The 7C(G) survey of radio sources at 151 MHz the Galactic plane at

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1 Mon. Not. R. Astron. Soc. 294, (1998) The 7C(G) survey of radio sources at 151 MHz the Galactic plane at 80 < l < 104 and 126 < l < 180, forjbj 5 :5 S. J. Vessey* and D. A. Green Mullard Radio Astronomy Observatory, Cavendish Laboratory, Madingley Road, Cambridge CB3 0HE Accepted 1997 September 10. Received 1997 June 30; in original form 1997 April 14 ABSTRACT Results from a survey of the northern Galactic plane (at declination 30 ) at 151 MHz made with the Cambridge Low Frequency Synthesis Telescope are presented. This survey is designated the 7C(G) i.e. the Galactic portion of the ongoing 7C surveys. This covers the regions 80 < l < 104 and 126 < l < 180, for jbj 5 : 5, and has some coverage to jbj 9, with a resolution of cosecðdþ arcsec 2 (RA Dec.). The observations, data reduction and calibration of this survey are described, and a catalogue of 6262 compact sources, with a completeness limit of 0:25 Jy over most of the survey region, is presented. The catalogue has an rms positional accuracy of better than 10 arcsec, and the flux densities are tied to the scale of Roger, Bridle & Costain with an accuracy of better than 10 per cent. Key words: catalogues surveys radio continuum: general. 1 INTRODUCTION The Cambridge Low Frequency Synthesis Telescope (CLFST) has been used to make a survey of much of the northern Galactic plane. This survey was made at 151 MHz, with a resolution of cosecðdþ arcsec 2 (RA Dec.). There is complete coverage over the Galactic longitude ranges 80 < l < 104 (at declination 35 ) and 126 < l < 180 (at declination 25 ), for jbj 5 : 5, and incomplete coverage to jbj 9. The average completeness limit of the compact source list is 0:25 Jy over most of the region observed, and the average source density is 4:8 deg ¹2, although in many regions it is > 6 deg ¹2. A catalogue of 6262 compact sources (i.e. those less than a few beamwidths in diameter) is presented here. This catalogue has an rms positional accuracy of 5 arcsec in right ascension, and 7 arcsec in declination, from comparison with the Texas 365-MHz survey (Douglas et al. 1973, 1980, 1996), and the flux densities are on the scale of Roger, Bridle & Costain (1973) to within 4 10 per cent. The observations are described in Section 2, with the positional and flux density calibrations discussed in Sections 3 and 4 respectively. Appendix A gives the log of the observations made for the survey, Appendix B discusses lobe-shift statistics for the Texas survey, and Appendix C presents the secondary flux density calibrators used for the survey. The compact source list is described in Section 5, and a sample from the full source list is given in Appendix D. *Present address: Corporate Value Associates, 70 Conduit Street, London W1R 9FD. Author for offprint requests. 2 OBSERVATIONS AND DATA REDUCTION The CLFST is an approximately east west Earth rotation aperture synthesis interferometer (see McGilchrist 1988; Rees 1989), which can operate at 38 or 151 MHz. The parameters of the telescope relevant to the Galactic plane survey observations, at 151 MHz, are given in Table 1. Twenty-four fields, covering 1700 square degrees, were observed during Some fields, for which the data were of poor quality, were re-observed in The Galactic coordinates of the field centres, together with their observation dates, are given in Appendix A. Fig. 1 shows the coverage of these fields. The region covered by the survey is from l 80 to l 180, but with a gap near the very bright source Cas A, which lies near l ¼ 111 : 7, b ¼¹2 :1. The CLFST has been used for many 7C surveys (McGilchrist et al. 1990; Lacy et al. 1995; Visser et al. 1995; Waldram et al. 1996; Pooley, Waldram & Riley 1998) of compact, extragalactic radio sources, in fields well way from the Galactic plane. The present survey, in the Galactic plane, is designated the 7C(G) survey. As the details of the standard data reduction procedures used for the CLFST are described in detail elsewhere (McGilchrist 1988; Rees 1989, 1990; Waldram & McGilchrist 1990; Waldram & Riley 1993), only an outline of the various steps in the data reduction process is given here, with emphasis on the problems associated with observations in the Galactic plane, as compared with the more usual observations made away from the plane. Full details of the data reduction are given in Vessey (1995). (i) Interference was removed both automatically during observations, and during the subsequent processing by the application of amplitude-based gates on the observed visibilities. (In some cases visibilities corresponding to a fictitious source at the North Pole 1998 RAS

