ARE PULSARS AND X-BINARIES STRANGE STARS ?

ARE PULSARS AND X-BINARIES STRANGE STARS ? The pulsars, of which more than one thousand are known today, are interpreted as being neutron stars in rotation. In the same way, the compact object which accretes the gas of its companion in X-ray binaries is interpreted as a neutron star (except in a few cases where it is rather thought to be a black hole). A neutron star is a very dense object (a solar mass in a sphere of 10 km in radius !), the central density being several times higher than the density of an atomic nucleus, i.e. 10 17 kg/m3. In 1984 the physicist E Witten have put forth the assumption that under such conditions, the fundamental state of matter is not the hadronic state that we know (that is a mixture of quark u and d, which constitute the proton and the neutron), but a state qualified of ``strange’ ’ because comprising, in addition to quark u and d, quark s (quark strange) . Some researchers of Paris Observatory, in collaboration with colleagues from Center Nicolas Copernic of Warsaw, have calculated some theoretical models of strange stars in fast rotation and have sought the observable characteristics allowing the distinction between "strange" and "standard" neutron stars, in the goal to confirm or to infirm the assumption of Witten, and to refine in the same time our knowledge of strong nuclear interaction. A significant qualitative difference found between strange and neutron stars is that, outside strange stars, there is a last stable orbit even for high disk speeds. That means that all the orbits around the star are not stable : a too close orbit is unstable : a particle could not rotate around the star but would be irremediably accreted by it. Such an area of instability, bounded by the last stable orbit, exists around neutron stars in slow rotation. But when they spin too quickly, neutron stars inflate (due to the centrifugal force) so much that they include the last stable orbit, which thus disappears. The existence of a last stable orbit is generally attributed to the very large gravitational field of this type of star, which requires to be described by General Relativity. But the researchers of Paris Observatory and their Polish colleagues showed that, in the case of strange stars, such a last stable orbit exists even for very low masses, therefore in nonrelativistic mode. This is due to the significant flatness of strange stars in fast rotation and to their high compactness.

Meridian cut of a strange star model of 1,6 solar mass in rotation at the frequency of 1210Hz (period 0,8 milliseconds). The hatched part is the solid crust, which consists of ordinary matter. This model was obtained by solving on computer the equations of General Relativity, known as Einstein equations.