Mon. Not. R. Astron. Soc. 310, 313±316 (1999)

Vela-size glitch rates in youthful pulsars

J. O. Urama1,2 and P. N. Okeke2 1Hartebeesthoek Radio Astronomy Observatory (HartRAO), PO Box 443, Krugersdorp 1740, South Africa 2Department of Physics & Astronomy, University of Nigeria, Nsukka, Enugu State, Nigeria

Accepted 1999 June 23. Received 1999 May 24; in original form 1998 December 30

ABSTRACT A total of 71 glitches Dn=n $ 1029† have been reported to date in 30 pulsars. 13 of these glitches come from the Vela pulsar, whose glitches are mostly of magnitude Dn=n , 1026. Only about 40 per cent of the total glitches are of this magnitude. While glitches of this size have been observed from pulsars aged 104±107 yr, 80 per cent of them come from the youthful pulsars, aged 104±105 yr. For pulsars older than 104 yr, the glitch activity is found to be proportional to the logarithm of the spin-down rate, nÇ. Based on this relationship, we identify six other pulsars that are most likely to yield frequent large glitches. Since these pulsars are of comparable ages to the Vela pulsar, real-time glitch detection on them and studies of their subsequent recovery would play a vital role in improving understanding of the neutron star interior and the pulsar glitch mechanisms. Key words: methods: statistical ± stars: neutron ± pulsars: general.

1 INTRODUCTION 2 GLITCH SIZE DISTRIBUTION In addition to a predictable general slow-down in their rotation Table 1 shows that 71 glitches have so far been reported in 30 rate (n), some pulsars exhibit timing irregularities in the form of pulsars, representing about 4 per cent of the total pulsar glitches or timing noise. A glitch is a spectacular step change in population. About two-thirds of these glitches occur in youthful rotation rate, usually accompanied by a change in spin-down rate pulsars, which constitute fewer than half of these 30. The (n_ ). While large glitches have a well-defined signature, fractional changes in spin-down rate have been measured only in Dn; Dn_ †ˆ 1; 2†; microglitches exhibit all possible signatures about two-thirds of the reported glitches. The distribution of these (Cordes, Downs & Krause-Polstorff 1988). Some of these jump sizes as a function of age is shown in Fig. 1. Smaller values microglitches have been explained as mere timing noise (e.g. of Dn/n and Dn_=n_ are seen to be more common in pulsars older Cordes & Helfand 1980), which is a fairly continuous erratic than 105 yr. The `Vela-size' jumps Dn=n , 1026† represent about behaviour in phase, frequency or frequency derivative. In this two-fifths of the total glitches in Table 1, the youthful pulsars paper, we have restricted the term glitch to jumps characterized by contributing over 80 per cent of them. None has been reported thus fractional changes in rotation rate, Dn=n $ 1029. It is known that far on very young , 104 yr† pulsars. For all the pulsars for which glitches are not due to sudden changes in the external ages have been determined (unpublished catalogue of Taylor et al. electromagnetic torques on the neutron star, and glitch models 1995), we also find that nearly 70 per cent of the youthful pulsars are therefore built on changes in the structure of the neutron star or have glitched (Fig. 2a). The average number of glitches per pulsar in the distribution and transfer of angular momentum in the is also highest for this age group (Fig. 2b). No glitch has been neutron star (Alpar 1995). There has been no lack of theories to observed in radio pulsars older than 107 yr and, out of five pulsars explain pulsar glitch trigger mechanisms and subsequent post- younger than 104 yr, only one (the Crab pulsar) has glitched. The glitch behaviour, as well as timing noise [see D'Alessandro (1996) surprisingly low level of glitch activity in these very young pulsars for a recent review]. has been attributed to their higher internal temperatures which Here, our focus is on the `large-glitch' rates of youthful (104± reduce the importance of pinning, resulting in a relatively smooth 105 yr) pulsars. In Section 2, we present the glitch magnitudes of transfer of angular momentum to their crusts (McKenna & Lyne all those observed to date. The glitch activity of those pulsars 1990). with repeated glitches is used in Section 3 to establish a Owing to long inter-observation gaps, fractional changes in relationship with observable pulsar parameters. Based on the spin-down rate are poorly measured, with errors in excess of relationship, we identify pulsars that would make the best targets 200 per cent in some cases (e.g. McKenna & Lyne 1990; Shemar for frequent observations to detect very large glitches soon after & Lyne 1996). This makes DnÇ/nÇ poorly suited to a statistical they happen. analysis of glitch behaviour. High-precision values of DnÇ/nÇ have q 1999 RAS 314 J. O. Urama and P. N. Okeke

