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1999MNRAS.315..839Minns& Riley
MNRAS 315 (4), 839-855
Compact radio sources variable at 151 MHz - I. Catalogue of sources
A. R. Minns and J. M. Riley
The Cambridge Low-Frequency Synthesis Telescope
has been used to produce a representative sample
of low-frequency variable sources. 20 fields, each
covering an area of approximately 9deg*9deg*cosec d,
have been observed at 151 MHz at between 2 and 10
epochs over the period from 1984 to 1996. At each
epoch, maps were made with rms noise levels of
typically 10-15 mJy beam-1. From a total of ~6000
sources detected on these maps, 207 are found for
which the flux density variations between at least
two epochs appear significant at greater than the
3s level. A numerical model is used to assess the
true significance of the variability, given the
analysis method adopted. This shows that for about
half of the sources which appear to vary by >3s
the variability is genuine. For the other half it
is caused by random statistical fluctuations; most
of the spurious variables vary by [Image]3.5s
between a single pair of epochs. A catalogue of
the variable sources is presented, which includes
an estimate of the probability that a given source
is a genuine variable. Fractional flux density
variations of between 5 and 100 per cent
(typically 15-25 per cent) have been detected on a
range of time-scales from 1 to 12 years.
2 Observations
The 20 7C fields used for variability studies are
listed in Table 1, which gives field centres and
observation epochs. Each field covers an area of
approximately 9deg*9deg*cosec(d). Fields were observed
when near to opposition to reduce the effects both
of the ionosphere and of the Sun. The exact dates
of observations differed from year to year, since
usable data could be obtained only under stable
ionospheric conditions. 7C surveys of some of
these fields have already been published by
McGilchrist et al. (1990),
Lacy et al. (1995) and
Pooley, Waldram & Riley (1998);
surveys of the remainder of the fields will be given in a
forthcoming paper.
5 The catalogue
The catalogue of 207 PLFV sources is presented in
Table 7. For each source, the CLFST field, radio
position, optical identification (where
available), variability characteristics, 151-MHz
flux density range, spectral indices and a
spectral classification are given. References for
positions and optical identifications are listed
in Tables 8 and 9, respectively.
The 151-MHz flux densities have been indirectly
scaled to the 6C flux scale. The required
scalefactors were determined from the final 7C
survey catalogue for each field using the method
described by Visser et al. (1995).
Where there was no published optical
identification, Digitized Sky Survey images were
examined at the radio positions. An identification
is reported if an optical object is seen with a
separation from the radio position less than or
equal to the error in the radio position or
5 arcsec, whichever is the larger.
As discussed in Section 2, the sensitivity of the
observations to flux density variations is a
function of the brightness of the source; for the
brightest sources (151-MHz flux density -1 Jy)
variations at the 5 per cent level can be
detected, whilst for the faintest sources the
limit is much higher than this. Significant
variations have been observed on a range of
time-scales from 1 to 12 years. Amongst the 72
sources for which the chance of being a true
variable is greater than 0.8, typical maximum flux
density variations of 15-25 per cent are seen;
eight objects show variations greater than 50 per
cent, with one source varying by more than 100 per
cent.
6 Conclusions
From a total of about 6000 sources mapped with the
CLFST at 151 MHz, 207 have been found for which
the flux density variations between at least two
epochs appear significant at greater than the 3sigma
level. Given the analysis method adopted, about
half of these sources are likely to be genuine
variables; a high proportion of these genuine
variables have varied by 3.5sigma over more than one
pair of epochs. Fractional flux density variations
of between 5 and 100 per cent (typically 15-25 per
cent) have been detected on a range of time-scales
between 1 and 12 years. The results will be
discussed in later papers.