@article{GonzalezDornerBretzetal.2019, author = {Gonz{\´a}lez, Mar{\´i}a Magdalena and Dorner, Daniela and Bretz, Thomas and Garc{\´i}a-Gonz{\´a}lez, Jos{\´e} Andr{\´e}s}, title = {Unbiased long-term monitoring at TeV energies}, series = {Galaxies}, volume = {7}, journal = {Galaxies}, number = {2}, issn = {2075-4434}, doi = {10.3390/galaxies7020051}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-197389}, year = {2019}, abstract = {For the understanding of the variable, transient and non-thermal universe, unbiased long-term monitoring is crucial. To constrain the emission mechanisms at the highest energies, it is important to characterize the very high energy emission and its correlation with observations at other wavelengths. At very high energies, only a limited number of instruments is available. This article reviews the current status of monitoring of the extra-galactic sky at TeV energies.}, language = {en} } @unpublished{Dandekar2022, author = {Dandekar, Thomas}, title = {Qubit transition into defined Bits: A fresh perspective for cosmology and unifying theories}, doi = {10.25972/OPUS-26641}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-266418}, pages = {42}, year = {2022}, abstract = {In this view point we do not change cosmology after the hot fireball starts (hence agrees well with observation), but the changed start suggested and resulting later implications lead to an even better fit with current observations (voids, supercluster and galaxy formation; matter and no antimatter) than the standard model with big bang and inflation: In an eternal ocean of qubits, a cluster of qubits crystallizes to defined bits. The universe does not jump into existence ("big bang") but rather you have an eternal ocean of qubits in free super-position of all their quantum states (of any dimension, force field and particle type) as permanent basis. The undefined, boiling vacuum is the real "outside", once you leave our everyday universe. A set of n Qubits in the ocean are "liquid", in very undefined state, they have all their m possibilities for quantum states in free superposition. However, under certain conditions the qubits interact, become defined, and freeze out, crystals form and give rise to a defined, real world with all possible time series and world lines. GR holds only within the crystal. In our universe all n**m quantum possibilities are nicely separated and crystallized out to defined bit states: A toy example with 6 qubits each having 2 states illustrates, this is completely sufficient to encode space using 3 bits for x,y and z, 1 bit for particle type and 2 bits for its state. Just by crystallization, space, particles and their properties emerge from the ocean of qubits, and following the arrow of entropy, time emerges, following an arrow of time and expansion from one corner of the toy universe to everywhere else. This perspective provides time as emergent feature considering entropy: crystallization of each world line leads to defined world lines over their whole existence, while entropy ensures direction of time and higher representation of high entropy states considering the whole crystal and all slices of world lines. The crystal perspective is also economic compared to the Everett-type multiverse, each qubit has its m quantum states and n qubits interacting forming a crystal and hence turning into defined bit states has only n**m states and not more states. There is no Everett-type world splitting with every decision but rather individual world trajectories reside in individual world layers of the crystal. Finally, bit-separated crystals come and go in the qubit ocean, selecting for the ability to lay seeds for new crystals. This self-organizing reproduction selects over generations also for life-friendliness. Mathematical treatment introduces quantum action theory as a framework for a general lattice field theory extending quantum chromo dynamics where scalar fields for color interaction and gravity have to be derived from the permeating qubit-interaction field. Vacuum energy should get appropriately low by the binding properties of the qubit crystal. Connections to loop quantum gravity, string theory and emergent gravity are discussed. Standard physics (quantum computing; crystallization, solid state physics) allow validation tests of this perspective and will extend current results.