Spectral and timing properties of the accreting millisecond X-ray pulsar IGR J17498‑2921 during its 2023 outburst

Illiano G.;Papitto A.;Marino A.;Strohmayer T. ~E.;Sanna A.

Writing – Review & Editing

;Di Salvo T.;La Placa R.;Ambrosino F.;Miraval Zanon A.;Coti Zelati F.;Ballocco C.;Malacaria C.;Ghedina A.;Cecconi M.;Gonzales M.;Leone F.

2024-01-01

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Abstract

We present a comprehensive study of the spectral properties of the accreting millisecond X-ray pulsar IGR J17498−2921 during its 2023 outburst. Similar to other accreting millisecond X-ray pulsars, the broadband spectral emission observed quasi-simultaneously by NICER and NuSTAR is well described by an absorbed Comptonized emission with an electron temperature of ∼17 keV plus a disk reflection component. The broadening of the disk reflection spectral features, such as a prominent iron emission line at 6.4–6.7 keV, is consistent with the relativistic motion of matter in a disk truncated at ∼21 Rg from the source, near the Keplerian corotation radius. From the high-cadence monitoring data obtained with NICER, we observed that the evolution of the photon index and the temperature of seed photons tracks variations in the X-ray flux. This is particularly evident close to a sudden ∼–0.25 cycle jump in the pulse phase, which occurs immediately following an X-ray flux flare and a drop in the pulse amplitude below the 3σ detection threshold. We also report on the non-detection of optical pulsations with TNG/SiFAP2 from the highly absorbed optical counterpart.