Spectral and timing properties of the accreting millisecond X-ray pulsar IGR J17498‑2921 during its 2023 outburst
Sanna, A.; Papitto, A.; Di Salvo, T.; Leone, F.; Cecconi, M.; Coti Zelati, F.; Ghedina, A.; Marino, A.; Miraval Zanon, A.; Strohmayer, T. E.; Illiano, G.; Ambrosino, F.; Malacaria, C.; La Placa, R.; Ballocco, C.; Gonzales, M.
Italy, Spain, United States
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.