The Fastest Rotating Pulsar : a Strange Star ?

According to the observational limits on the radius and mass, PSR 1937+21 is probably a strange star, or some neutron star equations of state should be ruled out, if we alternatively suggest that a dipole magnetic field (rather than a quadrupole one) is relevant to its radio emission. In order to find solid evidence to shown that PSR 1937+21 is a strange star, it is strongly suggested to measure precisely the maximum rate of position-angle swing by future polarization observations. It is conjectured that PSR 1937+21 and PSR 1257+12 (and possibly some or all of the isolated millisecond pulsars) may have weaker dynamo-originated magnetic field due to a less effective field magnification process. Pulsars are conventionally modeled as neutron stars, but a strange star model for pulsars is also proposed (see a review, e.g., by Xu et al. 1). Therefore it is of great importance to find observationally unambiguous features to distinguish strange stars from neutron stars. Up to now there are three hopeful ways to identify a strange star, which are based upon: 1, that strange stars (or neutron stars containing significant quantities of strange matter) may rotate more rapidly 2 ; 2, that the mass-radius relations of strange stars are in striking contrast to that of neutron stars 3 ; 3, The eccentric behavior of the quark surface condition 1 which results in distinguishable signatures in magnetospheric and polar radiation. Recently, Kapoor & Shukre 4 suggest to constrain the equations of state of neutron stars by the Rankin's experiential line 5 of the core emission of radio pulsars, and find that the pulsar masses M ≤ 2.5M ⊙ and radii R ≤ 10.5 km, indicating that puslars are strange stars. However Kapoor & Shukre's work 4 has at least two unseemly points: 1, the polar cap is defined for aligned rotators (rather than for orthogonal rotators) in their calculation; 2, the Rankin line is not doubtless true due to many observational and statistical uncertainties. By redefining the polar cap for orthogonal rotators, we find we can not get a conclusion that puslars are strange stars, or that some neutron stars equations of state are ruled out, if the Rankin line is used to constrain pulsar masses and radii. Nevertheless, when applying this method to the fastest rotating pulsar, PSR 1937+21, we conclude that the pulsar may be a strange star, or at least some equations of state should …