Welcome to our book review site go-pdf.online!
You may have to Search all our reviewed books and magazines, click the sign up button below to create a free account.
Are There Rotation Measure Gradients Across Active Galactic Nuclei Jets?
We report on multi-frequency polarimetry very long baseline interferometry observations of active galactic nuclei using the Very Long Baseline Array. These observations are used to construct images of the Faraday rotation measure (RM) in J1613+342, Mrk 501, 3C 371, and BL Lac. Despite having resolved the jets in total intensity and polarization for three of these sources no RM gradients are found. This is in contrast to the large fraction of sources with RM gradients now claimed in the literature and invoked as evidence in support of helical magnetic fields. We propose objective criteria for establishing what constitutes an RM gradient. Furthermore, although we note the absence of simple, monotonic gradients, comparison with simulations could reveal systematic changes in the RM that may be masked by a varying jet orientation.
Analyzing the [Greek Letter] Delta Sco Binary in Anticipation of a Disk-star Collision
A current investigation is underway into the possible collision between a circumstellar disk and the secondary star in the [Greek letter delta] Scorpii binary system. [Greek letter delta] Scorpii is a prime candidate for a disk-star collision since the primary star has a circumstellar disk and the secondary star has a highly elliptical orbit with a period of approximately 10.5 years making the periastron passage very close to the primary star. The Navy Prototype Optical Interferometer (NPOI) was used to spatially resolve the two stars as well as the cirumstellar disk around the primary. A revised orbit with new orbital parameters has been calculated using observations obtained between 2005 and 2010. Our results indicate periastron passage will occur on 2011-07-03.
A View Through Faraday's Fog
Rotation measure observations of nine quasars, four BL Lacertae objects, and three radio galaxies are presented. The rest-frame rotation measures in the cores of the quasars and the jets of the radio galaxy M87 are several thousand rad m-2. The BL Lac objects and the jets of the quasars have rest-frame rotation measures of a few hundred rad m-2. A core rotation measure of 500 rad m-2 in the rest frame is suggested as the dividing line between quasars and BL Lac objects. The substantial rotation measures of the BL Lac objects and quasars cast doubt on the previous polarization position angle investigations of these objects at frequencies of 15 GHz or less. BL Lac itself has a rotation measure that varies in time, similar to the behavior observed for the quasars 3C 273 and 3C 279. A simple model with magnetic fields of 40 uG or less can account for the observed rotation measures.
The Effects of Atmospheric Calibration Errors on Source Model Parameters
Optical long-baseline interferometric data is commonly calibrated with respect to an external calibrator, which is either an unresolved source or a star with a known angular diameter. A typical observational strategy involves acquiring data in a sequence of calibrator-target pairs, where the observation of each source is obtained separately. Therefore, the atmospheric variations that have time scales shorter than the cadence between the target-calibrator pairs are not always fully removed from the data even after calibration. This results in calibrated observations of a target star that contain unknown quantities of residual atmospheric variations. We describe how Monte Carlo simulations can be used to assess quantitatively the impact of atmospheric variations on fitted model parameters, such as angular diameters of uniform-disk models representing semi- and fully-resolved single stars.
The Algol Triple System Spatially Resolved at Optical Wavelengths
Interacting binaries typically have separations in the milliarcsecond regime, and hence it has been challenging to resolve them at any wavelength. However, recent advances in optical interferometry have improved our ability to discern the components in these systems and have now enabled the direct determination of physical parameters. We used the Navy Prototype Optical Interferometer to produce for the first time images resolving all three components in the well-known Algol triple system. Specifically, we have separated the tertiary component from the binary and simultaneously resolved the eclipsing binary pair, which represents the nearest and brightest eclipsing binary in the sky. We present revised orbital elements for the triple system, and we have rectified the 180° ambiguity in the position angle of Algol C. Our directly determined magnitude differences and masses for this triple star system are consistent with earlier light curve modeling results.
The Behavior of the Optical and X-Ray Emission from Scorpius X-1
In 1970, Hiltner & Mook reported the results of the first multiyear study of the optical emission from Sco X-1. They found that the Sco X-1 B-magnitude histograms changed from year to year. Subsequent multiwavelength campaigns confirmed the variable nature of these optical histograms and also found that the X-ray and optical emissions were only correlated when Sco X-1 was brighter than about B=12.6. Models had suggested that the optical emission from this source arose from X-rays reprocessed in an accretion disk surrounding the central neutron star. It was therefore difficult to explain why the optical and X-ray fluxes were not more closely correlated. In 1994 and 1995, two new simultaneous ...
Detection of a Geostationary Satellite with the Navy Prototype Optical Interferometer
"We have detected a satellite via optical interferometry for the first time, using a 16 m baseline of the Navy Prototype Optical Interferometer (NPOI) to observe the geostationary communications satellite DirecTV-9S during the "glint" seasons of February-March 2008 and 2009 when the sun-satellite-NPOI geometry was favorable for causing specular reflections from geostationary satellites. We used the US Naval Observatory Flagstaff Station 1 m telescope to generate accurate positions for steering the NPOI. Stars are the easiest targets for optical/infrared interferometers because of their high surface brightness. Low surface brightness targets are more difficult: if they are small enough not to...
Optical Observations with Milliarcsecond Resolution of Stars, Their Environments and Companions
Observations with milliarsecond resolution using the Navy Optical Interferometer have been obtained for a number of stellar systems which include high-mass binaries, eclipsing binaries, and radio stars. These observations also reveal the previously unseen companions in single-lined spectroscopic binaries via directly measured flux ratios. We will present examples of published and ongoing research efforts of these systems to illustrate how an optical interferometer contributes to our knowledge of stars, their environment, and companions. These studies include a conclusive revealing of the previously unseen companion in the single-lined binary [Greek letter Phi] Herculis, the direct determination of orbital parameters in the wide and close orbits of Algol, and revealing the orbit of [Greek letter beta] Lyrae with spatially resolved images of the H[Greek letter alpha] emission.
Recruiter Journal
None
