1. Sensitivity to keV-MeV dark matter from cosmic-ray scattering with current and the upcoming ground-based arrays CTA and SWGOIgor Reis, Saptashwa Bhattacharyya, Judit Pérez Romero, Samo Stanič, Veronika Vodeb, Serguei Vorobiov, Danilo Zavrtanik, Marko Zavrtanik, Miha Živec, 2023, published scientific conference contribution Abstract: A wealth of astrophysical and cosmological observational evidence shows that the matter content of the universe is made of about 85% of non-baryonic dark matter. Huge experimental efforts have been deployed to look for the direct detection of dark matter via their scattering on target nucleons, their production in colliders, and their indirect detection via their annihilation products. Inelastic scattering of high-energy cosmic rays off dark matter particles populating the Milky Way halo would produce secondary gamma rays in the final state from the decay of the neutral pions produced in such interactions, providing a new avenue to probe dark matter properties. We compute here the sensitivity for H.E.S.S.-like observatory, a current-generation ground-based Cherenkov telescopes, to the expected gamma-ray flux from collisions of Galactic cosmic rays and dark matter in the center of the Milky Way. We also derive sensitivity prospects for the upcoming Cherenkov Telescope Array (CTA) and Southern Wide-field Gamma-ray Observatory (SWGO). The expected sensitivity allows us to probe a poorly-constrained range of dark matter masses so far, ranging from keV to sub-GeV, and provide complementary constraints on the dark matter-proton scattering cross section traditionally probed by deep underground direct dark matter experiments.
Keywords: Cherenkov Telescope Array, CTA, very-high-energy gamma-ray astroparticle physics, instrument response functions, machine learning
Published in RUNG: 26.09.2023; Views: 592; Downloads: 6
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2. Detecting and characterizing pulsar halos with the Cherenkov Telescope Array ObservatoryChristopher Eckner, Saptashwa Bhattacharyya, Judit Pérez Romero, Samo Stanič, Veronika Vodeb, Serguei Vorobiov, Danilo Zavrtanik, Marko Zavrtanik, Miha Živec, 2023, published scientific conference contribution Abstract: The recently identified source class of pulsar halos may be populated and bright enough at TeV energies to constitute a large fraction of the sources that will be observed with the Cherenkov Telescope Array (CTA), especially in the context of the planned Galactic Plane Survey (GPS). In this study, we examine the prospects offered by CTA for the detection and characterization of such objects. CTA will cover energies from 20 GeV to 300 TeV, bridging the ranges already probed with the Fermi Large Area Telescope and High Altitude Water Cherenkov Observatory, and will also have a better angular resolution than the latter instruments, thus providing a complementary look at the phenomenon. From simple models for individual pulsar halos and their population in the Milky Way, we examine under which conditions such sources can be detected and studied from the GPS observations. In the framework of a full spatial-spectral likelihood analysis, using the most recent estimates for the instrument response function and prototypes for the science tools, we derive the spectral and morphological sensitivity of the CTA GPS to the specific intensity distribution of pulsar halos. From these, we quantify the physical parameters for which pulsar halos can be detected, identified, and characterized, and what fraction of the Galactic population could be accessible. We also discuss the effect of interstellar emission and data analysis systematics on these prospects.
Keywords: Cherenkov Telescope Array, CTA, very-high-energy gamma-ray astroparticle physics, instrument response functions, machine learning
Published in RUNG: 26.09.2023; Views: 587; Downloads: 7
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3. Performance update of an event-type based analysis for the Cherenkov Telescope ArrayJ. Bernete, Saptashwa Bhattacharyya, Judit Pérez Romero, Samo Stanič, Veronika Vodeb, Serguei Vorobiov, Danilo Zavrtanik, Marko Zavrtanik, Miha Živec, 2023, published scientific conference contribution Abstract: The Cherenkov Telescope Array (CTA) will be the next-generation observatory in the field of very-high-energy (20 GeV to 300 TeV) gamma-ray astroparticle physics. The traditional approach to data analysis in this field is to apply quality cuts, optimized using Monte Carlo simulations, on the data acquired to maximize sensitivity. Subsequent steps of the analysis typically use the surviving events to calculate one set of instrument response functions (IRFs) to physically interpret the results. However, an alternative approach is the use of event types, as implemented in experiments such as the Fermi-LAT. This approach divides events into sub-samples based on their reconstruction quality, and a set of IRFs is calculated for each sub-sample. The sub-samples are then combined in a joint analysis, treating them as independent observations. In previous works we demonstrated that event types, classified using Machine Learning methods according to their expected angular reconstruction quality, have the potential to significantly improve the CTA angular and energy resolution of a point-like source analysis. Now, we validated the production of event-type wise full-enclosure IRFs, ready to be used with science tools (such as Gammapy and ctools). We will report on the impact of using such an event-type classification on CTA high-level performance, compared to the traditional procedure.
Keywords: Cherenkov Telescope Array, CTA, very-high-energy gamma-ray astroparticle physics, instrument response functions, machine learning
Published in RUNG: 26.09.2023; Views: 606; Downloads: 6
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4. BCK codesHashem Bordbar, 2022, original scientific article Abstract: In this paper, we initiate the study of the notion of the BC K -function on an arbitrary set A, and providing connections with x -functions and x -subsets for x ∈ X where X is a B C K -algebra. Moreover, using the notion of order in a B C K -algebra, the BCK-code C is introduced and besides a new structure of order in C is investigated. Finally, we show that the structure of the B C K -algebra X and the B C K -code C which is generated by X , with their related orders are the same.
