Variability of 19 Millisecond Pulsars in 47 Tucanae with Chandra Hrc-S P
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The Astrophysical Journal, 660:587Y594, 2007 May 1 # 2007. The American Astronomical Society. All rights reserved. Printed in U.S.A. VARIABILITY OF 19 MILLISECOND PULSARS IN 47 TUCANAE WITH CHANDRA HRC-S P. B. Cameron,1 R. E. Rutledge,2 F. Camilo,3 L. Bildsten,4 S. M. Ransom,5 and S. R. Kulkarni 1 Received 2006 August 27; accepted 2006 December 24 ABSTRACT We present results from our 830 ks observation of the globular cluster 47 Tucanae with the Chandra X-ray Ob- servatory’s High Resolution Camera-S. We limit our analysis here to the 19 previously known, localized millisecond pulsars (MSPs) in the cluster. This work more than doubles the sample of X-rayYdetected MSPs observed with sen- sitivity to rotational variability; it is also the first survey of a large group of radio-discovered MSPs for which no previous X-ray pulsations have been detected and is therefore an unbiased survey of the X-ray properties of radio- discovered MSPs. We find that only 47 Tuc D, O, and R show significant pulsations at the k4 level, but there is statistical evidence for rotational variability in five additional MSPs. Furthermore, we constrain the pulsed magne- tospheric emission of seven more MSPs using Monte Carlo simulations. The result is that the majority of the 47 Tuc MSPs are characterized by low pulsed fractions, P50%. In cases where larger pulsed fractions are measured, the folded pulse profiles show relatively large duty cycles. When considered with previous spectroscopic studies, this suggests that the X-ray emission arises from the neutron star’s heated polar caps and, in some cases, from intrabinary shocks, but generally not directly from the star’s magnetosphere. We discuss the impact of these results on our un- derstanding of high-energy emission from MSPs. Subject headings:g globular clusters: individual (47 Tucanae) — pulsars: general — stars: neutron — X-rays: stars 1. INTRODUCTION phology and phase with large pulsed fractions, fp k 50% (see 3.1). Power-law spectral components can also be produced when Millisecond pulsars (MSPs) are old neutron stars spun-up by x the wind from the MSP interacts with material from the binary accretion of mass and angular momentum from the matter of a companion causing an intrabinary shock. These pulsars have sim- donor binary companion (Alpar et al. 1982). When compared to the canonical radio pulsar population, they are distinguished ilar properties to those above, with the exception that they lack strong rotational modulation (e.g., the ‘‘black widow’’ pulsar, by short spin periods, P P 25 ms, small spin-down rates, P˙ k PSR 1957+20). Finally, heating of the neutron star polar cap by 10À20, and thus low inferred dipole magnetic field strengths, the bombardment of relativistic particles provides a mechanism B / (PP˙ )1/2 108Y1010 G, with large characteristic ages, dipole for producing thermal X-ray emission. MSPs dominated by ther- P/2P˙ k1 Gyr. Studies of the 150 known MSPs are diffi- mal spectra (e.g., PSR J0437 4715 and J2124 3358; most of cult at wave bands outside of the radio due to their intrinsic faint- À À those below the first four entries in Table 1) are characterized by ness. The vast majority (80%) of MSPs have binary companions lower spin down energies (E˙ P1034 ergs sÀ1), lower X-ray lumi- that dominate at optical wavelengths; thus, X-rays are an impor- nosities (L P1032 ergs sÀ1), and pulse profiles with large duty tant avenue for studying MSPs. X cycles. As seen in Table 1, the pulsed fractions of these MSPs Currently, only 16 MSPs outside of 47 Tucanae (NGC 104, hereafter 47 Tuc) have been detected in X-rays, and only 12 of are usually poorly constrained, but generally show fp 50%. The emission of thermal cooling X-rays from the neutron star surface these have been observed with sufficient time resolution to explore and those from pulsar wind nebulae are not thought to be impor- variability on rotational timescales (see Table 1). There are several proposed physical mechanisms capable of generating X-rays from tant for these old objects, so we will not consider them here. The unprecedented spatial resolution of Chandra has enabled these MSPs. Nonthermal emission processes in the neutron star detailed studies of MSPs in globular clusters. Observations of magnetosphere generate power-law components in their X-ray M28, M4, NGC 6397, M30, and others have provided the first spectra with characteristic photon indices À 1:5Y2. Pulsars in census of low-luminosity X-ray sources in these clusters (Rutledge this class (e.g., PSR B1937+21 and B1821À24A; the first four en- et al. 2004; Becker et al. 2003; Bassa et al. 2004; Grindlay et al. tries in Table 1) have large spin-down energies (E˙ k1035 ergs sÀ1), 2002; Ransom et al. 2004). However, the largest endeavor has bright X-ray emission (L k1032 ergs sÀ1), low duty cycles, and X been Chandra’s observing campaign of 47 Tuc (Grindlay et al. pulse profiles that closely resemble the radio emission in mor- 2001, 2002; Heinke et al. 2005; Bogdanov et al. 2006). This work has shown that the spectral characteristics of the 47 Tuc MSPs 1 Division of Physics, Mathematics and Astronomy, California Institute of are relatively homogeneous. Their luminosities fall in a narrow Technology, MS 105-24, Pasadena, CA 91125; [email protected], srk@astro 30 31 À1 range, LX 10 Y10 ergs s , and are well described by ther- .caltech.edu. Y 2 Department of Physics, McGill University, Rutherford Physics Building, mal spectral models with small emission radii, ReA 0:1 3 km, 6 Montreal, QC H3A 2T8, Canada; [email protected]. and temperatures of TeA 1Y3 ; 10 K. The only exceptions are 3 Columbia Astrophysics Laboratory, Columbia University, New York, NY the radio-eclipsing binaries 47 Tuc J, O, and W, which require 10027; [email protected]. Y 4 additional power-law components above 2 keV,with À 1:0 1:5. Kavli Institute for Theoretical Physics and Department of Physics, Kohn Hall, These results are reinforced by the findings of detailed spectro- University of California, Santa Barbara, CA 93106; [email protected]. 5 National Radio Astronomy Observatory, Charlottesville, VA22903; sransom@ scopic studies by Chandra and XMM-Newton that have empha- nrao.edu. sized the dominant thermal components in nearby MSP X-ray 587 588 CAMERON ET AL. Vol. 660 TABLE 1 X-ray Properties of Millisecond Pulsars Outside of 47 Tuc P d logE˙ log LX fp PSR (ms) (kpc) (Gyr) (ergs sÀ1) (ergs sÀ1) (%) References B1937+21....................................................... 1.56 3.57 0.23 36.04 33.15 54 Æ 71,2 B1957+20....................................................... 1.61 2.49 2.22 35.04 31.81 <60 1, 3 J0218+4232.................................................... 2.32 2.67 0.48 35.38 32.54 59 Æ 74,5 B1821À24A (M28)....................................... 3.05 5.5a 0.03 36.35 33.22 85 Æ 36,7 J0751+1807.................................................... 3.48 1.15 7.08 33.86 30.84 52 Æ 8b 8, 9 J0030+0451.................................................... 4.87 0.32 7.71 33.53 30.40 69 Æ 18 10, 11 J2124À3358................................................... 4.93 0.25 6.01 33.83 30.23 56 Æ 14 1, 12 J1012+5307.................................................... 5.26 0.84 4.86 33.68 30.38 77 Æ 13b 13, 9 J0437À4715................................................... 5.76 0.14a 4.91 33.58 30.47 40 Æ 2 14, 12 J1024À0719................................................... 5.16 0.39 27 32.93 29.30 52 Æ 22 1, 12 J1744À1134................................................... 4.07 0.36a 9.1 33.62 29.49c ... 1, 3 J0034À0534................................................... 1.88 0.54 6.03 34.48 29.60c ... 12 No High Time Resolution Imaging B1620À26 (M4)............................................ 11.08 1.73a 0.26 34.28 30.08c ... 15, 16 J1740À5340 (NGC 6397) ............................ 3.65 2.55a 0.34 35.15 30.9c ... 17, 18 J1911À6000C (NGC 6752) .......................... 5.28 4.1a 38.1 32.77 30.34c ... 19 J2140À2310A (M30) .................................... 11.02 9.0a >0.08 <34.79 30.64c ... 20 Notes.—All distances are estimated from the pulsar dispersion measures and the model of Galactic distribution of free electrons (Cordes & Lazio 2002), except where noted. X-ray luminosities are quoted in the 0.2Y10 keV band, as adopted from Table 1 of Zavlin (2006) and references therein, except where noted. Pulsed fractions are quoted in roughly the HRC-S band (0.1Y2.0 keV), but see ref- erences for the specific bandpass. The spectra of the first four MSPs are dominated by nonthermal X-ray emission. The others have signif- icant thermal components or are indeterminate (and thus presumed to be thermal) in nature. a Accurate distance measurement. b Detection significance is low. c X-ray luminosity in the 0.5Y2.5 keV band. References.— (1) Toscano et al. 1999; (2) Nicastro et al. 2004; (3) Becker & Tru¨mper 1999; (4) Navarro et al. 1995; (5) Webb et al. 2004a; (6) Becker et al. 2003; (7) Rutledge et al. 2004; (8) Nice et al. 2005; (9) Webb et al. 2004b; (10) Lommen et al. 2000; (11) Becker & Aschenbach 2002; (12) Zavlin 2006; (13) Lange et al. 2001; (14) Zavlin et al. 2002; (15) Thorsett et al. 1999; (16) Bassa et al. 2004; (17) D’Amico et al. 2001; (18) Grindlay et al. 2002; (19) D’Amico et al. 2002; (20) Ransom et al. 2004. spectra over the fainter power-law features (Zavlin et al. 2002, Interactive Analysis of Observations6 software (CIAO), version 2006). Consequently, we expect the predominant X-ray emission 3.3, and CALDB, version 3.2.1.