ASTRONOMERS FIND MOST MASSIVE COMPANION TO RADIO PULSAR

The binary pulsar, PSR J1740-3052, was detected during the Parkes multibeam pulsar survey, a large-scale survey for pulsars currently being carried out using the 13-beam 1400-MHz receiver on the Parkes 64-m (210-ft) radio telescope operated by the Australia Telescope National Facility near the town of Parkes in eastern Australia. The survey has found 580 pulsars, more than 40% of all known. It covers a region within five degrees of the Galactic Plane and is designed to find young and distant pulsars.

Timing observations made with the 76-m Lovell Telescope at the Jodrell Bank Observatory, U.K., and at Parkes, show that PSR J1740-3052 is a 570 ms pulsar in a 231-day orbit. The orbit characteristics indicate that the pulsar is waltzing through space with a heavyweight companion. "It's at least 11 times the mass of the Sun, and probably more like 16 times," says Prof. Andrew Lyne of Jodrell Bank Observatory.

About 50 binary pulsar systems are known. In only two are the companions main-sequence stars: a Be star accompanies PSR B1259-63, and a B star tags along with PSR J0045-7319. Both have a mass about ten times that of the Sun.

Astronomers estimate that one neutron star in 1000 should be partnered by a black hole. The PSR J1740-3052 system holds some promise but the nature of the pulsar's companion is still tantalizingly unclear.

The pulsar's precise position was fixed with the 6-element Australia Telescope Compact Array near Narrabri in eastern Australia. It lies close to the Galactic Center (-0.13 degrees Galactic latitude and 357.8 degrees Galactic longitude), a highly obscured region at optical wavelengths. So the observing team searched for the companion star in the near-infrared, using the 3.9-m (150-inch) Anglo-Australian Telescope near Coonabarabran, Australia, pointing at the position determined by the Australia Telescope. This turned up a bright, late-type (red) star with a K-band magnitude of 10.05 that lies within 0.5 arcseconds of the pulsar's position. An image of the candidate star was also made with the 2.3-m (90-inch) Australian National University telescope near Coonabarabran.

"The probability of this late-type star being there by chance, and not being the pulsar companion, is only about 1.3%," says team member Dr. Fernando Camilo of Columbia University. And yet chance it appears to be. On many grounds the late-type star is not a suitable companion for the pulsar.

The pulsar is young and its orbit eccentric. It seems to have acquired no significant mass from its companion. "If the companion is a late-type supergiant it must be confined within its Roche lobe and have an unusually small radius for such a star," says Dr. Ingrid Stairs, a Jansky Post-Doctoral Fellow at the National Radio Astronomy Observatory. "A typical star of this mass and spectral type would extend beyond the pulsar's orbit. You would expect the pulsar to be eclipsed every orbit and it isn't."

Timing observations show a small advance of the periastron of the pulsar's orbit, a precessional effect attributable to a combination of general-relativistic and classical effects. "The value you'd predict for this number if the companion were a late-type supergiant just doesn't square with what we observe," says Dr. Richard Manchester of the Australia Telescope National Facility. But the observed value would be consistent with either a main-sequence companion with a radius only a few times that of the Sun, or a black hole.

The distance to the pulsar is about 36,000 light-years, with an uncertainty of about 25%. But the magnitude of the late-type star isn't right for that of an 11 solar-mass star unless the star is about 70,000 light-years away.

So what can the companion be? The smart money is on either a main-sequence B star or a black hole, with the former having better odds.

A B star lurking behind the late-type star would not significantly alter the observed K-band magnitude or spectrum.

At some phases of the pulsar's orbit, the arrival times of the radio pulses show frequency-dependent delays. These must be caused by the signal passing through a region with a higher density of free electrons. The most plausible explanation is that the companion star is emitting an ionized wind, and the pulsar passes behind this as it orbits the star. Yet the observation does not completely rule out a black hole companion. If the late-type star is between the pulsar and Earth, the pulsar signal could be delayed by passing through its atmosphere.

The pulsar team will continue to scrutinize these candidates with the care of zealous parents checking their child's prospective partner.

The team has applied for time on the European Southern Observatory's Very Large Telescope in Chile, to check out the spectrum of the late-type star, looking for Doppler-shifting that would be caused by the orbiting of the pulsar - the same technique used to hunt for planets around stars. No Doppler-shifting means that the late-type star can be ruled out as a companion. If the companion is a B star, these observations may also show some of its spectral features.

The team will also be doing long-term, precise timing of the pulsar. Measuring the precession of its orbit more precisely may allow the astronomers to determine if there is a 'classical' component to the effect, which would clearly imply a main sequence star rather than a black hole. "We will be keeping our eyes open for signatures in the timing data that could help determine what the companion star is, one way or the other," said Prof. Victoria Kaspi of McGill University.

The Parkes radio telescope and the Australia Telescope Compact Array are both part of the Australia Telescope, which is funded by the Commonwealth of Australia for operation as a National Facility by CSIRO (Commonwealth Scientific and Industrial Research Organisation). Ingrid Stairs received support from NSERC and Jansky postdoctoral fellowships. 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. This research has used the Astronomical Data Center at NASA Goddard Space Flight Center and the Simbad and Vizier services operated by CDS Strasbourg. It has also used data products from the Two Micron All Sky Survey, which is a joint project of the University of Massachusetts and the Infrared Processing and Analysis Center / California Institute of Technology, funded by the National Aeronautics and Space Administration and the National Science Foundation. The Digitized Sky Surveys were produced at the Space Telescope Science Institute under U.S. Government grant NAG W-2166.

For more information:

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

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. Fernando Camilo, Columbia University +1-212-854-2540 fernando@astro.columbia.edu

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