}, language = {en} } @phdthesis{Zenk2018, author = {Zenk, Markus}, title = {On Numerical Methods for Astrophysical Applications}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-162669}, school = {Universit{\"a}t W{\"u}rzburg}, year = {2018}, abstract = {Diese Arbeit befasst sich mit der Approximation der L{\"o}sungen von Modellen zur Beschreibung des Str{\"o}mungsverhaltens in Atmosph{\"a}ren. Im Speziellen umfassen die hier behandelten Modelle die kompressiblen Euler Gleichungen der Gasdynamik mit einem Quellterm bez{\"u}glich der Gravitation und die Flachwassergleichungen mit einem nicht konstanten Bodenprofil. Verschiedene Methoden wurden bereits entwickelt um die L{\"o}sungen dieser Gleichungen zu approximieren. Im Speziellen geht diese Arbeit auf die Approximation von L{\"o}sungen nahe des Gleichgewichts und, im Falle der Euler Gleichungen, bei kleinen Mach Zahlen ein. Die meisten numerischen Methoden haben die Eigenschaft, dass die Qualit{\"a}t der Approximation sich mit der Anzahl der Freiheitsgrade verbessert. In der Praxis werden deswegen diese numerischen Methoden auf großen Computern implementiert um eine m{\"o}glichst hohe Approximationsg{\"u}te zu erreichen. Jedoch sind auch manchmal diese großen Maschinen nicht ausreichend, um die gew{\"u}nschte Qualit{\"a}t zu erreichen. Das Hauptaugenmerk dieser Arbeit ist darauf gerichtet, die Qualit{\"a}t der Approximation bei gleicher Anzahl von Freiheitsgrade zu verbessern. Diese Arbeit ist im Zusammenhang einer Kollaboration zwischen Prof. Klingenberg des Mathemaitschen Instituts in W{\"u}rzburg und Prof. R{\"o}pke des Astrophysikalischen Instituts in W{\"u}rzburg entstanden. Das Ziel dieser Kollaboration ist es, Methoden zur Berechnung von stellarer Atmosph{\"a}ren zu entwickeln. In dieser Arbeit werden vor allem zwei Problemstellungen behandelt. Die erste Problemstellung bezieht sich auf die akkurate Approximation des Quellterms, was zu den so genannten well-balanced Schemata f{\"u}hrt. Diese erlauben genaue Approximationen von L{\"o}sungen nahe des Gleichgewichts. Die zweite Problemstellung bezieht sich auf die Approximation von Str{\"o}mungen bei kleinen Mach Zahlen. Es ist bekannt, dass L{\"o}sungen der kompressiblen Euler Gleichungen zu L{\"o}sungen der inkompressiblen Euler Gleichungen konvergieren, wenn die Mach Zahl gegen null geht. Klassische numerische Schemata zeigen ein stark diffusives Verhalten bei kleinen Mach Zahlen. Das hier entwickelte Schema f{\"a}llt in die Kategorie der asymptotic preserving Schematas, d.h. das numerische Schema ist auf einem diskrete Level kompatibel mit dem auf dem Kontinuum gezeigten verhalten. Zus{\"a}tzlich wird gezeigt, dass die Diffusion des hier entwickelten Schemas unabh{\"a}ngig von der Mach Zahl ist. In Kapitel 3 wird ein HLL approximativer Riemann L{\"o}ser f{\"u}r die Approximation der L{\"o}sungen der Flachwassergleichungen mit einem nicht konstanten Bodenprofil angewendet und ein well-balanced Schema entwickelt. Die meisten well-balanced Schemata f{\"u}r die Flachwassergleichungen behandeln nur den Fall eines Fluids im Ruhezustand, die so genannten Lake at Rest L{\"o}sungen. Hier wird ein Schema entwickelt, welches sich mit allen Gleichgewichten befasst. Zudem wird eine zweiter Ordnung Methode entwickelt, welche im Gegensatz zu anderen in der Literatur nicht auf einem iterativen Verfahren basiert. Numerische Experimente werden durchgef{\"u}hrt um die Vorteile des neuen Verfahrens zu zeigen. In Kapitel 4 wird ein Suliciu Relaxations L{\"o}ser angepasst um die hydrostatischen Gleichgewichte der Euler Gleichungen mit einem Gravitationspotential aufzul{\"o}sen. Die Gleichungen der hydrostatischen Gleichgewichte sind unterbestimmt und lassen deshalb keine Eindeutigen L{\"o}sungen zu. Es wird jedoch gezeigt, dass das neue Schema f{\"u}r eine große Klasse dieser L{\"o}sungen die well-balanced Eigenschaft besitzt. F{\"u}r bestimmte Klassen werden Quadraturformeln