520 Astronomie und zugeordnete Wissenschaften
Refine
Departments, institutes and facilities
Document Type
- Article (36)
- Conference Object (29)
- Preprint (2)
- Doctoral Thesis (1)
Year of publication
Keywords
4GREAT is an extension of the German Receiver for Astronomy at Terahertz frequencies (GREAT) operated aboard the Stratospheric Observatory for Infrared Astronomy (SOFIA). The spectrometer comprises four different detector bands and their associated subsystems for simultaneous and fully independent science operation. All detector beams are co-aligned on the sky. The frequency bands of 4GREAT cover 491-635, 890-1090, 1240-1525 and 2490-2590 GHz, respectively. This paper presents the design and characterization of the instrument, and its in-flight performance. 4GREAT saw first light in June 2018, and has been offered to the interested SOFIA communities starting with observing cycle 6.
Earth’s nearest candidate supermassive black hole lies at the centre of the Milky Way1. Its electromagnetic emission is thought to be powered by radiatively inefficient accretion of gas from its environment2, which is a standard mode of energy supply for most galactic nuclei. X-ray measurements have already resolved a tenuous hot gas component from which the black hole can be fed3. The magnetization of the gas, however, which is a crucial parameter determining the structure of the accretion flow, remains unknown. Strong magnetic fields can influence the dynamics of accretion, remove angular momentum from the infalling gas4, expel matter through relativistic jets5 and lead to synchrotron emission such as that previously observed6, 7, 8. Here we report multi-frequency radio measurements of a newly discovered pulsar close to the Galactic Centre9, 10, 11, 12 and show that the pulsar’s unusually large Faraday rotation (the rotation of the plane of polarization of the emission in the presence of an external magnetic field) indicates that there is a dynamically important magnetic field near the black hole. If this field is accreted down to the event horizon it provides enough magnetic flux to explain the observed emission—from radio to X-ray wavelengths—from the black hole.
Atomic oxygen in the mesosphere and lower thermosphere measured by terahertz heterodyne spectroscopy
(2021)
Atomic oxygen is a main component of the mesosphere and lower thermosphere (MLT). The photochemistry and the energy balance of the MLT are governed by atomic oxygen. In addition, it is a tracer for dynamical motions in the MLT. It is difficult to measure with remote sensing techniques. Concentrations can be inferred indirectly from the oxygen air glow or from observations of OH, which is involved in photochemical processes related to atomic oxygen. Such measurements have been performed with several satellite instruments such as SCIAMACHY, SABER, WINDII and OSIRIS. However, the methods are indirect and rely on photochemical models and assumptions such as quenching rates, radiative lifetimes, and reaction coefficients. The results are not always in agreement, particularly when obtained with different instruments.
Radio pulsars in relativistic binary systems are unique tools to study the curved space-time around massive compact objects. The discovery of a pulsar closely orbiting the super-massive black hole at the centre of our Galaxy, Sgr A⋆, would provide a superb test-bed for gravitational physics. To date, the absence of any radio pulsar discoveries within a few arc minutes of Sgr A⋆ has been explained by one principal factor: extreme scattering of radio waves caused by inhomogeneities in the ionized component of the interstellar medium in the central 100 pc around Sgr A⋆. Scattering, which causes temporal broadening of pulses, can only be mitigated by observing at higher frequencies. Here we describe recent searches of the Galactic centre region performed at a frequency of 18.95 GHz with the Effelsberg radio telescope.
To make best use of the exceptional good weather conditions at Chajnantor we developed CHAMP+, a two time seven pixel dual-color heterodyne array for operation in the 350 and 450 µm atmospheric windows. CHAMP+ uses state-of-the-art SIS-mixers provided by our collaborators at SRON. To maximize its performance, optical single sideband filter are implemented for each of the two subarrays, and most of the optics is operated cold (20K) to minimize noise contributions. The instrument can be operated remotely, under full computer control of all components. The autocorrelator backend, currently in operation with 2 × 1GHz of bandwidth for each of the 14 heterodyne channels, will be upgraded by a new technologies FFT spectrometer array in mid 2008. CHAMP+ has been commissioned successfully in late 2007. We will review the performance of the instrument "in the field," and present its characteristics as measured on-sky.
