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

Journal
Date Issued
2023
Author(s)
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Bult, Peter
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Coti Zelati, Francesco
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Altamirano, Diego
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BOZZO , ENRICO
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Di Salvo, Tiziana
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Ferrigno, Carlo
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Loktev, Vladislav
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Ng, Mason
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Poutanen, Juri
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Salmi, Tuomo
DOI
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 ${\dot{\nu }}_{\mathrm{SD}}=-(1.15\,\pm \,0.06)\times \,{10}^{-15}$ 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.
Volume
942
Issue
2
Start page
L40
Uri
Url
Issn Identifier
2041-8205
Ads BibCode
2023ApJ...942L..40I
Rights
open.access
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