Keywords: BCK -function, x-functions, x-subsets, BCK -codes, binary linear block codes
Published in RUNG: 04.01.2022; Views: 1464; Downloads: 8
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6. Meshless modeling of thermo-mechanics of low-frequency electromagnetic direct chill castingBoštjan Mavrič, 2017, doctoral dissertation Abstract: The aim of this dissertation is to devise a meshless model describing the thermomechanical phenomena, which occur during DC casting of aluminium alloys under the influence of electromagnetic stirring. The thermoemchanical phenomena are important, because they can cause several type of defects, which can significantly deteriorate the quality of the resulting billet. The two most important of them are the hot tearing, which causes cracks to appear in the mushy zone, and the porosity, which demonstrates itself as micrometer sized voids in the microstructure of the billet.
To calculate the stresses and strains, a computational model of the stationary state of the process, stated in axial symmetry, is formulated. It uses Eulerian formulation by fixing the computational domain to the mold of the casting device allowing the material to move through the computational domain. The stresses are calculated from the stress equilibrium equations. The small strain approximation is used to consider the three contributions to strain. The strain consists of the thermal strain, which is caused by the inhomogeneous thermal profile in the billet, the viscoplastic strain, which is caused by the irreversible deformation because of the large stresses occurring in the billet, and the elastic strain.
The spatial discretization of the governing equations is performed by local radial basis function collocation method (LRBFCM) and the temporal discretization is achieved by the method of lines with implicit Euler formula. The method used for spatial discretization uses radial basis functions augmented by monomials to approximate the solution values on localized stencils. This approximation is used to construct the discretization coefficients of the differential operators present in the model. A flexible framework for formulation of multiphysics problems is developed to use the obtained discretization coefficients to construct the temporal discretization of the governing equations. The node arrangement, on which the spatial discretization is performed, was generated by a point-repel algorithm.
The performance of the method is tested on several benchmark test cases. The accuracy of the discretization is estimated by comparing the analytic and the numerical solution to several stationary problems in thermomechancis. Of special interest is the performance of the method with respect to the choice of the shape parameter, which determines the spatial scale of the radial basis functions. Besides this, the dependence of the condition number of the interpolation matrix on the shape parameter is studied. The condition number is found fit to replace the condition number as the shape-determining free parameter of the method.
The implementation of the solver of time dependent problems is tested on problem of thermoelasticity, which couples the thermal transport with the elastic waves. The results of the problem are compared with the finite element method, showing good agreement of the two methods. The results are also compared with the results obtained by meshless local Petrov-Galerkin method and the proposed local collocation method demonstrated significantly better solution quality in the studied case.
The performance of the solver used to solve the system of nonlinear equations given by the viscoplastic constitutive equations is estimated on a quasi zero-dimensional problem. The results are found to match perfectly. Solution of a more complicated problem is obtained with the proposed method and the finite-element method, both methods giving practically the same solution, although some serious limitations of the chosen finite element solver are exposed during the selection of the problem parameters.
Finally, the devised method is applied to the problem of DC casting of aluminium alloys. The thermomechanical model relies on a model of heat and mass transfer to obtain the input fields needed in the solver. The required fields are: temperature, pressure, liquid
Keywords: thermomechanics, viscoplasticity, aluminium alloys, direct-chill casting, electromagnetic stirring, hot tearing, porosity, meshless methods, local collocation method, radial basis functions, shape parameter
Published in RUNG: 28.06.2017; Views: 5820; Downloads: 255
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7. Stability by linear approximation for time scale dynamical systemsSergey Kryzhevich, Alexander Nazarov, 2017, original scientific article Abstract: We study systems on time scales that are generalizations of classical differential or difference equations and appear in numerical methods. In this paper we consider linear systems and their small nonlinear perturbations. In terms of time scales and of eigenvalues of matrices we formulate conditions, sufficient for stability by linear approximation. For non-periodic time scales we use techniques of central upper Lyapunov exponents (a common tool of the theory of linear ODEs) to study stability of solutions. Also, time scale versions of the famous Chetaev’s theorem on conditional instability are proved. In a nutshell, we have developed a completely new technique in order to demonstrate that methods of non-autonomous linear ODE theory may work for time-scale dynamics.
Keywords: Time scale system, Linearization, Lyapunov functions, Millionschikov rotations, Stability
Published in RUNG: 15.03.2017; Views: 4007; Downloads: 160
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8. The Instrument Response Function Format for the Cherenkov Telescope ArrayJohn E. Ward, Andrej Filipčič, Gašper Kukec Mezek, Samo Stanič, Serguei Vorobiov, Lili Yang, Gabrijela Zaharijas, Danilo Zavrtanik, Marko Zavrtanik, 2015, published scientific conference contribution Keywords: Cherenkov Telescope Array (CTA), public observatory, instrument response functions
Published in RUNG: 02.03.2017; Views: 3551; Downloads: 177
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