Table 1. Jump parameters for all known glitches Dn=n $ 1029†.

PSR B Epoch Dn/n DnÇ/nÇ Ref. PSR B Epoch Dn/n DnÇ/nÇ Ref. (MJD) (1029) (1023) (MJD) (1029) (1023) 0355154 46079(1) 5.62(8) 1.6(4) 1,2 1641245 47589(4) 1.61(4) 1.1(1) 19 46496(8) 4368(2) 100(50) 1,2 1706244 48775(15) 2057(2) 4.0(1) 17 0525121 42057(7) 1.2(3) 3(5) 3 1727233 48000(10) 3033(8) 3.5(6) 17 0531121 40493.4 6(2) 0.035(2) 4 1727247 49387.68(3) 137.40(6) 1.28(2) 15 42448 38.3(7) 0.217(1) 4 50718(4) 3.2 1.4 10 46664.42(5) 9.2(1) 2.5(2) 5 1736229 46956(6) 2.9(2) 1.0(2) 20 47768.40(2) 67(2) 6 1737230 47003(50) 420(20) 2.8(8) 21 0611122 42650(100)* 35 7,8 47281(2) 33(5) 1.7(4) 21 0740228 47679(5) 2.63 20.40 9 47332(16) 7(5) 21(12) 21 48350(10) 1.5 20.22 9 47458(2) 30(8) 0(4) 21 49298(4) 1.9 0.9 10 47670.2(2) 600.9(6) 2(4) 21 50376(15) 1.3 20.7 10 48186(6) 642(16) 25(12) 20 0743253 43765(15) 27 11 48218(2) 48(10) 8(12) 20 0823126 42000(100)* 23 7,8 48431(2) 15.7(5) 0.8(3) 15,20 0833245 40280(4) 2336(9) 10(1) 12 49046(1) 9.2(3) 0.29(5) 15,20 41191(7) 2045(30) 15(6) 12 49239.4(1) 168.6(6) 0.96(6) 15,20 41312(4) 12(2) 2.5(6) 12 49456(3) 9(2) 20.2(5) 15 42682(3) 1986(8) 10.6(6) 12 49551(2) 7.8(5) 0.40(9) 15 43692(12) 3061(64) 18(9) 12 50936.803(4) 1446.0(6) 5(2) 10 44888.0707(2) 1145(3) 49(4) 13 1757224 49476(6) 1873 5.6 22 45192(1) 2051(3) 23(1) 13 1758223 46907(43) 217(7) 21(2) 20,23 46527.2284(8) 1601(1) 17(1) 13 47855(50) 231.9(2) 0.2(6) 20,23 47519.803(1) 1807.1(8) 120(10) 13 48454 347(2) 0(1) 20,23 48457.382(1) 2715(2) 600(60) 13 49709 64(2) 1(2) 20 49559.057 835(2) 13 1800221 48245(20) 4073(16) 9.1(2) 20 49591.158 199(2) 120(20) 13 1822209 49940(2) 5.21(7) 22.39(4) 24 50369.394 2150(20) 14 50557(6) 12.6(2) 0.2(2) 24 1046258 50791.485(5) 772(2) 26(7) 10 1823213 46507(60) 2718(26) 6.8(1) 20 1325243 43590(24) 116 11 49014(80) 3049(50) 9.0(5) 20 1338262 47989(22) 1504(3) 16 1830208 48041(20) 1865.9(4) 1.8(5) 20 48453(12) 23(6) 16 1859103 47200(100)* 21.3 7,8 48645(11) 993(1) 16 1859107 46859(6) 30(2) 46(50) 20 1358263 48305 2.49 3.5 10 1916114 42750(100)* 15(5) 7,8 1535256 48165(15) 2793(1) 1.1(6) 17 1929120 42860(60)* 3.5 7,8 1641245 43390(63) 191(1) 1.6(5) 18 2224165 43072(40) 1710(20) ,625 46453(35) 803.6(1) 0.5(3) 19 * Glitch epoch estimated from the plots in Gullahorn & Rankin (1982). References: (1) Shabanova (1990); (2) Lyne (1987); (3) Downs (1982); (4) Lohsen (1981); (5) Lyne, Pritchard & Graham-Smith (1993); (6) Lyne & Pritchard (1989); (7) Gullahorn & Rankin (1978); (8) Gullahorn & Rankin (1982); (9) D'Alessandro et al. (1993); (10) Urama & Flanagan (in preparation); (11) Newton, Manchester & Cooke (1981); (12) Cordes et al. (1988), and references therein; (13) Flanagan (1995), and references therein; (14) Flanagan (1996); (15) D'Alessandro & McCulloch (1997); (16) Kaspi et al. (1992); (17) Johnston et al. (1995); (18) Manchester et al. (1978); (19) Flanagan (1993); (20) Shemar & Lyne (1996); (21) McKenna & Lyne (1990); (22) Lyne et al. (1996); (23) Kaspi et al. (1993); (24) Shabanova (1998); (25) Backus, Taylor & Damashek (1982).

