The Timing of Nine Globular Cluster Pulsars

📝 Abstract

We have used the Robert C. Byrd Green Bank Telescope to time nine previously known pulsars without published timing solutions in the globular clusters M62, NGC 6544, and NGC 6624. We have full timing solutions that measure the spin, astrometric, and (where applicable) binary parameters for six of these pulsars. The remaining three pulsars (reported here for the first time) were not detected enough to establish solutions. We also report our timing solutions for five pulsars with previously published solutions, and find good agreement with past authors, except for PSR J1701-3006B in M62. Gas in this system is probably responsible for the discrepancy in orbital parameters, and we have been able to measure a change in the orbital period over the course of our observations. Among the pulsars with new solutions we find several binary pulsars with very low mass companions (members of the so-called “black widow” class) and we are able to place constraints on the mass-to-light ratio in two clusters. We confirm that one of the pulsars in NGC 6624 is indeed a member of the rare class of non-recycled pulsars found in globular clusters. We also have measured the orbital precession and Shapiro delay for a relativistic binary in NGC 6544. If we assume that the orbital precession can be described entirely by general relativity, which is likely, we are able to measure the total system mass (2.57190(73) M_sun) and companion mass (1.2064(20) M_sun), from which we derive the orbital inclination [sin(i) = 0.9956(14)] and the pulsar mass (1.3655(21) M_sun), the most precise such measurement ever obtained for a millisecond pulsar. The companion is the most massive known around a fully recycled pulsar.

💡 Analysis

We have used the Robert C. Byrd Green Bank Telescope to time nine previously known pulsars without published timing solutions in the globular clusters M62, NGC 6544, and NGC 6624. We have full timing solutions that measure the spin, astrometric, and (where applicable) binary parameters for six of these pulsars. The remaining three pulsars (reported here for the first time) were not detected enough to establish solutions. We also report our timing solutions for five pulsars with previously published solutions, and find good agreement with past authors, except for PSR J1701-3006B in M62. Gas in this system is probably responsible for the discrepancy in orbital parameters, and we have been able to measure a change in the orbital period over the course of our observations. Among the pulsars with new solutions we find several binary pulsars with very low mass companions (members of the so-called “black widow” class) and we are able to place constraints on the mass-to-light ratio in two clusters. We confirm that one of the pulsars in NGC 6624 is indeed a member of the rare class of non-recycled pulsars found in globular clusters. We also have measured the orbital precession and Shapiro delay for a relativistic binary in NGC 6544. If we assume that the orbital precession can be described entirely by general relativity, which is likely, we are able to measure the total system mass (2.57190(73) M_sun) and companion mass (1.2064(20) M_sun), from which we derive the orbital inclination [sin(i) = 0.9956(14)] and the pulsar mass (1.3655(21) M_sun), the most precise such measurement ever obtained for a millisecond pulsar. The companion is the most massive known around a fully recycled pulsar.

