Search

Seven new pulsars are reported from the Molonglo radio observatory. Some conclusions can be drawn about the distribution of pulsars in the galaxy.

The pulsed radio sources1,2 emit energy almost simultaneously over a wide frequency band3. Pulses from the source at 19h 19m 37s, + 21°47′02″ (epoch 1950) have been received with the Australian 210 ft telescope at frequencies of 85, 150, 630, 1,410 and 2,700 MHz. This report covers measurements of the spectrum of the pulses, the characteristics of the intensity variations at different frequencies, and observations of the structure and polarization of the pulses. A previous communication4 discussed measurements of the period of the pulses and of the dispersion between 85 and 1,410 MHz.

A NEW series of observations of the galactic radio emission has been conducted at the frequencies of 20, 30 and 85 Mc/s. These low-resolution observations made with scaled aerials were intended to investigate the spectral index of the galactic radio emission in the frequency region 10–100 Mc/s.

KNOWLEDGE of the intensity–frequency relation of extraterrestrial radio emission has been obtained from scaled aerial experiments at frequencies higher than 20 Mc/s (refs. 1 and 2) and from ground-based or satellite-borne radiometers operating below 10 Mc/s (refs. 3, 4 and 5). The low-frequency observations, whether ground-based or satellite-borne, are affected by the ionosphere. The consequence of these ionospheric effects has resulted in difficulties being experienced when reconciling the observations in frequency regions below 10 Mc/s with those above 20 Mc/s.

FOUR further pulsars have been discovered at the Molonglo radio observatory during a pulsar search at 408 MHz. One of the four pulsars, MP 0254, a high galactic latitude pulsar, was discovered in November 1968, but failed to appear on any of some twenty subsequent transits. In a recent observing session this pulsar was observed twice, allowing measurement of the dispersion and pulse width. Two of the pulsars (MP 1706 and MP 1911) were among many suspected pulsars noted during a close re-examination of the search records made since October 1968 using the two beam technique which led to the discovery of eighteen pulsars1–4. Recent observations have confirmed the existence of MP 1706 and MP 1911, and have enabled their properties to be measured. The fourth pulsar, MP 1154, was discovered during a search for high dispersion pulsars along the galactic plane using a two channel dispersion remover which was described in connexion with the discovery of MP 1240 (ref. 3).

THE pulsar CP 1919 (ref. 1) was observed at Parkes2 in March 1968 at a number of radio frequencies. On occasions the frequencies 85, 150, 150.9, 630 and 1,410 MHz were used simultaneously, with the 210 foot telescope tracking the source for some 4 h. The data have now been examined for statistical occurrence properties and a quantitative description of the source can be given. The statistics of pulse occurrence are of general interest because theories of emission (and pulse propagation) require these data. And anyway an observatory embarking on a programme of searching for such objects must know the statistical properties of pulse occurrence to aid in determining an observational programme.

WE recently described1 the discovery of seven pulsars found in a search at the Molonglo Radio Observatory using the 408 MHz radio telescope. A conclusion reached on the basis of this search was that pulsars are galactic objects associated with the disk or the spiral arms, but with a somewhat anomalous distribution. A zone of avoidance along the inner regions of the galactic plane was suggested, based on the argument that dispersion in the thin layer of an ionized medium closely confined to the galactic plane results in the loss of the distant pulsars. This communication describes the discovery of further pulsars which follow the same general pattern of galactic distribution.

PULSAR surveys carried out at the Molonglo Radio Observatory during February and March, 1969, have resulted in the discovery of three new pulsars. They have been designated MP 0450, MP 1642 and MP 1818. The last two were found in areas of the sky which had not previously been searched for pulsars using the Molonglo radio telescope. The position of MP 0450, however, had been searched twice before during the pulsar survey programme, but the pulsar was not detected presumably because of long term variations of its pulse amplitude.