been obtained in only a few of the Crab and Vela pulsar glitches coefficient ˆ 0:89). The simple Ag±nÇ relationship can be (Lohsen 1981; Flanagan 1995). More such data would be needed described by to establish the exact form of the relationship between these jump A < 41:4 ‡ 3:22 log n_: parameters and for a better understanding of the amount of g superfluid interior involved in glitches. Based on this relationship, we estimate the glitch activity for all the known youthful pulsars (Table 3). The values that we obtain are in good agreement with values obtained for some of these 3 GLITCH ACTIVITY OF THE YOUTHFUL pulsars in recent work by Ruderman, Zhu & Chen (1998). Few of PULSARS these youthful pulsars for which observed glitch activity has been measured also show a very good agreement ^1† with the Glitch activity (Ag) has been defined as the mean fractional change in period per year owing to glitches (McKenna & Lyne 1990). For theoretical prediction. Since the glitch activity for N glitches over pulsars with repeated glitches we have determined their activity an interval tg is obtained from (Table 2). It is observed from Fig. 3 that the glitch activity of the N Dn pulsars generally increases with the logarithm of the spin-down Ag ˆ ; tg n rate. The only outlier comes from the young Crab pulsar, aged about 1000 yr. This could be explained by the fact that pulsars we have also computed the average intervals at which these younger than 104 yr have `unusual' glitch behaviours (McKenna & youthful pulsars are expected to undergo very large glitches. The Lyne 1990). Pulsars older than 104 yr show a very good correlation intervals between such very large glitches (of average Dn=n ˆ of activity with frequency derivative in Fig. 3 (correlation 2  1026† are given in Table 3. Intervals of between 10 and 100 yr

q 1999 RAS, MNRAS 310, 313±316 Vela-size glitch rates in youthful pulsars 315

Table 2. Glitch activity of pulsars with repeated glitches.