2 608 S. J. Vessey and D. A. Green Table 1. The Cambridge Low Frequency Synthesis Telescope. observing frequency 151 MHz bandwidth 700 khz antenna 4 10-element Yagis primary beam (FWHM) 17 mount equatorial number of antennas 60 baseline interval 3l ( 6m) maximum baseline 779 ð3lþ ð 4:6kmÞ number of baselines 776 missing baselines ð0; 1; 3 and 778Þ ð3lþ synthesized beamwidth cosecðdþ arcsec 2 grating response radii cosecðdþ deg 2 were subtracted from the data, after the removal of bright sources see (iv) below to remove the effects of persistent low-level interference.) (ii) Correction for ionospheric effects was made by identifying bright, compact, isolated sources in each field to measure the phase gradient along the telescope. This observed phase gradient, averaged by typically a few minutes, was removed from the observed visibilities. (iii) The antenna-based amplitude and phase calibration of the CLFST was made regularly by observations of standard bright compact sources (3C 48, 3C 147 and 3C 295). In a few survey fields, further antenna-based phase calibrations using a bright, compact source within the field were applied. (iv) Many of the fields in the survey are close to the bright sources Cyg A, Cas A and Tau A (the Crab nebula), which are in the Galactic plane near l ¼ 78, 112 and 185 respectively. The effects of sidelobes and grating responses from these sources were greatly reduced by removing the averaged contribution to the observed visibilities from these sources on a baseline-by-baseline basis. Fig. 2 illustrates the effect of the removal of a bright source, Cyg A, near the field of observations. The proximity of one or other of the bright sources limits the quality of some fields, and also accounts for the gap in the Galactic longitude coverage of the survey near Cas A. In some cases other bright 3C sources in the field were also removed, to enable the parameters of compact sources nearby to be determined. (v) Maps were made both at the maximum resolution of the CLFST, cosecðdþ arcsec 2, and at a lower resolution of 4 4 cosecðdþ arcmin 2, to match the 6C survey (e.g. Baldwin et al. 1985; Hales et al and references therein). Where more than one observation was usable for a field, a weighted average of the individual maps was constructed to minimize the noise in the final map. At the higher resolution, in order to minimize synthesized beam distortions, four maps pixel in size were made, on a 2 2 grid with an overlap of 72 pixels between adjacent maps for the same field centre. A single pixel low-resolution map was sufficient to cover the useful field of view of the CLFST. (vi) The source fitting algorithm used to analyse the highresolution CLFST maps for sources was the beamset method of Waldram & McGilchrist (1990) and Waldram & Riley (1993). This method overcomes the distortions in the synthesized beam across the image by using a grid of theoretical synthesized beams. Synthesized beams were fitted to (interpolated) peaks at least 5 times the local noise level that lay more than 16 pixel from the map edges. Two flux densities were determined for each source: (1) the beam-fitted flux density, from the appropriate theoretical beam that best fits the observed source, and (2) the integrated flux density including map pixels down to 2.5 times the local noise level. In some cases the integration procedure includes several nearby local maxima. 3 POSITIONAL CALIBRATION Apart from random statistical errors, the major source of positional Figure 1. A plot showing, as shaded regions, the sky coverage of 24 fields of the 7C(G) survey. The radial lines are at constant right ascension, every hour, and the circles at constant declination, every 10. The Galactic plane at b ¼ 0 and at b ¼ 10 is also indicated.

3 The 7C(G) survey 609 Figure 2. Part of the field centred at l ¼ 83 : 5 before (top) and after (bottom) the removal of Cyg A, which lies 10 to the west. The grey-scale, white to black, corresponds to ¹0:1 to0:3 Jy beam ¹1.

