bluuebunny/crossref_metadata_2025_split huggingface dataset:
Dispersive Spectroscopy on AXAF
Authors: Markert T.H. 
Published: July 2016

AbstractThere are two transmission grating spectrometers and one Bragg crystal spectrometer being developed for the Advanced X-ray Astrophysics Facility (MIT is building the crystal spectrometer and one of the grating spectrometers; the Laboratory for Space Research in Utrecht is responsible for the other grating spectrometer). The gratings divide the AXAF energy band (80 eV – 10 keV) into three regions (the MIT instrument contains gratings with two different periods) and attain resolving powers for point sources between 100 and 1800. The gratings are composed of arrays of small facets mounted on plates which can be inserted immediately behind the AXAF telescope. The dispersed spectra from the grating arrays are read out by one of the AXAF imaging instruments.The Bragg Crystal Spectrometer (BCS) is a focal plane instrument. One of eight selectable curved diffractors intercepts the AXAF X-ray beam as it diverges beyond the focal point X-rays that satisfy Bragg’s law are reflected from the crystal which, because of its curvature, re-focuses the beam onto an imaging detector. Narrow spectral regions are scanned by rocking the crystal over a range ~0.1 to 1°. Nearly the entire AXAF energy range can be studied by selecting the appropriate crystal and rotating it to the proper Bragg angle. The BCS achieves the highest spectral resolutions of the AXAF spectrometers: for 500 eV < E < 1600 eV, the FWHM of a narrow line (ΔE) is ≲ 1 eV.
Self-noise in interferometers
Authors: Vivekanand M. , Kulkarni S.R. 
Published: July 2016

AbstractWe study the distribution of noise in images synthesized by radio interferometers, focusing particularly on the noise due to source power fluctuations. For a total power interferometer, the r.m.s. fluctuation in the image consists of a term that is constant all over the map, which is the uncorrelated system noise divided by the number of telescopes in the interferometer, and a term that is the image itself. For a correlation interferometer the expression is not so simple but is qualitatively similar. Our results are consistent with intuitive expectations in various observing situations. We find similarities in self-noise at optical and radio wavelengths.
The effect of spatial coherence of sources on synthetic aperture mapping
Authors: James Daniel F.V. 
Published: July 2016

The interferometric mapping of astronomical objects relies on the van-Cittert Zernike theorem, one of the major results of the theory of partially coherent light [see, Bom and Wolf (1980), chapter 10]. This theorem states that the degree of spatial coherence of the field from a distant spatially incoherent source is proportional to the Fourier transform of the intensity distribution across the source. Measurement of the degree of spatial coherence, by, for example, measuring the visibility of interference fringes, allows the object to be mapped by making an inverse Fourier transform. (For a full description of this technique see Thompson, Moran and Swenson, 1986.)In this paper I present a summary of the results an investigation into what happens when the distant source is not spatially coherent (James, 1990). Using a heuristic model of a spherically symmetric partially coherent source, an analytic expression for the error in the measurement of the effective radius, expressed as a function of coherence area, can be obtained.
The multi frequency front end: a new type of front end for the Westerbork Synthesis Radio Telescope
Authors: Tan G.H. 
Published: July 2016

AbstractThe Westerbork Synthesis Radio Telescope will be equipped with new front ends. These front ends will cover 8 frequency bands in the range from 250 MHz to 8.6 GHz. For the frequency bands above 1.2 GHz the sensitivity of the instrument will be drastically improved. Two independent local oscillator systems make it possible to observe in two frequency bands simultaneously.
Using sub-aperture illumination to solve the shortspacing problem
Authors: Woody David P. , Scoville Nick Z. , Mundy Lee G. 
Published: July 2016

AbstractA common problem in producing interferometer maps of objects with structure on the scale-size of the primary beam is the acquisition of UV data corresponding to telescope spacings of less than one telescope diameter. One technique for solving this “short-spacing” problem is to divide the aperture of one of the telescopes into smaller sub-apertures and measuring the visibilities between these sub-apertures. This technique is being tested using the Owens Valley Millimeter Interferometer.
Space Ground Interferometer
Authors: Savin A.I. , Zaxon M.B. , Matveyenko L.I. 
Published: July 2016

