TY - JOUR A1 - Romoli, Carlo A1 - Chakraborty, Nachiketa A1 - Dorner, Daniela A1 - Taylor, Andrew A1 - Blank, Michael T1 - Flux Distribution of Gamma-Ray Emission in Blazars: The Example of Mrk 501 JF - Galaxies N2 - Flux distribution is an important tool to understand the variability processes in activegalactic nuclei. We now have available a great deal of observational evidences pointing towards thepresence of log-normal components in the high energy light curves, and different models have beenproposed to explain these data. Here, we collect some of the recent developments on this topic usingthe well-known blazar Mrk 501 as example of complex and interesting aspects coming from its fluxdistribution in different energy ranges and at different timescales. The observational data we refer toare those collected in a complementary manner by Fermi-LAT over multiple years, and by the FirstG-APD Cherenkov Telescope (FACT) telescope and the H.E.S.S. array in correspondence of the brightflare of June 2014 KW - gamma rays KW - very high energy KW - active galactic nuclei KW - Markarian 501 KW - monitoring KW - flux distributions Y1 - 2018 U6 - http://nbn-resolving.de/urn/resolver.pl?urn:nbn:de:bvb:20-opus-197580 SN - 2075-4434 VL - 6 IS - 4 ER - TY - THES A1 - Wendel, Christoph T1 - Spectral Imprints from Electromagnetic Cascades in Blazar Jets T1 - Spektrale Merkmale elektromagnetischer Kaskaden in Jets von Blazaren N2 - The extragalactic gamma-ray sky is dominated by blazars, active galactic nuclei (AGN) with a relativistic jet that is closely aligned with the line of sight. Galaxies develop an active nucleus if the central supermassive black hole (BH) accretes large amounts of ambient matter and magnetic flux. The inflowing mass accumulates around the plane perpendicular to the accretion flow's angular momentum. The flow is heated through viscous friction and part of the released energy is radiated as blackbody or non-thermal radiation, with luminosities that can dominate the accumulated stellar luminosity of the host galaxy. A fraction of the accretion flow luminosity is reprocessed in a surrounding field of ionised gas clouds. These clouds, revolving around the central BH, emit Doppler-broadened atomic emission lines. The region where these broad-line-emitting clouds are located is called broad-line region (BLR). About one in ten AGN forms an outflow of radiation and relativistic particles, called a relativistic jet. According to the Blandford-Znajek mechanism, this is facilitated through electromagnetic processes in the magnetosphere of a spinning BH. The latter induces a magnetospheric poloidal current circuit, generating a decelerating torque on the BH and inducing a toroidal magnetic field. Consequently, rotational energy of the BH is converted to Poynting flux streaming away mainly along the rotational axis and starting the jet. One possibility for particle acceleration near the jet base is realised by magnetospheric vacuum gaps, regions temporarily devoid of plasma, such that an intermittent electric field arises parallel to the magnetic field lines, enabling particle acceleration and contributing to the mass loading of the jets. Magnetised structures, containing bunches of relativistic electrons, propagate away from the galactic nucleus along the jets. Assuming that these electrons emit synchrotron radiation and that they inverse-Compton (IC) up-scatter abundant target photons, which can either be the synchrotron photons themselves or photons from external emitters, the emitted spectrum can be theoretically determined. Additionally taking into account that these emission regions move relativistically themselves and that the emission is Doppler-boosted and beamed in forward direction, the typical