4 610 S. J. Vessey and D. A. Green Figure 3. Positional offsets in right ascension (DRA) and declination (DDec.) for 2149 CLFST sources with a Texas survey counterpart within 40 arcsec. error in this 151-MHz survey is the uncertainty in the assumed positions of the ionospheric calibrators. These and any unknown source of systematic positional errors (e.g. map scale errors, see McGilchrist et al.) were removed by comparison with the Texas 365-MHz survey (Douglas et al. 1973, 1980, 1996). [A prepublication version of the Douglas et al. (1996) catalogue was used, but subsequent comparison with the published version shows that the differences in position are less than 1 arcsec.] The Texas survey covers the entire CLFST survey region, contains many sources, and has a high resolution (10 arcsec), with positional errors ignoring lobe-shift uncertainties of better than an arcsecond. As the Texas survey is only sensitive to sources 1 arcmin in extent, a source that appears extended at CLFST resolution might be detected as two separate sources in the Texas survey. Therefore to minimize the possibility of confusing associations between the surveys, only sources that are unresolved in the CLFST survey were used in the positional calibration comparisons. The ratio of 151-MHz integrated to beam-fitted flux is a measure of source extension (Waldram & Riley 1993), so for these purposes a source was considered to be unresolved if this ratio was 1:3. Comparison of unresolved source positions between the surveys revealed offsets (DRA and DDec. in right ascension and declination respectively) of typically less than 10 arcsec, although in one case an offset of 2 arcmin was found, owing to a poor position being used for the ionospheric calibrator. After subtracting the simple offsets, further correlations were found between: (1) DRA and right ascension, DDec. and declination, and (2) DRA and declination, DDec. and right ascension. Where present, these correlations were also removed from the source lists, on a map-by-map basis. These effects are not limited to this Galactic survey, and have been noted on previous occasions in 7C observations of extragalactic fields made with the CLFST (e.g. Visser et al. 1995; Pooley et al. 1998). The magnitudes of these effects were typically less than 10 arcsec at the edge of a field. (Calibration of images was accomplished by adjusting the header entries giving the reference coordinate of each map. Because the sky is not planar, this is not exactly equivalent to the corrections applied to the source lists, but in practice the differences are small compared with the final uncertainties. For the field with the largest offset, the differences in the mean source positions are only a few arcsec, which is small compared with the resolution of the observations.) Fig. 3 shows a plot of the positional offsets (DRA and DDec.) of the final, calibrated CLFST source positions relative to the Texas survey, for 2149 CLFST sources with a Texas counterpart within 40 arcsec in both right ascension and declination. Only CLFST sources within 40 arcsec of Texas sources in each coordinate have been used, because of the presence of lobe-shifted sources within the Texas survey. The agreement with the Texas positions is excellent, with rms deviations over the entire 151-MHz survey of only 5.2 arcsec in right ascension and 6.8 arcsec in declination. The rms deviations are smaller for the brighter sources, as expected: e.g. for CLFST sources with a signal-to-noise ratio of better than 50, the rms deviations are 3.4 arcsec in RA and 3.6 arcsec in declination. The poorer accuracy in declination is expected, since the resolution of the CLFST in right ascension is higher. The statistics of lobeshifted sources in the Texas survey from a comparison of 151-MHz sources associated with Texas sources with offsets of up to 240 arcsec is given in Appendix B. The accuracy of corrected positions in the CLFST catalogue was also checked by comparisons with a number of other surveys, namely the 408-MHz Bologna B3 survey (Ficarra, Grueff & Tomassetti 1985), the 2.7-GHz Effelsberg survey (Reich et al. 1984, 1990; Fürst et al. 1990a,b) and the 4.85-GHz 87GB survey (Gregory & Condon 1991). These comparisons showed good statistical agreement, within a few arcsec, apart from a significant systematic offset in declination between the CLFST survey and the 2.7-GHz Effelsberg survey. The declinations of sources in the 2.7- GHz Effelsberg catalogue are lower than the CLFST declinations by 17 arcsec on average. This is in agreement with Fürst et al. (1990b), who found a systematic error in declination of about 20 arcsec, in the same sense, relative to fitted source positions from the 4.85-GHz Condon, Broderick & Seielstad (1989) survey images, over the range 100 < l < FLUX DENSITY CALIBRATION The primary beam of the CLFST is 17 full width to halfmaximum, and so its antenna temperature is dominated by largescale background emission (the receiver temperature is 200 K, whereas the average Galactic background in the CLFST primary beam varies from 450 to 850 K for this survey: see Landecker & Wielebinski 1970). Because the CLFST has an automatic gain control, these variations in antenna temperature produce variations in the sensitivity of the telescope, and these have to be determined, on a field-by-field basis, to calibrate the flux density scale of the survey. Three steps were required to carry out this calibration. (i) The expected 151-MHz flux densities from several secondary calibrators bright (3C) sources in the survey fields were determined from their spectra. Details of the 3C sources used as secondary calibrators are given in Appendix C. The calculated flux densities, on the scale of Roger et al. (1973), have accuracies of between 3.3 and 5.6 per cent. Comparison of these flux densities with those observed gives the flux scaling factors for the nine fields containing these secondary calibrators.

