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TEXAS: Help

TEXAS - Texas Survey of Radio Sources at 365 MHz


Overview

This catalog is from the Texas Survey of discrete radio sources between -35.5 degrees and +71.5 degrees declination (B1950), which was carried out at a frequency of 365 MHz (82 cm wavelength) with the Texas Interferometer during the period from 1974 to 1983. The Survey lists accurate positions with internal errors of about an arcsecond, flux densities (which for strong point sources have internal errors of about 1% and total errors of about 5%), simple structure models and indications of spectrum and variability for 66841 sources. Results of comparisons with other data show that the Survey is 90% complete at 0.4 Jy and 80% complete at 0.25 Jy, is nearly free from spurious sources, and has a lobeshift incidence which is reasonably described by quality flags associated with each source.

The University of Texas Radio Astronomy Observatory (UTRAO) carried out, with the Texas Interferometer, this 365 MHz survey of the sky, which was intended to be complete to a flux density level of 0.25 Jy, to provide positions with an accuracy of about 1 arcsec in both coordinates, to give accurate flux densities and indication of source variability, and to give rough structure models for each source. The observations began in 1974 and were completed in 1983. A preliminary version of one declination strip was published (Douglas et al., Publ. Dept. Astron. Univ. Texas, No. 17, Oct. 1980), and a number of intermediate versions of the survey were privately circulated for various purposes, pending completion of the final analysis and adjustment of the data.


Catalog Bibcode

1996AJ....111.1945D

References

The Texas survey of radio sources covering -35.5{deg} < {delta} < 71.5{deg}
at 365MHz
     Douglas J.N., Bash F.N., Bozyan F.A., Torrence G.W., Wolfe C.
    <Astron. J. 111, 1945 (1996)>
    =1996AJ....111.1945D

Provenance

This database was created by the HEASARC in February 2001, based on CDS/ADC catalog VIII/42.

Parameters

Strip
A code letter corresponding to the declination strip from which the particular entry was taken. The survey observations were carried out in ten overlapping declination strips. Each strip was surveyed for four observing cycles, at two or three frequencies. The catalog was originally decomposed into 10 tables, (3a to 3j), on the AAS CD-Rom accompanying the published version of this catalog, corresponding to the 10 declination strips, and the code letters A to J correspond to these:

          Strip      Range of Declination

          A   -26     [-35.7,-18.7]
          B   -12     [-18.6,-06.4]
          C   -01     [-06.3,+03.9]
          D   +09     [+04.0,+13.5]
          E   +18     [+13.6,+22.6]
          F   +27     [+22.7,+31.6]
          G   +36     [+31.7,+40.6]
          H   +45     [+40.7,+49.9]
          I   +55     [+50.0,+59.8]
          J   +65     [+59.9,+71.5]
  

Simple_Flag
This is a flag that is set to 'Y' if simpler model(s) exist for a source. 14964 of the 66841 sources in this catalog have one or more simpler models available: these 14964 simpler models are not listed in this catalog but appeared in a separate tabulation, which can be examined at ftp://cdsarc.u-strasbg.fr/cats/VIII/42/simpler.dat.gz

Name
The Texas Catalog designation in the form recommended in the Dictionary of Nomenclature of Celestial Objects maintained at the CDS (http://vizier.u-strasbg.fr/cgi-bin/Dic). The name is constructed from the hours and minutes of right ascension, the sign of declination, and truncated tenths of degrees of declination: hhmmsddd. Suffixes 'A', 'B', etc., are also used in cases where close sources would otherwise have duplicate names, e.g., 'TXS 0001-234A' and 'TXS 0001-234B'. Because of a final position system adjustment, the source name may not agree with the position now listed in the table, but should be used regardless.