two-hump spectral energy distribution (SED) of blazars is recovered. There are however findings that challenge this well-established model. Short-time variability, reaching down to minute scales at very high energy gamma rays, is today known to be a widespread phenomenon of blazars, calling for very compact emission regions. In most models of such optically thick emission regions, the gamma-ray flux is usually pair-absorbed exponentially, without considering the cascade evolving from the pair-produced electrons. From the observed flux, it is often concluded that emission emanates from larger distances where the region is optically thin, especially from outside of the BLR. Only in few blazars gamma-ray attenuation associated with pair absorption in the BLR was clearly reported. With the advent of sophisticated high-energy or very high energy gamma-ray detectors, like the Fermi Large Area Telescope or the Major Atmospheric Gamma-ray Imaging Cherenkov telescopes, besides the extraordinarily fast variability spectral features have been found that cannot be explained by conventional models reproducing the two-hump SED. Two such narrow spectral features are discussed in this work. For the nearby blazar Markarian 501, hints to a sharp peak around 3 TeV have been reported from a multi-wavelength campaign carried out in July 2014, while for 3C 279 a spectral dip was found in 2018 data, that can hardly be described with conventional fitting functions. In this work it is examined whether these spectral peculiarities of blazar jet emission can be explained, if the full radiation reprocessing through an IC pair cascade is accounted for. Such a cascade is the multiple concatenation of IC scattering events and pair production events. In the cascades generally considered in this work, relativistic electrons and high-energy photons are injected into a fixed soft target photon field. A mathematical description for linear IC pair cascades with escape terms is delivered on the basis of preliminary works. The steady-state kinetic equations for the electrons and for the photons are determined, whereby it is paid attention to an explicit formulation and to motivating the correct integration borders of all integrals from kinematic constraints. In determining the potentially observable gamma-ray flux, both the attenuated injected flux and the flux evolving as an effect of IC up-scattering, pair absorption and escape are incorporated, giving the emerging spectra very distinct imprints. Much effort is dedicated to the numerical solution of the electrons' kinetic equation via iterative schemes. It is explained why pointwise iteration from higher to lower Lorentz factors is more efficient than iterating the whole set of sampling points. The algorithm is parallelised at two positions. First, several workers can perform pointwise iterations simultaneously. Second, the most demanding integral is cut into a number of part integrals which can be determined by multiple workers. Through these measures, the Python code can be readily applied to simulate steady-state IC pair cascades with escape. In the case of Markarian 501 the developed framework is as follows. The AGN hosts an advection-dominated accretion flow with a normalised accretion rate of several \(10^{-4}\) and an electron temperature near \(10^{10}\) K. On the one hand, the accretion flow illuminates the few ambient gas clouds with approximate radius \(10^{11}\) m, which reprocess a fraction 0.01 of the luminosity into hydrogen and helium emission lines. On the other hand, the gamma rays from the accretion flow create electrons and positrons in a sporadically active vacuum gap in the BH magnetosphere. In the active gap, a power of roughly 0.001 of the Blandford-Znajek power is