5 The 7C(G) survey 611 (ii) Internal comparison of the observed flux densities of sources that are in the overlap region of adjacent fields (which typically contain about 20 sources) then allowed the flux scaling factors to be calculated for all but two fields not containing secondary calibrators. (iii) For the end fields of the survey at l ¼ 080 : 0 and l ¼ 178 : 0 there was an insufficient number of sources in the overlap regions to allow the flux scaling factor to be calculated with any degree of accuracy. For these two fields the flux scaling factors were estimated from values from the adjacent fields, using an extrapolation that varies smoothly with the estimated antenna temperature variation in l. In practice two flux density scaling factors are required for each field. This is because residual ionospheric smearing depresses the apparent flux density peak of a compact source, without decreasing its integrated flux density. (This also means that the depression of the peak flux is worse for sources far from the ionospheric calibrators. However, this effect is small compared with other uncertainites in the flux densities.) Thus slightly larger flux density scaling factors are required for beam-fitted than for integrated flux densities, and these then correct for residual ionospheric smearing. As most compact sources will be unresolved, the integrated flux density scale factors can be calculated by demanding that the median integrated to peak fitted flux density ratio for sources in each field is unity. The beam-fitted scaling factors vary from 1.45 for the l ¼ 157 : 0 field, which has low background emission, to 2.73 for the l ¼ 080 : 0 field, which has high background emission. Because of this variation, the sensitivity is much poorer near l ¼ 80. The integrated flux density scaling factors are slightly lower (typically by about 10 per cent) than the beam-fitted scaling factors. The accuracy of the beam-fitted flux density scaling factors, from the uncertainties in flux densities of the secondary calibrators and the cumulative uncertainties in the extrapolation process, is generally better than 8 per cent (except for the two end fields, for which it is estimated to be 10 per cent). For unresolved sources the random error in the beam-fitted flux density is lower than that in the integrated flux density (Waldram & McGilchrist 1990; Waldram & Riley 1993). Thus the accuracy of the integrated flux density scaling factors is slightly worse than for the beam flux density factors (it is generally better than 10 per cent or about 13 per cent for the two end fields). The l ¼ 153 : 5 field has an anomalously large beam-fitted flux scaling factor in relation to neighbouring fields, whereas the corresponding integrated flux factor is much more consistent with nearby values. The explanation for this is simply a rather large degree of residual ionospheric smearing in this field. An additional uncertainty in the flux density scale is that arising from possible systematic offsets in the pointing of the CLFST antennas during an observations. Several of the fields near l ¼ 140 overlap part of the 6C survey (Hales et al. 1993). Comparison with the 6C sources implies small systematic offsets of less than 0 : 5, which imply systematic flux scale errors across a field of less than 3 per cent. 5 THE FINAL COMPACT SOURCE LIST After correcting the positions, and calibrating the flux densities, as described above, the 96 individual compact source lists (four per field), containing 8192 sources, were combined into a single catalogue, by eliminating duplicate sources, and removing fictitious or erroneous entries. The sources found by the fitting routine routine are compact, being less than a few beamwidths in diameter. Two sources were considered to be duplicate entries for the same source if they were within 1 arcmin of each other, and they were from different maps. Once duplicates were identified, their entries were replaced by a single entry with weighted averages of their positions and flux densities duplicate entries were