AbstractA space project for studying ecological Earth problems is being carried out by means of radio techniques. A 30 m prototype antenna has already been deployed and tested. The radio telescope will be launched in 1994 into a circular orbit haying an altitude 600 km and an inclination of 65°. The planned mission time is ≥ 1.5 year; 25% of this will be available for VLBI observations at wavelengths 6 and 18 cm.
The requirement to the baseline Knowing of a Space VLBI
Authors: Kogan L.R. , Eysmont N.A. 
Published: July 2016

AbstractIt is shown that the influence of a baseline knowing error on the fringe rate change during coherent averaging time interval is extremely increasing for space VLBI due to more angle velocity of a space baseline rotation comparatively with ground VLBI. It is shown on the example of Radioastron orbit that the reguirement to its orbit for such a short wavelength as 1.35 cm is determined by unknowing of the fringe rate change during coherent averaging time interval rather than by delay ambiguity and this reguirement is equal several meters.
The use of redundancy in interferometry – a comparison of redundancy and selfcal
Authors: Wieringa Mark 
Published: July 2016

AbstractMany synthesis arrays currently in use have at least a few redundant baselines which can be used to derive model-independent telescope corrections. We discuss the relative merits of redundancy-based versus standard selfcalibration methods with respect to sensitivity to noise and interference. We then derive an approach to optimally combine the two methods for partially redundant arrays
Self-calibration of M.O.S.T. data
Authors: Gray A. D. 
Published: July 2016

AbstractThe Molonglo Observatory Synthesis Telescope (MOST), located near Canberra, Australia, is an east-west earth-rotation synthesis array which, unlike conventional synthesis arrays, generates multiple real-time fan-beams and forms images by the back-projection of the recorded intensities rather than by Fourier inversion of visibilities. The images produced in this fashion are often marred by the presence of radial artefacts emanating from strong sources due to residual calibration errors. An algorithm devised to self-calibrate these errors is discussed.
Low Frequency imaging and the non-isoplanatic atmosphere
Authors: Subrahmanya C. R. 
Published: July 2016

AbstractAt low frequencies, the typical fields of view over which imaging is required is several degrees across, for which (a) the visibility and brightness distribution cannot be assumed to be related by a two-dimensional Fourier Transform, and (b) the propagation delays introduced by the atmosphere cannot be assumed constant within the field of view. These are adequately represented in a method discussed in this paper. Although compute-intensive, the method is suitable for implementation on parallel processors.
VLA observations at 333 MHz: A Search for pancakes
Authors: Uson Juan M. , Bagri Durgadas S. , Cornwell T. J. 
Published: July 2016

AbstractThe P-band is the most recently completed observing band at the VLA. The aperture efficiency of the antennas is 0.4 and the system temperature is ∼ 130 K. The primary beam is slightly off-axis and its half-power width is steeply frequency dependent, which induces abnormal spectral indices on off-axis (≥ 1°) sources. We have searched for Zel’dovich “pancakes” and reached a level of ΔSrms ∼ 3.5 mJy for spectral channels of width Δν ∼ 100 kHz. We find no pancake with MHI ≳5 × 1014 M⊙.
Speckles in Interstellar Radio-Wave Scattering
Authors: Desai K. M. , Gwinn C. R. , Reynolds J. , King E. A. , Jauncey D. , Nicholson G. , Flanagan C. , Preston R.A. , Jones D.L. 
Published: July 2016

AbstractObservations of speckles in the scattering disk of the Vela pulsar are presented and speckle techniques for studying and circumventing scattering of radio waves by the turbulent interstellar plasma are discussed. The speckle pattern contains, in a hologrammatic fashion, complete information on the structure of the radio source as well as the distribution of the scattering material. Speckle observations of interstellar scattering of radio waves are difficult because of their characteristically short timescales (≈seconds) and narrow bandwidths (≈kHz). Here, we present first observations, taken at 13 cm wavelength with elements of the SHEVE VLBI network, of speckles in interstellar scattering.
Hot spots in Cygnus A; A mosaiced map at 86 GHz
Authors: Wright M.C.H. , Sault R.J. 
Published: July 2016