extracted from the rotating BH through a gap potential drop of several \(10^{18}\) V, generating ultra-relativistic electrons, which subsequently are multiplied by a factor of about \(10^6\) through interaction with the accretion flow photons. This electron beam propagates away from the central engine and encounters the photon field of one passing ionised cloud. The resulting IC pair cascade is simulated and the evolving gamma-ray spectrum is determined. Just above the absorption troughs due to the hydrogen lines, the spectrum exhibits a narrow bump around 3 TeV. When the cascaded emission is added to the emission generated at larger distances, the observed multi-wavelength SED including the sharp peak at 3 TeV is reproduced, underlining that radiation processes beyond conventional models are motivated by distinct spectral features. The dip in the spectrum of 3C 279 is addressed by a similar cascade model. Three types of injection are considered, varying in the ratio of the photon density to the electron density and varying in the spectral shape. The IC pair cascade is assumed to happen either in the dense BLR photon field with a luminosity of several \(10^{37}\) W and a radial size of few \(10^{14}\) m or in the diluted photon field outside of the BLR. The latter scenario is however rejected as the spectral slope around several 100 MeV and the dip at few 10 GeV cannot be reconciled within this model. The radiation cascaded in the BLR can explain the observational data, irrespective of the assumed injected rate. It is therefore concluded that for this period of gamma-ray emission, the radiation production happens at the edge of the BLR of 3C 279. Both investigations show that IC pair cascades can account for fine structure seen in blazar SEDs. It is insufficient to restrict the radiation transport to pure exponential absorption of an injection term. Pair production and IC up-scattering by all generations of photons and electrons in the optically thick regime critically shape the emerging spectra. As the advent of future improved detectors will provide more high-precision spectra, further observations of narrow spectral features can be expected. It seems therefore recommendable to incorporate cascading into conventional radiation production models or to extend the model developed in this work by synchrotron radiation. N2 - Beobachtet man das Firmament im Licht der Gammastrahlung, stellen Blasare die Mehrzahl extragalaktischer Objekte dar. Blasare sind aktive Galaxienkerne mit einem relativistischen Jet, der entlang der Sichtlinie ausgerichtet ist. Galaxien haben einen aktiven Kern, wenn das zentrale supermassereiche Schwarze Loch große Mengen an Umgebungsmaterie und magnetischem Fluss akkretiert. Die nach Innen strömende Masse sammelt sich nahe der Ebene an, die senkrecht zum Drehimpuls des Akkretionsflusses steht. Das akkretierte Material wird durch viskose Reibung aufgeheizt und ein Teil der freigesetzten Energie wird als Schwarzkörper- oder nicht-thermische Strahlung abgestrahlt, deren Leuchtkraft die gesamte stellare Leuchtkraft der Wirtsgalaxie übertreffen kann. Ein Teil der Leuchtkraft des Akkretionsflusses wird in einem umgebenden Feld von ionisierten Gaswolken reprozessiert. Diese Wolken, die um das zentrale Schwarze Loch kreisen, emittieren Doppler-verbreiterte Emissionslinien. Den Teil des aktiven Galaxienkerns, in dem sich diese Wolken befinden, bezeichnet man als BLR (englisch: broad-line region). Ihr Abstand zum zentralen Schwarzen Loch beträgt typischerweise etwa 0,1 pc. Etwa einer von zehn aktiven Galaxienkernen bildet einen Ausfluss von Strahlung und relativistischen Teilchen aus, einen sogenannten relativistischen Jet. Dies wird gemäß dem Blandford-Znajek-Mechanismus durch elektromagnetische