131 research outputs found
Structural thermal noise in gram-scale mirror oscillators
No full textThe thermal noise associated with mechanical dissipation is a ubiquitous limitation to the sensitivity of precision experiments ranging from frequency stabilization to gravitational wave interferometry. We report on the thermal noise limits to the performance of 1 gm mirror oscillators that are part of a cavity optomechanics experiment to observe quantum radiation pressure noise. Thermal noise limits the observed cavity displacement spectrum from 80 Hz to 5 kHz. We present a calculation of the thermal noise, based on finite element analysis of the dissipation due to structural damping, and find it to be in excellent agreement with the experimental result. We conclude with the predicted thermal noise for an improved oscillator design, which should be capable of revealing the noise that arises from quantum backaction in this system.National Science Foundation (U.S.) (Grant PHY-1068772
EFFECTS OF ANTENNA BEAM CHROMATICITY ON REDSHIFTED 21 cm POWER SPECTRUM AND IMPLICATIONS FOR HYDROGEN EPOCH OF REIONIZATION ARRAY
No full textUnaccounted for systematics from foregrounds and instruments can severely limit the sensitivity of current experiments from detecting redshifted 21 cm signals from the Epoch of Reionization (EoR). Upcoming experiments are faced with a challenge to deliver more collecting area per antenna element without degrading the data with systematics. This paper and its companions show that dishes are viable for achieving this balance using the Hydrogen Epoch of Reionization Array (HERA) as an example. Here, we specifically identify spectral systematics associated with the antenna power pattern as a significant detriment to all EoR experiments which causes the already bright foreground power to leak well beyond ideal limits and contaminate the otherwise clean EoR signal modes. A primary source of this chromaticity is reflections in the antenna-feed assembly and between structures in neighboring antennas. Using precise foreground simulations taking wide-field effects into account, we provide a generic framework to set cosmologically motivated design specifications on these reflections to prevent further EoR signal degradation. We show that HERA will not be impeded by such spectral systematics and demonstrate that even in a conservative scenario that does not perform removal of foregrounds, HERA will detect the EoR signal in line-of-sight k-modes, k[subscript ||] ≳ 0.2 h Mpc[superscript −1], with high significance. Under these conditions, all baselines in a 19-element HERA layout are capable of detecting EoR over a substantial observing window on the sky.National Science Foundation (U.S.) (Awards AST-1440343 and AST- 1410719)National Science Foundation (U.S.). Graduate Research Fellowship Program (Grant 1122374
Measuring phased-array antenna beampatterns with high dynamic range for the Murchison Widefield Array using 137 MHz ORBCOMM satellites
No full textDetection of the fluctuations in a 21 cm line emission from neutral hydrogen during the Epoch of Reionization in thousand hour integrations poses stringent requirements on calibration and image quality, both of which necessitate accurate primary beam models. The Murchison Widefield Array (MWA) uses phased-array antenna elements which maximize collecting area at the cost of complexity. To quantify their performance, we have developed a novel beam measurement system using the 137 MHz ORBCOMM satellite constellation and a reference dipole antenna. Using power ratio measurements, we measure the in situ beampattern of the MWA antenna tile relative to that of the reference antenna, canceling the variation of satellite flux or polarization with time. We employ angular averaging to mitigate multipath effects (ground scattering) and assess environmental systematics with a null experiment in which the MWA tile is replaced with a second-reference dipole. We achieve beam measurements over 30 dB dynamic range in beam sensitivity over a large field of view (65% of the visible sky), far wider and deeper than drift scans through astronomical sources allow. We verify an analytic model of the MWA tile at this frequency within a few percent statistical scatter within the full width at half maximum. Toward the edges of the main lobe and in the sidelobes, we measure tens of percent systematic deviations. We compare these errors with those expected from known beamforming errors.National Science Foundation (U.S.) (AST-0821321)National Science Foundation (U.S.) (AST-0457585)National Science Foundation (U.S.) (PHY-0835713)National Science Foundation (U.S.) (CAREER-0847753)National Science Foundation (U.S.) (AST-0908884)United States. Air Force Office of Scientific Research (FA9550-0510247
