arXiv subjects

Eric W. Greisen

Publications and source records attributed to Eric W. Greisen.

5 recordsLinked to original sources

arXiv2026

Correcting Ionospheric Faraday Rotation for the VLA and MeerKAT

We report here on studies to determine the accuracy of estimated corrections of Ionospheric Faraday Rotation Measure (IFRM) using observations of the Moon with the Very Large Array (VLA) and MeerKAT telescopes. To estimate the IFRM requires an estimate of the total electron content along the line-of-sight to the observed sources (the so-called Slant Total Electron Content, or STEC). Estimating the STEC requires an estimate of the global 2-D map of Vertical Total Electron Content (VTEC) along with the ray path from the telescope to the source. Traditionally, these global VTEC maps have been utilized along with an assumption that the electrons are in a thin shell at a given altitude to provide an estimate of the IFRM as a function of time. We find that this traditional technique significantly overestimates the IFRM - typically by 0.5 to 1.1 rad/m^2 for the VLA, and -0.3 rad/m^2 for MeerKAT. Alternatively, the software package ALBUS utilizes raw data from nearby Global Navigation Satellite System (GNSS) stations, to generate a local estimate of the IFRM as a function of time. ALBUS provides considerably better estimates of the IFRM - accurate to 0.1 rad/m^2 for both the VLA and MeerKAT, provided the stations utilized have known receiver bias values. A byproduct of our study is the establishment of the intrinsic electric vector position angle (EVPA) of the standard polarized calibrators 3C286 and 3C138 from 500 MHz to 50 GHz, using additional VLA observations of the Moon, Venus, and Mars.

astro-ph.IM↗

arXiv2009

The Deep SWIRE Field II. 90cm Continuum Observations and 20cm-90cm Spectra

We present one of the deepest radio continuum surveys to date at a wavelength ~1 meter, at 324.5 MHz. The data reduction and analysis are described and an electronic catalog of the sources detected above 5 sigma is presented. We also discuss the observed angular size distribution for the sample. Using our deeper 20cm survey of the same field, we calculate spectral indices for sources detected in both surveys. The spectral indices for 90cm-selected sources, defined as S ~nu^(-alpha}, shows a peak near 0.7 and only a few sources with very steep spectra. Thus no large population of very steep spectrum microJy sources seems to exist down to the limit of our survey. For 20cm-selected sources, we find similar mean spectral indices for sources with S_20>1 mJy. For weaker sources, below the detection limit for individual sources at 90cm, we use stacking to study the radio spectra. We find that the spectral indices of small (<3") 20cm-selected sources with S_20< 10 mJy have mean and median alpha(90,20)~0.3-0.5. This is flatter than the spectral indices of the stronger source population. We report log N-log S counts at 90cm which show a flattening below 5 mJy. Given the median redshift of the population, z~1, the spectral flattening and the flattening of the log N-log S counts occurs at radio luminosities normally associated with AGN rather than with galaxies dominated by star-formation.

astro-ph.CO↗

arXiv2009

Aperture Synthesis Observations of the Nearby Spiral NGC 6503: Modeling the Thin and Thick HI Disks

We present sensitive aperture synthesis observations of the nearby, late-type spiral galaxy NGC 6503, and produce HI maps of considerably higher quality than previous observations by van Moorsel & Wells (1985). We find that the velocity field, while remarkably regular, contains clear evidence for irregularities. The HI is distributed over an area much larger than the optical image of the galaxy, with spiral features in the outer parts and localized holes within the HI distribution. The absence of absorption towards the nearby quasar 1748+700 yields an upper limit of 5 10^{17} cm^{-2} for the column density of cold HI gas along a line of sight which should intersect the disk at a radius of 29 kpc. This suggests that the radial extent of the HI disk is not much larger than that which we trace in HI emission (23 kpc). The observed HI distribution is inconsistent with models of a single thin or thick disk. Instead, the data require a model containing a thin disk plus a thicker low column-density HI layer that rotates more slowly than the thin disk and that extends only to approximately the optical radius. This suggests that the presence of extra-planar gas in this galaxy is largely the result of star formation in the disk rather than cold gas accretion. Improved techniques for interferometric imaging including multi-scale Clean that were used in this work are also described.

astro-ph.GA↗

arXiv2002

Representations of world coordinates in FITS

The initial descriptions of the FITS format provided a simplified method for describing the physical coordinate values of the image pixels, but deliberately did not specify any of the detailed conventions required to convey the complexities of actual image coordinates. Building on conventions in wide use within astronomy, this paper proposes general extensions to the original methods for describing the world coordinates of FITS data. In subsequent papers, we apply these general conventions to the methods by which spherical coordinates may be projected onto a two-dimensional plane and to frequency/wavelength/velocity coordinates.

astro-ph↗

arXiv2002

Representations of celestial coordinates in FITS

In Paper I, Greisen & Calabretta (2002) describe a generalized method for assigning physical coordinates to FITS image pixels. This paper implements this method for all spherical map projections likely to be of interest in astronomy. The new methods encompass existing informal FITS spherical coordinate conventions and translations from them are described. Detailed examples of header interpretation and construction are given.

astro-ph↗