It has been demonstrated, through the example of the deconfinement transition from hadronic to quark matter, that the rotational characteristics of neutron stars (braking index, spin-down rate) are sensitive to changes of their inner structure and can thus be investigated in order to detect structural phase transitions.
The theoretical basis for the present work was a perturbation method for the solution of the Einstein equations for axial symmetry which allows us to calculate the contribution of different rotational effects to the change of the moment of inertia. This quantity can be used as a tool for the investigation of the changes in the rotation timing for different scenarios of the neutron star evolution.
The deviation of the braking index from the value (magnetic dipole radiation) as a function of the angular velocity has been suggested as a possible signal for the deconfinement transition and the occurrence of a quark matter core in pulsars. We have reinvestigated this signal within our approach and could show that the magnitude of this deviation is correlated with the size of the quark core, since the influence of the Ae phase crust on such processes is negligible.
For neutron stars with mass accretion, we have suggested that under the assumption of total angular momentum conservation a flip from spin-down to spin-up behaviour signals the appearance of a quark matter core. A more detailed investigation is necessary to identify possible candidates of rotating compact objects with mass accretion (see e.g. Lamb et al. (1998)) for which the suggested deconfinement signal could be relevant.
Although the quantitative details of the reported deconfinement signals are quite model-dependent and might change when one uses more realistic equations of state, e.g. including the strangeness degree of freedom (Glendenning et al. 1997; Weber 1999), the relation between the magnitude of the effect and the size of the quark core which has been found here is expected to be model-independent and should be confirmed by subsequent studies.
© European Southern Observatory (ESO) 2000
Online publication: June 5, 2000