Globular Cluster Observations
Projects P427 & P778 @Parkes Radio Telescope, NSW, AustraliaTeam members @Cagliari Astronomical ObservatoryMarta BurgayAlessandro Corongiu Nichi D'Amico Andrea Possenti Team members @other institutesMattew Bailes - CSIRO Astronomy and Space Science, AustraliaFernando Camilo - Columbia Astrophysical Laboratory, United States Michael Kramer - Max Plank Institute fuer Radioastronomie, Germany Andrew Lyne - Jodrell Bank Observatory, United Kingdom Dick Manchester - CSIRO Astronomy and Space Science, Australia John Sarkissian - CSIRO Astronomy and Space Science, Australia Willem van Straten - CSIRO Astronomy and Space Science, Australia | ||||
Scientific RationaleGlobular clusters (GCs) are a fertile environment for the formation of recycled pulsars: besides evolution from a primordial system, exchange interactions in the ultra-dense core of the cluster favor the formation of various types of binary systems suitable for spinning up the neutron stars they host. Because of this, about 60% of all known millisecond pulsars (MSPs) are in GCs. Being point-like objects and, often, also extremely stable clocks, the MSPs in GCs allow one to investigate: (i) the gravitational potential of the GC, (ii) the dynamical interactions in the GC core, (iii) the gas content in a GC, (iv) the neutron star retention, (v) the binary evolution in a dense stellar environment, (vi) the equation of state of nuclear matter.  | ||||
Project status65 Globular Clusters have been observed using the Parkes radio telescope (NSW, Australia). So far 12 millisecond pulsars have been discovered in six clusters which did not contain previously known pulsars. These discoveries contribute about 25% to the total number of globular clusters (26 so far) hosting a known millisecond pulsar. | ||||
 Observed Globular Clusters | ||||
Discovered Pulsars: HighlightsNGC 6397 NGC 6397 hosts a MSP (PSR J1740-5340) whose characteristics are unique: it is a member of a binary with orbital period 1.35 days and suffers eclipses for about 40% of the orbital phase at 1.4 GHz. Light curves and spectroscopic observations of the optically identified companion suggest that it must have a mass in the interval 0.22-0.32 Msun and completely (or almost completely) fills its Roche lobe. These facts, combined with radio measurements, indicate that the companion may be either (i) the remnant of the star that spun up the pulsar and is now experiencing the so-called radio-ejection phase (in which the accretion is quenched by the radio emission), (ii) a star which exchanged its position in the binary with that of the star which originally spun-up the pulsar, (iii) a subsubgiant resulting from a binary-binary encounter in the cluster core, or (iv) a combination of (i) and (ii), hence making this binary the archetype of the black-widow systems whose mass loss is driven by the nuclear evolution of the companion.
Related publications: NGC 6752 hosts 5 MSPs. PSR-B and PSR-E (located within the cluster core) have very high negative spin derivatives. If these negative derivatives are ascribed to the overall effect of the cluster potential (as customarily assumed), the resulting central mass-to-light ratio (M/L) is much larger than that reported in the literature and at least twice as large as the typical value M/L~ 2-3 observed in other core-collapsed clusters. The unusually high M/L is confirmed by the determination of the cluster-F¢s dynamical mass obtained from the measurement of three dimensional velocity for PSR-A with respect to the cluster center. This peculiarity could be explained with ~ 1000 Msun of low-luminosity unseen matter enclosed within the central 0.08 pc of the cluster. Given the observed shape of the cluster density profile it could be due to: (a) a very high concentration of white dwarfs, or (b) a central (single or binary) intermediatemass black hole. On the other hand, (b) is a viable hypothesis to account for the unprecedented positions of PSR A and PSR C. The timing analysis of PRS A also allowed the determination of the companion mass and the orbital inclination in this binary system.
Related publications:
In NGC 6266, six MSPs have been discovered, three by our team and three more by Jacoby et al. The inferred luminosities of the MSPs A, B and C place them at the bright end of the luminosity function of the MSPs in GC, entailing the possible existence of many slightly dimmer MSPs in NGC 6266. A recent Chandra pointing revealed the presence in the inner part of the cluster of a few tens of X-ray sources whose colours suggest that NGC 6266 should harbour tens of MSPs. In contrast to the other GCs in which at least five pulsars have been discovered, all the 6 known MSPs in NGC 6266 are in binaries. The absence of isolated pulsars cannot be explained by invoking a selection effect since, for a given spin period and flux density, an isolated MSP is easier to detect than a binary MSP. If this absence of isolated pulsars in NGC 6266 is not a statistical fluctuation, it must relate to the mechanisms of formation of these objects and their interplay with the dynamical state of the cluster.
Related publications: NGC 6441 hosts PSR J1750-3703A, which is a mildly recycled radio pulsar orbiting a companion whose minimum mass is 0.58Msun. This fact and the large orbital eccentricity (e = 0.71) favor two interpretations for the nature of the system: (a) a pulsar orbiting a star (a heavy white dwarf or a main-sequence star) acquired during an exchange interaction in the cluster core, or (b) a double neutron star binary.  New Discoveries |
