N. D’Amico

The Parkes Multibeam Pulsar Survey – II. Discovery and timing of 120 pulsars

The Parkes Multibeam Pulsar Survey is a sensitive survey of a strip of the Galactic plane with |b| < 5 ◦ and 260 ◦ < l < 50 ◦ at 1374 MHz. Here we report the discovery of 120 new pulsars and subsequent timing observations, primarily using the 76-m Lovell radio telescope at Jodrell Bank. The main features of the sample of 370 published pulsars discovered during the multibeam survey are described. Furthermore, we highlight two pulsars: PSR J1734−3333, a young pulsar with the second highest surface magnetic field strength among the known radio pulsars, Bs = 5.4 × 10 13 G, and PSR J1830−1135, the second slowest radio pulsar known,

Detection of Ionized Gas in the Globular Cluster 47 Tucanae

We report the detection of ionized intracluster gas in the globular cluster 47 Tucanae. Pulsars in this cluster with a negative period derivative, which must lie in the distant half of the cluster, have significantly higher measured integrated electron column densities than the pulsars with a positive period derivative. We derive the plasma density within the central few parsecs of the cluster using two different methods that yield consistent values. Our best estimate of ne = 0.067 ± 0.015 cm-3 is about 100 times the free electron density of the interstellar medium in the vicinity of 47 Tucanae, and the ionized gas is probably the dominant component of the intracluster medium.

PSR J1847–0130: A Radio Pulsar with Magnetar Spin Characteristics

We report the discovery of PSR J18470130, a radio pulsar with a 6.7 s spin period, in the Parkes Multibeam Pulsar Survey of the Galactic plane. The slowdown rate for the pulsar, s s 1 , is high and implies a 12 1.3 # 10 surface dipole magnetic field strength of G. This inferred dipolar magnetic field strength is the highest 13 9.4 # 10 by far among all known radio pulsars and over twice the “quantum critical field” above which some models predict radio emission should not occur. The inferred dipolar magnetic field strength and period of this pulsar are in the same range as those of the anomalous X-ray pulsars, which have been identified as being “magnetars” whose luminous X-ray emission is powered by their large magnetic fields. We have examined archival ASCA data and place an upper limit on the X-ray luminosity of J18470130 that is lower than the luminosities of all but one anomalous X-ray pulsar. The properties of this pulsar prove that inferred dipolar magnetic field strength and period cannot alone be responsible for the unusual high-energy properties of the magnetars and create new challenges for understanding the possible relationship between these two manifestations of young neutron stars. Subject headings: pulsars: individual (PSR J18470130) — stars: magnetic fields — stars: neutron — X-rays: stars

AN ECLIPSING MILLISECOND PULSAR WITH A POSSIBLE MAIN-SEQUENCE COMPANION IN NGC 6397

We present the results of 1 yr of pulse timing observations of PSR J1740-5340, an eclipsing millisecond pulsar located in the globular cluster NGC 6397. We have obtained detailed orbital parameters and a precise position for the pulsar. The radio pulsar signal shows frequent interactions with the atmosphere of the companion and suffers significant and strongly variable delays and intensity variations over a wide range of orbital phases. These characteristics and the binary parameters indicate that the companion may be a bloated main-sequence star or the remnant (still overflowing its Roche lobe) of the star that spun up the pulsar. In both cases, this would be the first binary millisecond pulsar system known with such a companion.

Discovery of Short-Period Binary Millisecond Pulsars in Four Globular Clusters

We report the discovery, using the Parkes radio telescope, of binary millisecond pulsars in four clusters for which no associated pulsars were previously known. The four pulsars have pulse periods lying between 3 and 6 ms. All are in circular orbits with low-mass companions and have orbital periods of a few days or less. One is in a 1.7 hr orbit with a companion of planetary mass. Another is eclipsed by a wind from its companion for 40% of the binary period despite being in a relatively wide orbit. These discoveries result from the use of improved technologies and prove that many millisecond pulsars remain to be found in globular clusters.

