Astronomy and Astrophysics – Astronomy
Scientific paper
Sep 2000
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2000phdt........55f&link_type=abstract
Ph. D. Thesis
Astronomy and Astrophysics
Astronomy
3
Scientific paper
Radio observations of the globular cluster 47 Tucanae have been performed over the last 11 years. These observations have resulted in the discovery of 20 millisecond pulsars, nine of which are presented in this work. We find that thirteen of the pulsars are members of binary systems, including all the nine new discoveries. Prior to this work only three binary systems had their orbital parameters determined; we now know the orbital parameters for twelve binary systems.
Three of the new discoveries are eclipsing binary pulsars. One of these (47 Tuc R) has the shortest orbital period measured for a radio pulsar (96 minutes). For two of the new binary pulsars (47 Tuc S and T), the orbital parameters were determined using a new procedure, described in this work, that uses the detected pulsar accelerations to determine the orbital parameters of the binary system.
We also present improved pulse profiles and luminosities for these pulsars, and find that their pulsewidth and flux density distributions are typical of galactic millisecond pulsars.
Prior to this work, only two of the pulsars in 47 Tuc had published timing solutions. Here we improve upon these two and present 13 new solutions. This new database is used to determine a variety of physical properties of the pulsars and of the cluster as a whole.
We find that all 15 pulsars with timing solutions are located within 1.2 arcmin of the cluster core. Their spatial density within that distance is consistent with a distribution of the type n(r) ∝ 1/r^2 with a cutoff distance of four core radii. Two pulsars (J0024-7204G and J0024-7204I) have a projected separation of only 0.12 arcsec, and could be part of a triple system containing these two observable pulsars.
We have measured the proper motions of four of the pulsars. The weighted mean of these motions is found to be a good approximation of the proper motion motion of 47 Tuc, and it is μ_{α} = (5.9 ± 0.6) mas yr^{-1} and μ_{δ} = (-4.1 ± 0.6) mas yr^{-1}. This value is more precise than the value for the proper motion of 47 Tuc measured by Hipparcos, which still is in good agreement with the value we derived.
The measured period derivatives of the pulsars are used to estimate limits for the surface mass density of the cluster. We verified that the average of the pulsar accelerations and their distribution are well described by a King model of 47 Tuc with nominal parameters for the distance and the stellar line-of-sight velocity dispersion at the centre of the cluster. All pulsars have characteristic ages greater than 170 Myr and have magnetic fields smaller than 2.3×10^5 T; their average characteristic age is larger than 1 Gyr. We also find an apparent relation between the DM of the pulsars and dot{P}, suggesting that the observed variations in DM are due to plasma in the cluster, with a total column density of ˜ 0.5 cm^{-3} pc.
We have also measured the rate of advance of periastron for the binary pulsar J0024-7204H, dot{ω} = (0.073 ± 0.005)° yr^{-1}. This implies that the binary system has a total mass of (1.9 ± 0.2) M_{&sun;}, at the 95% confidence level. We also discovered a modulation of DM with orbital phase for PSR J0023-7203J, this information is used to study the plasma envelope of the companion object and to improve the coherent timing solutions for this pulsar.
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