Prozesse in den Magnetosphären rotierender Schwarzer Löcher bewerkstelligt. Letztere induzieren einen poloidalen magnetosphärischen Stromkreis, der ein abbremsendes Drehmoment auf das Schwarze Loch ausübt und ein toroidales Magnetfeld erzeugt. Folglich wird die Rotationsenergie des Schwarzen Lochs in Poynting-Fluss umgewandelt, der hauptsächlich entlang der Rotationsachse abfließt und den Jet entstehen lässt. Durch Prozesse, die noch nicht eindeutig identifiziert wurden, werden geladene Teilchen in der Nähe der Jetbasis beschleunigt. Eine Möglichkeit dafür ist Teilchenbeschleunigung in magnetosphärischen Vakuum-Lücken. Dies sind Regionen, die vorübergehend nahezu frei von Plasma sind, sodass zeitweise ein elektrisches Feld parallel zu den Magnetfeldlinien entsteht, das die Teilchenbeschleunigung ermöglicht und zur Aufladung der Jets mit massebehafteten Teilchen beiträgt. Magnetisierte Strukturen, die relativistische Elektronen enthalten, bewegen sich entlang der Jets vom Galaxienkern weg. Unter der Annahme, dass diese Elektronen Synchrotronstrahlung aussenden und dass sie vorhandenen weichen Photonen, die entweder die Synchrotronphotonen selbst oder Photonen von externen Emittern sein können, durch inverse Compton-Streuung höhere Energien verleihen, kann das emittierte Spektrum berechnet werden. Berücksichtigt man zusätzlich, dass sich diese Emissionsgebiete selbst relativistisch bewegen und dass die Emission Doppler-verstärkt ist und bevorzugt in Vorwärtsrichtung abgestrahlt wird, erhält man die typische zweihöckrige spektrale Energieverteilung von Blasaren. Es gibt jedoch Erkenntnisse, die dieses bewährte Modell in Frage stellen. Kurzzeit-Variabilität, die bei sehr hochenergetischer Gammastrahlung bis zu Minuten-Skalen hinunterreicht, ist ein weit verbreitetes Phänomen bei Blasaren und setzt sehr kompakte Emissionsregionen voraus. In den meisten Modellen für solche optisch dicken Emissionsregionen wird der Gammastrahlenfluss durch Paarbildung lediglich exponentiell absorbiert, ohne die Kaskade zu berücksichtigen, die sich durch die erzeugten Elektronen entwickelt. Aus den Beobachtungen wird oft gefolgert, dass die Emission aus optisch dünnen Regionen bei größeren Entfernungen stammt, insbesondere von außerhalb der BLR. Nur bei wenigen Blasaren wurde eine Abschwächung der Gammastrahlung durch Absorption in der BLR eindeutig nachgewiesen. Durch moderne Gammastrahlen-Detektoren, wie das Fermi Large Area Telescope oder den Major Atmospheric Gamma-ray Imaging Cherenkov Teleskopen, wurden neben der Kurzzeit-Variabilität auch spektrale Merkmale gefunden, die nicht durch konventionelle Modelle, die die zweihöckrigen spektralen Energieverteilungen wiedergeben können, erklärt werden können. Zwei solcher besonderen spektralen Merkmale werden in dieser Arbeit diskutiert. Für den Blasar Markarian 501 wurden bei einer im Juli 2014 durchgeführten Multiwellenlängenkampagne Hinweise auf einen schmalen Buckel bei 3 TeV gefunden, während für 3C 279 in Daten von 2018 eine Mulde im Spektrum gefunden wurde, die mit oft verwendeten Fit-Funktionen nur schlecht beschrieben werden kann. In dieser Arbeit wird untersucht, ob diese spektralen Besonderheiten der Blasar-Jet-Emission erklärt werden können, wenn die vollständige Reprozessierung der Strahlung durch eine inverse Compton-Paar-Kaskade berücksichtigt wird. Eine solche Kaskade ist die mehrfache Aneinanderreihung von inverser Compton-Streuung und Paarproduktion. Bei den in dieser Arbeit allgemein betrachteten Kaskaden werden relativistische Elektronen und hochenergetische Photonen in eine Region mit niederenergetischen Photonen konstanter Dichte injiziert. Auf der Grundlage von Vorarbeiten wird eine mathematische Beschreibung für lineare inverse Compton-Paar-Kaskaden mit