R. N. Manchester
, PASA, 18 (1), in press.
Next Section: Millisecond Pulsars in 47
Title/Abstract Page: Finding Pulsars at Parkes
Previous Section: The Early Years
The Parkes Multibeam Pulsar Survey
The Parkes multibeam receiver, while primarily designed for HI surveys (Staveley-Smith et al. 1996), is a superb instrument for pulsar surveys. Its 13 beams allow the sky to be covered roughly 13 times as fast, or alternatively, much longer to be spent on a given point. Also, its receivers have excellent sensitivity, with an average system noise of only 21 K. With major contributions from Jodrell Bank Observatory and Osservatorio Astronomica di Bologna, a filterbank system capable of handling the data from the 13 beams was installed at Parkes in early 1997 and the Parkes multibeam pulsar survey commenced in August 1997. The survey is covering the regionwith
. The filterbank has 96 3-MHz channels for each polarisation of each beam and all outputs are one-bit digitised at 250 s intervals and recorded to tape. Each pointing of the 13 beams is of 35 min duration, giving a sensitivity for long-period pulsars away from the hot regions of the Galactic background of about 0.15 mJy. This is about seven times better than the previous best survey of this type (Johnston et al. 1992a), and so a large increase in the number of detected pulsars was expected. These expectations have already been fully realised. With about 80% of the survey completed, more than 570 previously unknown pulsars have been discovered, making this by far the most successful pulsar survey ever. When finished, the survey will come close to doubling the number of known pulsars. Fig. 4 shows the distribution of known pulsars projected on to the Galactic plane, where distances have been computed using the Taylor & Cordes (1993) electron density model. In contrast to the previously known pulsars which are clustered around the Sun, many of the multibeam pulsars are at large distances, with some apparently on the other side of the Galactic Centre. There is some indication of a deficit of detected pulsars within a couple of kpc of the Galactic Centre. The electron density model is not well determined at these large distances though, and the distances may have systematic biases. Also, many of the multibeam pulsars are concentrated in spiral arms, but this may simply be a result of the increased model electron density in the arms. The multibeam sample will be important in helping to refine the electron density model.
5.5 x 1013 G. These parameters place the pulsar near the so-called `anomalous X-ray pulsars' (AXPs) on the plane (Fig. 5). AXPs are believed to be slowly rotating neutron stars, but they have no detectable radio emission. On the other hand, PSR J1814-1744 has no detectable X-ray emission (Pivovaroff, Kaspi & Camilo 2000). The reason(s) for these very different properties are not well understood.
yr) in a much tighter ( h) and eccentric orbit (Kaspi et al. 2000). Precession of the longitude of periastron has been observed for this system, and interpreting this as due to the effects of general relativity gives a value for the total mass of the system of
M. The pulsar and orbit properties suggest that the companion is a heavy white dwarf formed before the supernova explosion that created the pulsar. This is unusual. In most binary systems the neutron star is formed from the heavier binary companion which evolves faster. As shown in Fig. 6, PSR J1740-3052 is in a highly eccentric long-period orbit. The interesting thing about this system is that the minimum companion mass is 11 M, implying that the companion is either a massive star or a black hole. Unfortunately the pulsar lies close to the direction of the Galactic Centre and probably at about the same distance, so optical searches for the companion are unlikely to be productive. However, 2.2 m infrared observations with the Siding Spring 2.3-m telescope and the Anglo-Australian Telescope have revealed a K-supergiant star whose position agrees with that of the pulsar to better than 0.3 arcsec (Stairs et al. 2000). The infrared spectrum of this star shows Brackett- emission, consistent with the presence of a compact binary companion, and the star's colours are consistent with a distance comparable to that of the pulsar. Furthermore, DM and rotation measure changes were observed over the last periastron passage, in February 2000. All of these observations point toward this star being the binary companion. However, there a couple of puzzling features. The pulsar comes to within 1.25 stellar radii of the companion star at periastron. One might expect the radio emission to be eclipsed by the stellar atmosphere or wind, but no eclipses are observed. Also, it should raise large tides on the companion, causing a large precession in the longitude of periastron. This is not observed. Either we do not understand winds and tides in supergiant stars very well, or all the other observations are misleading and the companion is really a black hole. Although the latter is an attractive option (this would be the first known neutron star - black hole system), the former is more likely.
Next Section: Millisecond Pulsars in 47
Title/Abstract Page: Finding Pulsars at Parkes
Previous Section: The Early Years
© Copyright Astronomical Society of Australia 1997