PSR B Log age No. of Ag (yr) glitches (1027 yr21) 0355 ‡ 54 5.75 2 2 0531 ‡ 21 3.1 4 0.05 0740 2 28 5.2 4 0.01 0833 2 45 4.05 13 8 1338 2 62 4.08 3 4 1641 2 45 5.55 3 0.5 1727 2 47 4.91 2 0.1 1737 2 30 4.32 13 3 1758 2 23 4.77 4 1 1822 2 09 5.37 2 0.01 1823 2 13 4.33 2 7

Figure 1. Glitch magnitudes (Dn/n and DnÇ/nÇ) as a function of pulsar age.

Figure 3. Glitch activity, Ag, as a function of spin-down rate, nÇ, for pulsars with repeated glitches.

Amongst these low-activity youthful pulsars, it is only PSR 2334+61 that is not known to have glitched at least once even after 7 yr of timing (Shemar & Lyne 1996). These low-activity pulsars are all older than 40  103 yr; which agrees with the postulated reduction of glitch activity in pulsars older than 30  103 yr (Lyne, Pritchard & Shemar 1995). PSR 1737230; the most frequently glitching pulsar known, has in the past 11 yr had only one glitch Figure 2. Distribution of glitches with pulsar age. The characteristic ages 2 with magnitude up to 10 6. This is consistent with the 7-yr Vela- were obtained from the unpublished pulsar catalogue ofTaylor et al. A . (1995). Panel (a) shows the percentage of pulsars that have glitched, while size glitch interval found for this pulsar. Pulsars with g 5 2 2 2 2 2 panel (b) gives the average number of glitches per pulsar for the different (PSRs 1046 58; 1338 62; 1706 44; 1757 24; 1800 21 and age ranges. 1823213) could have very large glitches every 3±4 yr. Because of their frequent large glitches, they make the best candidates for are found for pulsars of Ag # 2. These are low-activity youthful studying pulsar glitch recovery. Among this class it is only in PSR pulsars. We should note that this does not represent the actual 1338262 that multiple glitches have been observed, with two glitch intervals in these pulsars, as such pulsars exhibit more large glitches within an interval of 2 yr. The rest have not been frequent small glitches as may be the case in PSR 1727247. timed long enough $ 8yr† to yield the predicted glitches. This q 1999 RAS, MNRAS 310, 313±316 316 J. O. Urama and P. N. Okeke

Table 3. Predicted interval between Vela-size glitches in the youthful pulsars ± the PSR B name is given except otherwise ACKNOWLEDGMENTS stated. We thank Mike Gaylard for critically reading through the

