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The Cambridge IPS survey at 81.5 MHz

Purvis, A., Tappin, S. J., Rees, W. G., Hewish, A., Duffett-Smith, P. J.

Monthly Notices RAS, vol. 229, Dec. 15, 1987, p. 589-619.


A catalog of 1789 radio sources which exhibit interplanetary scintillation (IPS) at 81.5 MHz is presented. The angular diameters of scintillating components in the range 0.2-2 arcsec are listed together with values of the scintillating flux density at a solar elongation of 90 deg. IPS selects those sources which are highly compact, such as pulsars and some unusual extragalactic sources, or those in which energy is being released from active beams in the outer lobes of intrinsically powerful radio galaxies and quasars.

1. Introduction.

We present the results of a survey of radio sources which exhibit Interplanetary scintillation (IPS) at 81.5 MHz. The survey was made with the 3.6-hectare array at Cambridge and covers the area of sky between declinations -10 deg and +83 deg at all values of right ascension. The faintest sources in the catalogue have scintillating flux densities of about 0.3 Jy rms at a solar elongation of 90 , and total flux densities of about 5 Jy at 81.5 MHz. The sensitivity of the survey is not uniform over the sky, being determined largely by the galactic background emission. Data were collected continuously during the periods 1978 August to 1979 September, and 1980 January to 1981 March, so that the records presented here are the result of many days of averaging. Perturbations from day-to-day fluctuations in the scattering power of the solar wind have been averaged out and false deflections due to manmade interference have been removed.

The presence of IPS indicates that there is structure in the brightness distribution of a source on scales less than 2" in extent. The scintillating flux density, dS, is the rms variation in the total flux density S, and is a function of angular size, the fraction of the total emission from regions within that size, and the solar elongation, e. By observing the systematic variation in dS as a function of a, we determine the IPS angular diameter, e, assuming that the source has a circularly symmetrical gaussian brightness distribution. This determination is based upon a Model of the solar wind by Readhead, Kemp & Hewish (1978). The data are presented in the form of daily Measurements of dS plotted against solar elongation for each source, and we list values of e and dS(90deg), the rms scintillating flux density at an elongation of 90 deg. When an IPS observation is free from confusion from neighbouring sources the fraction of the total emission within the conpact component can also be determined if the total flux density is known. The flux density corresponding to one source per beam area in this survey is about 2.3 Jy at 81.5 MHz so that confusion errors are likely to be significant for S<=20 Jy. This corresponds to a level near the bottom of the 3C catalogue.

IPS provides the only method currently available for the detection of sub-arcsec structure in a large number of weak radio sources at metre wavelengths. Apart from a small number of highly compact sources having an overall angular size <1", scintillation normally identifies sources characterized by hot-spots in the outer lobes of intrinsically powerful radio galaxies and quasars. It therefore labels those sources in which energy is being released from active beams. Scintillation from galactic sources has also drawn attention to steep-spectrum, compact objects in the galactic plane such as the millisecond pulsar.

This survey is similar to one carried out by Readhead & Hewish (1974) but is more sensitive; the 3.6-hectare array has twice the collecting area and the whole sky was observed each day, as compared to once per week in the earlier work. The more frequent observations of each source provide a better average over day-to-day perturbations of the interplanetary medium and allow weakly scintillating sources to be detected more reliably. Observations of the perturbations themselves have enabled transients to be mapped and the results have been of special value to solar-terrestrial physics (See eg. Gapper et al., 1982).

2. The antenna

The 3.6-hectare array consists of 4096 full-wave dipoles and is an extension of the antenna used by Readhead & Hewish in their IPS survey (1974). Its geometry is shown in Fig. 1. The dipoles are arranged in 32 east-west rows of 128 dipoles, the signals being combined in each row by a branched feeder network. The antenna is divided into two sections of 16 rows and operated as a north-south phase-switching interferometer observing sources near meridian transit. The 16 row-outputs from each half of the antenna are combined in 'Butler matrices', in effect the analogue equivalents of fast Fourier transforms, to produce 16 declination beams (solid curves in Fig. 2) of half-power width 5.5 sec(52.16-DEC) degrees, where DEC is the declination corresponding to the peak response. The separation between the rows (0.65 wavelengths) is such as to cause multiple responses on beams 1, 2, 3 and 16.

