Astronomers have found a radio pulsar with a companion at least 11 times the mass of the Sun -- the most massive pulsar companion known. The identity of the companion is uncertain: it may be a massive late-type (red) star, a massive but compact blue star, or possibly a black hole. If it is a black hole then this will be the first pulsar -- black hole binary system found, and a superb natural laboratory for testing general relativity.

The finding was presented in a poster paper at the American Astronomical Society meeting in San Diego, CA, by an international research team lead by Dr. Ingrid Stairs (University of Manchester, Jodrell Bank Observatory, U.K., and the National Radio Astronomy Observatory, U.S.A.).

The star field showing the late type red star within 0.5 arcseconds of the pulsar position. This is not thought possible to be the true companion of the pulsar. Photo: Jodrell Bank Observatory

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.

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.

With a mass of at least 11 solar masses the companion is either a massive star or possibly a black hole -- an exciting prospect!

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, measured with the Australia Telescope Compact Array, is near the Galactic Centre, which is a highly obscured region at optical wavelengths. Searching for the companion star in the near-infrared, using the 3.9-m (150-inch) Anglo-Australian Telescope, Australia, the research team found a late-type (red) star 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.

"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 76m Lovell telescope. Photo: Jodrell Bank Observatory

"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 change of the orbital position where the pulsar and its companion have their closest approach. This is the result of an 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.

In addition, the distance to the pulsar is about 36,000 light-years, with an uncertainty of about 25%. But in order to reconcile the measured magnitude of the observed star with a star of that type and mass, the system would have to be about 70,000 light-years away.

So what can the companion be? If it has to be smaller than a giant star, it must be either a main-sequence B-star or a black hole, with the former being somewhat more likely.

B-stars are less luminous and would hence not easily be detected with a late-type star at an apparently similar position.

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 the star's atmosphere.

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 continue the long-term, precise timing of the pulsar. Measuring the change 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 observations leading to the discovery were made by Dr. Ingrid Stairs (University of Manchester, Jodrell Bank Observatory, U.K., and the National Radio Astronomy Observatory, U.S.A.); Drs Richard Manchester and Jon Bell (Australia Telescope National Facility, Australia); Prof. Andrew Lyne, Dr. Michael Kramer, Mr. Dominic Morris and Ms Nuria McKay (University of Manchester, Jodrell Bank Observatory, U.K.); Prof. Victoria Kaspi (McGill University, Canada); Dr. Fernando Camilo (Columbia University, U.S.A.); Drs Nichi D'Amico and Andrea Possenti (Osservatorio Astronomico di Bologna, Italy); Dr. Froney Crawford (Massachusetts Institute of Technology, U.S.A.); Dr. Stuart Lumsden (University of Leeds, U.K.); Dr. Lowell Tacconi-Garman (Max-Planck-Institut fuer Extraterrrestrische Physik, Germany); Dr. Russell Cannon (Anglo-Australian Observatory, Australia); Dr. Nigel Hambly (University of Edinburgh, U.K.) and Dr. Peter Wood (Australian National University, Australia).

Background information
A pulsar is the collapsed core of a massive star that has ended its life in a supernova explosion. Weighing more than our Sun, yet only 20 kilometres across, these incredibly dense objects produce beams of radio waves which sweep round the sky like a lighthouse, often hundreds of times a second. Radio telescopes receive a regular train of pulses as the beam repeatedly crosses the Earth so the objects are observed as a pulsating radio signal.

Pulsars make exceptional clocks, which enable a number of unique astronomical experiments. Some very old pulsars, which have been "spun up" to speeds of over 600 rotations per second by material flowing onto them from a companion star, appear to be rotating so smoothly that they may be even "keep time" more accurately than the best atomic clocks here on Earth. Very precise timing observations of systems in which a pulsar is in orbit around another neutron star have been able to prove the existence of gravitational radiation as predicted by Albert Einstein and have provided very sensitive tests of his theory of General Relativity -- the theory of gravitation which supplanted that of Isaac Newton.