found, leaving 6402 entries in the source list. Multiple peak sources with the same number of peaks in each field were inspected visually, and their components averaged manually. Multiple peak sources with a different number of peaks in each field were left in the catalogue (see below). In the original source list there were 316 entries representing components of 152 multiple peak sources. After averaging coordinates and fluxes of the individual components as described above, there remain 282 entries representing components of 135 sources. The total number of duplicate entries eliminated is therefore 1824, leaving 6368 entries in total. In other words, about 29 per cent of sources appear in more than one survey field. 67 sources out of the remaining 6368 were found to have a size estimate of zero, indicating integration of a residual sidelobe artefact, or an unusually high peak in the noise, and these were discarded after a visual inspection. There were a further 36 sources with a negative beam flux, which is produced by fitting to peaks in sidelobes very close to bright sources, and these sources were also discarded. In a few cases this meant that multiple peak sources which previously had a different number of peaks in each field now had the same number of peaks, and their components were averaged manually. The result was a final list of 6262 sources. This catalogue is designated 7C(G), for the 7th Cambridge survey of radio sources (Galactic plane region). Entries in the source catalogue contain only the information below. Further details about these parameters are given by Waldram & Riley (1993). (i) The right ascension and declination of the beam-fitted position of the source at epoch B1950.0; the rms accuracies are 5 and 7 arcsec respectively. (ii) The right ascension and declination of the centroid position of the source at epoch B Multiple peaks integrated together have the same centroid positions. (iii) The Galactic longitude and latitude of the beam-fitted position of the source. (iv) The beam-fitted (S bf ) and integrated (S in ) flux densities, in Jy. The beam-fitted value is the preferred flux density unless the source is clearly extended, in which case the integrated flux density should be used. (v) The signal-to-noise ratio (S/N) as defined by Waldram & Riley, i.e. the interpolated peak height of the source on the map (after subtraction of the local zero level), divided by the local noise. (vi) The extent of the source size in units of the local beam area. This estimate is influenced by residual ionospheric smearing of the synthesized beam. Another measure of whether a source is resolved or not is the ratio S in =S bf. The flux density calibration is performed in such a way as to ensure that the median value of this ratio for unresolved sources is unity. Thus any source with this ratio q1 is resolved, and for this survey a ratio of S in =S bf 1:2 is a good indication that the source is resolved. (vii) The number of individual peaks that were included in the flux integration, together with a single character flag indicating whether the source is the main ( M ) or a component ( C ) peak in the integration. If no number and flag are given then the source has a

6 612 S. J. Vessey and D. A. Green Figure 4. Galactic survey latitude and longitude coverage, each dot indicating a source in the survey. single peak only. For multiple sources each peak has a separate entry in the source list, with its own beam-fitted flux density and position, although the integrated flux and the centroid position are the same for each. As noted above, some multiple peaked sources appear with different numbers of components in overlapping fields. These entries have not been averaged together, and care should be taken with such entries. The coverage of this survey is shown in Fig. 4. About one-quarter of the Galactic plane is covered in l, although the presence of Cas A near l ¼ 112 and the consequent difficulty of observing there result in a gap in coverage that splits the survey into two parts. The average source