The influence of rotation deceleration on the shape of a pulsar may reflect on the star's braking index

Abstract

Pulsars are stars whose electromagnetic radiation is observed to pulsate in well-defined time intervals while they rotate as long as the electromagnetic signal is not emitted in the direction of the rotation axis. The frequencies of the pulses decay with time as quantified by the braking index (n). In the canonical model, n = 3 for all pulsars, but observational data yield n < 3. In this work, this model is modified to allow time change in the moment of inertia. According to that model, as the neutron star loses energy by the emission of electromagnetic radiation, its rotation decelerates under constant moment of inertia. Here, we propose that, as the rotation decelerates, the shape of the star changes due to an increasingly weaker centrifugal force that reduces its moment of inertia, a change that, under normal circumstances, would accelerate the star's rotation. In the case of pulsars, as the rotation decreases in view of electromagnetic energy loss, the rotation would decelerate less than in the canonical model, yielding a braking index less than three. This is a good result from our model, especially regarding stars with indices close to three. For lower indices, our model requires refinements as a complete explanation of braking index values probably demands that a series of phenomena be incorporated into the process.