. 157
3. C.J. Clarke, J.E. Pringle., Mon. Not. R. Astron. Soc. (2003)
4. Titarchuk l., Lapidus I., Muslimov A. The Astrophysical Journal, 499:315-328, 1998 May 20
5. Turolla, R., Zampieri, L., Colpi, M., & Treves, A. 1994, ApJ, 426, L35
Primary author
Dr
Giovanny Bernal
(IA-UNAM)