density over the whole survey is 4:8 deg ¹2 ; however, if the three lowest longitude fields are not included in this average, the mean source density rises to 5:3 deg ¹2, and many regions have a source density of > 6 deg ¹2. This source density is significantly larger than in the Texas, Effelsberg and 87GB surveys, which have source densities (sampled over a similar region of sky to that observed at 151 MHz) of 2.2, 2.4 and 3:0 deg ¹2 respectively. There is complete area coverage of the Galactic plane over the latitude range jbj 5 : 5, for 80 < l < 104 and 126 < l < 180, and incomplete coverage (see Fig. 4) to jbj 9. The faintest sources detected in each field of the survey are at approximately the average 5j noise level of that field, since the source fitting algorithm fits interpolated map peaks down to this Table 2. 5j noise levels for the Galactic survey fields. l= 5j limit/jy l= 5j limit/jy l= 5j limit/jy level. These noise levels are given in Table 2. The overall average 5j limit of the survey, not including the three lowest l fields (which have significantly higher noise levels than the rest of the survey), is 0:20 Jy. From source counts, the completeness limit of the survey is estimated to be 25 per cent higher than these values, so the average completeness limit for most of the survey is 0:25 Jy. 6 CONCLUSIONS The 7C(G) catalogue contains details of 6262 compact radio sources in the Galactic plane, detected by the CLFST at 151 MHz. The source density is significantly larger than other, large-scale,

7 The 7C(G) survey 613 surveys such as the 365-MHz Texas, 2.7-GHz Effelsberg and GHz 87GB surveys. The properties of survey are as follows. (i) Coverage: 80 < l < 104 and 126 < l < 180, for jbj 5 : 5 (with partial coverage out to jbj 9 ). (ii) Resolution: cosecðdþ arcsec 2. (iii) Positional accuracy: 5 arcsec rms in right ascension and 7 arcsec rms in declination, relative to the Texas survey. (iv) Flux densities: on the scale of Roger et al. (1973) to within 4 10 per cent. (v) Mean completeness limit: 0:25 Jy, not including the three lowest longitude fields. The catalogue and the other products from the survey the 96 high-resolution [70 70 cosecðdþ arcsec 2 ] and 24 low-resolution [4 4 cosecðdþ arcmin 2 ] maps will be available on the MRAO World Wide Web server ( ACKNOWLEDGMENTS We thank our present and former colleagues at MRAO for their help in maintaining and operating the CLFST and its associated data analysis programs. SJV thanks the UK Particle Physics and Astronomy Research Council for financial support. REFERENCES Baars J. W. M., Genzel R., Pauliny-Toth I. I. K., Witzel A., 1977, A&A, 61, 99 Baldwin J. E., Boysen R. C., Hales S. E. G., Jennings J. E., Waggett P. C., Warner P. J., Wilson D. M. A., 1985, MNRAS, 217, 717 Condon J. J., Broderick J. J., Seielstad G. A., 1989, AJ, 97, 1064 Douglas J. N., Bash F. N., Ghigo F. D., Moseley G. F., Torrence G. W., 1973, AJ, 78, 1 Douglas J. N., Bash F. N., Torrence G. W., Wolfe C., 1980, Univ. Texas Publ. Astron. No. 17 Douglas J. N., Bash F. N., Bozyan F. A., Torrence G. W., Wolfe C., 1996, AJ, 111, 1945 Ficarra A., Grueff G., Tomassetti G., 1985, A&AS, 59, 255 Fürst E., Reich W., Reich P., Reif K., 1990a, A&AS, 85, 691 Fürst E., Reich W., Reich P., Reif K., 1990b, A&AS, 85, 805 Gregory P. C., Condon J. J., 1991, ApJS, 75, 1011 Hales S. E. G., Masson C. R., Warner P. J., Baldwin J. E., Green D.A., 1993, MNRAS, 262, 1057 Lacy M., Riley J. M., Waldram E. M., McMahon R. S., Warner P. J., 1995, MNRAS, 276, 614 Laing R. A., Peacock J. A., 1980, MNRAS, 190, 903 Landecker T. L., Wielebinski R., 1970, Aust. J. Phys., Astrophys. Suppl. No. 16, 1 McGilchrist M. M., 1988, PhD thesis, University of Cambridge McGilchrist M. M., Baldwin J. E., Riley J. M., Titterington D. J., Waldram E. M., Warner P. J., 1990, MNRAS, 246, 110 Pooley D. M., Waldram E. M., Riley J. M., 1998, MNRAS, submitted Rees N., 1989, PhD thesis, University of Cambridge Rees N., 1990, MNRAS, 243, 637 Reich W., Fürst E., Steffen P., Reif K., Haslam C. G. T., 1984, A&AS, 58, 197 Reich W., Fürst