zur Approximation des Quellterms entwickelt. Es wird auch gezeigt, dass das Schema robust, d.h. es erh{\"a}lt die Positivit{\"a}t der Masse und Energie, und stabil bez{\"u}glich der Entropieungleichung ist. Die numerischen Experimente konzentrieren sich vor allem auf den Einfluss der Quadraturformeln auf die well-balanced Eigenschaften. In Kapitel 5 wird ein Suliciu Relaxations Schema angepasst f{\"u}r Simulationen im Bereich kleiner Mach Zahlen. Es wird gezeigt, dass das neue Schema asymptotic preserving und die Diffusion kontrolliert ist. Zudem wird gezeigt, dass das Schema f{\"u}r bestimmte Parameter robust ist. Eine Stabilit{\"a}t wird aus einer Chapman-Enskog Analyse abgeleitet. Resultate numerische Experimente werden gezeigt um die Vorteile des neuen Verfahrens zu zeigen. In Kapitel 6 werden die Schemata aus den Kapiteln 4 und 5 kombiniert um das Verhalten des numerischen Schemas bei Fl{\"u}ssen mit kleiner Mach Zahl in durch die Gravitation geschichteten Atmosph{\"a}ren zu untersuchen. Es wird gezeigt, dass das Schema well-balanced ist. Die Robustheit und die Stabilit{\"a}t werden analog zu Kapitel 5 behandelt. Auch hier werden numerische Tests durchgef{\"u}hrt. Es zeigt sich, dass das neu entwickelte Schema in der Lage ist, die Dynamiken besser Aufzul{\"o}sen als vor der Anpassung. Das Kapitel 7 besch{\"a}ftigt sich mit der Entwicklung eines multidimensionalen Schemas basierend auf der Suliciu Relaxation. Jedoch ist die Arbeit an diesem Ansatz noch nicht beendet und numerische Resultate k{\"o}nnen nicht pr{\"a}sentiert werden. Es wird aufgezeigt, wo sich die Schw{\"a}chen dieses Ansatzes befinden und weiterer Entwicklungsbedarf besteht.}, subject = {Str{\"o}mung}, language = {en} } @phdthesis{Langejahn2022, author = {Langejahn, Marcus}, title = {Hard X-ray Properties of Relativistically Beamed Jets from Radio- and Gamma-Ray-Bright Blazars}, doi = {10.25972/OPUS-28200}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-282009}, school = {Universit{\"a}t W{\"u}rzburg}, year = {2022}, abstract = {In this work I characterize the hard X-ray properties of blazars, active galactic nuclei with highly beamed emission, which are notoriously hard to detect in this energy range. I employ pre-defined samples of beamed AGN: the radio-selected MOJAVE and TANAMI samples, as well as the most recent gamma-ray-selected Fermi/LAT 4LAC catalog. The hard X-ray data is extracted from the 105-month all-sky survey maps of the Swift/BAT (Burst Alert Telescope) in the energy band of 20 keV to 100 keV. A great majority of both the MOJAVE and TANAMI samples are significantly detected, with signal-to noise ratios of the sources often just below the X-ray catalog signal thresholds. All blazar sub-types (FSRQs, BL Lacs) and radio galaxies show characteristic ranges of X-ray flux, luminosity, and photon index. Their properties are correlated with the corresponding SED's shape / peak frequency. The LogN-LogS distributions of the samples show a scarcity of blazars in the middle and lower X-ray flux range, indicating differing evolutionary paths between radio and X-ray emission, which is also suggested by the corresponding luminosity functions. Compared to the radio samples, the 4LAC sources are on average significantly less bright in the BAT band since this range often coincides with the spectral gap region between the two big SED emission bumps. Also, the spectral shapes differ notably, especially for the sub-type of BL Lacs. Using the parameter space of X-ray and gamma-ray photon indices, 35 blazar candidate sources can be assigned to either the FSRQ or BL Lac type with high certainty. The reason why many blazars are weak in this energy band can be traced back to a number of factors: the selection bias of the initial sample, differential evolution of the X-rays and the wavelengths in which the sample is defined, and the limited sensitivity of the observing instruments.