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

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 similar or equal to 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) X 10(-15) Hz s(-1) , compatible with the spin-down torque 15 of a approximate 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.

Automated summary

We analyzed NICER observations of SAX J1808.4-3658 and found similar behavior as previous outbursts. After peaking at a luminosity of about 1 x 10^36 erg/s, the pulsar entered a month-long reflaring stage. The average pulsar spin frequency during the outburst was confirmed to match previous measurements, indicating a long-term spin derivative and a rotating magnetic dipole. Furthermore, for the first time in twenty years, the orbital phase evolution showed evidence of a decreasing orbital period, which is influenced by a modulation with a period of approximately 21 years.