E., Reich P., Reif K., 1990, A&AS, 85, 633 Roger R. S., Bridle A. H., Costain C. H., 1973, AJ, 78, 1030 Vessey S. J., 1995, PhD thesis, University of Cambridge Visser A. E., Riley J. M., Röttgering H. J. A., Waldram E. M., 1995, A&AS, 110, 419 Waldram E. M., McGilchrist M. M., 1990, MNRAS, 245, 532 Waldram E. M., Riley J. M., 1993, MNRAS, 265, 853 Waldram E. M., Yates J. A., Riley J. M., Warner P. J., 1996, MNRAS, 282, 779 (Erratum: 1997, MNRAS, 284, 1007) Table A1. Observation dates of the 24 Galactic survey fields. field centre l= b= observation date(s) 080:0 þ3 93 Sep Oct Oct :5 ¹3 91 Oct Oct Sep :0 þ3 91 Aug Oct Nov Sep :5 ¹3 91 Oct Sep :0 þ3 91 Aug Sep :5 ¹3 91 Aug Sep Oct Oct Sep :0 þ3 91 Sep Sep Sep Oct Nov Oct :5 ¹3 91 Sep Oct Oct Oct Oct :5 ¹3 90 Dec Oct :0 þ3 90 Nov Nov :5 ¹3 90 Dec Dec Nov :0 þ3 90 Dec Dec Nov :5 ¹3 90 Dec Dec Dec Jan Nov :0 þ3 90 Dec Nov :5 ¹3 93 Dec Dec :0 þ3 93 Nov Nov :5 ¹3 90 Dec Jan :0 þ3 90 Dec Jan :5 ¹3 90 Dec :0 þ3 94 Sep Sep Sep :5 ¹3 91 Jan Jan :0 þ3 90 Dec :5 ¹3 90 Dec Mar Mar :0 þ3 90 Dec Jan 02 APPENDIX A: LOG OF OBSERVATIONS Table A1 gives the log of the observations in this survey. APPENDIX B: TEXAS SURVEY LOBE-SHIFTS The Texas survey is known to suffer from a problem with lobeshifting (Douglas et al. 1973, 1980, 1996), with offsets of 51:8 m arcsec in right ascension and 51:8 secðzþ n arcsec in declination (where m and n are integers such that m þ n is even, and z is the zenith distance for the Texas interferometer). As the rms errors of the calibrated CLFST list are much smaller than the lobe-shift offsets, comparison of the CLFST and Texas source positions allows an assessment of the statistical likelihood of lobe-shifts in the Texas survey. Fig. B1 shows the right ascension and declination offsets (DRA and DDec.) for 2448 Texas survey sources that lie within 240 arcsec of a CLFST source. Six clusters arising from lobe-shifted sources can clearly be seen above a weak background of (presumably) chance associations. These clusters correspond to ðm; nþ values of ð 1; 1Þ, ð 1; 1Þ and ð 2; 0Þ, with possibly a seventh towards the top of the diagram [the ð0; ¹2Þ lobe-shift]. The fraction of lobe-shifted sources calculated by counting the number of points in each of the boxes in Fig. B1, assuming that all associations with non-lobe-shifted Texas sources lie within the central box (2098 matches), while those in the others represent only associations with lobe-shifted sources (a total of 225 matches), is 9.5 per cent. This is in agreement with the results given by Douglas et al. (1996), based on a comparison of the Texas sources with sources in the 408-MHz Molonglo catalogue and the 87GB 4.85-GHz catalogue.

8 614 S. J. Vessey and D. A. Green Table C1. Estimated 151-MHz flux densities of the secondary flux density calibrators. source flux density/jy uncertainty/jy 3C C C C C C Figure B1. Positional offsets in right ascension (DRA) and declination (DDec.) for 2448 sources in the CLFST survey that have a Texas survey counterpart within 240 arcsec. APPENDIX C: SECONDARY FLUX DENSITY CALIBRATORS Six 3C sources within the survey region were selected as secondary calibrators: 3C 69, 91, 125, 141, 428 and 431. These are compact sources, unresolved in the CLFST survey, are away from confusing sources, and are sufficiently well studied that accurate flux density measurements at a variety of frequencies, both above and below 151 MHz, are available in the literature. Flux densities at frequencies below 400 MHz were placed on the Roger et al. (1973) scale (using scaling factors given by Laing & Peacock 1980), and those at frequencies above 400 MHz were placed on the scale of Baars et al. (1977). Some obviously discordant observations were discarded (see Vessey 1995 for details). Accurate spectra for these calibrators were constructed, from which their 151-MHz flux densities, which are given in Table C1, were estimated. APPENDIX D: A SAMPLE OF THE CATALOGUE Table D1. A sample from the 7C(G) catalogue. This paper has been typeset from a T E X=L A T E X file prepared by the author.