3,978 research outputs found

    A cryogenic seven-element HEMT front end for DSS 13

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    A cryogenically cooled Ka-band (33.6-GHz), seven-element front-end array for the DSN was built and tested. This system uses seven high electron mobility transistor (HEMT) low-noise amplifiers cooled by a two-stage closed-cycle refrigerator. All system components from the polarizers to the output isolators are cooled to a physical temperature between 18 and 35 K. The noise temperatures of the individual elements range from 64 to 84 K over a 2.75-GHz bandwidth

    A 32-GHz reflected-wave maser amplifier with wide instantaneous bandwidth

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    An eight stage, 32 GHz reflected wave ruby maser was built. The maser operates in a 3 watt closed cycle refrigerator at 4.5 K and is capable of 21 dB of net gain with an instantaneous bandwidth of 400 MHz. The input noise temperature referred to the room temperature flange is approximately 21 K

    Radio Emission Associated with Ultraluminous X-ray Sources in the Galaxy Merger NGC 3256

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    We present new 6, 3.6, and 2 cm VLA radio observations of the nearby merger system NGC 3256, with resolutions of ~100 pc, which reveal compact radio sources embedded in more diffuse emission at all three wavelengths. The two radio nuclei are partially resolved, but the two dominant compact sources that remain coincide with the two most powerful compact Ultraluminous X-ray sources (ULXs) recently reported by Lira et al. The radio/X-ray ratios for these two sources are too high by factors of >100-1000 to be normal X-ray binaries. However, their radio and X-ray powers and ratios are consistent with low-luminosity active galactic nuclei (LLAGNs), and optical emission lines suggest the presence of a nuclear disk around the northern nucleus. If the two nuclear ULXs are LLAGNs, their associated black holes are separated by only \~1kpc, about 6 times closer to one another than those found recently in the merger galaxy NGC 6240. A third ULX on the outskirts of the merger is also a radio source, and probably is a collection of supernova remnants. The remaining ULXs are not coincident with any source of compact radio emission, and are consistent with expectations for beamed X-ray binaries or intermediate-mass black holes.Comment: Accepted for ApJ (10 pages, 5 figures

    Multiple origins of the yucca-yucca moth association.

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    Spitzer Observations of Centaurus A: Infrared Synchrotron Emission from the Northern Lobe

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    We present measurements obtained with the Spitzer Space Telescope in five bands from 3.6-24 microns of the northern inner radio lobe of Centaurus A, the nearest powerful radio galaxy. We show that this emission is synchrotron in origin. Comparison with ultraviolet observations from GALEX shows that diffuse ultraviolet emission exists in a smaller region than the infrared but also coincides with the radio jet. We discuss the possibility, that synchrotron emission is responsible for the ultraviolet emission and conclude that further data are required to confirm this.Comment: 4 pages, accepted by ApJ

    The Emergence of the Modern Universe: Tracing the Cosmic Web

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    This is the report of the Ultraviolet-Optical Working Group (UVOWG) commissioned by NASA to study the scientific rationale for new missions in ultraviolet/optical space astronomy approximately ten years from now, when the Hubble Space Telescope (HST) is de-orbited. The UVOWG focused on a scientific theme, The Emergence of the Modern Universe, the period from redshifts z = 3 to 0, occupying over 80% of cosmic time and beginning after the first galaxies, quasars, and stars emerged into their present form. We considered high-throughput UV spectroscopy (10-50x throughput of HST/COS) and wide-field optical imaging (at least 10 arcmin square). The exciting science to be addressed in the post-HST era includes studies of dark matter and baryons, the origin and evolution of the elements, and the major construction phase of galaxies and quasars. Key unanswered questions include: Where is the rest of the unseen universe? What is the interplay of the dark and luminous universe? How did the IGM collapse to form the galaxies and clusters? When were galaxies, clusters, and stellar populations assembled into their current form? What is the history of star formation and chemical evolution? Are massive black holes a natural part of most galaxies? A large-aperture UV/O telescope in space (ST-2010) will provide a major facility in the 21st century for solving these scientific problems. The UVOWG recommends that the first mission be a 4m aperture, SIRTF-class mission that focuses on UV spectroscopy and wide-field imaging. In the coming decade, NASA should investigate the feasibility of an 8m telescope, by 2010, with deployable optics similar to NGST. No high-throughput UV/Optical mission will be possible without significant NASA investments in technology, including UV detectors, gratings, mirrors, and imagers.Comment: Report of UV/O Working Group to NASA, 72 pages, 13 figures, Full document with postscript figures available at http://casa.colorado.edu/~uvconf/UVOWG.htm

    Biogeochemical Response of Alpine Lakes to a Recent Increase in Dust Deposition in the Southwestern, US

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    The deposition of dust has recently increased significantly over some regions of the western US. Here we explore how changes in dust deposition have affected the biogeochemistry of two alpine watersheds in Colorado, US. We first reconstruct recent changes in the mass accumulation rate of sediments and then we use isotopic measurements in conjunction with a Bayesian mixing model to infer that approximately 95% of the inorganic fraction of lake sediments is derived from dust. Elemental analyses of modern dust indicate that dust is enriched in Ca, Cr, Cu, Mg, Ni, and in one watershed, Fe and P relative to bedrock. The increase in dust deposition combined with its enrichment in certain elements has altered the biogeochemisty of these systems. Both lakes showed an increase in primary productivity as evidenced by a decrease in carbon isotopic discrimination; however, the cause of increased primary productivity varies due to differences in watershed characteristic. The lake in the larger watershed experienced greater atmospheric N loading and less P loading from the bedrock leading to a greater N:P flux ratio. In contrast, the lake in the smaller watershed experienced less atmospheric N loading and greater P loading from the bedrock, leading to a reduced N:P flux ratio. As a result, primary productivity was more constrained by N availability in the smaller watershed. N-limited primary productivity in the smaller watershed was partly ameliorated by an increase in nitrogen fixation as indicated by reduced nitrogen isotopic values in more contemporary sediments. This study illustrates that alpine watersheds are excellent integrators of changes in atmospheric deposition, but that the biogeochemical response of these watersheds may be mediated by their physical (i.e. watershed area) and chemical (i.e. underlying geology) properties
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