A note on solid pulsars

After a brief introduction to the necessary of quark stars in modelling pulsars, I present a qualitative analysis of the solidification of quark matter with low-temperature but high-density. The reason, that a solid neutron star could not be possible, is also given. Subject headings: pulsars: general — stars: neutron — elementary particles What’s the nature of pulsars? This question seems to have been answered: pulsars are simply neutron stars — a kind of boring big “nuclei” bound dominantly by gravity rather nuclear force. However, this view is criticized in both theoretical and experimental grounds: (1) nucleons (neutron and proton) are in fact not structureless points although they were thought to be elementary particles in Landau’s time (1930s), they (and other hadrons) have been actually composed of quarks proposed since 1964; (2) the neutron star model does sometimes not fit well the recent observations of advanced X-ray facilities in space, and there is still no convincing evidence that pulsars are just neutron stars. Therefore, we have to pay attention to this issue, and to reconsider the question again, even more than 35 years later. The neutron star ideal appeared in 1932, soon after the discovery of neutrons by Chadwick. After the first radio pulsars was discovered in 1967 and as the quark model of hadrons developed in 1960s, a new ideal was proposed: there may exist a kind of compact stars to be similar to neutron stars, so-called Quark stars, that are composed mainly of quark matter, rather than neutron matter. Since quark stars can also reproduce the behaviors of pulsar-like stars, it is argued then that both neutron and quark star models are possible. But nature may choose only the second one Quark matter is defined as a state in which quarks and gluons are de-confined (to be thus called also as quarkgluon plasma), whereas neutron matter as a state of free nucleons (neutrons and a few parts of protons). of these two if quark matter with strangeness (i.e., strange quark matter) is absolutely stable. Quark stars with strangeness are called to be strange stars (Xu 2003a). Are pulsars really neutron or strange stars? Because we can not now determine whether strange quark matter is absolutely stable from the first principles, we have to try to answer the question via astrophysical experiments: observations! Strange stars, if they are radio pulsars, were usually considered to be crusted (i.e., there is a ∼ 10−5∼−6M⊙ crust around a strange quark matter core) until 1998 when Xu & Qiao (1998) addressed that bare strange stars (i.e., strange stars without crusts) being chosen as the interior of radio pulsars have three advantages: 1, the spectral features; 2, the bounding energy; and 3, the core collapse process during supernova. The formation of bare strange stars is possible in principle, and their emission can generally explain various observations of pulsar-like stars (Xu 2003b). Furthermore, it is suggested that pulsar-like stars could be solid quark stars with strangeness, sometimes exposing their quark surfaces, in order to fit the continuum thermal X-ray spectra and to understand the free-precession behaviors (Xu 2003c). In this note, two selected points relevant to solid pulsars are discussed. 1. Can a solid neutron star be possible? The answer is no. Free precession of isolated pulsars could be common in nature; the gradual, longterm variation of RX J0720.4-3125 (a radio-quiet isolated neutron star) observed in X-ray band