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LORCAT - Low-Frequency Radio Catalog of Flat-Spectrum Sources



A well-known property of the gamma-ray sources detected by Cos-B in the 1970s, by the Compton Gamma-Ray Observatory in the 1990s, and recently by the Fermi Gamma-ray Observatory is the presence of radio counterparts, particularly for those associated with extragalactic objects. This observational evidence is the basis of the radio/gamma-ray connection established for the class of active galactic nuclei known as blazars. In particular, the main spectral property of the radio counterparts associated with gamma-ray blazars is that they show a flat spectrum in the GHz frequency range. The authors' recent analysis dedicated to search for blazar-like candidates as potential counterparts for the unidentified gamma-ray sources allowed them to extend the radio/gamma-ray connection in the MHz regime. They also showed that blazars below 1 GHz maintain flat radio spectra. Thus, on the basis of these new results, the authors have assembled a low-frequency radio catalog of flat-spectrum sources built by combining the radio observations of the Westerbork Northern Sky Survey (WENSS) and of the Westerbork in the southern hemisphere (WISH) catalogs with those of the NRAO Very Large Array Sky survey (NVSS). This catalog could be used in the future to search for new, unknown blazar-like counterparts of gamma-ray sources. First, the authors found NVSS counterparts of Westerbork Synthesis Radio Telescope (WSRT) radio sources, and then they selected flat-spectrum radio sources according to a new spectral criterion, specifically defined for radio observations performed below 1 GHz. In their paper, they also describe the main properties of the catalog listing 28,358 radio sources with spectral indices between 1400 and 325/352 MHz between -1.0 and +0.4, and their log N - log S distributions. Finally, a comparison with the Green Bank 6 cm radio source catalog was performed so as to investigate the spectral shape of the low-frequency flat-spectrum radio sources at higher frequencies.

Catalog Bibcode



The Low-frequency Radio Catalog of Flat-spectrum Sources.
     Massaro F., Giroletti M., D'Abrusco R., Masetti N., Paggi A.,
     Cowperthwaite Philip S., Tosti G., Funk S.
    <Astrophys. J. Suppl. 213, 3 (2014)


This table was created by the HEASARC in July 2014 based on a machine-readable version of Table 1 from the reference paper which was obtained from the ApJS web site.


The WSRT source identifier using the B1950.0 source coordinates and the 'WN' prefix for sources from the WENSS and the 'WISH' prefix for sources from that survey, e.g., 'WN2357.9+3900', 'WISH B2357.4-1408'. Both of these formats are the styles recommended by the CDS Dictionary of Nomenclature of Celestial Objects.

The NVSS source identifier using the J2000.0 source coordinates and the NVSS prefix, e.g., 'NVSS J000011+751002'.

The Right Ascension of the NVSS radio source in the selected equinox. This was given in J2000.0 sexagesimal coordinates to a precision of 0.01 seconds of time in the original table.

The Declination of the NVSS radio source in the selected equinox. This was given in J2000.0 sexagesimal coordinates to a precision of 0.1 arcseconds in the original table.

The Galactic Longitude of the NVSS radio source.

The Galactic Latitude of the NVSS radio source.

The angular separation between the WSRT and NVSS source positions, in arcseconds. The authors chose a maximum value of 95 arcseconds for the maximum angular separation between the WSRT and the NVSS positions in order to consider a 1.4-GHz radio source a reliable counterpart for a WSRT object.

The low-frequency radio spectral index, alphalow, defined as alphalow = -k1 * log (S_1400/Slow), where Slow is the integrated flux density at 325 MHz from the WENSS or the integrated flux density at 352 MHz as reported in the WISH, i.e., S325 or S352, respectively, S1400 is the integrated flux density of the corresponding NVSS source at 1.4 GHz, the k1 factor is equal to 1.58 and 1.67 (i.e., [log(1400/325)]-1 and [log(1400/352)]-1 ) for the WENSS and the WISH surveys, respectively, and all flux densities in units of mJy.

The uncertainty in the low-frequency radio spectral index, alphalow, calculated using equation (5) in the reference paper (q.v.).

The WSRT original survey name, either WENSS or WISH.

Contact Person

Questions regarding the LORCAT database table can be addressed to the HEASARC User Hotline.

Page Author: Browse Software Development Team
Last Modified: 16-Jul-2014