THE UNIVERSITY OF TEXAS PUBLICATIONS IN ASTRONOMY No. 17 (1980)

THE TEXAS SURVEY: THE PRELIMINARY +18^o STRIP

by J.N.Douglas, F.N. Bash, G.W. Torrence and C. Wolfe

• 1 is the source name in IAU format. A preceding asterisk means that the source is listed with a simpler model in Texasadd. A letter is appended where two sources have the same numeric name; letters are assigned in order of increasing RA. It is suggested that the letters TP ( for "Texas Preliminary") precede the name in external references to sources from this catalogue.
• 2 contains a three-character code: The FIRST character refers to the goodness of the structure model fit: + denotes acceptable, C means caution, W warning and N means that the model clearly does not correctly des- cribe the source ("no model"). This flag is based on the re- siduals of each baseline to the listed model, taking signal- to-noise ratio into account, and permitting small fractional variations which might be expected for a source substantial- ly but not perfectly modelled. The SECOND character is the source environment flag, and de- scribes the expected worst-case effect produced by nearby catalogued sources; +, C and W have the same qualitative significance as with the model flag; an X in this column in- dicates that in the worst case the listed source flux may be accounted for by sidelobes of nearby sources. Such sources are omitted from this catalogue unless the model flag is +, which is taken as evidance that the worst case did not occur and the source is real. The THIRD flag indicates the possibility that the listed po- sition is lobe-shifted, based on the contrast of the various lobe-selection parameters at the alternative lobe positions. The symbols +, C, W have the same qualitative significance as before.
• 3&4 contain the right ascension and declination of the CENTROID of the source and the rms errors in these quantitives, refer- red to the epoch 1950.0. The reference system of Wade & Johnston (1978, Astron.J., 82, 791) has been used.
• 5 is the flux density of the source as modelled and its error (in mJy, referred to 365 MHz) on the flux scale of B.Wills (1976, Astrophys. J., 180, 335).
• 6 lists the spectral index a (in the sense S~f**a) and its rms error whenever the error is less than or equal to 0.8; in other cases the entry is blank. Spectral indices are given for 1756 of the 7250 sources (dec=+18. strip) and refer only to the interval 335-380 MHz.
• 7 is the summed-squared standardized residual to the model flux flux and spectrum over the four observing cycles, and is thus an indicator of flux variability from one cycle to the next. This parameter has a chi-square distribution with two degrees of freedom; values greater than 9 (corresponding to P(chi**2 ) <= 0.01) are flagged with an asterisk to indicate possible source variability. An asterisk is also appended if the spectral index is significantly outside the range of normal source spectra, inasmuch as this can also indicate variability. The parameter is not printed unless all four observing cycle fluxes appear unaffected by problems, and have sufficient data. The parameter is printed for 5679 of the 7250 sources (dec=+18. strip), and an asterisk is assigned in 147 cases; about 100 of these are real variables. In the alternative model file this column is not recalculated and is blank.
• 8-10 describe the model adopted. A point model P is chosen unless a double D or an asymmetric double AD is clearly better. Column 8 has the separation of the components of a double in arc-seconds (and its error). The double models assume that the components are gaussian with full width to half-maximum equal to 0.19 times the separation; the AD model further assumes that one component is stronger than the other. The number appended to the AD code is 100*(S_a-S_b)/(S_a+S_b) and its error.Column 10 has the position angle in degrees (and its error).
• 11 contains the (k_alpha,k_delta) offset to the second most likely position lobe choice (see equations 2 below). This offset is most likely lobe is clearly better than the remaining seven. printed when there is doubt about the lobe choice AND when the second. Thus, (+1,+1) means the second most likely lobe is 51.8" LATER in RA, and 51.8"*sec(z) HIGHER in DEC than the position listed.

             B_x' = B_x + k_x*51.8"
                                                                    (1)
             B_y' = B_y + k_y*51.8"*sec(z) 

 
         RA'*cos(DEC) =  RA*cos(DEC) + k_alpha*51.8" - k_x*25.9"
                                                                    (2)
         DEC' =  DEC  + k_delta*51.8"sec(z)  - k_y*25.9"*sec(z)

SAMPLE LINE (note the toggling of lines at character 81):
          1         2         3         4         5         6         7         8         9         0         1         2         3         4
0123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234
*IAU name* Flags  Right Ascension       Declination  rms      Flux   rms  spec. index var. separation"model  pos.angle    Ka,Kd     l2       b2  
 0801-310   C+W    8  1 47.162  .211   -31  4 31.36  2.07      426    57    *.*  .*   *.*     28( 7)    D      105(20)   (+1,-1)  248.545  -0.005 
            |||                                                                       *-variable        |                  | 
            |||                                                                                        D double           RA+51.8"
             +  acceptable                                                                            AD asym.double 
            C   caution
              W warning
            N   no model