}, subject = {Aktiver galaktischer Kern}, language = {en} } @article{DornerMostafaSatalecka2021, author = {Dorner, Daniela and Mostaf{\´a}, Miguel and Satalecka, Konstancja}, title = {High-energy alerts in the multi-messenger era}, series = {Universe}, volume = {7}, journal = {Universe}, number = {11}, issn = {2218-1997}, doi = {10.3390/universe7110393}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-248496}, year = {2021}, abstract = {The observation of electromagnetic counterparts to both high energy neutrinos and gravitational waves marked the beginning of a new era in astrophysics. The multi-messenger approach allows us to gain new insights into the most energetic events in the Universe such as gamma-ray bursts, supernovas, and black hole mergers. Real-time multi-messenger alerts are the key component of the observational strategies to unravel the transient signals expected from astrophysical sources. Focusing on the high-energy regime, we present a historical perspective of multi-messenger observations, the detectors and observational techniques used to study them, the status of the multi-messenger alerts and the most significant results, together with an overview of the future prospects in the field.}, language = {en} } @article{KrausMouchaRoth2022, author = {Kraus, Daniela and Moucha, Annika and Roth, Oliver}, title = {A sharp Bernstein-type inequality and application to the Carleson embedding theorem with matrix weights}, series = {Analysis and Mathematical Physics}, volume = {12}, journal = {Analysis and Mathematical Physics}, number = {1}, issn = {1664-235X}, doi = {10.1007/s13324-021-00639-5}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-270485}, year = {2022}, abstract = {We prove a sharp Bernstein-type inequality for complex polynomials which are positive and satisfy a polynomial growth condition on the positive real axis. This leads to an improved upper estimate in the recent work of Culiuc and Treil (Int. Math. Res. Not. 2019: 3301-3312, 2019) on the weighted martingale Carleson embedding theorem with matrix weights. In the scalar case this new upper bound is optimal.}, language = {en} } @article{RoelofsBlackburnLindahletal.2023, author = {Roelofs, Freek and Blackburn, Lindy and Lindahl, Greg and Doeleman, Sheperd S. and Johnson, Michael D. and Arras, Philipp and Chatterjee, Koushik and Emami, Razieh and Fromm, Christian and Fuentes, Antonio and Knollm{\"u}ller, Jakob and Kosogorov, Nikita and M{\"u}ller, Hendrik and Patel, Nimesh and Raymond, Alexander and Tiede, Paul and Traianou, Efthalia and Vega, Justin}, title = {The ngEHT analysis challenges}, series = {Galaxies}, volume = {11}, journal = {Galaxies}, number = {1}, issn = {2075-4434}, doi = {10.3390/galaxies11010012}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-304976}, year = {2023}, abstract = {The next-generation Event Horizon Telescope (ngEHT) will be a significant enhancement of the Event Horizon Telescope (EHT) array, with ∼10 new antennas and instrumental upgrades of existing antennas. The increased uv-coverage, sensitivity, and frequency coverage allow a wide range of new science opportunities to be explored. The ngEHT Analysis Challenges have been launched to inform the development of the ngEHT array design, science objectives, and analysis pathways. For each challenge, synthetic EHT and ngEHT datasets are generated from theoretical source models and released to the challenge participants, who analyze the datasets using image reconstruction and other methods. The submitted analysis results are evaluated with quantitative metrics. In this work, we report on the first two ngEHT Analysis Challenges. These have focused on static and dynamical models of M87* and Sgr A* and shown that high-quality movies of the extended jet structure of M87* and near-horizon hourly timescale variability of Sgr A* can be reconstructed by the reference ngEHT array in realistic observing conditions using current analysis algorithms. We identify areas where there is still room for improvement of these algorithms and analysis strategies. Other science cases and arrays will be explored in future challenges.