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NCP21CMA - North Celestial Pole Region Radio Sources Detected by the 21cm Array

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Overview

This table contains the catalog of 624 radio sources detected around the North Celestial Pole (NCP) with the 21 Centimeter Array (21CMA), a radio interferometer dedicated to the statistical measurement of the epoch of reionization (EoR). The data are taken from a 12-hr observation made on 2013 April 13, with a frequency coverage from 75 to 175 MHz and an angular resolution of ~4 arcminutes. The catalog includes flux densities at eight sub-bands across the 21CMA bandwidth and provides the in-band spectral indices for the detected sources. To reduce the complexity of interferometric imaging from the so-called "w" term and ionospheric effects, the present analysis is restricted to the east-west baselines within 1500 m only. 624 radio sources are found within 5 degrees around the NCP down to ~0.1 Jy (100 mJy). These source counts are compared, and also exhibit a good agreement, with deep low-frequency observations made recently with the GMRT and MWA. In particular, for fainter radio sources below ~1 Jy, the authors find a flattening trend of source counts toward lower frequencies. While the thermal noise (~0.4 mJy) is well controlled to below the confusion limit, the dynamical range (~104) and sensitivity of current 21CMA imaging are largely limited by calibration and deconvolution errors, especially the grating lobes of very bright sources, such as 3C061.1, in the NCP field, which result from the regular spacings of the 21CMA. The authors note that particular attention should be paid to the extended sources, and their modeling and removal may constitute a large technical challenge for current EoR experiments. Their analysis may serve as a useful guide to the design of next generation low-frequency interferometers like the Square Kilometre Array (SKA).

The 21CMA is a ground-based radio interferometer dedicated to the detection of the EoR. The array, sited in the Ulastai valley of western China, consists of 81 pods or stations, and a total of 10,287 log-periodic antennas are deployed in two perpendicular arms along the east-west (6.1 km) (see Figure 1 in the reference paper) and north-south (4 km) directions, respectively. The spacing of these 81 pods is chosen such that a sufficiently large number of redundant baselines and a good uniform UV coverage can both be guaranteed. Each antenna element has 16 pairs of dipoles with lengths varying from 0.242 to 0.829 m, optimized to cover a frequency range of 50-200 MHz, which gives rise to an angular resolution of 3 arcminutes at 200 MHz. All of the antennas are fixed on the ground and point at the NCP for the sake of simplicity and economy.

In the current work, the radio point sources observed with the 40 pods of the 21 Centimeter Array (21CMA) E-W baselines in an integration of 12 hours made on 2013 April 13 centered on the North Celestial Pole (NCP) are presented. An extra deep sample with a higher sensitivity from a longer integration time of up to years will be published later. The authors have detected a total of 624 radio sources over the central field within 3 degrees in a frequency range of 75-175 MHz band and in the outer annulus from 3-5 degrees in the 75-125 MHz band. By performing a Monte-Carlo simulation, the authors estimate a completeness of 50% at a flux density of ~0.2 Jy.


Catalog Bibcode

2016ApJ...832..190Z

References

Radio sources in the NCP region observed with the 21 Centimeter Array.
    Zheng Q., Wu X.-P., Johnston-Hollitt M., Gu J.-H., Xu H.
   <Astrophys. J., 832, 190-190 (2016)>
   =2016ApJ...832..190Z    (SIMBAD/NED BibCode)

Provenance

This table was created by the HEASARC in May 2017 based upon the CDS Catalog J/ApJ/832/190 file table3.dat. Some of the values for the name parameter in the HEASARC's implementation of this table were corrected in April 2018.

Parameters

Name
The source designation created by the HEASARC in the style recommended by the CDS Dictionary of Nomenclature of Celestial Objects for J2000.0 position-based names, viz., '[ZWJ2016] 21CMA JHHMM.m+DDMM', where the prefix stands for 'Zheng, Wu, Johnston-Hollitt 2016, 21 Centimeter Array'.

RA
The Right Ascension of the radio source centroid in the selected equinox. This was given in J2000 equatorial coordinates to an accuracy of 0.1 seconds of time in the original reference. The CDS ReadMe file notes that 'according to the author, the resolution is ~arcminutes, so the [tabulated] Right Ascension is too accurate and the precision should be several seconds, not 0.1 seconds [of time]'. The authors estimate that the positional accuracy is ~5 arcminutes.

