Relativistic X-ray reflection from the accreting millisecond X-ray pulsar IGR J17498-2921

The accreting millisecond X-ray pulsar IGR J17498-2921 went into X-ray outburst on April 13-15, 2023, for the first time since its discovery on August 11, 2011. