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

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 = 1.67 R 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)×10 8 G.