Shilpa Sarkar (Harish-Chandra Research Institute (HRI), India)
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.
Piotr Wielgórski (CAMK PAN, Warsaw)
Radially pulsating stars, like Cepheids (Classical, Type II) and RR Lyrae are widely used as precise distance indicators thanks to their period-luminosity relations which serve as a standard candle. However, there is another, semi-geometrical method which can be used to derive distances of such stars, known as the Baade-Wesselink or parallax-of-pulsation technique. In this method, change of the angular diameter of the star, which is calculated from photometric or interferometric measurements, is compared to the physical radius displacement obtained from integrating the pulsational velocity curve (velocity of the atmosphere of the star during pulsations). Pulsational velocity can be obtained from radial velocity measured from doppler shift of absorption lines in spectra once the so-called projection factor (p-factor) is known. The value of this parameter is primarily the result of the geometrical projection of the pulsational velocity of different parts of the stellar disk onto the line of sight, but factors like limb darkening, velocity gradient in the atmosphere etc. modify the value of p-factor. Precision of the Baade-Wesselink method is currently limited to ~5-10% due to poor knowledge of this parameter. During my talk I will present the efforts of the Araucaria group in calibrating the projection factor.
Arkadiusz Orłowski (Department of Artificial Intelligence, Institute of Information Technology, SGGW, Warsaw)
Iftikhar Ahmad (CAMK/AstroCeNT, Warsaw)
Dark matter's existence is a key topic in fundamental physics, with Weakly Interacting Massive Particles (WIMPs) being a leading candidate. Direct detection experiments, like those of the DarkSide collaboration, require highly pure target materials to achieve the necessary sensitivity. DarkSide-50 (DS-50) used argon due to its pulse shape discrimination (PSD), ease of purification, and scalability. During transport from Colorado to LNGS (Italy), cosmic rays interacted with the liquid argon, producing an impurity of Argon-37. A study was conducted to determine the activation of Argon-37 using DS-50 data, which was compared with production estimates during transport. The results were in agreement within 1 sigma, validating cosmic activation estimates for Argon-39 and future detectors. The DarkSide-20k (DS-20k) experiment, under construction at INFN-LNGS, will use a dual-phase liquid argon time projection chamber (LAr-TPC) to achieve high sensitivity. To suppress background noise, DS-20k will incorporate cryogenic silicon photomultipliers (SiPMs) and a sophisticated neutron veto. Key to this is the development and testing of Veto PhotoDetection Units (vPDUs), which must detect individual photons with high precision. At Astrocent, two vPDUs were tested, meeting DS-20k’s specifications with results including breakdown voltages (~54 V), dark count rates (<0.1 Hz/mm²), and signal-to-noise ratios (<8).
Günther Hasinger
