GB87 (E0): 1987 Green Bank 300-foot (91m) 4.85 GHz Radio Survey (19941130)
FILES:
gb87_23.e0 (27723 sources; 23 <= peak flux < 50 mJy)
gb87_50.e0 (16179 sources; 50 <= peak flux < 100 mJy)
gb87_100.e0 (10677 sources; peak flux >= 100 mJy)
CONTENTS: 1987 Green Bank 300-foot (91m) 4.85 GHz Radio Survey, -0 <= Dec <= +75
DATASET: These files come in HyperSky dataset E-5, version 19941130 Availability on HyperSky CD-ROM: Vol. 2 (1995)
DESCRIPTION: This datafile contains the 87GB Catalog of Radio Sources. It is based on a survey carried out by the Green Bank 300-foot (91 m) telescope to map 6.0 square radians of sky in the declination band 0 <= Dec <= +75, at 4.85 GHz. The 87GB catalog consists of 54,579 discrete sources with angular sizes < 10.5 arcmin and flux densities larger than S ~ 25-40 mJy, as derived from the survey maps (Gregory and Condon 1991, Astrophys. J. Suppl., 77, 1). 87GB catalog positions and peak flux densities were corrected for known map biases. Estimated RMS uncertainties were verified by comparisons with more accurate positions and flux densities available for some sources.
The survey maps are confusion limited with RMS confusion plus noise 25-30 mJy except near the galactic plane, so that individual point sources stronger than about 150 mJy should be reliable. Fainter "sources" are usually "real" in the sense that they would appear in any noiseless map of the sky made with the same resolution, but they are often unresolved blends of two or more physically unrelated radio sources.
Note that for HyperSky file-naming purposes, the original survey name, 87GB, has been converted to GB87. This is because file names starting with numbers are often frowned upon, and are not legal on some non-DOS systems.
DATA: The HyperSky datafiles contain the following data from the 87GB catalog:
POSITION: Right Ascension and Declination are based on the catalog J2000 positions. Catalog positions were provided to an accuracy of 0.1 second in Right Ascension and 1" in declination.
MAG: No magnitudes or equivalents are provided in the catalog. A set of artificial magnitudes is provided, however, in the HyperSky catalog strictly to allow use of HyperSky's limiting-magnitude controls. In this way, users can display only those sources above a desired threshold. The artificial magnitude system is based on the relation (from Pogson) used to define visual magnitudes:
m1 - m2 = 2.5 * log10(S1/S2)
where { m1, m2 } are magnitudes, and { S1, S2 } are source strengths. For the HyperSky catalog, the brightest source (S1 = 65829 mJy) is arbitrarily assigned the magnitude m1 = 1.0. Given this, any other source S2 can have its relative magnitude, m2, derived using the relation:
m2 = m1 + 2.5 * log10 (S1/S2);
In this system, the faintest catalog source (23 mJy) has a magnitude of 9.6. The following table summarizes the relation between magnitude and source strength for catalog extremes and intermediate magnitudes:
Magnitude Source Strength (mJy)
----------|----------------------
1.0 65829 (brightest catalog source)
1.5 37500
2.5 16000
3.0 10400
4.0 4200
5.0 1600
6.0 650
7.0 260
8.0 100
9.0 40
9.6 23 (faintest catalog source)
With the above table, you can set HyperSky's limiting magnitude (for emission sources) to mask out catalog sources below a desired strength. Except for this use, the derived source magnitudes have NO significance. It should be emphasized again that the magnitudes are NOT provided in the original catalog; only the source strengths are fundamental catalog values.
IDENTIFICATION: The derived source name, based on the J2000 position. Such names take the IAU-adopted form "Jhhmm+ddmm", where 'J' indicates J2000, hhmm is the RA in hours and minutes, and +ddmm is the declination in degrees and minutes. For complete identification, this form would be preceded by the catalog abbreviation. Thus for example, a source at position RA 12h 34m, declination +56 deg 01 min, would be identified as source "87GB J1234+5601".
COMMENTS: The comment lines contain the following information, provided if available for each radio source identification:
* A line giving the source dimensions and position angle, derived from catalog values as follows: Catalog dimensions (major and minor axes) for each source are given in units of beamwidth. Because this beam was a 3.7x3.3' Gaussian, beamwidth dimensions can be converted to approximately equivalent linear dimensions using a multiplier of 3.5. The resulting dimensions appear in the HyperSky files. To retrieve original beamwidth dimensions, simply divide each value by 3.5. The fitted major axis position angle, measured in degrees East from North, is as provided in the 87GB catalog.
* A line providing the source's 4.85 GHz peak flux density and its uncertainty, both in mJy (milli-Janskys). This line has the the general form "FFF:UUU mJy", where FFF is the peak flux density and UUU is the uncertainty.
* On the same line, following the above information, may be a string of the general form "!CEW", containing up to three catalog flags associated with the source. These flags can include the following:
'C' for confusion
'E' for significant extension
'W' for warning