Four milestones of pulsar astronomy were established in year 2001 by the Bologna Pulsar Group, and these are: a) The discovery of the most massive companion to a radio pulsar; b) The discovery of a substantial sample of new young and energetic radio pulsars, possible counterparts of the unidentified gamma ray sources; c) The first detection of gas in a globular cluster; d) The discovery of a probable new-born millisecond pulsar.
(a) The discovery of the most massive
companion to a pulsar
We 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.
This 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, which is at least 11 times the mass of the Sun.
(b) The discovery of new young and energetic pulsars
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 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.
We have now found, as a result of the Multibeam Pulsar Survey carried out at Parkes, about 30 young and energetic pulsars which may be the counterparts of otherwise unidentified Galactic gamma-ray sources.
(c) The first detection of gas in a globular cluster
Using the 64-m Parkes radio telescope in Australia, we have discovered more than 20 millisecond pulsars in the GC 47 Tuc. We have then made very precise observations of the minute changes in the observed rotation rates due to the Doppler shift caused by the gravitational pull of the cluster. This has enabled us to determine their positions within the cluster. A further measurement, made for each pulsar, measures the amount of material and gas in the line of sight to us. We have found that pulsars on the far side of the cluster have more gas in front of them than those on the near side, thus proving the presence of gas within the cluster.
(d) The discovery of a probable newborn
millisecond pulsar
Millisecond pulsars (MSP) are old, slow pulsars that have been recycled. The pulsar's gravity sucks off gas from a larger companion star. As the gas hits the pulsar surface, it transfers angular momentum to it, making it spin faster. The result is a turbo-charged pulsar spinning at hundreds of times a second. Meanwhile, the enfeebled companion dwindles into a white dwarf.
We have started a long-term programme which takes advantage of the current generation of astronomical instruments (from ground and from space) in a coordinated effort to fully understand the formation mechanism and evolution of MSP's in GCs. In particular, our search for optical counterparts to MSP companions in binary systems in GCs has been very fruitful. Two optical counterpart candidates to MSP companions have been identified in the core of 47 Tuc (Ferraro et al. 2001) using deep HST observations in the UV.
However, the most exciting result has been obtained in NGC 6397: during a
systematic search of the galactic globular cluster system for millisecond
pulsars (carried out with the Parkes radiotelescope) we have found an eclipsing
binary millisecond pulsar (PSR J1740
5340, D'Amico et al. 2001) whose gravity
has deformed its companion into a red teardrop.
The optical identification (Ferraro et al. 2001)
of the companion revealed a bright red star (a
tidally deformed Main Sequence star) instead of a white dwarf, and the
peculiar modulation of the light curves showed that it had filled and
overflowed its 'Roche lobe'.
This discovery has had a large "echo" on international science journals
(see Public Outreach publications).
This is the first known binary system in which the overflow stage is not ended yet and could represent the first detection of a new-born MSP i.e., we are seeing the system in a fleeting phase of its life - the point at which a new millisecond pulsar has just been fully spun up by its companion star. Otherwise, it's also possible that the pulsar has picked up the red star after parting from its original partner.
A coordinated spectroscopic study of this object is currently in progress to determine its origin and true nature. In particular, phase resolved spectroscopy at low (with EMMI@NTT), and high resolution (with UVES@UT2), and high precision multiband photometry (with SUSI@NTT) is going to be secured at ESO Telescopes in the next months. This will allow us to determine the detailed light curve shape, the radial velocity curve, the accurate abundance pattern, and to map the emission of the donor star as a function of the orbital phase. The complete data-set will finally allow a complete characterization of the system and will shed light on the formation mechanism and the evolution of MSP binary systems in Globular Clusters.