Xpvr_Flag
This flag is left blank if the fringe visibility of the adopted structure model is within the first minimum on all three subbeams. Such a model is in the Principal Value Region (PVR) of the catalog authors' structure-fitting algorithm. If the fringe visibility of the adopted structure model is near or beyond the first minimum in one or more subbeams, the value of this flag is set to the number of such subbeams, and thus may be 1, 2, or 3. Such models are called XPVR models, and although they are often perfectly accurate, they are more subject to structure lobeshift than are the PVR models.

Model_Flag
This flag describes the goodness of the model fit. Parameters for each entry in this catalog were calculated from 32 independent interferometer baselines, subject to the model listed, which was itself chosen to minimize residuals. If the residuals to the fit are consistent with the system errors, including both noise- and flux-dependent terms, this flag is assigned the value of '+'. For cases where the residuals are inconsistent with the expected errors, it is assigned the value of 'C' for Caution, for a marginal inconsistency, or 'W' for Warning, for a serious inconsistency. When the residuals are such that the model is clearly incorrect, it is set to 'N' for No model, although the least bad model has been used for the entry. Many sources with flag values of 'N' are, in fact, well-modeled, based on comparison to other work. In such cases, the large residuals are often the results of nearby sources or of interference which happened to increase the rms residuals greatly, while affecting the mean tabulated model quantities by much smaller amounts. But of course, many 'N' models are truly bad, and a few are attached to entries which do not represent sources at all, but which are spurious responses.

Environ_Flag
This flag is an estimate of the influence of the environment for a listed source. It is set to '+' if the sky surrounding the source (as represented in the Texas Survey itself) contains no other source which will contribute significantly to the fringe amplitudes at the position of the particular source. The contributions of other sources were calculated, and if their flux at the source position was sufficient to contribute significantly to the residuals in one or more subbeams, this flag was set to 'C' (Caution) or 'W' (Warning) if the contribution of the other source(s) were expected to dominate the residuals. Occasionally, the interfering sources have a higher calculated flux density at the source position than the catalog source itself, in which case the flag is set to 'X'. Some sources with flag values of 'X' survived to the final Texas Survey Catalog, when other tests suggested that the sources were indeed real.

Lobe_Flag
This lobeshift flag is an estimate of the reliability that the correct lobe was chosen. Each tentative source model was used to reduce the data on a grid of possible lobeshifted positions, and the best-fitting position was chosen.In many cases, this position was the only decent fit to the data, and in such cases the flag is set to '+'. If another lobe position is possible, the flag is set to 'C' (Caution), and if it produces only a slightly worse fit to the data than the most probable position, it is set to 'W' (Warning). In the event that the second most likely position is the only remaining lobe position that is at all likely, its offset from the best position is given in the parameters lobe2_offset_ra and lobe2_offset_dec.

Low_Vis_Flag
This is the low-visibility flag,indicating the number of subbeams on which the source model has fringe visibilities less then 0.2. Possible values are blank (for 0, presumably), 1, and 2: it is impossible for all three subbeams of those models attempted to have visibilities less than 0.2. Sources that are fit by models with essentially no flux on one or two subbeams may be questionable.

RA
The Right Ascension of the centroid of the listed structure model for the radio source, in the selected equinox. This was given to a precision of 0.001 seconds of time and in B1950 equatorial coordinates in the original catalog tables.

RA_Error
The error in the Right Ascension, in seconds of time. This is the standard deviation of the fitted RA, based on the observed residuals to the fitted model, or based on propagated noise, whichever is larger. It is thus purely an internal error, and does not reflect the error in adjustment of the survey position system to the position calibrators (typically 0.2" total), nor does it include estimates of other types of excess error, such as residual systematic error or the centroid variance found when comparing to catalogs made at other frequencies.

Dec
The Declination of the centroid of the listed structure model for the radio source, in the selected equinox. This was given to a precision of 0.01 arcseconds and in B1950 equatorial coordinates in the original catalog tables.