The hydrogen epoch of reionization array dish. II. Characterization of spectral structure with electromagnetic simulations and its science implications
No full textWe use time-domain electromagnetic simulations to determine the spectral characteristics of the Hydrogen Epoch of Reionization Arrays (HERA) antenna. These simulations are part of a multi-faceted campaign to determine the effectiveness of the dish's design for obtaining a detection of redshifted 21 cm emission from the epoch of reionization. Our simulations show the existence of reflections between HERA's suspended feed and its parabolic dish reflector that fall below −40 dB at 150 ns and, for reasonable impedance matches, have a negligible impact on HERA's ability to constrain EoR parameters. It follows that despite the reflections they introduce, dishes are effective for increasing the sensitivity of EoR experiments at a relatively low cost. We find that electromagnetic resonances in the HERA feed's cylindrical skirt, which is intended to reduce cross coupling and beam ellipticity, introduces significant power at large delays (−40 dB at 200 ns), which can lead to some loss of measurable Fourier modes and a modest reduction in sensitivity. Even in the presence of this structure, we find that the spectral response of the antenna is sufficiently smooth for delay filtering to contain foreground emission at line-of-sight wave numbers below k ∥ ≲ 0.2 h Mpc⁻¹, in the region where the current PAPER experiment operates. Incorporating these results into a Fisher Matrix analysis, we find that the spectral structure observed in our simulations has only a small effect on the tight constraints HERA can achieve on parameters associated with the astrophysics of reionization.National Science Foundation (U.S.) (AST-1440343)National Science Foundation (U.S.) (AST-1410484)National Science Foundation (U.S.) (CAREER award 1352519)National Science Foundation (U.S.) (1122374
The Hydrogen Epoch of Reionization Array Dish. I. Beam Pattern Measurements and Science Implications
No full textThe Hydrogen Epoch of Reionization Array (HERA) is a radio interferometer aiming to detect the power spectrum of 21 cm fluctuations from neutral hydrogen from the epoch of reionization (EOR). Drawing on lessons from the Murchison Widefield Array and the Precision Array for Probing the EOR, HERA is a hexagonal array of large (14 m diameter) dishes with suspended dipole feeds. The dish not only determines overall sensitivity, but also affects the observed frequency structure of foregrounds in the interferometer. This is the first of a series of four papers characterizing the frequency and angular response of the dish with simulations and measurements. In this paper, we focus on the angular response (i.e., power pattern), which sets the relative weighting between sky regions of high and low delay and thus apparent source frequency structure. We measure the angular response at 137 MHz using the ORBCOMM beam mapping system of Neben et al. We measure a collecting area of 93 m[superscript 2] in the optimal dish/feed configuration, implying that HERA-320 should detect the EOR power spectrum at z ∼ 9 with a signal-to-noise ratio of 12.7 using a foreground avoidance approach with a single season of observations and 74.3 using a foreground subtraction approach. Finally, we study the impact of these beam measurements on the distribution of foregrounds in Fourier space.National Science Foundation (U.S.) (AST-1440343)Kathy and Curt Marble Cancer Research FundMassachusetts Institute of Technology. School of ScienceNational Science Foundation (U.S.). Graduate Research Fellowship Program (Grant No. 1122374
Mapmaking for precision 21 cm cosmology