}, language = {en} } @article{JohnsonAkiyamaBlackburnetal.2023, author = {Johnson, Michael D. and Akiyama, Kazunori and Blackburn, Lindy and Bouman, Katherine L. and Broderick, Avery E. and Cardoso, Vitor and Fender, Rob P. and Fromm, Christian M. and Galison, Peter and G{\´o}mez, Jos{\´e} L. and Haggard, Daryl and Lister, Matthew L. and Lobanov, Andrei P. and Markoff, Sera and Narayan, Ramesh and Natarajan, Priyamvada and Nichols, Tiffany and Pesce, Dominic W. and Younsi, Ziri and Chael, Andrew and Chatterjee, Koushik and Chaves, Ryan and Doboszewski, Juliusz and Dodson, Richard and Doeleman, Sheperd S. and Elder, Jamee and Fitzpatrick, Garret and Haworth, Kari and Houston, Janice and Issaoun, Sara and Kovalev, Yuri Y. and Levis, Aviad and Lico, Rocco and Marcoci, Alexandru and Martens, Niels C. M. and Nagar, Neil M. and Oppenheimer, Aaron and Palumbo, Daniel C. M. and Ricarte, Angelo and Rioja, Mar{\´i}a  J. and Roelofs, Freek and Thresher, Ann C. and Tiede, Paul and Weintroub, Jonathan and Wielgus, Maciek}, title = {Key science goals for the next-generation Event Horizon Telescope}, series = {Galaxies}, volume = {11}, journal = {Galaxies}, number = {3}, issn = {2075-4434}, doi = {10.3390/galaxies11030061}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-313525}, year = {2023}, abstract = {The Event Horizon Telescope (EHT) has led to the first images of a supermassive black hole, revealing the central compact objects in the elliptical galaxy M87 and the Milky Way. Proposed upgrades to this array through the next-generation EHT (ngEHT) program would sharply improve the angular resolution, dynamic range, and temporal coverage of the existing EHT observations. These improvements will uniquely enable a wealth of transformative new discoveries related to black hole science, extending from event-horizon-scale studies of strong gravity to studies of explosive transients to the cosmological growth and influence of supermassive black holes. Here, we present the key science goals for the ngEHT and their associated instrument requirements, both of which have been formulated through a multi-year international effort involving hundreds of scientists worldwide.}, language = {en} } @article{ChatterjeeChaelTiedeetal.2023, author = {Chatterjee, Koushik and Chael, Andrew and Tiede, Paul and Mizuno, Yosuke and Emami, Razieh and Fromm, Christian and Ricarte, Angelo and Blackburn, Lindy and Roelofs, Freek and Johnson, Michael D. and Doeleman, Sheperd S. and Arras, Philipp and Fuentes, Antonio and Knollm{\"u}ller, Jakob and Kosogorov, Nikita and Lindahl, Greg and M{\"u}ller, Hendrik and Patel, Nimesh and Raymond, Alexander and Traianou, Efthalia and Vega, Justin}, title = {Accretion flow morphology in numerical simulations of black holes from the ngEHT model library: the impact of radiation physics}, series = {Galaxies}, volume = {11}, journal = {Galaxies}, number = {2}, issn = {2075-4434}, doi = {10.3390/galaxies11020038}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-304084}, year = {2023}, abstract = {In the past few years, the Event Horizon Telescope (EHT) has provided the first-ever event horizon-scale images of the supermassive black holes (BHs) M87* and Sagittarius A* (Sgr A*). The next-generation EHT project is an extension of the EHT array that promises larger angular resolution and higher sensitivity to the dim, extended flux around the central ring-like structure, possibly connecting the accretion flow and the jet. The ngEHT Analysis Challenges aim to understand the science extractability from synthetic images and movies to inform the ngEHT array design and analysis algorithm development. In this work, we compare the accretion flow structure and dynamics in numerical fluid simulations that specifically target M87* and Sgr A*, and were used to construct the source models in the challenge set. We consider (1) a steady-state axisymmetric radiatively inefficient accretion flow model with a time-dependent shearing hotspot, (2) two time-dependent single fluid general relativistic magnetohydrodynamic (GRMHD) simulations from the H-AMR code, (3) a two-temperature GRMHD simulation from the BHAC code, and (4) a two-temperature radiative GRMHD simulation from the KORAL code. We find that the different models exhibit remarkably similar temporal and spatial properties, except for the electron temperature, since radiative losses substantially cool down electrons near the BH and the jet sheath, signaling the importance of radiative cooling even for slowly accreting BHs such as M87*. We restrict ourselves to standard torus accretion flows, and leave larger explorations of alternate accretion models to future work.