📄 Content

arXiv:1112.2612v2 [astro-ph.HE] 9 Jan 2012 The Timing of Nine Globular Cluster Pulsars Ryan S. Lynch1,2, Paulo C. C. Freire3, Scott M. Ransom4, and Bryan A. Jacoby5 ABSTRACT We have used the Robert C. Byrd Green Bank Telescope to time nine previ- ously known pulsars without published timing solutions in the globular clusters M62, NGC 6544, and NGC 6624. We have full timing solutions that measure the spin, astrometric, and (where applicable) binary parameters for six of these pulsars. The remaining three pulsars (reported here for the first time) were not detected enough to establish solutions. We also report our timing solutions for five pulsars with previously published solutions, and find good agreement with past authors, except for PSR J1701−3006B in M62. Gas in this system is proba- bly responsible for the discrepancy in orbital parameters, and we have been able to measure a change in the orbital period over the course of our observations. Among the pulsars with new solutions we find several binary pulsars with very low mass companions (members of the so-called “black widow” class) and we are able to place constraints on the mass-to-light ratio in two clusters. We confirm that one of the pulsars in NGC 6624 is indeed a member of the rare class of non- recycled pulsars found in globular clusters. We also have measured the orbital precession and Shapiro delay for a relativistic binary in NGC 6544. If we assume that the orbital precession can be described entirely by general relativity, which is likely, we are able to measure the total system mass (2.57190(73) M⊙) and companion mass (1.2064(20) M⊙), from which we derive the orbital inclination (sin i = 0.9956(14)) and the pulsar mass (1.3655(21) M⊙), the most precise such measurement ever obtained for a millisecond pulsar. The companion is the most massive known around a fully recycled pulsar. 1Physics Department, McGill University, 3600 Rue University, Montr´eal, QC, Canada H3A 2T8, rlynch@physics.mcgill.ca 2Department of Astronomy, University of Virginia, P.O. Box 400325, Charlottesville, VA 22904-4325 3Max-Planck-Institut f¨ur Radioastronomie, Auf dem H¨ugel 69, D-53121 Bonn, Germany, pfreire@mpifr-bonn.mpg.de 4National Radio Astronomy Observatory, 520 Edgemont Road, Charlottesville, VA 22903-4325, sransom@nrao.edu 5Affiliated with The Aerospace Corporation, 15049 Conference Center Drive, Chantilly, VA 20151-3824, bryan.jacoby@gmail.com – 2 – Subject headings: globular clusters: individual (M62, NGC 6544, NGC 6624)— pulsars: individual (J1701−3006D, J1701−3006E, J1701−3006F, J1807−2459A, J187−2500B, J1823−3021C, J1823−3021D, J1823−3021E, J1823−3021F) 1. Introduction Millisecond pulsars (MSPs) form by “recycling” a dormant neutron star through the accretion of mass and angular momentum from a binary companion (Alpar et al. 1982). This leads to a very rapidly rotating and stable pulsar with a relatively low magnetic field (∼109 G) and long lifetime (≳109 yr). MSPs form naturally in the dense environments of globular clusters (GCs), thanks to frequent exchange interactions that may lead to the formation of mass transferring binaries (Camilo & Rasio 2005). Sensitive searches of GCs have uncovered 144 pulsars in 28 clusters6, and the vast majority of these are recycled MSPs. Indeed, nearly half of all MSPs have been discovered in GCs7. The same interactions that form MSPs so efficiently in clusters also lead to many exotic systems that are rarely seen in the disk of the Galaxy. These include the fastest spinning MSP (Hessels et al. 2006), highly eccentric binaries (Ransom et al. 2005; Freire et al. 2007), massive neutron stars (Freire et al. 2008), pulsar-main sequence binaries (D’Amico et al. 2001b), and many “black widow” systems (King et al. 2005). These discoveries demonstrate the huge scientific payoffthat can come from the discovery of unique pulsars thanks to two factors—the extreme nature of neutron stars, which opens windows on otherwise inaccessible realms of physics, and the extraordinary clock-like nature of MSPs. The bedrock of pulsar astronomy is timing, the process of creating a model that unambiguously accounts for every rotation of the pulsar, thus probing the pulsar and its environment. For MSPs, the arrival time of a pulse can typically be measured to within a few microseconds or better (e.g. Verbiest et al. 2008), which enables very precise timing models. Timing GC pulsars leads to unique challenges, though. Because GCs are usually at a distance of several kiloparsecs, the flux density of their constituent MSPs is usually very low, necessitating very long integration times. Another consequence of cluster distances is very high dispersion measures, which necessitate moving to higher observing frequencies (e.g. 2 GHz) where pulsars tend to be weaker. Luckily, since many clusters contain several MSPs that can each be observed during a single observation, the required observing time is well spent. 6See http://www.naic.edu/~pfreire/GCpsr.html for an up-t

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