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