Dec_Error
The error in the Declination, in arcseconds. This is the standard deviation of the fitted Declination, based on the observed residuals to the fitted model, or based on propagated noise, whichever is larger. It is thus purely an internal error, and does not reflect the error in adjustment of the survey position system to the position calibrators (typically 0.2" total), nor does it include estimates of other types of excess error, such as residual systematic error or the centroid variance found when comparing to catalogs made at other frequencies.

LII
The Galactic Longitude of the radio source.

BII
The Galactic Latitude of the radio source.

Low_Flux_Flag
This low-flux flag is set to 'X' if the source flux density is less than 0.15 Jy (925 sources), and is left blank otherwise (65916 sources).

Flux_82_cm
The total 365 MHz (82 cm) flux density, in milliJanskies (mJy), of the source, given the listed model. A fitted spectral slope over the operating frequency range (335 - 380 MHz) has been removed. Note that the value of this flux will be a serious overestimate if a large 'D' or 'AD' model is erroneously used for a point source, and will be a serious underestimate if the reverse is true.

Flux_82_cm_Error
The internal standard deviation error in the the total 365 MHz (82 cm) flux density, in mJy, given the observed residuals, or the propagated noise, whichever is greater.

SNR_Flag
This is a Signal-to-Noise (SNR) flag that is set to 'Y' if 4 <= SNR < 5 (2475 sources) and to 'Z' if 5 <= SNR < 7 (8480 sources), and is left blank if SNR >=7 (55886 sources). All sources with SNR < 4 were discarded from this final catalog.

Spectral_Index
The source spectral index, gam, defined in the sense: log flux ~ gam * log nu, where nu is the frequency. Since the Texas Survey was carried out in at least two of the three frequencies of 335, 365, and 380 MHz, estimation of the flux density at the standard frequency of 365 MHz involved fitting a spectral index to the data. A number of sources have blanks in this field; they represent instances where the spectral index could not be determined or, in the case of sources from the TS6 strip, are sources whose spectral index error is greater than 0.8. For the Survey as a whole, useful spectral indications are given for 24558 of the 66841 sources.

Spectral_Index_Error
The error in the spectral index, being the standard deviation of the fit based on observed residuals. Since the Texas Survey was carried out in at least two of the three frequencies of 335, 365, and 380 MHz, estimation of the flux density at the standard frequency of 365 MHz involved fitting a spectral index to the data. Naturally, the error in the spectral index is high, given the small frequency range involved, but a useful indication of the spectrum is afforded in many cases. When this error is greater than 0.9, this field has been left blank, and the value of the spectral index (if present) should be treated with suspicion. If both the spectral index and spectral index error fields are left blank, this means that the spectral index could not be determined or, in the case of the 343 sources from the TS6 strip, that the source has a spectral index error that is greater than 0.8. For the Survey as a whole, useful spectral indications are given for 24558 of the 66841 sources.

Flux_Chi2_Res
This is a variability indicator, being the sum of the squared standardized residuals to the flux density of the modeled source on each of the four observing cycles. It has a Chi-Squared distribution with 2 degrees of freedom (2 were lost by fitting flux and spectrum). Values of this parameter > 5.99 would be expected for 5% of non-variable sources, > 9.21 for 1% of non-variable sources, and > 13.82 for 0.1% of non-variable sources. Since a spectral slope has been removed, a variable source may also show up by virtue of having a very odd spectral index, notice (see the var_flag parameter description).

Flux_Chi2_Res_Susp
This is a flag that is set to 'X' rather than left blank, if the value of the flux_chi2_res parameter is suspect (if the value of the model_flag parameter is not '+', or fewer than 6 baselines are present in any one observing cycle, or the normalized Chi-Squared residual for any one observing cycle was greater than twice the number of baselines present). If this flag is set to 'X', any variability indication in the var_flag parameter should be treated with due caution.