Entweichtermen ausgearbeitet. Es werden die zeit-unabhängigen kinetischen Gleichungen für Elektronen und Photonen hergeleitet, wobei auf eine vollständige Formulierung und auf die Begründung der korrekten Integrationsgrenzen aller Integrale durch die kinematischen Vorgaben geachtet wird. Bei der Bestimmung des potentiell beobachtbaren Gammastrahlenflusses werden sowohl der teilweise absorbierte, injizierte Fluss als auch der Fluss, der sich als Effekt der inversen Compton-Streuung, der Paar-Absorption und des Entweichens ergibt, einbezogen, was den entstehenden Spektren charakteristische Formen aufprägt. Die kinetische Gleichung der Elektronen wird durch iterative Vorgehensweisen numerisch gelöst. Es wird erklärt, warum eine punktweise Iteration von höheren zu niedrigeren Lorentz-Faktoren effizienter ist als die Iteration des gesamten Satzes von Stützstellen. Der Algorithmus wird an zwei Stellen parallelisiert. Erstens können mehrere Prozessor-Kerne gleichzeitig punktweise Iterationen durchführen. Zweitens wird das rechenintensivste Integral in mehrere Teilintegrale zerlegt, die von mehreren Kernen berechnet werden können. Durch diese Maßnahmen kann der Python-Code zur Simulation von zeitunabhängigen inversen Compton-Paar-Kaskaden eingesetzt werden. Im Fall von Markarian 501 wird folgendes Modell bemüht. Der aktive Galaxienkern hat einen advektionsdominierten Akkretionsfluss mit einer normalisierten Akkretionsrate von mehreren \(10^{-4}\) und einer Elektronentemperatur um \(10^{10}\) K. Einerseits bestrahlt der Akkretionsfluss die wenigen umgebenden Gaswolken mit ungefährem Radius von \(10^{11}\) m, die einen Faktor 0,01 der Leuchtkraft in Form von Wasserstoff- und Helium-Emissionslinien wieder abstrahlen. Andererseits erzeugen die vom Akkretionsfluss stammenden Gammaphotonen in einer zeitweise aktiven Vakuum-Lücke in der Magnetosphäre des Schwarzen Lochs Elektronen und Positronen. In der geöffneten Lücke wird dem rotierenden Schwarzen Loch durch einen Potentialunterschied von mehreren \(10^{18}\) V eine Leistung von etwa 0,001 der Blandford-Znajek-Leistung entzogen, wodurch ultra-relativistische Elektronen erzeugt werden, die anschließend durch Wechselwirkung mit den Photonen des Akkretionsflusses um einen Faktor von etwa \(10^6\) multipliziert werden. Dieser Elektronenstrahl verlässt die Magnetosphäre und trifft auf das Photonenfeld einer vorbeiziehenden ionisierten Wolke. Die daraus resultierende inverse Compton-Paar-Kaskade wird simuliert und das sich ergebende Gammastrahlenspektrum wird berechnet. Unmittelbar oberhalb der durch die Wasserstofflinien verursachten Absorptionströge erscheint bei rund 3 TeV ein schmaler Höcker. Wenn die Strahlung der Kaskade der aus größerer Entfernung stammenden Strahlung überlagert wird, wird die gesamte spektrale Energieverteilung einschließlich des scharfen Buckels bei 3 TeV reproduziert. Das bedeutet, dass schmale spektrale Merkmale für die Relevanz von Strahlungsprozessen sprechen, die über konventionelle Modelle hinausgehen. Der Trog im Spektrum von 3C 279 wird mit einem ähnlichen Kaskadenmodell untersucht. Es werden drei Fälle der Injektion betrachtet, die sich im Verhältnis der Photonen-Anzahl zur Elektronen-Anzahl und im spektralen Verlauf unterscheiden. Es wird angenommen, dass die Kaskade entweder im dichten Photonenfeld der BLR mit einer Leuchtkraft von mehreren \(10^{37}\) W und einer radialen Ausdehnung von einigen \(10^{14}\) m oder im ausgedünnten Photonenfeld außerhalb der BLR stattfindet. Das letztgenannte Szenario muss jedoch verworfen werden, da die spektrale Steigung bei einigen 100 MeV und der Absorptionstrog bei einigen 10 GeV innerhalb dieses Modells nicht miteinander in Einklang gebracht werden können. Die innerhalb der BLR kaskadierte