(Indian Academy of Sciences, 2020) Isaac, Habumugisha; Solomon, B. Tessema; Edward, Jurua; Simon, K. Anguma
In this study, we investigated the time-independent dynamics (disc structure, forces and torques)
of a quasi-Keplerian disc around a millisecond pulsar (MSP) with an internal dynamo. We considered the
disc around a MSP to be divided into the inner, middle and outer regions. By assuming that the disc matter
flows in a quasi-Keplerian motion, we derived analytical equations for a complete structure (temperature,
pressure, surface density, optical depth and magnetic field) of a quasi-Keplerian thin accretion disc, and the
pressure gradient force (PGF). In our model, the MSP-disc interaction results into magnetic and material
torques, such that for a given dynamo ( ) and quasi-Keplerian (n) parameter, we obtained enhanced spin-up
and spin-down torques for a chosen star spin period. Results obtained reveal that PGF results into episodic
torque reversals that contribute to spinning-up or spinning-down of a neutron star, mainly from the inner
region. The possibility of a quasi-Keplerian disc is seen and these results can explain the observed spin
variations in MSPs like SAX J1808.4-3658 and XTE J1814-338.