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

Illiano Giulia;Papitto Alessandro;Sanna Andrea

Conceptualization

;Bult Peter;Ambrosino Filippo;Miraval Zanon;Arianna;Coti Zelati;Francesco Stella;Luigi Altamirano;Diego Baglio;Maria Cristina;Bozzo Enrico;Burderi Luciano;de Martino;Domitilla;Di Marco;Alessandro;di Salvo;Tiziana Ferrigno;Carlo Loktev;Vladislav Marino;Alessio Ng;Mason Pilia;Maura Poutanen;Juri Salmi;Tuomo

2023-01-01

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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.