Timing Analysis of the 2022 Outburst of the Accreting Millisecond X-Ray Pulsar SAX J1808.4-3658: Hints of an Orbital Shrinking

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 ≃1 × 1036 erg s−1 in about a week, the pulsar entered a ∼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 ν ̇ SD = − ( 1.15 ± 0.06 ) × 10 − 15 Hz s−1, compatible with the spin-down torque of a ≈1026 G cm3 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 ∼11 s modulation of the orbital phase epoch consistent with a ∼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.

NICER Discovery that SRGA J144459.2–604207 Is an Accreting Millisecond X-Ray Pulsar

“…By comparing the expected accretion luminosity produced by different orbital angular momentum sinks (e.g., magnetic braking and/or gravitational radiation) and the observed accretion luminosity during outbursts, AMXPs can be used to test mass transfer scenarios in these extreme systems (e.g., Marino et al 2019a;Bult et al 2021a;Ng et al 2021). For example, in the latest outburst of SAX J1808.4−3658, the observed orbital evolution was proposed to be due to ejected material having a specific angular momentum equal to or greater than that of the companion (Applegate & Shaham 1994;Illiano et al 2023). We can further refine the physics governing these systems through the discovery of additional AMXPs with all-sky multiwavelength monitoring of any transient outburst activity.…”