Документ взят из кэша поисковой машины. Адрес
оригинального документа
: http://www.sao.ru/cats/doc/Texas_old.html
Дата изменения: Wed Mar 18 12:25:11 2015
Дата индексирования: Sun Apr 10 02:57:31 2016
Кодировка:
Поисковые слова: stonehenge
|
[ CATS home ] [ Back to CATS list ]
[ ftp ]
TEXAS SURVEY: THE PRELIMINARY
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
The Texas Survey of discrete radio sources north of DEC> -35 and stronger
than 0.25 Jy is intended to provide accurate (one arc-second rms error)
positions, flux densities and structure models for each of the ~70000
sources expected to be listed. The preliminary +18 deg strip covers the
range of declination 13d35' to 22d42' (1975.4) and lists 7250 sources in
0.949 steradians to a lower limiting flux density of 0.15 Jy and is
usefully complete to 0.25 Jy. Average rms position accuracy for point
sources is 1" in RA and 0.5" in DEC.
Column
- 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.
Alternative position lobe coordinates and structure lobe separations and
possitions angles cannot be precisely calculated without re-analysis,
since weighted means of many baselines are involved. However, an appoxi-
mate algorithm may be used: suppose the catalogue position is RA,DEC and
the listed double model has cartesian component separation B_x=R*sin(chi)
in RA, B_y=R*cos(chi) in DEC. Then, alternative structures :
B_x' = B_x + k_x*51.8"
(1)
B_y' = B_y + k_y*51.8"*sec(z)
where k_x and k_y are integers whose sum k_x + k_y is even. For the structure
lobe (k_x,k_y), the alternative position lobes RA',DEC' are given by:
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)
where k_alpha and k_delta are integers whose sum k_alpha + k_delta is even.
In both algorithms z is the zenith distance of the source at the UTRAO
latitude: 30d06'35" North.
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