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

Giulia Illiano, Alessandro Papitto, Alessio Marino, Tod E. Strohmayer, Andrea Sanna, Tiziana Di Salvo, Riccardo La Placa, Filippo Ambrosino, Arianna Miraval Zanon, Francesco Coti Zelati, Caterina Ballocco, Christian Malacaria, Adriano Ghedina, Massimo Cecconi, Manuel Gonzales, Franco Leone

Published 2024-08-13Version 1

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 $\sim$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 $\sim$$21 \, \mathrm{R_g}$ 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 $\sim$-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 $3\sigma$ detection threshold. We also report on the non-detection of optical pulsations with TNG/SiFAP2 from the highly absorbed optical counterpart.