Based on our reconfigurable FPGA spectrometer technology, we have developed a read-out system, operating in the frequency domain, for arrays of Microwave Kinetic Inductance Detectors (MKIDs). The readout consists of a combination of two digital boards: A programmable DAC-/FPGA-board (tone-generator) to stimulate the MKIDs detectors and an ADC-/FPGA-unit to analyze the detectors response. Laboratory measurement show no deterioration of the noise performance compared to low noise analog mixing. Thus, this technique allows capturing several hundreds of detector signals with just one pair of coaxial cables.
Obwohl bis zum heutigen Tage mehr als 1500 Radio-Pulsare in unserer Galaxie entdeckt wurden, konnte bislang nicht ein einziger Pulsar im direkten Umfeld des Galaktischen Zentrums gefunden werden. Dies ist um so mehr erstaunlich, da die statistische Pulsar-Verteilung nicht nur eine deutliche Zunahme der Pulsare zum Zentrum unserer Galaxie zeigt, sondern dieser Himmelsbereich auch schon mehrfach in verschiedenen Pulsar-Suchen beobachtet wurde.
Das Defizit von Pulsaren im Galaktischen Zentrum wird heute allgemein durch Selektionseffekte bei der Suche erklärt, die aufgrund von Inhomogenitäten und der erhöhten Dichte des Interstellaren Mediums im Zentrumsbereich hervorgerufen werden. Diese Einflüsse bewirken eine frequenzabhängige Phasenverschiebung (Dispersion) sowie eine Pulsverbreiterung durch Mehrwegeausbreitung (Scattering) der zeitvarianten Strahlung von Pulsaren. Während die Dispersion durch geeignete Maßnahmen bei der Beobachtung nahezu vollständig beseitigt werden kann, ist die Pulsverbreiterung durch Scattering, die einen negativen Einfluß auf die Suchempfindlichkeit hat, nur mit Beobachtungen bei höheren Frequenzen zu mindern. Weil die Strahlungsintensität von Pulsaren jedoch zu höheren Frequenzen steil abfällt, kann die optimale Beobachtungsfrequenz nur ein Kompromiß der beiden gegensätzlichen Forderungen sein.
Im Rahmen dieser Arbeit wurde daher die erste Suche nach Pulsaren im Galaktischen Zentrum bei der für Pulsar-Beobachtungen ungewöhnlich hohen Frequenz von 5 GHz mit dem 100-m Radioteleskop des Max-Planck-Instituts für Radioastronomie durchgeführt und analysiert. Der wissenschaftliche Teil dieser Dissertation umfasst eine ausführliche Diskussion über die zu erwartende Anzahl detektierbarer Zentrumspulsare für zwei unterschiedliche Sternentstehungs-Szenarien im Galaktischen Zentrum und ferner eine gründliche Untersuchung der erzielten Empfindlichkeit zur durchgeführten Pulsar-Suche. Die technischen Kapitel beschreiben die Entwicklung des Datenaufnahmesystems (Backends) und der Suchsoftware zur Auswertung der Beobachtungsdaten, die beide speziell für dieses Suchprojekt entworfen wurden.
Atomic oxygen is a key species in the mesosphere and thermosphere of Venus. It peaks in the transition region between the two dominant atmospheric circulation patterns, the retrograde super-rotating zonal flow below 70 km and the subsolar to antisolar flow above 120 km altitude. However, past and current detection methods are indirect and based on measurements of other molecules in combination with photochemical models. Here, we show direct detection of atomic oxygen on the dayside as well as on the nightside of Venus by measuring its ground-state transition at 4.74 THz (63.2 µm). The atomic oxygen is concentrated at altitudes around 100 km with a maximum column density on the dayside where it is generated by photolysis of carbon dioxide and carbon monoxide. This method enables detailed investigations of the Venusian atmosphere in the region between the two atmospheric circulation patterns in support of future space missions to Venus.