Intermediate declination beams can be obtained by inserting a phase gradient in the north-south direction across the array shifting the whole pattern by half a beam width. This results in a response shown by the dotted curves in Fig. 2. We conducted the IPS survey in two stages, using the unshifted beams in 'year 1' (August 1978 to September 1979) and the shifted beams in 'year 2' (February 1980 to March 1981). The peak response of beam number N (year 1) is given by

60 ' 52.16 + arcsin ((N - 10)/10.4) degrees.

The corresponding intermediate positions (year 2) can be found by inserting N+0.5 in place of N. Tappin (1984) has shown that the declination power response, D, of the antenna follows well that expected for an array of dipoles a quarter of a wavelength above a horizontal reflecting screen, Ve: <==?

D = (4 Do sinZ((X/2) cos ) (sin2 16t)/sinZt)2 ,

where + d sin + (N -10)w/16, 5 52.16-6, d is the spacing of the rows and Do is the peak response. This is the function plotted in Fig. 2. It was checked by observing the deflections caused by the bright radio source Cygnus A (Purvis, 1981).

Individual phase-switching receivers having IP bandwidths 10.7 MHz and post-detection low-pass filters with bandwidths of 3 Hz were used on each declination beam. The IF gain was adjusted automatically 80 as to keep the post-detection noise power constant. The galactic background radiation dominated the system temperature everywhere except when a very bright 3C source passed through the antenna beam. Small gain corrections were applied in a few cases to allow for this. The tine constant of the automatic gain control circuit (AGC) was always Much longer than IPS tine scales. Typical traces of the sine and cosine (phase quadrature) outputs of one receiver for the passage of a scintillating source through the antenna pattern are shown in Figs. 3a & b. These outputs were Combined in a 'total power scintillometer' (Duffett-Smith, 1980), a device which first filtered out the low-frequency Components produced by the beams response leaving only the IPS signal and receiver noise. These signals were then squared, added, and finally integrated in a single RC stage with a tire constant of lose The output (Fig. 3c) was proportional to the mean square scintillating flux density, Multiplied by the square of the antenna power response. All three outputs (sine, cosine and scintillometer) of every receiver were monitored continuously so that we observed the whole sky above declination -10 every 24 hours throughout the period of the survey. The outputs were sampled, digitized, and recorded onto magnetic tape for later analysis Paper chart records of 20 of the possible 48 outputs were also made. These proved invaluable for checking the systez performance etc. As an additional check of the system, we used a device which made daily measurements of the contribution to the total system noise by each of the 256 first-stage preamplifiers (distributed over the 36,000 m^2 of the antenna). Faulty preamplifiers were quickly detected and replaced.

The right ascension power response of the antenna to a source at declination delta has a half-power beam width of 107 sec(delta) s. As the ground on which the telescope is built is not level, the peak response does not occur on the celestial meridian but is shifted earlier by about 70s we determined these pointing errors for each declination beam by minimising the residuals for 3C and 4C identifications, and have corrected for them in the data published here.

3. Production of the source list

A detailed description of the reduction of the first-year data has been given by Purvis (1981), and modifications made for the second-year data have been described by Tappin (1984) Here we give a brief description of the procedure

The digitised records were scrutinized for man-made interference, solar radio emission, and periods of intense ionospheric scintillation All such events were removed using both an automatic and manual method Although the former was much faster, the latter proved to be more reliable, unwanted signals being 80 diverse in character as to make automatic detection very difficult. Zero levels (the deflections caused only by system noise) were then determined for each of the cleaned scintillometer records and subtracted.

