Shilpa Sarkar: Two-temperature accretion flows around compact objects

Invitation to the talk by Dr. Shilpa Sarkar from the Harish-Chandra Research Institute (HRI), India. The talk is organized by the Stellar department of the Astronomical institute CAS in Ondřejov.

Shilpa Sarkar: Two-temperature accretion flows around compact objects

Accretion is one of the most efficient processes by which the gravitational potential energy of matter can be converted into radiation. This phenomenon provides us with an explanation of the huge amount of energy liberated and high luminosities observed in Active Galactic Nuclei, X-ray binaries, etc. Therefore, modelling of these accretion flows is necessary to understand the underlying physical processes present in these systems. The soup of protons and electrons in these ionised flows are bound together by weak Coulomb force. Additionally, in most of the astrophysical cases, the infall timescales are much shorter. This makes the species settle down into two different temperature distributions, hence, the name two-temperature flows.

However, this theory suffers from a serious problem of degeneracy. Compared to one-temperature flows, there is one more variable in the two-temperature system -- the extra temperature. However, there is no increase in the number of equations of motion. Thus, no unique solution exists, for a given set of constants of motion; or in other words, the system is degenerate! Different values of Tp/Te ratio supplied at any boundary, would generate different kinds of solutions with drastically different topologies as well as spectra. In addition, there is no known principle dictated by plasma physics which may constrain the relation between these two-temperatures. This degeneracy is irrespective of the type of the central object and is generic to two-temperature flows.

We propose for the first time, an entropy maximisation formulation using the first principles. Using this methodology, we were able to constrain degeneracy and a unique solution with maximum entropy was selected following the second law of thermodynamics. Thereafter, we analysed the spectrum of these unique solutions for different accreting systems like black holes and neutron stars.