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

¹ Key Laboratory of Stars and Interstellar Medium, Xiangtan University, Xiangtan, 411105 Hunan, P.R. China
² SRON-Netherlands Institute for Space Research, Sorbonnelaan 2, 3584 CA Utrecht, The Netherlands
³ International Space Science Institute (ISSI), Hallerstrasse 6, 3012 Bern, Switzerland
⁴ Physikalisches Institut, University of Bern, Sidlerstrasse 5, 3012 Bern, Switzerland
⁵ Department of Physics and Astronomy, FI-20014 University of Turku, Finland
⁶ Space Research Institute of the Russian Academy of Sciences, Profsoyuznaya str. 84/32, 117997 Moscow, Russia
⁷ Department of Astronomy, School of Physics, Peking University, Beijing 100871, People’s Republic of China
⁸ Kavli Institute for Astronomy and Astrophysics, Peking University, Beijing 100871, People’s Republic of China
⁹ Key Laboratory of Particle Astrophysics, Institute of High Energy Physics, Chinese Academy of Sciences, 19B Yuquan Road, Beijing 100049, China

^⋆ Corresponding author; lizhaosheng@xtu.edu.cn

Received: 26 June 2024
Accepted: 9 September 2024

Abstract

We report on the broadband spectral and timing properties of the accreting millisecond X-ray pulsar IGR J17498−2921 during its April 2023 outburst. We used data from NICER (1–10 keV), NuSTAR (3–79 keV), Insight-HXMT (2–150 keV), and INTEGRAL (30–150 keV). We detected significant 401 Hz pulsations across the 0.5–150 keV band. The pulse fraction increases from ∼2% at 1 keV to ∼13% at 66 keV. We detected five type-I X-ray bursts, including three photospheric radius expansion bursts, with a rise time of ∼2 s and an exponential decay time of ∼5 s. The recurrence time is ∼9.1 h, which can be explained by unstable thermonuclear burning of hydrogen-deficient material on the neutron star surface. The quasi-simultaneous 1–150 keV broadband spectra from NICER, NuSTAR and INTEGRAL can be aptly fitted by an absorbed reflection model, relxillCp, and a Gaussian line of instrumental origin. The Comptonized emission from the hot corona is characterized by a photon index Γ of ∼1.8 and an electron temperature kT_e of ∼40 keV. We obtained a low inclination angle i ∼ 34°. The accretion disk shows properties of strong ionization, log(ξ/erg cm s⁻¹)∼4.5, over-solar abundance, A_Fe ∼ 7.7, and high density, log(n_e/cm⁻³)∼19.5. However, a lower disk density with normal abundance and ionization could also be possible. Based on the inner disk radius of R_in = 1.67R_ISCO and the long-term spin-down rate of −3.1(2)×10⁻¹⁵ Hz s⁻¹, we were able to constrain the magnetic field of IGR J17498−2921 to the range of (0.9 − 2.4)×10⁸ G.