New-found Pulsars Start to Crack Gamma-Ray Source Mystery
10 April 2001
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.
Astronomers from the University of Manchester and other members of an international team 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. The positions of these sources can be uncertain by as much as a degree so making it difficult to identify their origin. Two of the newly discovered pulsars have positions which coincide within the positional uncertainties to hitherto unidentified gamma-ray sources detected by the EGRET (Energetic Gamma Ray Experiment) instrument on the Compton Gamma Ray Observatory satellite.
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 - and both of these new pulsars linked to gamma-ray positions 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. The increasing rotation periods observed for both pulsars indicates 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. Nichi D'Amico of the Osservatorio Astronomico di Bologna, Italy, as he presented the results at the "Gamma-Ray Astrophysics 2001" Symposium in Baltimore.
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.
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 EGRET have been identified. 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 the positional uncertainties," added Dr. D'Amico.
Pulsars are neutron stars formed in the collapse of massive stars in supernova explosions. As Professor Andrew Lyne of the University of Manchester explains: "their intense magnetic fields are expected to make them profilic sources of high-energy radiation."
The pulsars were found in the Parkes multibeam survey using a 13-beam, 1400-MHz receiver on the Parkes radio telescope to search for young, distant pulsars within five degrees of the Galactic Plane. 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 found in 30 years of observations. 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 in order to help future gamma-ray observatories detect any gamma-ray pulses from these objects.
"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." With this encouragement the team plans to use the multibeam system to target more of these sources with even higher sensitivity and will expect to find many more gamma-ray emitting pulsars. If so, the nature of these mysterious galactic gamma-ray sources may soon be solved.
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).
Prof. Andrew Lyne, University of Manchester, Jodrell Bank Observatory
Dr. Nichi D'Amico, Osservatorio Astronomico di Bologna, Italy
Prof. Victoria Kaspi, McGill University, Canada
Dr. Richard Manchester, Australia Telescope National Facility
Dr. Ingrid Stairs, National Radio Astronomy Observatory, U.S.A.
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.