In the last ten years, 20 millisecond pulsars have been discovered in the globular cluster 47 Tucanae. Hitherto, only three of these pulsars had published timing solutions. Here we improve upon these three and present 12 new solutions. These measurements can be used to determine a variety of physical properties of the pulsars and of the cluster. The positions of the 15 pulsars have been determined with typical errors of only a few mas and they are all located within 1.2 arcmin of the cluster centre. Their spatial density within that region is consistent with a distribution of the type n(r) proportional to r(-2), with a sudden cut-off outside four core radii. Two pulsars have a projected separation of only 0.12 arcsec, and could be part of a triple system containing two observable pulsars. We have measured the proper motions of five of the pulsars: the weighted mean of these, mu (alpha) = (6.6 +/- 1.9) mas yr(-1) and mu (delta) = (-3.4 +/- 0.6) mas yr(-1), is in agreement with the proper motion of 47 Tucanae based on Hipparcos satellite data. The period derivatives measured for many of the pulsars are dominated by the dynamical effects of the cluster gravitational field, and are used to constrain the surface mass density of the cluster. The pulsar accelerations inferred from the observed period derivatives are consistent with those predicted by a King model using accepted cluster parameters. We derive limits on intrinsic pulsar parameters: all the pulsars have characteristic ages greater than 170 Myr and have magnetic fields smaller than 2.4 x 10(9) Gauss; their average characteristic age is greater than similar to1 Gyr. We have also measured the rate of advance of periastron for the binary pulsar J0024-7204H, (omega) over dot = (0.059 +/- 0.012)degrees yr(-1), implying a total system mass of 1.4(-0.8)(+0.9) M. with 95 per cent confidence.
Timing the millisecond pulsars in 47 Tucanae
D'AMICO, NICOLO'
2001-01-01
Abstract
In the last ten years, 20 millisecond pulsars have been discovered in the globular cluster 47 Tucanae. Hitherto, only three of these pulsars had published timing solutions. Here we improve upon these three and present 12 new solutions. These measurements can be used to determine a variety of physical properties of the pulsars and of the cluster. The positions of the 15 pulsars have been determined with typical errors of only a few mas and they are all located within 1.2 arcmin of the cluster centre. Their spatial density within that region is consistent with a distribution of the type n(r) proportional to r(-2), with a sudden cut-off outside four core radii. Two pulsars have a projected separation of only 0.12 arcsec, and could be part of a triple system containing two observable pulsars. We have measured the proper motions of five of the pulsars: the weighted mean of these, mu (alpha) = (6.6 +/- 1.9) mas yr(-1) and mu (delta) = (-3.4 +/- 0.6) mas yr(-1), is in agreement with the proper motion of 47 Tucanae based on Hipparcos satellite data. The period derivatives measured for many of the pulsars are dominated by the dynamical effects of the cluster gravitational field, and are used to constrain the surface mass density of the cluster. The pulsar accelerations inferred from the observed period derivatives are consistent with those predicted by a King model using accepted cluster parameters. We derive limits on intrinsic pulsar parameters: all the pulsars have characteristic ages greater than 170 Myr and have magnetic fields smaller than 2.4 x 10(9) Gauss; their average characteristic age is greater than similar to1 Gyr. We have also measured the rate of advance of periastron for the binary pulsar J0024-7204H, (omega) over dot = (0.059 +/- 0.012)degrees yr(-1), implying a total system mass of 1.4(-0.8)(+0.9) M. with 95 per cent confidence.
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