Timing Analysis of the 2022 Outburst of SAX J1808.4-3658: Hints of Orbital Decay

Abstract

We present a pulse timing analysis of NICER observations of the accreting millisecond X-ray pulsar SAX J1808.4 -3658 during the outburst that started on 2022 August 19. Similar to previous outbursts, after decaying from a peak luminosity of (sic)1 x 10(36 )erg s(-1 )in about a week, the pulsar entered a similar to 1 month long reflaring stage. Comparison of the average pulsar spin frequency during the outburst with those previously measured confirmed the long-term spin derivative of v(SD) =-(1.15 +/- 0.06) & acute; 10(-15) Hz s(-1) , compatible with the spin-down torque 15 of a asymptotic to 10(26) G cm(3) rotating magnetic dipole. For the first time in the last twenty years, the orbital phase evolution shows evidence for a decrease of the orbital period. The long-term behavior of the orbit is dominated by an similar to 11 s modulation of the orbital phase epoch consistent with a similar to 21 yr period. We discuss the observed evolution in terms of a coupling between the orbit and variations in the mass quadrupole of the companion star.