Deep Radio Imaging of Globular Clusters and the Cluster Pulsar Population

Abstract

We have obtained deep multifrequency radio observations of seven globular clusters using the Very Large Array and the Australia Telescope Compact Array. Five of these, NGC 6440, NGC 6539, NGC 6544, NGC 6624 and Terzan 5 had previously been detected in a shallower survey for steep spectrum radio sources in globular clusters (Fruchter and Goss 1990). The sixth, the rich globular cluster, Liller 1, had heretofore been undetected in the radio, and the seventh, 47 Tucanae, was not included in our original survey. High resolution 6 and 20 cm images of three of the clusters, NGC 6440, NGC 6539, NGC 6624 reveal only point sources coincident with pulsars which have been discovered subsequent to our first imaging survey. 21 and 18 cm images reveal several point sources within a few core-radii of the center of 47 Tuc. Two of these are identified pulsars, and a third, which is both variable and has a steep spectrum, is also most likely a pulsar previously identified by a pulsed survey. However, the 6, 20 and 90 cm images of NGC 6544, Liller 1 and Terzan 5 display strong steep-spectrum emission which cannot be associated with known pulsars. The image of the rich cluster Terzan 5 displays numerous point sources within $30^{\prime \prime }$, or 4 core radii of the cluster center. The density of these objects rises rapidly toward the core, where an elongated region of emission is found. The brightest individual sources, as well as the extended emission, possess the steep spectra expected of pulsars. Furthermore, the flux distribution of the sources agrees well with the standard pulsar luminosity function. The total luminosity and number of objects observed suggest that Terzan 5 contains more pulsars than any other Galactic globular cluster.

33 pages, 6 Postscript figures; Accepted for publication in the Astrophysical Journal; abstract abridged. PDF version also available at http://nemesis.stsci.edu/~fruchter/fg99/fg99.pdf

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• Fermi Detection of a Luminous Gamma-ray Pulsar in a Globular Cluster
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• A Chandra Study of the Dense Globular Cluster Terzan 5
• Luminosities of recycled radio pulsars in globular clusters
• Very-high-energy gamma-ray emission from the direction of the Galactic globular cluster Terzan 5
• Predictions of Gamma-ray Emission from Globular Cluster Millisecond Pulsars Above 100 MeV
• HST-NICMOS Observations of Terzan 5: Stellar Content and Structure of the Core
• Six New Recycled Globular Cluster Pulsars Discovered with the Green Bank Telescope
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• Searching for Pulsars in Close Binary Systems
• A Search for Sub-Millisecond Pulsars
• The Pulsar Population in Globular Clusters and in the Galaxy
• The Rapid Burster in Liller 1: the Chandra X-ray Position and a Search for an IR Counterpart
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• Luminosities of radio Pulsars
• High energy gamma-ray emission from compact galactic sources in the context of observations with the next generation Cherenkov Telescope Arrays
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• Compact objects in globular clusters
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• A 34 Yr Timing Solution of the Redback Millisecond Pulsar Terzan 5A
• Relativistic scattering of a fast spinning neutron star by a massive black hole
• Gamma-rays from millisecond pulsars in Globular Clusters
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