Astrophysics, Vol. 54, No. 1, March, 2011
OSCILLATIONS IN THE ANGULAR VELOCITY OF PULSARS
K. M. Shahabasyan and M. K. Shahabasyan
Observational data on long-period oscillations in the angular velocity of pulsars are examined. The characteristic radii of the superfluid regions of pulsars are determined assuming that these oscillations are collective elastic oscillations of a superfluid vortex lattice (Tkachenko oscillations). These radii are compared with values obtained in various theoretical models of neutron stars which assume the existence of a superfluid quark “CFL”-core or a superfluid hyperon core. This method can be used to estimate the radii of pulsars. Keywords: pulsars: radii of superfluid regions: Tkachenko oscillations
1. Introduction
Ever more observational data on long-period oscillations in the rotation of pulsars has become available recently. Thus, oscillations in the angular velocity of the pulsar B1828-11 with periods of 256 days, 511 days, and, less reliably, 1009 days, have been observed [1]. Studies of the variations in the rotation period of 366 pulsars over 36 years have revealed long-period sinusoidal oscillations in the pulsars PSR B1540-06, PSR B1826-17, PSR B1828-11, and PSR B2148+63, along with quasiperiodic oscillations of the pulsars PSR B1642-03 and PSR B1818-04 [2]. After the largest-ever glitch in the angular velocity of a pulsar, DW/W=20.5·10 -6, was observed in the pulsar PSR B2334+61, oscillations with a period of 364 days were observed [3]. Oscillations with periods of 800 and 1600 days have been observed in the pulsar PSR B1557-50 [4]. Periodic oscillations with a period of 200 days have been observed in the pulsar PSR B0531+21 in the Crab nebula [5]. The position angle Ψ and linear polarization of 81 pulsars have been measured. It turns out that for 19 of the pulsars the changes in this angle with time are sinusoidal to a good approximation. The periods of the sinusoids range from 185-1250 days. The radiation pulses, linear polarization position angle Ψ , and arrival time of the pulses
Erevan State University, Armenia; e-mail:
[email protected],
[email protected]
Original article submitted October 1, 2010; accepted for publication December 16, 2010. Translated from Astrofizika, Vol. 54, No. 1, pp. 131-137 (February 2011). 0571-7256/11/5401-0111
©
2011 Springer Science+Business Media, Inc.
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for the pulsar PSR B0833-45 oscillate with a period of 330 days [7]. The x-ray emission from the nebula that contains this pulsar varies in the same way [8]. One mechanism for explaining these oscillations is free precession of a neutron star [9]. However, free precession of a neutron star is incompatible with superfluidity of its “npe”-phase [10,11]. Another such mechanism is superfluidity of the neutron fluid. As a neutron star rotates, a two-dimensional triangular lattice of quantized neutron vortex filaments develop in the “Aen”- and “npe”-phases.
Collective elastic oscillations (Tkachenko
oscillations), in which the vortices move parallel to one another, can develop in this lattice [12].
Undamped
propagation of these filaments leads to a change in the angular momentum of the fluid and to periodic variations in the angular velocity and rate of slowing-down of the rotation. It has been shown [13,14] that the Tkachenko oscillations have periods on the order of 100 days and can explain the observed period oscillations in the angular velocity of the pulsars B0531+21 and PSR B1828-11. In the central part of a neutron star, however, a superfluid quark “CFL” core can develop, where massless “u”, “d”, and “s” quarks of all three colors are paired [15,16]. A “CFL”-condensate has both superfluid and superconducting properties. This is because of the breakdown of both local symmetries (color SU(3) and electromagnetic U(1) EM) and global (flavor SU(3) F and baryon U(1) B) symmetries. Thus, it is natural that singular solutions should appear: quark abelian superfluid U(1)B-vortex filaments [17,18], quark abelian magnetic vortex filaments [19], and quark nonabelian semi-superfluid vortex filaments [20]. filaments are topologically stable [20].
