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VLANEP: Help

VLANEP - 1.5-GHz VLA-NEP Survey


Overview

The VLANEP database contains the VLA-NEP survey of 29.3 square degrees around the North Ecliptic Pole mapped with the VLA at 20 cm (1.5 GHz) in the `C-configuration`. The database table contains 2435 radio sources with flux densities ranging from 0.3 to 1000 mJy, including over 200 fainter than 1 mJy. Source positions have been corrected for instrumental effects, and most positions are accurate to less than 2 arcseconds. The sensitivity varies from field to field, with the 1 sigma level being approximately 0.06 mJy at the center of the inner fields and 0.12 mJy at the center of the outer fields. Sensitivity drops with distance from the center of each field due to the primary beam response of the VLA antennas and interferometer effects. Source flux densities have been corrected for these effects. The spatial resolution varies from field to field, with the typical HPBW being 20 arcseconds. Source positions have been corrected for instrumental effects, and most positions are accurate to less than 2 arcseconds. Approximately 6% of the sources were found to be extended with angular sizes greater than 30 arcseconds.

Catalog Bibcode

1994ApJS...93..145K

References

'A Sensitive 1.5 GHz Radio Survey around the North Ecliptic Pole', R. I. Kollgaard et al. 1994, ApJS, 93, 145.

Provenance

This catalog was recreated at the HEASARC in February 2001 (replacing a previous version: some parameters were renamed, and the source names were changed to the format recommended for VLA-NEP sources in the Dictionary of Nomenclature of Celestial Objects maintained at the CDS) based on CDS/ADC Catalog J/ApJS/93/145/table2.

Parameters

Name
The VLANEP Catalog designation based on the J2000 coordinates of the source in the form recommended in the Dictionary of Nomenclature of Celestial Objects maintained at the CDS: (http://vizier.u-strasbg.fr/cgi-bin/Dic). In a few cases, two sources which otherwise would have the same name are distinguished by the suffixes `A` or `B`.

Field_Num
The numerical designation of the VLA field where the source is located. The VLA-NEP Survey field parameters are listed in Table 1 of the published paper.

RA
The Right Ascension of the radio source in the selected equinox.

Dec
The Declination of the radio source in the selected equinox.

LII
The galactic latitude of the radio source.

BII
The galactic longitude of the radio source.

Pos_Error
The positional uncertainty of the source, in arcseconds. The positional uncertainty was estimated from a quadratic sum of two terms. The first term arises from the use of the parabolic fitting routine MAXFIT and depends on the signal-to-noise of the source and the size of the CLEAN restoring beam for that field. The second error term arises from residual errors remaining from the application of the radial expansion correction.

Distance
The angular distance, in arcminutes, of the source from the center of the VLA field in which it was identified. This angular distance was used to determine positional and flux density corrections and uncertainties.

Int_Flux_20_cm
The corrected integrated flux density of the source, in mJy. The total flux densities were determined with the AIPS tasks TVSTAT and IMSTAT which sum the pixels in a small region around the peak of the source. Regions were selected which best enclosed the source.

Int_Flux_20_cm_Error
The rms error in the corrected integrated flux density, in mJy. The uncertainty was determined from a quadratic sum of 5 terms. These take into account: (1) the statistical error based on the rms noise level of the image, (2) calibration uncertainties, (3) pointing errors from individual antennas, (4) uncertainties in the primary beam correction, and (5) uncertainties in the the total flux determination for faint sources.

Flux_20_cm
The corrected peak flux density of a source, in mJy. The peak flux density (and position) was determined with the AIPS task MAXFIT which fits a quadratic function to a 3 x 3 pixel area around each peak. (On these fields 1 pixel = 5 arcseconds.) The raw values were corrected for the primary beam response of the VLA antennas and for bandwidth smearing (chromatic aberration).

Flux_20_cm_Error
The rms error in the corrected peak flux density, in mJy. The uncertainty was determined from a quadratic sum of 5 terms. These take into account (1) statistical error based on the rms noise level of the image, (2) calibration uncertainties, (3) pointing errors from individual antennas, (4) uncertainties in the primary beam correction, and (5) uncertainties in the bandwidth smearing correction.

SNR
The signal-to-noise ratio of the detection.

ID_Flag
This parameter flags those VLA-NEP sources which have been tentatively identified with objects in other catalogs with a 'Y'. A full list of these possibly identified sources is available as Table 3 of the published paper containing this catalog, or in electronic form at ftp://cdsarc.u-strasbg.fr/pub/cats/J/ApJS/93/145/table3

Sus_Flag
This parameter flags those VLA-NEP sources which are `suspect` because they lie on or near an image processing artifact on the VLA image with an 'S'.

Ext_Flag
This parameter flags those VLA-NEP sources which are resolved. These include ('D') double sources, either a blend of two comparable sources or two resolved peaks with a common envelope, and ('E') extended sources with an elongated, asymmetric, or complex structure. ('D?') and ('E?') indicate possible doubles and extended sources whose resolved features appear at a significance level of less than 8 sigma.

Fig_Flag
This parameter flags those VLA-NEP sources having complex structure and which have maps available in the published paper (Kollgaard et al. 1994) either by 'A' if in Figure 5A or by 'B'if in Figure 5B.

Separation
This is the angular separation, in arcminutes, between the peaks of double sources (entries with the parameter ext_flag = 'D'), or the angular size of extended sources (entries with the parameter ext_flag = 'E'), in arcminutes, as measured from the peak to the most distant feature (minus a small resolution correction).

Position_Angle
The position angle, in degrees, of an extended feature or a secondary lobe, as measured from the brightest peak in a direction from north eastwards. A value of 999 indicates that the source has a symmetric halo.

Rel_Name
In some instances, both peaks of a large, resolved double source were bright enough to be included as individual entries. The `Rel Name` parameter lists the name of the other peak associated with the catalog entry. (Note: in these instances, the peak flux density is that for an individual lobe, while the integrated flux density is that for the entire object, i.e., for both lobes.)


Contact Person

Questions regarding the VLANEP database table can be addressed to the HEASARC User Hotline.
If you have any problems, please consult the help page or mail ledas-help@star.le.ac.uk
 
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