Beam squint and Stokes V with off-axis feeds

Radio telescopes with off-axis feeds, such as the (E)VLA, suffer from "beam squint" in which the two orthogonal circular polarizations sampled have different pointing centers on the sky. Its effects are weak near the beam center but become increasingly important towards the edge of the antenna power pattern where gains in the two polarizations at a given sky position are significantly different. This effect has limited VLA measurements of circular polarization (Stokes V) and introduced dynamic range limiting, wide-field artifacts in images made in Stokes I. We present an adaptation of the visibility-based deconvolution CLEAN method that can correct this defect "on the fly" while imaging, correcting as well the associated self-calibration. We present two examples of this technique using the procedure "Squint" within the Obit package which allows wide-field imaging in Stokes V and reduced artifacts in Stokes I. We discuss the residual errors in these examples as well as a scheme for future correction of some of these errors. This technique can be generalized to implement temporally- and spatially-variable corrections, such as pointing and cross-polarization leakage errors.Comment: 9 pages, 6 figures (five of them double), to appear in Astronomy & Astrophysics (accepted: May 9, 2008). High-resolution versions of the figures (gzipped, tar,gzipped) can be downloaded from http://www.cv.nrao.edu/~juson/technical/squint/squint_figures.g

High Dynamic-Range Radio-Interferometric Images at 327 MHz

Radio astronomical imaging using aperture synthesis telescopes requires deconvolution of the point spread function as well as calibration of the instrumental characteristics (primary beam) and foreground (ionospheric/atmospheric) effects. These effects vary in time and also across the field of view, resulting in directionally-dependent (DD), time-varying gains. The primary beam will deviate from the theoretical estimate in real cases at levels that will limit the dynamic range of images if left uncorrected. Ionospheric electron density variations cause time and position variable refraction of sources. At low frequencies and sufficiently high dynamic range this will also defocus the images producing error patterns that vary with position and also with frequency due to the chromatic aberration of synthesis telescopes. Superposition of such residual sidelobes can lead to spurious spectral signals. Field-based ionospheric calibration as well as "peeling" calibration of strong sources leads to images with higher dynamic range and lower spurious signals but will be limited by sensitivity on the necessary short-time scales. The results are improved images although some artifacts remain.Comment: to appear in Comptes Rendus Physique (2011

Rotation Measure synthesis study and polarized properties of PSR J1745-2900 at 7 mm

We present results of interferometric polarization observations of the recently discovered magnetar J1745-2900 in the vicinity of the Galactic center. The observations were made with the Karl G. Jansky Very Large Array (VLA) on 21 February 2014 in the range 40-48 GHz. The full polarization mode and A configuration of the array were used. The average total and linearly polarized flux density of the pulsar amounts to 2.3$\pm$0.31 mJy/beam and 1.5$\pm$0.2 mJy/beam, respectively. Analysis shows a rotation measure (RM) of (-67$\pm$3)x10$^3$ rad/m$^2$, which is in a good agreement with previous measurements at longer wavelengths. These high frequency observations are sensitive to RM values of up to ~2x10$^7$ rad/m$^2$. However, application of the Faraday RM synthesis technique did not reveal other significant RM components in the pulsar emission. This supports an external nature of a single thin Faraday-rotating screen which should be located close to the Galactic center. The Faraday corrected intrinsic electric vector position angle is 16$\pm$9 deg East of North, and coincides with the position angle of the pulsar's transverse velocity. All measurements of the pulsar's RM value to date, including the one presented here, well agree within errors, which points towards a steady nature of the Faraday-rotating medium.Comment: 6 pages, 4 figures, accepted for publication in MNRAS; "Faraday synthesis" has been changed to "Faraday RM synthesis"/"RM synthesis

A Data Exchange Standard for Optical (Visible/IR) Interferometry

This paper describes the OI Exchange Format, a standard for exchanging calibrated data from optical (visible/infrared) stellar interferometers. The standard is based on the Flexible Image Transport System (FITS), and supports storage of the optical interferometric observables including squared visibility and closure phase -- data products not included in radio interferometry standards such as UV-FITS. The format has already gained the support of most currently-operating optical interferometer projects, including COAST, NPOI, IOTA, CHARA, VLTI, PTI, and the Keck Interferometer, and is endorsed by the IAU Working Group on Optical Interferometry. Software is available for reading, writing and merging OI Exchange Format files.Comment: 26 pages, 1 figur

The 2~μ\mum spectrum of the auroral emission in the polar regions of Jupiter

We report observations of the high (R$\sim$18000) and medium (R$\sim$5900) resolution, near-infrared spectra of Jupiter's polar regions with the GNIRS instrument at the Gemini North telescope. The observations correspond to the area of main auroral oval in the South and the main spot of the Io footprint in the North. We detected and assigned 18 emission lines of the H$_{3}^{+}$, 2$\nu_{2}\rightarrow 0$ overtone band in the region from 4800 to 4980 cm$^{-1}$ and 5 additional lines in the extended low-resolution spectrum. We use our new modelling scheme, ATMOF to remove telluric absorption bands of CO$_2$ that feature strongly in the 2 $\mu$m region. The H${_2}$ 1-0 S(1), S(2) and S(3) emission lines are also detected in the observed spectral region. We found the rotational temperature and column density of H$_{3}^{+}$ emission at the peak intensity for both northern and southern auroral regions to be the same within the measurement errors (T$_{rot} \sim950$K and N(H$_{3}^{+}$) $\sim$ 4.5$\times10^{16}$m$^{-2}$). The estimates of T$_{rot}$ from H$_{2}$ are consistent within much higher uncertainties with temperatures derived from H$_{3}^{+}$ emissions. We derived the profiles of the H$_{3}^{+}$ emissivity and ion density for both auroral regions providing the first such measurement for the emission associated with the main spot of the Io footprint. We also found a number of weaker lines in the high-resolution spectra that could be associated with emission from high excitation levels in neutral iron, which could be deposited in Jupiter's atmosphere as a result of meteor ablation.Comment: Accepted for publication in Icaru

Rotation Measures in AGN jets seen by VLA at 21 cm to 6 mm

We present Faraday Rotation Measure (RM) properties of seven active galactic nuclei (AGN), observed with the NRAO VLA at three epochs in 2012-2014. Data was taken at 1.4, 2.2, 5.0, 8.2, 15.4, 22.4, 33.5 and 43.1 GHz quasi simultaneously in full polarization mode. For the first time RMs were calculated in a range of wavelengths covering more than one order of magnitude: from 21 cm up to 6 mm. We measured RM for each source and showed a tendency to increase its value toward high frequencies according to the law |RM|~f^a with a=1.6+/-0.1. For 0710+439, we observed an increase over the frequency range of 4 orders of magnitude and measured one of the highest RM ever, (-89+/-1)*10^3 rad/m^2. Analysis of different epochs shows variations of the value and the sign of RM on short and long time-scales. This may be caused by changing physical conditions in the compact regions of the AGN jets, e.g. strength of magnetic field, particle density and so on.Comment: 5 pages, 4 figures, 2 tables. To appear in the Proceedings of the IAU Symposium No. 313 Extragalactic jets from every angle, Galapagos, Ecuador, 15-19 September 2014, F. Massaro, C. C. Cheung, E. Lopez, and A. Siemiginowska (Eds.), Cambridge University Pres