AbstractWe describe a new software package, MIRIAD, for making multichannel images of multiple-field aperture synthesis observations. As an example of the use of the new software we present a map of the radio galaxy Cygnus A. We used the BIMA array to map Cygnus A at 86 GHz with 2.28 × 3.06 arcsec resolution in 3 fields centered on the central radio source and the radio hot spots in each radio lobe. The 3 fields were combined and deconvolved to find the positions and flux densities of the radio source components.
Errors in high precision source position determination
Authors: Elosegui Pedro , Marcaide Juan-Maria , Shapiro Irwin I. 
Published: July 2016

AbstractWe have made a detailed analysis of the systematic errors in the determination, from two sets of VLBI observations, of the position of the quasar 1038+528 A relative to the quasar 1038+528 B. This analysis confirms an apparent proper motion at λ=3.6cm of 26±8 μas/yr of the core of the quasar 1038+528 A relative to the quasar 1038+528 B.
MERLIN observations of superluminal motion in the jet of 3C120
Authors: Muxlow T.W.B. , Wilkinson P.N. 
Published: July 2016

AbstractWe present evidence for changes in the radio structure of the jet in 3C120 between December 1980 and December 1988 derived from MERLIN observations at 1.67 GHz. The changes are consistent with outward motion of 19±2 mas (2.4 ± 0.3 mas yr−1, i.e. 3.5 ± 0.4h−1 c, for H0 = 100h km s−1 Mpc−1) in the inner jet, about 0.15 arcsec (∼ 75h−1 pc) from the core. This confirms the suggestion by Benson etal. (1988) that the jet does not slow down quickly just outside the core region usually imaged by VLBI. However, we find no evidence for motion at the position of the knot 4 arcsec (∼ 2h−1 kpc) from the nucleus as suggested by Walker et al. (1988). Our limit is 1.26 mas yr−1, i.e. <1.8h−1 c, compared with Walker et al. ’s measurement of 2.4 ± 0.8 mas yr−1 i.e. 3.7 ± 1.2h−1 c.
The link to extragalactic sources: observations of radio stars
Authors: Baudry Alain , Requieme Yves 
Published: July 2016

AbstractOptical positions obtained with the Bordeaux automatic meridian circle (accuracy – 0.05”) are compared with VLA positions taken from the literature. Alignment of the FK5 and VLA frames are discussed, and other projects (HIPPARCOS, VLBI) are briefly presented.
High-accuracy equation of equinoxes and VLBI astrometric Modelling
Authors: Gontier A.M. , Capitaine N. 
Published: July 2016

AbstractIn the course of implementing the use of nonrotating origin (Guinot 1979) in astrometric VLBI, for the transformation between celestial and terrestrial frames, we have compared the new approach with the classical one. We have shown that a difference exists between the two procedures at a few milliarcsecond level; this difference is due to the terms generally neglected when considering the equation of equinoxes in the classical representation of the Earth angle of rotation.
Mark-III VLBI astrometry of pulsars
Authors: Jones Dayton L. , Dewey Rachel J. , Gwinn Carl R. , Davis Michael M. 
Published: July 2016

AbstractDifferential 18-cm VLBI measurements of the position of PSR 1937+214 with respect to the quasar 1923+210 have been made to improve our knowledge of the offset between the planetary ephemeris reference frame and the extragalactic radio reference frame. Our preliminary frame-tie result (VLBI – Timing position) is +21 ± 2 milli-arcseconds in right ascension and +14 ± 9 milli-arcseconds in declination. We describe the data analysis methods used and compare our results with other radio interferometric position measurements of PSR 1937+214. Our results illustrate the value of combining the large collecting area of the Arecibo telescope with the Mk-III VLBI recording system.
Imaging Optical interferometry
Authors: Simon R.S. , Johnston K.J. , Mozurkewich D. , Weiler K.W. , Hutter D.J. , Armstrong J.T. , Brackett T.S. 
Published: July 2016

AbstractInterferometry at optical wavelengths is very similar to radio interferometry, once the fundamental differences in detectors are accounted for. The Mount Wilson Mark III optical interferometer has been used for optical interferometry of stars and stellar systems. Success with the Mark III has lead to the current program at the Naval Research Laboratory to build the Big Optical Array (BOA), which will be an imaging interferometer. Imaging simulations show that BOA will be able to produce images of complex stellar systems, with a resolution as fine as 0.2 milliarcseconds.
Monitoring source variability with the Fast All Sky Telescope
Authors: Johnston Kenneth J. , Fiedler Ralph L. , Simon Richard S. 
Published: July 2016

