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Timing of Millisecond Pulsars in NGC 6752-II. Proper Motions of The

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Timing of Millisecond Pulsars in NGC 6752-II. Proper Motions of The DRAFT VERSION AUGUST 22, 2018 Preprint typeset using LATEX style emulateapj v. 6/22/04 TIMING OF MILLISECOND PULSARS IN NGC6752 - II. PROPER MOTIONS OF THE PULSARS IN THE CLUSTER OUTSKIRTS A. CORONGIU1,2,A.POSSENTI1, A.G. LYNE3, R.N. MANCHESTER4, F. CAMILO5, N. D’AMICO1,2 AND J.M. SARKISSIAN6 Draft version August 22, 2018 ABSTRACT Exploiting a five-year span of data, we present improved timing solutions for the five millisecond pulsars known in the globular cluster NGC6752. They include proper motion determinations for the two outermost pulsars in the cluster, PSR J1910-5959A and PSR J1910-5959C.The values of the proper motions are in agree- ment with each other within current uncertainties, but do not match (at 4σ and 2σ level respectively) with the value of the proper motion of the entire globular cluster derived in the optical band. Implications of these re- sults for the cluster membership of the two pulsars are investigated. Prospects for the detection of the Shapiro delay in the binary system J1910-5959A are also discussed. Subject headings: pulsars: individual (PSR J1910−5959A; PSR J1910−5959B; J1910−5959C; J1910−5959D; J1910−5959E) — globular clusters: individual (NGC6752) 1. INTRODUCTION explain the unusual position of PSR A, invoking a dynami- The globular cluster (GC) NGC6752 is known to host cal encounter in the inner region of the GC. The most prob- five millisecond pulsars (MSPs) (D’Amico et al. 2001; able picture is that PSR A was originally in the GC central D’Amico et al. 2002, hereafter PaperI). PSR J1910−5959B regions and it has been expelled to the outskirts by the in- and PSRJ1910−5959E (hereafter PSRB and E respectively) teraction with either a single massive black hole (BH) or a reside in the central region of the GC and show large neg- binary [BH+BH] of unequal mass. Timing results in PaperI indicated a low-mass white dwarf as the most probable com- ative P˙ values, which are interpreted as an effect of the GC panion for PSRA. This has been confirmed by Bassa et al. gravitational potential well (PaperI). This in turn implies a (2003) and Ferraro et al. (2003a), who identified with Hubble large mass-to-light ratio in the central region of NGC6752. Space Telescope observations the companion of PSRA with a Ferraro et al. (2003b) recalculated the center of gravity and helium white dwarf star whose mass is M ≃ 0 17 − 0 20M studied the luminosity profile of this cluster: combining co . ⊙ ˙ and whose photometric properties are compatible with its be- their HST data with the P value of PSR B and PSR E, they longing to NGC6752. put a firm lower limit on the central mass-to-light ratio of > The issue of the association of PSRA to NGC6752 has M/LV ∼ 5.5M⊙/L⊙. Also PSR J1910−5959D (PSRD) is ˙ been recently revisited using spectroscopic observations of located close to the GC center. Its P value is positive and the optical companion to the pulsar, performed with the ESO- of the same order of magnitude of PSRB and E, suggesting ˙ VLT. Cocozza et al. (2006) found full agreement (at 1σ) be- that also for PSR D the P value is dominated by the gravita- tween the radial velocity of the center of mass of the binary tional potential well (PaperI). PSR J1910−5959C7 (PSR C) is γ = −28.1 ± 4.9kms−1 and the overall cluster radial velocity ′ −1 located at a projected distance θ⊥ = 2.6 from the GC cen- v6752 = −27.9 ± 0.8km s , obtained by Harris (1996) (cata- ter (PaperI), which is much larger than the cluster’s core ra- log revision 2003) averaging various determinations. This is a ′′ ′′ arXiv:astro-ph/0609154v1 6 Sep 2006 dius rc = 5. 2 ± 2. 4 (Ferraro et al. 2003b). The binary pulsar strong indication in favour of the association of the pulsar with J1910−5959A (PSRA) is located at an even larger distance NGC6752. However, using the same data set, Bassa et al. ′ from the GC center (θ⊥ = 6.4, PaperI), the largest offset (2006) compared the systemic velocity of the binary with that known for a GC pulsar. of nearby stars which certainly belong to the cluster and con- The positions of PSRA and PSRC are unexpected since cluded that they are only marginally consistent at 2σ level. mass segregation should have driven the two neutron stars In this paper we present timing results based on more than < close to the GC center in a time scale (∼ 1Gyr) much shorter five years of regular observations. In particular, with a much than the time since their formation (∼ 10Gyr). In particu- longer available data span we have been able to measure lar, Colpi et al. (2002, 2003) explored various scenarios to proper motions of PSRA and PSRC. The new timing solu- tions as well as the pulse profiles for all the millisecond pul- 1 INAF - Osservatorio Astronomico di Cagliari, Loc. Poggio dei Pini, sars are presented in §2, §3 reports on the proper motion de- Strada 54, 09012 Capoterra (CA), Italy 2 Università degli Studi di Cagliari, Dip. di Fisica, S.S. Monserrato-Sestu terminations and implications for the cluster membership of km 0,700, 09042 Monserrato (CA), Italy the two pulsars are discussed in § 4. 