Blue Stragglers, Young White Dwarfs, and UV-Excess Stars in the Core of 47 Tucanae*

We used a set of archived Hubble Space Telescope/WFPC2 images to probe the stellar population in the core of the nearby galactic globular cluster (GGC) 47 Tuc. From the ultraviolet (UV) color magnitude diagrams (CMDs) obtained for ~4000 stars detected within the Planetary Camera (PC) field of view we have pinpointed a number of interesting objects: (1) 43 blue stragglers stars (BSSs), including 20 new candidates; (2) 12 bright (young) cooling white dwarfs (WDs) at the extreme blue region of the UV-CMD; and (3) a large population of UV-excess (UVE) stars, lying between the BSS and the WD sequences. The colors of the WD candidates identified here define a clean pattern in the CMDs, which define the WD cooling sequence. Moreover, both the location on the UV-CMDs and the number of WDs are in excellent agreement with the theoretical expectations. The UVE stars discovered here represent the largest population of anomalous blue objects ever observed in a globular cluster—if the existence of such a large population is confirmed, we have finally found the long-searched-for population of interacting binaries predicted by the theory. Finally, we have investigated the feasibility of the optical identification of the companions of the binary X-ray sources recently detected by Chandra and of binary millisecond pulsars (MSPs) residing in the core of 47 Tuc. Unfortunately, the extreme faintness expected for the MSP companions, together with the huge stellar crowding in the cluster center, prevents statistically reliable identifications based only on positional coincidences.

The Puzzling Dynamical Status of the Core of the Globular Cluster NGC 6752

We have used high-resolution Hubble Space Telescope Wide Field Planetary Camera 2 and ground-based wide-field images to determine the center of gravity and construct an extended radial density and brightness profile of the cluster NGC 6752 including, for the first time, detailed star counts in the very inner region. The barycenter of the nine innermost X-ray sources detected by Chandra is located only 19 off the new center of gravity. Both the density and the brightness profile of the central region are best fitted by a double King model, suggesting that NGC 6752 is experiencing a post-core-collapse bounce. Taking advantage of our new optical data, we discuss the puzzling nature of the accelerations displayed by the innermost millisecond pulsars detected in this cluster. We discuss two possible origins to the accelerations: (1) the overall cluster gravitational potential, which would require a central projected mass-to-light ratio of the order of 6-7 and the existence of a few thousand solar masses of low-luminosity matter within the inner 0.08 pc of NGC 6752, and (2) the existence of a local perturber(s) of the pulsar dynamics, such as a recently proposed binary black hole of intermediate (100-200 M☉) mass.

Discovery of a Young Radio Pulsar in a Relativistic Binary Orbit

We report on the discovery of PSR J1141-6545, a radio pulsar in an eccentric, relativistic 5 hr binary orbit. The pulsar shows no evidence of being recycled, having a pulse period P = 394 ms, a characteristic age τc = 1.4 × 106 yr, and an inferred surface magnetic dipole field strength B = 1.3 × 1012 G. From the mass function and measured rate of periastron advance, we determine the total mass in the system to be 2.300 ± 0.012 M☉, assuming that the periastron advance is purely relativistic. Under the same assumption we constrain the pulsars mass to be Mp ≤ 1.348 M☉, and the companions mass to be Mc > 0.968 M☉ (both with 99% confidence). Given the total system mass and the distribution of measured neutron star masses, the companion is probably a massive white dwarf that formed prior to the birth of the pulsar. Optical observations can test this hypothesis.

The Bright Optical Companion to the Eclipsing Millisecond Pulsar in NGC 6397

We report the optical identification of the companion to the eclipsing millisecond pulsar PSR J1740 5340 in the globular cluster NGC 6397. A bright variable star with an anomalous red color and optical variability (∼0.2 mag) that nicely correlates to the orbital period of the pulsar ( ∼1.35 days) has been found close to the pulsar position. The peculiar shape of the optical light curves is unprecedented for a millisecond pulsar companion and is a clear signature of tidal distortions. Subject headings: binaries: close — globular clusters: individual (NGC 6397) — pulsars: individual (PSR J17405340) — stars: evolution — stars: mass loss — stars: neutron

Discovery of three wide-orbit binary pulsars: Implications for binary evolution and equivalence principles

We report the discovery of three binary millisecond pulsars during the Parkes Multibeam Pulsar Survey of the Galactic plane. The objects are highly recycled and are in orbits of many tens of days about low-mass white dwarf companions. The eccentricity of one object, PSR J1853+1303, is more than an order of magnitude lower than predicted by the theory of convective fluctuations during tidal circularization. We demonstrate that under the assumption that the systems are randomly oriented, current theoretical models of the core-mass-orbital-period relation for the progenitors of these systems likely overestimate the white dwarf masses, strengthening previous concerns about the match of these models to the data. The new objects allow us to update the limits on the violation of relativistic equivalence principles to 95% confidence upper limits of 5.6 ? 10-3 for the strong equivalence principle parameter |?| and 4.0 ? 10-20 for the Lorentz-invariance/momentum-conservation parameter |3|.