27 21 manuscript and for his useful comments, and the referee for PSR Log age Ag (10 yr ) Interval (yr) Observed Predicted (yr) helpful suggestions. JOU acknowledges the support and hospital- ity of HartRAO. He is also grateful for the IAU Commission 38 0833245 4.05 8 7 3 grant that enabled him visit HartRAO. 1338262 4.08 4 5 4 1757224 4.19 5 4 1800221 4.2 5 4 1706244 4.24 6 3 REFERENCES 1046258 4.31 5 4 1853101 4.31 4 5 Alpar M. A., 1995, in Alpar M. A., Kiziloglu UÈ ., van Paradijs J., eds, 1737230 4.32 3 3 7 NATO ASI Series C450, The Lives of the Neutron Stars. Kluwer, 2 1823 13 4.33 7 6 4 Dordrecht, p. 185 2 1727 33 4.41 4 5 Alpar M. A., Chau H. F., Cheng K. S., Pines D., 1993, ApJ, 409, 345 1643243 4.51 3 6 Alpar M. A., Chau H. F., Cheng K. S., Pines D., 1994, ApJ, 427, L29 1930122 4.6 4 5 2334161 4.61 2 10 Backus P. R., Taylor J. H., Damashek M., 1982, ApJ, 255, L63 J063111036 4.64 3 7 Cordes J. M., Helfand D. J., 1980, ApJ, 239, 640 1758223 4.77 1 2 10 Cordes J. M., Downs G. S., Krause-Polstorff J., 1988, ApJ, 330, 847 J110526107 4.8 4 5 D'Alessandro F., 1996, Ap&SS, 246, 73 1727247 4.9 0.1 0.8 25 D'Alessandro F., McCulloch P. M., 1997, MNRAS, 292, 879 0611122 4.95 2 10 D'Alessandro F., McCulloch P. M., King E. A., Hamilton P. A., McConnel 1 1916 14 4.95 0.2 100 D., 1993, MNRAS, 261, 883 Downs G. S., 1982, ApJ, 257, L67 Flanagan C. S., 1993, MNRAS, 260, 643 Flanagan C. S., 1995, PhD thesis, Rhodes University, Grahamstown Flanagan C. S., 1996, IAU Circ., 6491 Gullahorn E. G., Rankin J. M., 1978, AJ, 83, 1219 3 class of pulsars are aged , 10±20 Â 10 yr; and this age bracket Gullahorn E. G., Rankin J. M., 1982, ApJ, 260, 520 probably marks the peak of glitch activity in pulsars. Johnston S., Manchester R. N., Lyne A. G., Kaspi V. M., D'Amico N., 1995, A&A, 293, 795 Kaspi V. M., Manchester R. N., Simon J., Lyne A. G., D'Amico N., 1992, 4 DISCUSSION AND CONCLUSION ApJ, 399, L155 Kaspi V. M., Lyne A. G., Manchester R. N., Johnston S., D'Amico N., The intensive monitoring of post-glitch behaviour in the Vela and Shemar L., 1993, ApJ, 409, L57 Crab pulsars has revealed recovery over a wide range of time- Lohsen E. H. G., 1981, A&AS, 44, 1 scales, from hours to years. These observations have been used to Lyne A. G., 1987, Nat, 326, 569 study the pulsar interior, in particular the various components of Lyne A. G., Pritchard R. S., 1989, IAU Circ., 4845 the interior and the manner in which they couple with each other Lyne A. G., Pritchard R. S., Graham-Smith F., 1993, MNRAS, 265, 1003 (e.g. Alpar et al. 1993, 1994). Similarly frequent observations of a Lyne A. G., Pritchard R. S., Shemar S., 1995, JA&A, 16, 179 larger sample of pulsars are required to distinguish between Lyne A. G., Kaspi V. M., Bailes M., Manchester R. N., Taylor H., competing theories of the neutron star interior. The few pulsars of Arzoumanian Z., 1996, MNRAS, 281, L14 comparable ages to the Crab are glitch-inactive. However, for the McKenna J., Lyne A. G., 1990, Nat, 343, 349 Manchester R. N., Newton L. M., Hamilton P. A., Goss W. M., 1978, Vela-like pulsars, we have identified six other pulsars of MNRAS, 184, 35p comparable ages, for which the sizes and likely frequency of Newton L. M., Manchester R. N., Cooke D. J., 1981, MNRAS, 194, 841 glitches could permit a detailed study of their glitch behaviours. Ruderman R., Zhu T., Chen K., 1998, ApJ, 492, 267 At the Hartebeesthoek Radio Astronomy Observatory (Hart- Shabanova T. V., 1990, SvA, 34, 372 RAO), glitch monitoring commenced on one of these `Vela-like' Shabanova T. V., 1998, A&A, 337, 723 pulsars, PSR 1046258; in 1997 July. A large glitch has already Shemar S. L., Lyne A. G., 1996, MNRAS, 282, 677 been observed in this pulsar and its post-glitch behaviour is being Taylor J. H., Manchester R. N., Lyne A. G., Camilo F., 1995, catalogue monitored (Urama & Flanagan, in preparation). Similar monitor- available via anonymous ftp from pulsar.princeton.edu ing of the other glitch-active youthful pulsars would drastically increase both the quantity and quality of glitch data needed for a more comprehensive analysis of glitch trends. This paper has been typeset from a TEX/LATEX file prepared by the author.

q 1999 RAS, MNRAS 310, 313±316