Illiano, Giulia, Papitto, Alessandro, Sanna, Andrea, Bult, Peter, Ambrosino, Filippo, Zanon, Arianna Miraval, Zelati, Francesco Coti, Stella, Luigi, Altamirano, Diego, Baglio, Maria Cristina, Bozzo, Enrico, Burderi, Luciano, Martino, Domitilla de, Marco, Alessandro Di, Salvo, Tiziana di, Ferrigno, Carlo, Loktev, Vladislav, Marino, Alessio, Ng, Mason, Pilia, Maura, Poutanen, Juri and Salmi, Tuomo (2023) Timing analysis of the 2022 outburst of the accreting millisecond x-ray pulsar SAX J1808.4-3658: hints of an orbital shrinking. The Astrophysical Journal Letters, 942 (2), [L40]. (doi:10.3847/2041-8213/acad81).
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
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.
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