Strahlung kann die Beobachtungsdaten unabhängig von der angenommenen Injektionsrate erklären. Daraus folgt, dass die Gammastrahlung während dieses Emissionsereignisses am Rande der BLR von 3C 279 produziert wird. Beide Untersuchungen zeigen, dass inverse Compton-Paar-Kaskaden Feinstrukturen in der spektralen Energieverteilung von Blasaren erklären können. Es reicht nicht aus, den Strahlungstransport auf reine exponentielle Absorption eines Injektionsterms zu beschränken. Paarbildung und inverse Compton-Streuung im optisch dicken Bereich und über alle Generationen von Photonen und Elektronen hinweg prägen die entstehenden Spektren entscheidend. Da künftige, verbesserte Detektoren detailliertere Spektren liefern werden, darf man weitere Berichte über schmale spektrale Merkmale erwarten. Es erscheint daher empfehlenswert, die Kaskadierung in konventionelle Modelle der Strahlungsproduktion mit einzubeziehen oder das in dieser Arbeit entwickelte Modell um Synchrotronstrahlung zu erweitern. KW - Active galactic nucleus KW - Blazar KW - BL Lacertae objects KW - Compton-Streuung KW - Paarbildung KW - inverse-Compton pair cascades KW - radiative processes KW - Markarian 501 KW - 3C 279 Y1 - 2022 U6 - http://nbn-resolving.de/urn/resolver.pl?urn:nbn:de:bvb:20-opus-290076 ER - TY - JOUR A1 - Dashkovskiy, Sergey A1 - Slynko, Vitalii T1 - Stability conditions for impulsive dynamical systems JF - Mathematics of Control, Signals, and Systems N2 - In this work, we consider impulsive dynamical systems evolving on an infinite-dimensional space and subjected to external perturbations. We look for stability conditions that guarantee the input-to-state stability for such systems. Our new dwell-time conditions allow the situation, where both continuous and discrete dynamics can be unstable simultaneously. Lyapunov like methods are developed for this purpose. Illustrative finite and infinite dimensional examples are provided to demonstrate the application of the main results. These examples cannot be treated by any other published approach and demonstrate the effectiveness of our results. KW - lyapunov methods KW - stability KW - robustness KW - impulsive systems KW - infinite-dimensional systems KW - nonlinear systems KW - input-to-state stability Y1 - 2022 U6 - http://nbn-resolving.de/urn/resolver.pl?urn:nbn:de:bvb:20-opus-268390 SN - 1435-568X VL - 34 IS - 1 ER - TY - JOUR A1 - González, María Magdalena A1 - Dorner, Daniela A1 - Bretz, Thomas A1 - García-González, José Andrés T1 - Unbiased long-term monitoring at TeV energies JF - Galaxies N2 - 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. KW - monitoring KW - very high energies (VHE) KW - TeV energies Y1 - 2019 U6 - http://nbn-resolving.de/urn/resolver.pl?urn:nbn:de:bvb:20-opus-197389 SN - 2075-4434 VL - 7 IS - 2 ER - TY - JOUR A1 - Roelofs, Freek A1 - Blackburn, Lindy A1 - Lindahl, Greg A1 - Doeleman, Sheperd S. A1 - Johnson, Michael D. A1 - Arras, Philipp A1 - Chatterjee, Koushik A1 - Emami, Razieh A1 - Fromm, Christian A1 - Fuentes, Antonio A1 - Knollmüller, Jakob A1 - Kosogorov, Nikita A1 - Müller, Hendrik A1 - Patel, Nimesh A1 - Raymond, Alexander A1 - Tiede, Paul A1 - Traianou, Efthalia A1 - Vega, Justin T1 - The ngEHT analysis challenges JF - Galaxies N2 - 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. KW - very long baseline interferometry KW - black holes KW - active galactic nuclei KW - radio astronomy KW - imaging KW - instrument design KW - telescopes KW - algorithms KW - data analysis Y1 - 2023 U6 - http://nbn-resolving.de/urn/resolver.pl?urn:nbn:de:bvb:20-opus-304976 SN - 2075-4434 VL - 11 IS - 1 ER - TY - JOUR A1 - Johnson, Michael D. A1 - Akiyama, Kazunori A1 - Blackburn, Lindy A1 - Bouman, Katherine L. A1 - Broderick, Avery E. A1 - Cardoso, Vitor