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NEW-FOUND PULSARS START TO CRACK GAMMA-RAY SOURCE MYSTERY

Astronomers using the Parkes 64-m radio telescope in Australia have found about 30 young, energetic pulsars, which may be the counterparts of otherwise unidentified Galactic gamma-ray sources. Two have already been found to be good matches for sources detected by EGRET. The results are presented today [Thursday 5 April] at the "Gamma-Ray Astrophysics 2001" Symposium in Baltimore by Dr. Nichi D'Amico of the Osservatorio Astronomico di Bologna, Italy. The members of the research team are Dr. D'Amico, Professor Victoria Kaspi (McGill University, Canada), Dr. Richard Manchester (Australia Telescope National Facility, Australia), Dr. Fernando Camilo (Columbia University, USA), Professor Andrew Lyne (University of Manchester, Jodrell Bank Observatory), Dr. Andrea Possenti (Osservatorio Astronomico di Bologna, Italy), Dr. Ingrid Stairs (National Radio Astronomy Observatory, USA), Dr. Michael Kramer and Mr George Hobbs (University of Manchester, Jodrell Bank Observatory), and Dr. Jon Bell (Australia Telescope National Facility, Australia).

Gamma-ray observations were pioneered in the 1970s by the SAS-2 and COS-B satellites, launched by NASA and ESA respectively. But three decades later many gamma-ray sources still defy identification. To date, less than half of the gamma-ray sources observed with the EGRET (Energetic Gamma Ray Experiment) instrument on the Compton Gamma Ray Observatory satellite have been identified. The main difficulty in finding counterparts to gamma-ray sources is that gamma rays are very hard to localize, and so the uncertainties in the source positions can be as much as a degree on the sky.

Young pulsars have long been favoured as potential counterparts. The two most powerful gamma-ray sources in the sky are the Crab and Vela pulsars. The third-strongest source, Geminga, is a pulsar, but has been detected only once at radio wavelengths.

Seven gamma-ray sources near the Galactic plane have previously been identified as pulsars. "We've now found a further two pulsars that coincide with unidentified EGRET sources, within position uncertainties," said Dr. D'Amico.

Both pulsars are young and energetic. PSR J1420-6048, with a period of 68 ms, appears to be 13,000 years old. PSR J1837-0604, with a period of 96 ms, is estimated to be 34,000 years old.

An increasing rotation period for both signals that they are losing energy at a significant rate. "For this reason alone one would expect that they should be observable as gamma-ray sources," said Dr. D'Amico.

The Parkes survey has found 30 radio pulsars with ages less than 100,000 years. The survey team will now do precise, long-term timing of their periods. This will help future gamma-ray observatories such as AGILE, GLAST and INTEGRAL, detect any gamma-ray pulses from these objects.

The Parkes multibeam survey is a large survey for young, distant pulsars within five degrees of the Galactic Plane currently being done with a 13-beam, 1400-MHz receiver on the Parkes radio telescope.

Although not yet complete, this survey is already the most successful survey for radio pulsars ever made, having found more than 600 previously unknown pulsars, nearly doubling the number previously known.

"The Parkes multibeam pulsar survey has been very good at finding faint, fast pulsars," said Professor Victoria Kaspi of McGill University. "It turned up these two energetic young objects even without deliberately targeting the gamma-ray-source error boxes."

"To find more counterparts, we could use the multibeam system to target more error boxes, but with even higher sensitivity."

The Parkes radio telescope is part of the Australia Telescope, which is funded by the Commonwealth of Australia for operation as a National Facility managed by CSIRO (Commonwealth Scientific and Industrial Research Organisation). Nichi D'Amico and Andrea Possenti received support from the Italian Space Agency and from the Italian Minister of the University and Technological and Scientific Research (MURST). Victoria Kaspi is an Alfred P. Sloan Research Fellow and received support from an NSF CAREER award (AST-9875897) and an NSERC grant (RGPIN228738-00). Fernando Camilo is supported by NASA grant NAG 5-3229. Ingrid Stairs received support from NSERC and Jansky postdoctoral fellowships.

Fig.1: Positions of all known pulsars in our Galaxy. Parkes multibeam survey pulsars and the young energetic radio pulsars are indicated. Positions are plotted in `Galactic coordinates' with the direction towards the Galactic Center at the center of the plot. The horizontal axis is the Galactic Plane or Milky Way.

Fig.2: Pulsar ages are given by the ratio of the pulsar spin period to the rate at which the pulsar is slowing down, measured by the period derivative. Also, electrodynamic theory shows that the surface magnetic field on the neutron star is proportional to the square root of the product of the spin period times the period derivative. A representation of pulsars in a period derivative versus period diagram helps in the classification of pulsars according to their age and magnetic field strength. In this diagram, young, high magnetic field radio pulsars are located in the upper part.

CONTACTS:

Dr. Nichi D'Amico, Osservatorio Astronomico di Bologna, Italy +39-051-209-5719 DAMICO@astbo3.bo.astro.it

Prof. Victoria Kaspi, McGill University, Canada +1-514-398-6412 vkaspi@physics.mcgill.ca

Dr. Richard Manchester, Australia Telescope National Facility +61-2-9372-4313 rmanches@atnf.csiro.au

Prof. Andrew Lyne, University of Manchester, Jodrell Bank Observatory +44-1477-572-640 agl@jb.man.ac.uk

Dr. Ingrid Stairs, National Radio Astronomy Observatory, U.S.A. +1-304-456-2213 istairs@nrao.edu

Helen Sim Communications Manager CSIRO Australia Telescope National Facility PO Box 76 Epping NSW 1710 Australia hsim@atnf.csiro.au tel: +61 2 9372 4251 fax: +61 2 9372 4444