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

¹ INAF-Osservatorio Astronomico di Roma, Via Frascati 33, I-00076 Monte Porzio Catone, (RM), Italy
² Tor Vergata University of Rome, Via della Ricerca Scientifica 1, I-00133 Roma, Italy
³ Sapienza Universitá di Roma, Piazzale Aldo Moro 5, I-00185 Rome, Italy
⁴ Institute of Space Sciences (ICE, CSIC), Campus UAB, Carrer de Can Magrans s/n, E-08193 Barcelona, Spain
⁵ Institut d’Estudis Espacials de Catalunya (IEEC), E-08860 Castelldefels (Barcelona), Spain
⁶ Astrophysics Science Division, NASA Goddard Space Flight Center, Greenbelt, MD 20771, USA
⁷ Joint Space-Science Institute, NASA Goddard Space Flight Center, Greenbelt, MD 20771, USA
⁸ Dipartimento di Fisica, Università degli Studi di Cagliari, SP Monserrato-Sestu km 0.7, I-09042 Monserrato, Italy
⁹ Università degli Studi di Palermo, Dipartimento di Fisica e Chimica, via Archirafi 36, 90123 Palermo, Italy
¹⁰ ASI - Agenzia Spaziale Italiana, Via del Politecnico snc, 00133 Roma, Italy
¹¹ INAF–Osservatorio Astronomico di Brera, Via Bianchi 46, I-23807 Merate, (LC), Italy
¹² Fundación Galileo Galilei – INAF, La Palma, Spain
¹³ Dipartimento di Fisica e Astronomia, Sezione Astrofisica, Università di Catania, Via S. Sofia 78, I-95123 Catania, Italy
¹⁴ INAF–Osservatorio Astrofisico di Catania, Via S. Sofia 78, I-95123 Catania, Italy

^⋆ Corresponding author; giulia.illiano@inaf.it

Received: 29 July 2024
Accepted: 2 October 2024

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 R_g 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.