A1 - Fender, Rob P. A1 - Fromm, Christian M. A1 - Galison, Peter A1 - Gómez, José L. A1 - Haggard, Daryl A1 - Lister, Matthew L. A1 - Lobanov, Andrei P. A1 - Markoff, Sera A1 - Narayan, Ramesh A1 - Natarajan, Priyamvada A1 - Nichols, Tiffany A1 - Pesce, Dominic W. A1 - Younsi, Ziri A1 - Chael, Andrew A1 - Chatterjee, Koushik A1 - Chaves, Ryan A1 - Doboszewski, Juliusz A1 - Dodson, Richard A1 - Doeleman, Sheperd S. A1 - Elder, Jamee A1 - Fitzpatrick, Garret A1 - Haworth, Kari A1 - Houston, Janice A1 - Issaoun, Sara A1 - Kovalev, Yuri Y. A1 - Levis, Aviad A1 - Lico, Rocco A1 - Marcoci, Alexandru A1 - Martens, Niels C. M. A1 - Nagar, Neil M. A1 - Oppenheimer, Aaron A1 - Palumbo, Daniel C. M. A1 - Ricarte, Angelo A1 - Rioja, María  J. A1 - Roelofs, Freek A1 - Thresher, Ann C. A1 - Tiede, Paul A1 - Weintroub, Jonathan A1 - Wielgus, Maciek T1 - Key science goals for the next-generation Event Horizon Telescope JF - Galaxies N2 - 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. KW - black holes KW - general relativity KW - interferometry KW - accretion KW - relativistic jets KW - very-long-baseline interferometry KW - EHT KW - ngEHT Y1 - 2023 U6 - http://nbn-resolving.de/urn/resolver.pl?urn:nbn:de:bvb:20-opus-313525 SN - 2075-4434 VL - 11 IS - 3 ER - TY - JOUR A1 - Chatterjee, Koushik A1 - Chael, Andrew A1 - Tiede, Paul A1 - Mizuno, Yosuke A1 - Emami, Razieh A1 - Fromm, Christian A1 - Ricarte, Angelo A1 - Blackburn, Lindy A1 - Roelofs, Freek A1 - Johnson, Michael D. A1 - Doeleman, Sheperd S. A1 - Arras, Philipp A1 - Fuentes, Antonio A1 - Knollmüller, Jakob A1 - Kosogorov, Nikita A1 - Lindahl, Greg A1 - Müller, Hendrik A1 - Patel, Nimesh A1 - Raymond, Alexander A1 - Traianou, Efthalia A1 - Vega, Justin T1 - Accretion flow morphology in numerical simulations of black holes from the ngEHT model library: the impact of radiation physics JF - Galaxies N2 - 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. KW - black holes KW - general relativity KW - accretion KW - relativistic jets KW - very-long-baseline interferometry Y1 - 2023 U6 - http://nbn-resolving.de/urn/resolver.pl?urn:nbn:de:bvb:20-opus-304084 SN - 2075-4434 VL - 11 IS - 2 ER - TY - JOUR A1 - Renaut, Léo A1 - Frei, Heike A1 - Nüchter, Andreas T1 - Lidar pose tracking of a tumbling spacecraft using the smoothed normal distribution transform JF - Remote Sensing N2 - 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. KW - pose tracking KW - uncooperative space rendezvous KW - lidar KW - normal distribution transform Y1 - 2023 U6 - http://nbn-resolving.de/urn/resolver.pl?urn:nbn:de:bvb:20-opus-313738 SN - 2072-4292 VL - 15 IS - 9 ER - TY - JOUR A1 - Wang, Xiaoliang A1 - Liu, Xuan A1 - Xiao, Yun A1 - Mao, Yue A1 - Wang, Nan A1 - Wang, Wei A1 - Wu, Shufan A1 - Song, Xiaoyong A1 - Wang, Dengfeng A1 - Zhong, Xingwang A1 - Zhu, Zhu A1 - Schilling, Klaus A1 - Damaren, Christopher T1 - On-orbit verification of RL-based APC calibrations for micrometre level microwave ranging system JF - Mathematics N2 - 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. KW - reinforcement learning KW - antenna phase center calibration KW - K band ranging (KBR) KW - micrometre level microwave ranging KW - MSC: 49M37 KW - MSC: 65K05 KW - MSC: 90C30 KW - MSC: 90C40 Y1 - 2023 U6 - http://nbn-resolving.de/urn/resolver.pl?urn:nbn:de:bvb:20-opus-303970 SN - 2227-7390 VL - 11 IS - 4 ER - TY - JOUR A1 - Dorner, Daniela A1 - Mostafá, Miguel A1 - Satalecka, Konstancja T1 - High-energy alerts in the multi-messenger era JF - Universe N2 - 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. KW - multi-messenger KW - real-time KW - high-energy KW - alerts Y1 - 2021 U6 - http://nbn-resolving.de/urn/resolver.pl?urn:nbn:de:bvb:20-opus-248496 SN - 2218-1997 VL - 7 IS - 11 ER -