The selection of the primary grid of sources was carried out manually using an interactive programme running on a mini computer. Sections of record averaged over the elongation interval 20*-90 were displayed and the ?== ?? positions of possible sources marked by the operator using a movable cursor. The declination of each putative source was then determined automatically by an algorithm which used deflections for the same source on adjacent declination beams. We used ratios only between adjacent beams recorded simultaneously (ie. in the same 'year') in order to eliminate possible systematic effects due to gain variations, interplanetary disturbances, residual interference etc. For each source a search was made near to the marked position in right ascension, and the best deflection common to both beams used. We assumed the theoretical response of the antenna given in the previous section.

The final list of scintillating radio sources was generated from the initial source grid. Some putative sources were fictitious, and some had declinations which were in error where the declination algorithm had failed. The selection was done by inspection using averaged records plotted for 3 adjacent beams (at half beam spacing) with source positions and declinations marked. Any serious errors in right ascension and declination were immediately apparent, as were noise spikes marked as sources or deflections too small to be significant. Our list was now more reliable, but contained many duplicate entries, the same source having been selected on adjacent beams. We therefore plotted all the records on a single sheet (similar to Fig. 4) and chose the strongest deflection for duplicate entries as the 'true' entry. Right ascension corrections were then made where necessary to align the quoted value with the corresponding peak on the record.

For those sources where the declination algorithr had failed we examined the records by eye and determined the best position possible. Where we could not make a reasonable estimate, the central declination of the beam itself was selected as the appropriate value. The method by which each entry in the catalogue was determined is noted by a letter following the declination (see below).

A further improvement in the scintillating source list was made using plots of daily measurements of dS against S, such as those shown in Fig 5. IPS signals show a characteristic variation. As the solar elongation decreases from its maximum value, the scintillating flux density rises, reaching a maximum at about 35 deg where the scattering becomes strong. Nearer the sun the scintillating flux falls again as the scintillations become blurred due to the finite angular size of the source and the finite bandwidth of the receiver. Most of the sources remaining in the list displayed normal curves. Sources which did not have a maximum at e = 35 deg but showed instead a steady increase in scintillation away from the Sun were considered to be ionospheric scintillators (ionospheric scintillation is expected to increase beyond e = 90deg). Sources having a flat curve lying well above the noise level may either have been pulsars or instrumental in origin (eg breakthrough from bright non-scintillating sources) we reflected the latter and listed the rest as having "pulsar-like curves". <== ?? Sources at high ecliptic latitudes covered only a small range of elongations. These were retained in the list where the signals were well above the noise level. Sources with sparse curves (because of editing or instrumental failure) were also retained on the same basis.

4. Calibration

The interferoveter was calibrated by comparing the deflections on the sine and cosine records of bright 3C sources with their known flux densities, due account being taken of the declination response of the antenna The variation in gain caused by the action of the AGC on the galactic background emission was measured directly as described by Purvis (1981) we used the flux density scale of Baars et at (1977) with the observations of Scott & Shakeshaft (1971) as secondary calibrators we also took into account the measurements of Artyukh et al (1969), Collins (1968), Smith (1968), Slee and Higgins (1973, 1975) and of Branson (1967). The calibration curves of the scintillometers were measured directly by inflecting known signals and measuring the response.

We believe that the quoted values of scintillating flux densities measured in this survey are accurate to 10 per cent.

5. Presentation of the sclatillation data

As a supplement to the source list, we present the scintillation measurements themselves in two forms. The first of these (Fig. 4) displays the scintillometer records in 1 hour segments averaged over the elongation range 20 deg - 90 deg. Each point is therefore the mean of many days of observation, Figure 4 shows a representative section; the rest are given on the microfiche which accompanies this paper. The vertical deflection is proportional to the mean square scintillating flux density and zero levels (shown by the dashed horizontal lines) have been subtracted. The records have been corrected for variations in system gain so that the vertical scale is the same at all right ascensions. The coordinates have been corrected for pointing errors, but have not been adjusted for precession. Sources are marked at the positions where they were measured ie. at mean epoch 1980. Apparent sources which are not marked are those which we have discarded in the compilation of the source list.

