F. Camilo (Columbia University, USA); B. M. Gaensler (Harvard University, USA); D. R. Lorimer (University of Manchester, UK); R. N. Manchester (ATNF)

2002 yielded a remarkable vintage of young pulsar discoveries, with the Parkes telescope playing a central role.

Young pulsars are objects of interest for a variety of reasons. The physics of core collapse in evolved stars can be inferred from observations which provide the initial spin period, space velocity, and magnetic field distributions of neutron stars, while measurements of their beaming properties, luminosities, and spectra are crucial for determining the Galactic population and birthrate of pulsars. Young pulsars frequently exhibit period glitches, and emit substantial amounts of X- and gamma rays these can be observed to learn about the internal composition of neutron stars, and the pulse emission mechanisms. In addition, young neutron stars are embedded in compact non-thermal radio and/or X-ray pulsar wind nebulae where the ambient medium confines the relativistic pulsar wind, or otherwise interact with their host supernova remnants. The embedded pulsars are unique probes of their immediate environment and the local interstellar medium.

A natural location to search for young pulsars is the Galactic plane and more specifically in supernova remnants. However, establishing bona fide associations between Galactic pulsars and supernova remnants has been a painfully slow business: two were known by 1970 (those of the Crab and Vela), five by 1985, and only 10 by 2000, although more than 200 supernova remnants and 1,400 pulsars are known. Searches for associations have not been immensely productive in this regard a survey of 88 supernova remnants in the 1990s netted zero associated pulsars!

More recently the Parkes multibeam pulsar survey of the Galactic plane, using a 13-beam receiver system at a frequency of 1,400 MHz, covered a very large area with sensitivity broadly comparable to that of the best previous surveys of supernova remnants. This survey has been extraordinarily successful, discovering approximately 700 pulsars, but to date has yielded only one new association between a pulsar and a supernova remnant (for a discussion of this pulsar, PSR J1119-6127, in the supernova remnant G292.2-0.5 see the ATNF Annual Report 2000).

We have used a method to detect young pulsars that relies on the premise that the existence of a pulsar wind nebula must also indicate the presence of an energetic and reasonably young pulsar. We decided to search pulsar wind nebulae as deeply as possible, to look for new detections of young pulsars. A complication with this approach is that it is not always clear whether a particular compact object is a pulsar wind nebula. However, recent X-ray images from the Chandra X-ray Observatory can, in some cases, unambiguously identify a pulsar wind nebula and its embedded pulsar even when pulsations are not detected.

One such example is provided by the beautiful Chandra observation of SNR G292.0+1.8, shown in Figure 1. The X-ray data of this source reveal a two arcminute pulsar wind nebula within which is located a point source. While this supernova remnant had been searched previously without a pulsar detection, the Chandra results encouraged new efforts. In a 10-hour integration at Parkes using the central beam of the multibeam system, we detected PSR J1124-5916, with a pulse period of 135 milliseconds, a moderate dispersion measure (330 pc cm^-3), and a characteristic age of 2,900 years. PSR J1124-5916 is a very weak source, with a flux density at 1,400 MHz of 80 microJansky and a low radio luminosity. X-ray pulsations were detected subsequent to and with the help of the radio discovery.

Following this discovery, many more pulsar wind nebulae were searched as deeply as possible with the Arecibo, Green Bank, Jodrell Bank and Parkes telescopes. In all, six very weak and energetic young pulsars, associated with wind nebulae (in some cases within supernovae), have been detected in the past two years, and the searches continue. The newly discovered objects are being investigated in follow-up studies at radio and X-ray wavelengths, and a careful analysis of the sensitivity and selection effects relevant to these searches is underway and should yield useful constraints on the combination of luminosity distribution and beaming fraction of young pulsars.

Our study has shown that many young pulsars beam towards the Earth. However, many of them do so with luminosities that are detectable with the best present-day radio telescopes only when using integration times of a day or so, if at all! While X-ray astronomers have long been used to spending a day obtaining a handful of photons from astronomical sources of interest, such as young pulsars, this is a relatively new lesson for radio pulsar observers to learn.

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