Var_Flag
This is a source variability flag, based both on the value of the flux_chi2_res parameter and on the values of the spectral index and its error. (Since a spectral slope has been removed, a variable source may show up by virtue of having a very odd spectral index). This flag is set to 'Y' if the flux_chi2_res value is greater than 9.21 (the 1% level), or if the spectral index gam is less than -1.5 - 2 * sigma(gam) or greater than 0.5 + 2 * sigma(gam), otherwise it is left blank. For the Survey as a whole, 49144 sources have variability parameters which are not suspect: 904 of these are marked as possibly variable (at the 1% level), and 17697 sources have supspect variability parameters, and 432 of these are marked as possibly variable.

Comp_Separation
The double component separation, in integer arcseconds, for sources fit using a symmetric Double model (model_type = 'D') or an Asymmetric Double model (model_type = 'AD').

Comp_Separation_Error
The error in the double component separation, in integer arcseconds, for sources fit using a symmetric Double model (model_type = 'D') or an Asymmetric Double model (model_type = 'AD').

Model_Type
The structure model adopted to fit the source: 'P' if a point source model was adopted, 'D' if a symmetric Double model was adopted, and 'AD' if an Asymmetric Double model was adopted.

Asymmetry
The asymmetry parameter q for sources fit using an Asymmetric Double model (model_type = 'AD'), in %.

Asymmetry_Error
The error in the asymmetry parameter q for sources fit using an Asymmetric Double model (model_type = 'AD'), in %.

Position_Angle
The Position Angle of the double components, in degrees and increasing eastwards from North, for sources fit using a symmetric Double model (model_type = 'D'), or an Asymmetric Double model (model_type = 'AD').

Position_Angle_Error
The error in the Position Angle of the double components, in degrees, for sources fit using a symmetric Double model (model_type = 'D'), or an Asymmetric Double model (model_type = 'AD').

RA_Overlap_Flag
The Right Ascension overlap zone flag. The Survey was reduced in ten declination strips, in which regions near the edges overlapped with adjacent strips, and each declination strip was reduced in RA segments, with intersegment boundaries also having overlap. Only one version of such overlap sources was chosen for inclusion in the catalog, and the ra_overlap_flag is set to 'Y' to denote a source is in such an RA overlap region, and is left blank otherwise.

Dec_Overlap_Flag
The Declination overlap zone flag. The Survey was reduced in ten declination strips, in which regions near the edges overlapped with adjacent strips, and each declination strip was reduced in RA segments, with intersegment boundaries also having overlap. Only one version of such overlap sources was chosen for inclusion in the catalog, and the dec_overlap_flag is set to 'Y' to denote a source is in such a Dec overlap region, and is left blank otherwise.

Lobe2_Offset_RA
The offset of the second most likely lobe in RA from the actual lobe choice, in multiples of the lobe interval lambda (defined in the published paper), for those cases where the lobe selection routines found that the second most likely lobe choice, although inferior to the most probable choice, was nonetheless far better than any of the other possibilities.

Lobe2_Offset_Dec
The offset of the second most likely lobe in Dec from the actual lobe choice, in multiples of the lobe interval lambda (defined in the published paper), for those cases where the lobe selection routines found that the second most likely lobe choice, although inferior to the most probable choice, was nonetheless far better than any of the other possibilities.

RA_Overlap_Zone
The particular Right Ascension overlap zone, with possible values from A to E, or blank if the source is not in an RA overlap zone.

Dec_Overlap_Zone
The particular Declination overlap zone, with possible values from A to K, or blank if the source is not in a Dec overlap zone.

Source_Strip
The observation strip and segment from which the source was taken. For example, looking at the entry for the source 'TXS 0000+178', the quoted value for source_strip of 181 means that the source came from the +18 degee observation strip, RA segment 1 (although since it is RA overlap zone A, this source might have come from RA segment 5). Furthermore, some sources in the +18 degree observation strip came from an entirely different reduction from the rest of the Survey (although using the same observational data and the same position and flux system): these are designated with source_strip = TS6, for Version 6 of the Texas Survey (the main Survey reduction was Version 7).


Contact Person

Questions regarding the TEXAS 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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