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97.80.Jp X-ray binaries—97.60.Gb pulsars—97.10.Gz accretion and accretion discs.

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 ($\varepsilon$) and quasi-Keplerian ($\xi$) 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.

ISAAC HABUMUGISHA^{1 2} SOLOMON B. TESSEMA³ EDWARD JURUA¹ SIMON K. ANGUMA⁴

1. Department of Physics, Mbarara University of Science and Technology, Mbarara, Uganda.
2. Department of Physics, Kabale University, Kabale, Uganda.
3. Astronomy and Astrophysics Research and Development Department, Entoto Observatory and Research Center, Addis Ababa, Ethiopia.
4. Department of Physics, Muni University, Arua, Uganda.

Published

12 October 2020

Manuscript received

30 January 2020

Accepted

9 July 2020

Final version published online

12 October 2020