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NORTH6CM: Help

NORTH6CM - 6cm Radio Catalog


Overview

The NORTH6CM database is a catalog of 53,522 4.85 GHz sources generated by Becker, R. H., White, R. L., Edwards, A. L. 1991, ApJS 75, 1. It covers between 0 degrees and 75 degrees declination using observations taken with the NRAO Greenbank 300-ft telescope by Condon, Broderick and Seielstad (1989). The flux limit of the catalog is dependent on declination and ranges from approximately 40 mJy at 0 degrees to 20 mJy at 60 degrees. The source positions given in the catalog have a 95% confidence radius of approximately 50 arcsec. Spectral indices have been calculated for 29,051 sources which have counterparts in the Texas 365 MHz Northern Sky survey.

Catalog Bibcode

1991ApJS...75....1B

References

Bennett, C. L., Lawrence, C. R., Burke, B. F., Hewitt, J. N., and Mahaney, J. 1986, ApJS 61, 1.

Condon, J. J., Broderick, J. J. and Seielstad, G. A. 1989, AJ 97, 1064.

Langston, G. I., Heflin, M. B., Conner, S. R., Lehar, J., Carrilli, C. L., and Burke, B. F. 1990, ApJS 72, 621.

White, R. L., Becker, R. H., and Helfand, D. J. 1991, ApJ 371, 148.


Description

This is a catalog of 53,522 4.85 GHz radio sources based on the northern sky survey of Condon, Broderick, and Seielstad (1989), hereafter referred to as CBS. The NRAO 91 m telescope in Greenbank, West Virginia was used by CBS to survey the sky between declinations of 0 degrees--75 degrees at 4.85 GHz. The observations were made in October, 1987. The resulting data were gridded into 285 1,024 by 1,024 FITS images with 40 arcsec pixel size. The RMS noise in the images varied from 5--8 mJy. The images had an angular resolution of approximately 3.5 arcmin. Sources with peak fluxes brighter than 70 Jy were truncated in the CBS images, so they usually have been omitted from this catalog. The extraction of a source catalog from the images was limited to compact sources smaller than approximately 5 arcmin in diameter.

The goal was to identify sources with peak intensities greater than 5 times the image RMS intensity level. Near declination +70 degrees the threshold is 20 mJy, while at 0 degrees it is typically 40 mJy. In fact, the RMS noise level varies on scales of 5 degrees or less. For instance, there is a ring of high noise at 64 degrees +/- 1 degrees declination which is approximately 30% higher than that just to the immediate north or south. Aside from these global effects, the completeness of the catalog suffers from local effects. The sensitivity to weak sources decreases in the vicinity of strong sources (> approximately 10 Jy) due to the sidelobe pattern of the 300-ft telescope. This is a serious problem along the entire galactic plane (abs(b) < 5 degrees) as well as near a number of isolated sources which are listed in Becker et al.


Accuracy

For each radio source in the catalog a position and a 4.85 GHz flux density are given. The errors are dominated by systematic errors in the data. An estimate of the accuracy of the catalog was made through comparisons to other data sets as well as through checks on internal consistency. This is most readily accomplished for the source positions using data collected at other frequencies. In particular, a comparison was made to the Texas 80 cm Survey (Douglas, priv. comm.) which covers nearly the entire 6 cm survey area (the Texas survey reaches Dec approximately 71.5 degrees). Using a matching criterion of 160 arcsec 29,051 matches were found between the two catalogs with an expectation of 300 false matches.

The separation between matched pairs is a measure of the combined positional errors in both catalogs. The Texas Survey has much better angular resolution in principle (approximately 5 arcsec) but has positional ambiguity due to interferometric sidelobes. To reduce this effect, we can limit ourselves to the 7,531 matched Texas source which are point sources with high quality flags. For this subset, 67% of the matches have separations of 25 arcsec or less while 95% are separated by less than 51 arcsec. The positional accuracy also depends on the 4.85 GHz flux density of the source in question. For sources with a 4.85 GHz flux density greater than 100 mJy, the 67% and 95% confidence error circles decrease to 18 and 38 arcsec respectively. These values are consistent with the positional accuracy for the survey estimated by CBS. To the extent that Texas sources contribute some of uncertainty in position, the above values should be taken as upper limits to the positional uncertainty in this catalog.

Determining the accuracy of the flux densities in the catalog is more problematic. A comparison between this catalog and the MIT--Greenbank 5 GHz Surveys (Langston et al. 1990; Bennett et al. 1986) indicates a general agreement in flux scales for brighter sources (>150 mJy) but some systematic differences for weak or extended sources.


Detection Thresholds

                Declination  Threshold (mJy)
                ----------------------------
                 0 degrees        40
                10 degrees        30
                20 degrees        25
                30 degrees        25
                40 degrees        22
                50 degrees        22
                60 degrees        20
                70 degrees        20

Parameters

BII
The galactic latitude of the source.

BV_Color
The B-V color.

Class
BROWSE classification flag.

Dec
The Declination of the source.

Ext_Flag
Ext_flag is "*" if the source is extended, else it is blank.

Flux_6_cm
The 4.85 GHz flux (mJy) density of each source in mJy (the peak flux density for point sources and the integrated flux density for extended sources).

LII
The galactic longitude of the source.

Name
The 6cm name, constructed from RA and Dec in the format HHMM+DDSS, with A or B added at the end if needed to make the name unique.

NH
NH Galactic Absorption from 21 cm (Note: All values are zero.)

RA
The Right Ascension of the source.

Redshift
The redshift of the source.

Spectral_Index
The spectral index between 80 cm and 6 cm for all sources with a counterpart in the Texas Survey. Apart from the uncertainty in the flux measurements, the spectral indices may also be affected by the nonsimultaneity of the two measurements and by the tendency of the Texas interferometric survey to underestimate the fluxes of sources larger than approximately 15 arc sec. Spectral indices are flagged using the wid_flag parameter with an asterisk for the 910 sources with 6 cm/80 cm position differences greater than 100 arc sec ; such matches have a reliability less than 80%.

This is blank if the source is not detected at 0.365 GHz.

UB_Color
The U-B color.

Vmag
The V magnitude of the source.

Wid_Flag
This is "*" if the separation between 4.85 and 0.365 GHz positions is greater than 100 arc sec; otherwise, it is blank.


Contact Person

Questions regarding the NORTH6CM 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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