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_ 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 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.