Broad-band X-ray spectral and timing properties of the accreting millisecond X-ray pulsar IGR J17498$-$2921 during the 2023 outburst

Abstract

We report on the broadband spectral and timing properties of the accreting millisecond X-ray pulsar IGR J17498$-$2921 during its April 2023 outburst using data from NICER (1$-$10 keV), NuSTAR (3$-$79 keV), Insight-HXMT (2$-$150 keV), and INTEGRAL (30$-$150 keV). We detect significant 401 Hz pulsations across the 0.5$-$150 keV band. The pulse fraction increases from $\sim$2% at 1 keV to $\sim$13% at 66 keV. Five type-I X-ray bursts have been detected, including three photospheric radius expansion bursts, with a rise time of $\sim$2 s and an exponential decay time of $\sim$5 s. The recurrence time is $\sim$9.1 h, which can be explained by unstable thermonuclear burning of hydrogen-deficient material on the neutron star surface. The quasi-simultaneous 1$-$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 $\Gamma$ of $\sim$1.8 and an electron temperature $k{T}_{\mathrm{e}}$ of $\sim$40 keV. We obtain a low inclination angle $i\sim 34^{\circ }$. The accretion disk shows properties of strong ionization, $\log (\xi /\mathrm{erg}\mathrm{cm}{\mathrm{s}}^{-1})\sim 4.5$, over-solar abundance, $A_{\mathrm{Fe}}\sim 7.7$, and high density, $\log (n_{\mathrm{e}}/\mathrm{c}{\mathrm{m}}^{-3})\sim 19.5$. However, a lower disk density with normal abundance and ionization could also be possible. From the inner disk radius $R_{\mathrm{in}}=1.67R_{\mathrm{ISCO}}$ and the long-term spin-down rate of $-3.1(2)\times 10^{-15}\mathrm{Hz}{\mathrm{s}}^{-1}$, we constrain the magnetic field of IGR J17498$-$2921 in the range of $(0.9-2.4)\times 10^8$ G.

arXiv commentPublished in A&A

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