No full textIn order to study the “Cosmic Dawn” and the Epoch of Reionization with 21 cm tomography, we need to statistically separate the cosmological signal from foregrounds known to be orders of magnitude brighter. Over the last few years, we have learned much about the role our telescopes play in creating a putatively foreground-free region called the “EoR window.” In this work, we examine how an interferometer’s effects can be taken into account in a way that allows for the rigorous estimation of 21 cm power spectra from interferometric maps while mitigating foreground contamination and thus increasing sensitivity. This requires a precise understanding of the statistical relationship between the maps we make and the underlying true sky. While some of these calculations would be computationally infeasible if performed exactly, we explore several well-controlled approximations that make mapmaking and the calculation of map statistics much faster, especially for compact and highly redundant interferometers designed specifically for 21 cm cosmology. We demonstrate the utility of these methods and the parametrized trade-offs between accuracy and speed using one such telescope, the upcoming Hydrogen Epoch of Reionization Array, as a case study.National Science Foundation (U.S.) (Grant AST-0457585)National Science Foundation (U.S.) (Grant AST-0821321)National Science Foundation (U.S.) (Grant AST-0804508)National Science Foundation (U.S.) (Grant AST-1105835)National Science Foundation (U.S.) (Grant AST-1125558)National Science Foundation (U.S.) (Grant AST-1129258)National Science Foundation (U.S.) (Grant AST-1410484)National Science Foundation (U.S.) (Grant AST-1411622)Mount Cuba Astronomical AssociationMIT School of ScienceMarble Astrophysics Fun
Philon von Alexandria
No full textPhilons Schrift De Migratione Abrahami ist ein allegorischer Kommentar zu Gen 12. Der Auszug Abrahams aus seiner chaldäischen Heimat wird von Philon allegorisch auf den Auszug der Seele aus der Sinnenwelt und ihre Rückkehr zu Gott gedeutet. Die vorliegende Ausgabe bietet neben einer ausführlichen Einführung in Leben und Denken des alexandrinischen Religionsphilosophen den Text mit Übersetzung und erläuternden Anmerkungen. Kommentierende Essays aus unterschiedlichen Fachperspektiven ordnen die Schrift in den Kontext des zeitgenössischen Judentums sowohl in Alexandria wie im palästinischen Mutterland ein, würdigen Philon als Vertreter des kaiserzeitlichen Platonismus und machen mit der Gotteserkenntnis und philosophisch gedeuteten Exilserfahrungen zwei zentrale Topoi der Schrift zum Thema. Vergleichende Untersuchungen zur Figur des Abraham im Neuen Testament und in Thomas Manns Joseph und seine Brüder runden den Band ab
Abrahams Aufbruch
No full textPhilons Schrift De Migratione Abrahami ist ein allegorischer Kommentar zu Gen 12. Der Auszug Abrahams aus seiner chaldäischen Heimat wird von Philon allegorisch auf den Auszug der Seele aus der Sinnenwelt und ihre Rückkehr zu Gott gedeutet. Die vorliegende Ausgabe bietet neben einer ausführlichen Einführung in Leben und Denken des alexandrinischen Religionsphilosophen den Text mit Übersetzung und erläuternden Anmerkungen. Kommentierende Essays aus unterschiedlichen Fachperspektiven ordnen die Schrift in den Kontext des zeitgenössischen Judentums sowohl in Alexandria wie im palästinischen Mutterland ein, würdigen Philon als Vertreter des kaiserzeitlichen Platonismus und machen mit der Gotteserkenntnis und philosophisch gedeuteten Exilserfahrungen zwei zentrale Topoi der Schrift zum Thema. Vergleichende Untersuchungen zur Figur des Abraham im Neuen Testament und in Thomas Manns Joseph und seine Brüder runden den Band ab
Parametrizing Epoch of Reionization foregrounds: a deep survey of low-frequency point-source spectra with the Murchison Widefield Array
No full textExperiments that pursue detection of signals from the Epoch of Reionization (EoR) are relying on spectral smoothness of source spectra at low frequencies. This article empirically explores the effect of foreground spectra on EoR experiments by measuring high-resolution full-polarization spectra for the 586 brightest unresolved sources in one of the Murchison Widefield Array (MWA) EoR fields using 45 h of observation. A novel peeling scheme is used to subtract 2500 sources from the visibilities with ionospheric and beam corrections, resulting in the deepest, confusion-limited MWA image so far. The resulting spectra are found to be affected by instrumental effects, which limit the constraints that can be set on source-intrinsic spectral structure. The sensitivity and power-spectrum of the spectra are analysed, and it is found that the spectra of residuals are dominated by point spread function sidelobes from nearby undeconvolved sources. We release a catalogue describing the spectral parameters for each measured source
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