Unlike superfluid and magnetic vortex filaments, semi-superfluid vortex They are also dynamically stable, since their quantum of circulation
κ = πD mB is a third of the quantum of circulation for a superfluid U(1)B vortex and their linear strain (kinetic energy per unit length) is smaller by a factor of 9 than that of a superfluid U(1) B vortex [21]. It has been shown [22] that a long-range repulsive force acts between two distance parallel semi-superfluid vortices. It was found that a superfluid U(1)B-vortex could decay into three semi-superfluid vortices and that a stable lattice of these vortices could exist. Abelian magnetic vortex filaments are also dynamically unstable since their quantum of circulation and magnetic flux are 3 times those for semi-superfluid vortices [21]. It has been shown [21] that rotation in a quark “CFL”-core leads initially to formation of a lattice of superfluid U(1)B-vortex filaments which then is converted into a stable lattice of semi-superfluid vortices because of the decay of each superfluid vortex into three semi-superfluid vortices. Later, because the quanta of circulation for neutron vortex filaments and quark semi-superfluid vortices are equal, at the boundary of the quark and “npe”-phases this lattice joins with the lattice of neutron superfluid vortices to form a unified vortex structure. This ensures continuity of the chemical potential of the baryons at the phase boundary. Variations observed in the rotation of the pulsars PSR B0531+21 and PSR B1828-11 have been explained [23] in terms of Tkachenko oscillations of a unified vortex lattice of this type. Oscillations of this kind are also possible in strange quark stars consisting mainly of strange quark matter and a very thin shell. Λ-hyperons can also appear in the core of a neutron star at densities of order 2ρ 0 ( ρ0 = 2.8 ⋅1014 g/cm 3 is the normal core density) [24,25].
Attraction between Λ-hyperons leads to formation of a superfluid
condensate of S0 Cooper pairs [26]. The energy gap of the Λ-hyperons is of order 0.1-0.2 MeV, depending on the 1
potential energy of the interaction [27]. Thus, a superfluid hyperon fluid can be formed in the core of a neutron star.
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As the star rotates, a lattice of hyperon vortex filaments develops in this liquid.
2. Determining the radius of the superfluid phase
The radius of the superfluid region of a pulsar, where Tkachenko oscillations develop, is given by [13]
R=
0.01812 T P
,
(1)
where T is the period of the Tkachenko oscillations and P is the rotation period of the pulsar. Table 1 lists the observed periods T of the sinusoidal oscillations of ten pulsars [2-5, 7,8], the rotation periods P of the pulsars, and the radii of the superfluid region calculated using Eq. (1). Note that the oscillations of PSR B1642-03 and PSR B1818-04 are quasiperiodic [2]. The other pulsars have periods T of the sinusoidal oscillations ranging from 200 to 1600 days, while the calculated radii R of the superfluid region range over 6.3 to 29.8 km. Table 2 lists the observed periods T of the sinusoidal oscillations in the position angle Ψ of the linear polarization for 21 pulsars [6], along with the periods P of these pulsars and the calculated radii R of the superfluid region. These pulsars can be divided into three classes, depending on the degree of agreement between the observed data and the sinusoidal approximation [6]. Class 1 includes the 4 pulsars for which this agreement is best. Class 2 includes 15 pulsars, for which the agreement is not so good, and class 3 includes 2 pulsars for which there is no such agreement at all. Most of the periods of the sinusoidal variations for the 19 pulsars lie within a range of 185-450 days, with a maximum at 200 days. Three of the periods, 787, 1047, and 1250 days, lie outside this range. In Ref. 6 these variations are explained in terms of free precession of the neutron stars. Here, on the other hand, we explain these variations in terms of Tkachenko oscillations of a lattice of superfluid vortex filaments. The calculated radii R of the superfluid region range over 2.26 to 32.85 km. Most of these lie within a range TABLE 1.
