X-ray spectral properties of the accreting millisecond pulsar IGR J17498-2921 during its 2023 outburst

We present a comprehensive study of the X-ray spectral properties of the accreting millisecond pulsar IGR J17498-2921 during its 2023 outburst. Similar to other accreting millisecond pulsars, the broad-band 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 co-rotation radius. From the high-cadence monitoring data obtained with NICER, we observe 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 cycles jump in the pulse phase, which occurs immediately following an X-ray flux flare and a drop in the pulse amplitude below the 3 detection threshold. We also report on the non-detection of optical pulsations with TNG/SiFAP2 from the highly absorbed optical counterpart.