3 University of Manchester, Jodrell Bank Observatory, Jodrell Bank, Mac- clesfield, Cheshire, SK11 9DL, UK 2. OBSERVATIONS AND IMPROVED TIMING PARAMETERS 4 Australia Telescope National Facility, Commonwealth Scientific and In- dustrial Research Organization, P.O. Box 76, Epping, NSW 1710, Australia Regular pulsar timing observations of NGC6752 have been 5 Columbia Astrophysics Laboratory, Columbia University, 550 West, carried out since September 2000 with the Parkes 64m radio 120th Street, New York, NY 10027 telescope at a central frequency of 1390MHz, using the cen- 6 Australia Telescope National Facility, CSIRO, Parkes Observatory, P.O. tral beam of the multibeam receiver or the H-OH receiver. Box 276, Parkes, New South Wales 2870, Australia The hardware system is the same as that used in the discov- 7 Note that, to conform with currently accepted practice, all pulsars asso- ciated with the cluster have been given a J2000 name with the same rounded ery observations (D’Amico et al. 2001). The effects of inter- coordinates, corresponding approximately to the cluster centre. stellar dispersion are minimised by using a filterbank having 2 Corongiu et al. 512×0.5MHz frequency channels for each polarization. Af- The small eccentricity of PSR A’s binary system is typical ter detection, the signals from individual channels are added of fully recycled binary millisecond pulsars and is consistent in polarization pairs, integrated, 1 bit-digitized every 125 µs with the effects of random encounters with other cluster stars (80 µs in recent observations), and recorded to magnetic tape (Rasio & Heggie 1995). The upper limit on e is also compat- for off-line analysis. Pulse times of arrival (TOAs) are de- ible with the offset position of PSR A resulting from an inter- termined by fitting a template profile to the observed mean action which occurred ∼ 1 Gyr ago between the already re- pulse profiles and analysed using the program TEMPO8 and cycled binary system including PSRA and a WD companion the DE405 solar system ephemeris. [PSR A+WD] with a binary black hole of a few tens of solar Table1 summarizes the best fit values and uncertainties masses (Colpi et al. 2003, see§1). We note that the value of (chosen to be twice the nominal TEMPO errors) for the pa- e also fits with the hypothesis (Bassa et al. 2003; Colpi et al. rameters entering our timing solutions, whose residuals are 2003) that a dynamical encounter with a single BH, whose displayed in Figure1. The same figure presents a high signal- mass is higher than a few hundred M⊙, may have simultane- to-noise profile obtained for each of the pulsars by folding the ously ejected the progenitor of [PSR A+WD] and triggered best available data according to the reported ephemerides. the recycling process in the binary, which in turn circularized The new positional and rotational parameters at the refer- the system and removedany informationon its post-encounter ence epoch are all compatibile with those reported in PaperI eccentricity. However, the value of e does not agree with an (assuming 3σ uncertainties for the values quoted in PaperI). ejection event involving the already formed [PSR A+WD] bi- However, the MJD range of the available TOAs is now ∼ 3.5 nary and a single BH. In this case, Colpi et al. (2003) showed times longer than for PaperI and hence the accuracy of the so- that the post-encounter eccentricity of [PSRA+WD] would − − lutions has improved correspondingly. Orbital parameters for be significantly larger, up to values of 10 4 − 10 2 and only PSR A, obtained using the ELL1 model of TEMPO, have also slightly affected by subsequent random encounters with nor- been measured with a higher precision than in PaperI. Figure mal stars of the cluster (Rasio & Heggie 1995). 2 shows that no trend is evident in the timing residuals plotted The mean flux densities at 1400MHz (S1400) in Table 1 are with respect to the orbital phase for the timing solution given average values, derived from the system sensitivity, the ob- in Table 1. An additional constraint on the orbit of PSRA served signal-to-noise ratio, the shape of the pulse profile, has resulted from the recent optical observations of the pulsar the displacement of the pulsars with respect to the center of companion. Spectroscopy (Cocozza et al. 2006; Bassa et al.
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