Dec
The Declination of the radio source centroid in the selected equinox. This was given in J2000 equatorial coordinates to an accuracy of 1 arcminute in the original reference. The authors estimate that the positional accuracy is ~5 arcminutes.

LII
The Galactic Longitude of the radio source centroid.

BII
The Galactic Latitude of the radio source centroid.

Flux_81p25_MHz
The integrated flux density of the radio source in the 81.25-MHz sub-band image, in mJy (converted by the HEASARC from the Jansky units used in the original table).

Flux_81p25_MHz_Error
The uncertainty in the integrated flux density of the radio source in the designated sub-band, in mJy (converted by the HEASARC from the Jansky units used in the original table).

Flux_93p75_MHz
The integrated flux density of the radio source in the 93.75-MHz sub-band image, in mJy (converted by the HEASARC from the Jansky units used in the original table).

Flux_93p75_MHz_Error
The uncertainty in the integrated flux density of the radio source in the designated sub-band, in mJy (converted by the HEASARC from the Jansky units used in the original table).

Flux_106p25_MHz
The integrated flux density of the radio source in the 106.25-MHz sub-band image, in mJy (converted by the HEASARC from the Jansky units used in the original table).

Flux_106p25_MHz_Error
The uncertainty in the integrated flux density of the radio source in the designated sub-band, in mJy (converted by the HEASARC from the Jansky units used in the original table).

Flux_118p75_MHz
The integrated flux density of the radio source in the 118.75-MHz sub-band image, in mJy (converted by the HEASARC from the Jansky units used in the original table).

Flux_118p75_MHz_Error
The uncertainty in the integrated flux density of the radio source in the designated sub-band, in mJy (converted by the HEASARC from the Jansky units used in the original table).

Flux_131p25_MHz
The integrated flux density of the radio source in the 131.25-MHz sub-band image, in mJy (converted by the HEASARC from the Jansky units used in the original table).

Flux_131p25_MHz_Error
The uncertainty in the integrated flux density of the radio source in the designated sub-band, in mJy (converted by the HEASARC from the Jansky units used in the original table).

Flux_143p75_MHz
The integrated flux density of the radio source in the 143.75-MHz sub-band image, in mJy (converted by the HEASARC from the Jansky units used in the original table).

Flux_143p75_MHz_Error
The uncertainty in the integrated flux density of the radio source in the designated sub-band, in mJy (converted by the HEASARC from the Jansky units used in the original table).

Flux_156p25_MHz
The integrated flux density of the radio source in the 156.25-MHz sub-band image, in mJy (converted by the HEASARC from the Jansky units used in the original table).

Flux_156p25_MHz_Error
The uncertainty in the integrated flux density of the radio source in the designated sub-band, in mJy (converted by the HEASARC from the Jansky units used in the original table).

Flux_168p75_MHz
The integrated flux density of the radio source in the 168.75-MHz sub-band image, in mJy (converted by the HEASARC from the Jansky units used in the original table).

Flux_168p75_MHz_Error
The uncertainty in the integrated flux density of the radio source in the designated sub-band, in mJy (converted by the HEASARC from the Jansky units used in the original table).

Spectral_Index
The spectral index (alpha) of the radio source obtained by fitting the observed sub-band flux densities to a power law Snu = S0 * nualpha. Note that for sources within 3 degrees of the NCP, the authors fit the integrated fluxes in all eight sub-bands spanning the 75-175 MHz frequency coverage of the 21CMA observation, while for sources in the 3-5 degrees annulus around the NCP, the authors only use the integrated fluxes measured in the four lower frequency sub-bands. Moreover, data points in some of the sub-bands have not been taken if the integrated fluxes of images cannot be precisely measured due to poor resolution and/or significant overlap. Of the 624 sources, 322 sources lie between 3 and 5 degrees from the NCP field center for which only the four lower frequency sub-bands (81.25, 93.75, 106.25, and 118.75 MHz) were used.

Spectral_Index_Error
The uncertainty in the spectral index (alpha) of the radio source.


Contact Person

Questions regarding the NCP21CMA database table can be addressed to the HEASARC User Hotline.
Page Author: Browse Software Development Team
Last Modified: Thursday, 12-Apr-2018 11:57:51 EDT