Refereed journal article or data article (A1)
Timing Analysis of the 2022 Outburst of SAX J1808.4-3658: Hints of Orbital Decay
List of Authors: Illiano Giulia, Papitto Alessandro, Sanna Andrea, Bult Peter, Ambrosino Filippo, Zanon Aarianna M, Zelati Francesco Coti, Stella Luigi, Altamirano Domitilla, 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
Publisher: IOP Publishing Ltd
Publication year: 2023
Journal: Astrophysical Journal Letters
Journal name in source: ASTROPHYSICAL JOURNAL LETTERS
Journal acronym: ASTROPHYS J LETT
Volume number: 942
Issue number: 2
Number of pages: 8
ISSN: 2041-8205
DOI: http://dx.doi.org/10.3847/2041-8213/acad81
URL: https://www.doi.org/10.3847/2041-8213/acad81
Self-archived copy’s web address: https://research.utu.fi/converis/portal/detail/Publication/178724441
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.
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