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A search for fast pulsars in globular clusters Begin, Steve

Abstract

Millisecond pulsars (MSP) are old neutron stars that have been spun up to high spin frequencies(as fast as 716 Hz) through the accretion of matter from a companion star. The extreme stellar densities in the core of globular clusters creates numerous accreting neutron star systems through exchange interactions; this leads to the formation of MSPs in larger numbers than in the galactic disk. Over the course of this project, we have collected over 17 TB of data on the 3 globular clusters M28, NGC6440 and NGC6441 plus 2 observations on NGC6522 and NGC6624 as part of the recently begun S-band survey using the Green Bank telescope. I have analyzed and conducted acceleration searches on 70% of the data and discovered 7 of the 23 new millisecond pulsars reported in this work. One year of timing observations of the pulsars in M28 and NGC6440 has led to the phase connected solution for 12 of the 15 new pulsars in those two clusters, 7 of which are in binaries. We have measured the rate of advance of periastron for two highly eccentric binaries and assuming this is purely due to general relativity, this leads to total system masses of (1.616 - 0.014)M and (2.2 - 0.8)M for M28C and NGC6440B respectively. The small mass function combined with this information imply that the most likely neutron star mass of NGC6440B is either very large or else there could be significant contribution to the advance of periastron from a nonzero quadrupole moment due to tidal interaction with the companion. Measurements of the period derivatives for many of the pulsars show that they are dominated by the dynamical effect of the gravitational field of the clusters. Finally, we have discovered the potential presence of a Mars-mass planet orbiting the pulsar NGC6440C with a period of 21 days. A dedicated timing campaign will be necessary to confirm the presence of such an object.

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Attribution-NonCommercial-NoDerivatives 4.0 International

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