AbstractThe proposed Fast All Sky Telescope (FAST) is an interferometer which is intended to monitor the northern four-fifths of the celestial sphere every two days at 8.1 GHz and daily at 2.7 GHz. The design goal is to have a rms sensitivity of 10 mJy/beam at both frequencies. The array is planned to comprise 20 3-meter diameter antennas with a maximum baseline of 0.7 km. FAST will provide a valuable database that may be used to study time variability in a sensitivity limited sample of radio sources. This will significantly impact on the understanding of active Galactic and extragalactic radio sources, as well as on the understanding of radio wave scattering in the interstellar medium.
Optimal antenna configurations for the BIMA Array
Authors: Thatte Niranjan , Welch Wm. J. 
Published: July 2016

AbstractWe attempt to determine optimal configurations of six antennas for the proposed Berkeley Illinois Maryland Array. The instrumental response is used as a performance measure. A two dimensional sine function is chosen as the ideal beam, supported by theoretical considerations. A mean square comparison with this ideal is used to perform optimizations for arrays of various resolutions, using the current station locations at Hat Creek. Configurations for larger fields of view are also discussed, employing two days of coverage with six antennas.
Understanding Changes in Period Ratios
Authors: Petersen J. O. 
Published: July 2016

The discrepancies between observed and theoretically calculated period ratios of the double mode, classical Cepheids and other groups of Cepheid type variables have played a prominent role in investigations of the important physical characteristics of these stars during the last twenty years. Today, there is growing consensus that the drastic increases in astrophysical opacities proposed by Simon (1982) provide the correct solution of all the period ratio problems (e.g. Petersen, 1989; Iglesias and Rogers, 1992; Moskalik et al., 1991).It seems at first strange that the optimal opacity change (see Andreasen, 1988) can produce both the decrease in period ratio for double mode classical Cepheids and the increased period ratio in models of δ Scuti stars. Here we use ĸ-effect-functions (Refsdal and Stabell, 1972) to provide both a detailed description of the effects of opacity changes and an improved and more direct understanding of these effects.
Long-Term Amplitude and Period Variations of δ Scuti Stars: A Sign of Chaos?
Authors: Breger M. 
Published: July 2016

AbstractOn short time-scales of under a year, the vast majority of δ Scuti stars studied in detail show completely regular multiperiodic pulsation. Nonradial pulsation is characterized by the excitation of a large number of modes with small amplitudes. Reports of short-term irregularity or nonperiodicity in the literature need to be examined carefully, since insufficient observational data can lead to an incorrect impression of irregularity. Some interesting cases of reported irregularities are examined.A few δ Scuti stars, such as 21 Mon, have shown stable variations with sudden mode switching to a new frequency spectrum. This situation might be an indication of deterministic chaos. However, the observational evidence for mode switching is still weak.One the other hand, the case for the existence of long-term amplitude and period changes is becoming quite convincing. Recently found examples of nonradial pulsators with long-term changes are 4 CVn, 44 Tau, τ Peg and HD 2724. (We note that other δ Scuti pulsators such as X Cae and θ2 Tau, have shown no evidence for amplitude variations over the years.) Neither the amplitude nor the period changes are periodic, although irregular cycles with time scales between a few and twenty years can be seen. While the amplitude changes can be very large, the period changes are quite small. This property is common in nonlinear systems which lead to chaotic behavior. There exists observational evidence for relatively sudden period jumps changing the period by about 10−5 and/or slow period changes near dP/dt ≤ 10−9. These period changes are an order of magnitude larger than those expected from stellar evolution.The nonperiodic long-term changes are interpreted in terms of resonances between different nonradial modes. It is shown that a large number of the nonradial acoustic modes can be in resonance with other modes once the mode interaction terms, different radial orders and rotational m-mode splitting are considered. These resonances are illustrated numerically by the use of pulsation model. Observational evidence is presented that these interaction modes exist in the low-frequency domain.
The Delta Scuti Star GX Pegasi: A Theoretical Investigation of its Power Spectrum
Authors: Michel E. , Goupil M. J. , Lebreton Y. , Baglin A. 
Published: July 2016