}, language = {en} } @article{RenautFreiNuechter2023, author = {Renaut, L{\´e}o and Frei, Heike and N{\"u}chter, Andreas}, title = {Lidar pose tracking of a tumbling spacecraft using the smoothed normal distribution transform}, series = {Remote Sensing}, volume = {15}, journal = {Remote Sensing}, number = {9}, issn = {2072-4292}, doi = {10.3390/rs15092286}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-313738}, year = {2023}, abstract = {Lidar sensors enable precise pose estimation of an uncooperative spacecraft in close range. In this context, the iterative closest point (ICP) is usually employed as a tracking method. However, when the size of the point clouds increases, the required computation time of the ICP can become a limiting factor. The normal distribution transform (NDT) is an alternative algorithm which can be more efficient than the ICP, but suffers from robustness issues. In addition, lidar sensors are also subject to motion blur effects when tracking a spacecraft tumbling with a high angular velocity, leading to a loss of precision in the relative pose estimation. This work introduces a smoothed formulation of the NDT to improve the algorithm's robustness while maintaining its efficiency. Additionally, two strategies are investigated to mitigate the effects of motion blur. The first consists in un-distorting the point cloud, while the second is a continuous-time formulation of the NDT. Hardware-in-the-loop tests at the European Proximity Operations Simulator demonstrate the capability of the proposed methods to precisely track an uncooperative spacecraft under realistic conditions within tens of milliseconds, even when the spacecraft tumbles with a significant angular rate.}, language = {en} } @article{WangLiuXiaoetal.2023, author = {Wang, Xiaoliang and Liu, Xuan and Xiao, Yun and Mao, Yue and Wang, Nan and Wang, Wei and Wu, Shufan and Song, Xiaoyong and Wang, Dengfeng and Zhong, Xingwang and Zhu, Zhu and Schilling, Klaus and Damaren, Christopher}, title = {On-orbit verification of RL-based APC calibrations for micrometre level microwave ranging system}, series = {Mathematics}, volume = {11}, journal = {Mathematics}, number = {4}, issn = {2227-7390}, doi = {10.3390/math11040942}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-303970}, year = {2023}, abstract = {Micrometre level ranging accuracy between satellites on-orbit relies on the high-precision calibration of the antenna phase center (APC), which is accomplished through properly designed calibration maneuvers batch estimation algorithms currently. However, the unmodeled perturbations of the space dynamic and sensor-induced uncertainty complicated the situation in reality; ranging accuracy especially deteriorated outside the antenna main-lobe when maneuvers performed. This paper proposes an on-orbit APC calibration method that uses a reinforcement learning (RL) process, aiming to provide the high accuracy ranging datum for onboard instruments with micrometre level. The RL process used here is an improved Temporal Difference advantage actor critic algorithm (TDAAC), which mainly focuses on two neural networks (NN) for critic and actor function. The output of the TDAAC algorithm will autonomously balance the APC calibration maneuvers amplitude and APC-observed sensitivity with an object of maximal APC estimation accuracy. The RL-based APC calibration method proposed here is fully tested in software and on-ground experiments, with an APC calibration accuracy of less than 2 mrad, and the on-orbit maneuver data from 11-12 April 2022, which achieved 1-1.5 mrad calibration accuracy after RL training. The proposed RL-based APC algorithm may extend to prove mass calibration scenes with actions feedback to attitude determination and control system (ADCS), showing flexibility of spacecraft payload applications in the future.}, language = {en} }