Declination beams are identified by the 3-figure numbers to right and left. The first figure (1 or 2) indicates that the data were collected between August 1978 and September 1979 (1) or between February 1980 and March 1981 (2). The last 2 figures indicate the beam number as shown in Fig. 2, except that beam 16 was not used for technical reasons. The second period on beam 5 is indicated by 216. The gaps in the second-period records following hours divisible by 6 are connected with the automatic determination of the zero levels; those in all beams following hours divisible by 4 are calibration periods when no data were collected. Fig. 4 gives a good indication of the quality of the scintillation data on which this survey is based. It is apparent that residual interference is negligible and that the data are limited mainly by confusion. No convolution with the beam pattern has been applied. Beam-shaped deflections are therefore due to celestial scintillating signals and cannot be artefacts generated on the Earth. We also present plots showing the daily measurements of dS as a function of solar elongation for each source in the list. Representative plots are shown in Fig. 5 (a-f); the rest are on the microfiche. Scintillating flux density (Jy rms) is plotted vertically on a linear scale. The top line has the value given by 'plot peak'. The solar elongation (0-180) is plotted horizontally on a linear scale. Superposed on the individual daily measurements is a 5 deg median computed at 2.5 deg intervals. It is there simply to guide the eye and has not been used in the computations of angular sizes. The first five plots in the figure are arranged in order of increasing IPS angular size (a-e) while the last plot is that of a pulsar (f). Note that the slope of the data between elongations 35 deg and 90 deg is a good indicator of the angular size.

All the data presented on the microfiche are also available in book form on application to the appropriate author (PJD-S).

6. The catalogue

The catalogue is presented as a list of 1789 scintillating radio sources (Table 1). Each source is designated by an A0 number (column 1) which serves as its name. The positions (columns 2 and 3) are those derived from the scintillation measurements precessed to epoch 1950. Right ascensions (column 2) are correct to 5 s of time. The declination (column 3) is followed by a code which indicates the method by which it was derived as follows:

I successful determination by the computer algorithm; H determined by hand.

In each of the following categories, the central declination of the beam was used:

C source confused; N insufficient data to measure declination; B adjacent beam record apparently contradictory.

The errors in declination for sources determined by methods I and H are typically 30 arcminutes, dependent on the environment of the source. Categories B and C have errors of about half a declination beam width, while for category N the error is about 1 beam width.

The column dS_9O (4) gives the scintillating flux density of the source at elongation 90 deg, determined as the median scintillating flux density in the range 80 deg - 100 deg. This elongation was selected as the only elongation common to sources at all ecliptic latitudes.

The angular diameters given in column 5 are in arcseconds and have been derived from the data between elongations 40 and so by fitting theoretical curves from the model of Readhead, Kemp and Hewish (1978). One sigma formal error limits follow in square brackets (the square brackets were omitted in the electronic version). In a few cases it was not possible to make adequate estimates of the errors and a note has been inserted. Where an angular size is meaningless, either because the source is a pulsar or because it covers too small a range of elongations, a note to that effect replaces the figure. A dagger follows the error estimate for the small number of sources where the data were too sparse for a reliable estimate of the angular diameter to be made.

The compact flux density (column 6) is that inferred for the scintillating component in fitting the model curves to derive the angular diameters. The formal errors (square brackets) are one sigma values.

Identifications (column 7) are given from the 4C survey (with 3C numbers where applicable), from a 1-Jy sample of the 6C survey where it currently exists (Hales et al., 1988), and the pulsar list of Seiradakis (1978, private communication). As noted above, the antenna pattern was too broad for reliable identifications in most cases so that several possibilities are listed as 'contributing sources'. Those which make only a minor contribution to the measured signal are marked with superscript 1. In the case of confusion by a system artifact a superscript 2 appears. Sources from the 4C catalogue for which positional agreement would be improved by 'lobe shifting' are marked with a superscript asterisk.


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