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Register a new userThe Very Large Array Low-frequency Sky Survey Redux (VLSSr)
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We present the results of a recent re-reduction of the data from the Very Large Array (VLA) Low-frequency Sky Survey (VLSS). We used the VLSS catalog as a sky model to correct the ionospheric distortions in the data and create a new set of sky maps and corresponding catalog at 73.8 MHz. The VLSS Redux (VLSSr) has a resolution of 75 arcsec, and an average map RMS noise level of $sigmasim0.1$ Jy beam$^{-1}$. The clean bias is $0.66timessigma$, and the theoretical largest angular size is 36 arcmin. Six previously un-imaged fields are included in the VLSSr, which has an unbroken sky coverage over 9.3 sr above an irregular southern boundary. The final catalog includes 92,964 sources. The VLSSr improves upon the original VLSS in a number of areas including imaging of large sources, image sensitivity, and clean bias; however the most critical improvement is the replacement of an inaccurate primary beam correction which caused source flux errors which vary as a function of radius to nearest pointing center in the VLSS.
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The results of a climatological study of ionospheric disturbances derived from observations of cosmic sources from the Very Large Array (VLA) Low-frequency Sky Survey (VLSS) are presented. We have used the ionospheric corrections applied to the 74 MHz interferometric data within the VLSS imaging process to obtain fluctuation spectra for the total electron content (TEC) gradient on spatial scales from a few to hundreds of kilometers and temporal scales from less than one minute to nearly an hour. The observations sample nearly all times of day and all seasons. They also span latitudes and longitudes from 28 deg. N to 40 deg. N and 95 deg. W to 114 deg. W, respectively. We have binned and averaged the fluctuation spectra according to time of day, season, and geomagnetic (Kp index) and solar (F10.7) activity. These spectra provide a detailed, multi-scale account of seasonal and intraday variations in ionospheric activity with wavelike structures detected at wavelengths between about 35 and 250 km. In some cases, trends between spectral power and Kp index and/or F10.7 are also apparent. In addition, the VLSS observations allow for measurements of the turbulent power spectrum down to periods of 40 seconds (scales of ~0.4 km at the height of the E-region). While the level of turbulent activity does not appear to have a strong dependence on either Kp index or F10.7, it does appear to be more pronounced during the winter daytime, summer nighttime, and near dusk during the spring.
The Very Large Array Sky Survey (VLASS) is a synoptic, all-sky radio sky survey with a unique combination of high angular resolution ($approx$2.5), sensitivity (a 1$sigma$ goal of 70 $mu$Jy/beam in the coadded data), full linear Stokes polarimetry, time domain coverage, and wide bandwidth (2-4 GHz). The first observations began in September 2017, and observing for the survey will finish in 2024. VLASS will use approximately 5500 hours of time on the Karl G. Jansky Very Large Array (VLA) to cover the whole sky visible to the VLA (Declination $>-40^{circ}$), a total of 33,885 deg$^2$. The data will be taken in three epochs to allow the discovery of variable and transient radio sources. The survey is designed to engage radio astronomy experts, multi-wavelength astronomers, and citizen scientists alike. By utilizing an on the fly interferometry mode, the observing overheads are much reduced compared to a conventional pointed survey. In this paper, we present the science case and observational strategy for the survey, and also results from early survey observations.
We present a survey of the radio sky accessible from the first station of the Long Wavelength Array (LWA1). Images are presented at nine frequencies between 35 and 80 MHz with spatial resolutions ranging from $4.7^circ$ to $2.0^circ$, respectively. The maps cover the sky north of a declination of $-40^circ$ and represent the most modern systematic survey of the diffuse Galactic emission within this frequency range. We also combine our survey with other low frequency sky maps to create an updated model of the low frequency sky. Due to the low frequencies probed by our survey, the updated model better accounts for the effects of free-free absorption from Galactic ionized Hydrogen. A longer term motivation behind this survey is to understand the foreground emission that obscures the redshifted 21 cm transition of neutral hydrogen from the cosmic dark ages ($z>10$) and, at higher frequencies, the epoch of reionization ($z>6$).
In almost 30 years of operation, the Very Large Array (VLA) has proved to be a remarkably flexible and productive radio telescope. However, the basic capabilities of the VLA have changed little since it was designed. A major expansion utilizing modern technology is currently underway to improve the capabilities of the VLA by at least an order of magnitude in both sensitivity and in frequency coverage. The primary elements of the Expanded Very Large Array (EVLA) project include new or upgraded receivers for continuous frequency coverage from 1 to 50 GHz, new local oscillator, intermediate frequency, and wide bandwidth data transmission systems to carry signals with 16 GHz total bandwidth from each antenna, and a new digital correlator with the capability to process this bandwidth with an unprecedented number of frequency channels for an imaging array. Also included are a new monitor and control system and new software that will provide telescope ease of use. Scheduled for completion in 2012, the EVLA will provide the world research community with a flexible, powerful, general-purpose telescope to address current and future astronomical issues.
The Very Large Array Sky Survey (VLASS) is observing the entire sky north of $-40^{circ}$ in the S-band ($2< u<4,$GHz), with the highest angular resolution ($2.5$) of any all-sky radio continuum survey to date. VLASS will cover its entire footprint over three distinct epochs, the first of which has now been observed in full. Based on Quick Look images from this first epoch, we have created a catalog of $1.9times10^{6}$ reliably detected radio components. Due to the limitations of the Quick Look images, component flux densities are underestimated by $sim 15,%$ at $S_{text{peak}}>3,$mJy/beam and are often unreliable for fainter components. We use this catalog to perform statistical analyses of the $ u sim 3,$GHz radio sky. Comparisons with the Faint Images of the Radio Sky at Twenty cm survey (FIRST) show the typical $1.4-3,$GHz spectral index, $alpha$, to be $sim-0.71$. The radio color-color distribution of point and extended components is explored by matching with FIRST and the LOFAR Two Meter Sky Survey. We present the VLASS source counts, $dN/dS$, which are found to be consistent with previous observations at $1.4$ and $3,$GHz. Resolution improvements over FIRST result in excess power in the VLASS two-point correlation function at angular scales $lesssim 7$, and in $18,%$ of active galactic nuclei associated with a single FIRST component being split into multi-component sources by VLASS.
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