Pulsars
T days
P s
R km
PSR B1540-06
1599
0.7090
29.6
PSR B1642-03
1242
0.3876
31.2
PSR B1818-04
2557
0.5980
51.6
PSR B1826-17
1055
0.3071
29.8
PSR B1828-11
256
0.4050
6.3
PSR B2148+63
766
0.3801
19.4
PSR B0531+21
200
0.0334
17.1
PSR B0833-45
330
0.0893
16.6
PSR B1557-50
800
0.1925
28.5
PSR B2334+61
364
0.4953
8.1
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TABLE 2. Pulsars
T, days
P, s
R, km
Class 1 PSR B0523+11
1250
0.3544
32.85
PSR B0611+22
314
0.3349
8.47
PSR B0656+14
1047
0.3848
26.4
PSR B2053+21
444
0.8151
7.7
Class 2 PSR B0626+14
356
0.4766
8.06
PSR B1133+16
364
1.1879
5.22
PSR B1737+13
185
0.8030
3.23
PSR B1802+03
209
0.2187
6.99
PSR B1821+05
206
0.7529
3.71
PSR B1842+14
221
0.3754
5.65
PSR B1854+00
270
0.3569
7.06
PSR B1913+10
443
0.4045
10.9
PSR B1914+09
454
0.2702
13.65
PSR B1930+22
787
0.1444
32.4
PSR B1952+29
213
0.4266
5.1
PSR B2020+28
176
0.3434
4.7
PSR B2034+19
208
2.0743
2.26
PSR B2122+13
289
0.6940
5.43
PSR B2210+29
310
1.0045
4.83
Class 3 PSR B1604-00
380
0.4218
9.2
PSR B1918+26
869
0.7855
15.3
of 4.7-10.9 km. Note that we have also calculated the radii R of the superfluid region for the pulsars in class 3. Note that two pulsars in class 1, PSR B0523+11 and PSR B0656+14, and one pulsar in class 2, PSR B1930+22, have large radii for their superfluid regions, 32.85, 26.4, and 32.4 km, respectively. The pulsars PSR B1540-06, PSR B1557-50, PSR B1826-17, PSR B2148+63, PSR B0531+21 and PSR B0833-45 also have large radii, as listed in Table 1: 29.6, 28.5, 29.8, 19, 17.1, and 16.6 km. Assuming that the radiation from the x-ray neutron star RXJ1856.5-3754 is black-body radiation implies a radius R = 14.4 km with a mass of M = 1.4 M ¤ [28]. During giant x-ray bursts from the soft x-ray flare sources SGR 1900+14 (August 27 1998) and SGR 180620 (December 27, 2004), periodic oscillations with frequencies of 28, 54, 84, and 155 Hz [29] and 18, 30, 92.5, 155, 625, and 1840 Hz [30], respectively, were observed. These frequencies are modes of shear wave oscillations caused
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by deformation of the solid core of the neutron stars owing to catastrophic redistribution of magnetic fields of order 1014-1015 G. Thus, global seismic waves propagate in the neutron star. Observations of these waves can be used to estimate the thickness of a pulsar’s crust; for SGR 1806-20, it is 1.6 km [31]. A series of models of neutron stars with strange quark cores has been constructed [32]. For high mass stars ( M ≥ 1.44 M ¤ ), the radius of the quark core is on the order of 10 km and the star’s radius is of order 11 km. Based on an equation of state employing a quark bag model, quark stars consisting entirely of quarks with three flavors (u, d, s) have been modelled [33], with stars of mass M = 1.3 M ¤ , M = 1.42 M ¤ , and M = 1.75 M ¤ having radii of 9.2, 10.6, and 14 km, respectively. An equation of state based on a quantized field formulation of the Nambu-Jona-Lazinio type including the presence of a scalar condensate of diquarks, an isoscalar vector mean field, and a determinant Kobayashi-Maskawat’Hooft interaction has been used in the quark region [34]. In the hadron phase a Dirac-Brukner-Hartree-Fock equation of state was used. A neutron star with a mass of M = 2 M ¤ has a radius of 12 km. The quark core of the star is in a “2SC” phase, which is superconducting with no semi-superfluid vortex filaments and in which Tkachenko oscillations are not possible. An equation of state including hyperons in the core of a neutron star yields stellar configurations with a mass of M = 1.4 M ¤ and radius R = 13 km [35]. However, a rigid equation of state including the presence of Λ and Σ − hyperons in the core of a neutron star examined in Ref. 36 yielded stellar configurations with a mass of M = 1.2 M ¤ and radius R = 12 km, while a soft equation of state reduces the mass to 0.9 M ¤ and the radius to 11 km. The theoretical models of neutron stars examined above do not explain the large radii of the superfluid regions derived from an analysis of the periods of Tkachenko oscillations. Since observational data indicate the presence of a solid core in neutron stars, the radii of neutron stars will be even larger. Thus, pulsars with large radii of their superfluid regions cannot have quark cores or hyperon cores. On the other hand, pulsars with smaller radii of their superfluid regions, can have semi-superfluid quark “CFL” cores or superfluid L-hyperon cores. This method of analyzing the periods of Tkachenko oscillations offers the additional possibility of estimating the radii of pulsars. It also shows that there is a need for systematic observations of pulsars over prolonged time intervals. We thank D. M. Sedrakian for useful discussions.
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