Target of a STEPHI multisite campaign, the Delta Scuti star GX Pegasi has been found to oscillate with at least five simultaneous, close frequencies (table I).Mode identification together with informations about the star that such an identification can provide are outlined below (see also Michel et al, 1992b).The mode identification is carried out by means of a comparison between the observed frequencies and the adiabatic frequencies of models appropriate to this star. Models that match GX Peg’s position in a Hertzsprung-Russell diagram have masses in the range 1.9 – M⊙. When included, convective core overshoot is handled as in Maeder and Meynet (1989). According to these models, GX Peg is a rather evolved, main sequence star.
Short-Term Amplitude Variation of FM Com (=HR 4684)
Authors: Pararó M. , Pena J. , Peniche R. , Ibanoglu C. , Tunca Z. , Evren S. 
Published: July 2016

AbstractUsing unpublished data obtained in 1982 at Mexico and Turkey combined with Hungarian observations the process of amplitude variation of the frequency at 18.5 c/d is shown. The time scale of the amplitude increase is less than 100 days.
Physical Characters of HD 93044
Authors: Zhiping Li 
Published: July 2016

AbstractThe measurements of uvby Hβ of HD 93044 were obtained in April 1991. and the observational results that the star locates nearly in the middle of δ Scuti instability strip with somewhat deviation to red edge. According to Crawford (1979) and Philip’s (1979) calibrations, the effective temperature, absolute visual magnitude and surface gravity are obtained to be Teff = 7300±200 K, Mv = l.m33±0.39 and logg = 3.7±0.15, respectively. The observational results of Δmi = 0.01 give an estimate of [Fe/H] = —0.003 ± 0.18, so the opinion of metallic deficient is not supported obviously. The observations show the reddening index E(b — y) to be 0.014 which is 1.4 times as large as the standard deviation of Crawford’s (1979) statistics.
Observation of a Variable, ZZ Ceti White Dwarf: GD154
Authors: Pfeiffer B. , Vauclair G. , Dolez N. , Chevreton M. , Fremy J. R. , Herpe G. , Barstow M. , Kleinman S. J. , Watson T. K. , Belmonte J. A. , Kepler S. O , Kanaan A. , Giovannini O. , Nather R. E. , Winget D. E. , Provencal J. , Clemens J. C. , Bradley P. , Dixson J. , Grauer A. D. , Fontaine G. , Bergeron P. , Wesemael F. , Claver C. F. , Matzeh T. , Leibowitz E. , Moskalik P. 
Published: July 2016

The ZZ Ceti stars form a class of variable white dwarfs: the hydrogen dominated atmosphere ones, which do pulsate in an instability strip in the effective temperature range 13000K-11500K. We know 22 such ZZ Ceti white dwarfs. Their variations are caused by nonradial g-mode pulsations with periods are in the range 100-1000 seconds.A subsample of the ZZ Ceti stars shows amplitude variations on time scales of the order of one month. These variations could be driven by nonlinear phenomena.
Wind Variability of LBV Stars
Authors: Israelian G. L. 
Published: July 2016

AbstractGeneral properties of Luminous Blue Variables (LBV) have been reviewed by Lamers (1987). The LBV’s are all close to the Humphreys-Davidson luminosity upper limit. The semi periods of the photometric microvariations with ΔV ~ 0.1m are about twice as large as for normal supergiants of the same L and Teff, and 4-20 times larger than the fundamental mode for radial pulsations. So, it is likely due to nonradial pulsations. During shell ejections, which accompanied by moderate photometric variations of ΔV ~ 1m, Lbol remains constant. This fact has been explained by the quasiperiodic variations of Teff and the radius of pseudo-photosphere (Lamer 1986). LBV’s are less stable than normal supergiants so that any internal instability has more effect on their envelopes. The nature of these instabilities remains unknown. We have found some interesting peculiarities which can throw light to this problem.
Long-Term V/R Variations of be Stars Due to Global One-Armed Oscillations of Equatorial Disks
Authors: Okazaki Atsuo T. 
Published: July 2016

AbstractWe study the long-term variations of Balmer line profiles due to global one-armed oscillations in Be-star disks. In order to examine the qualitative effects of oscillations on line profiles, we assume that the eigenfunctions of one-armed nonlinear oscillations are similar to those of linear oscillations. Computing the line profiles for various values of disk parameters, we find that in small disks or in disks with steep density gradients the one-armed fundamental modes cause remarkable variabilities similar to the observed V/R variations.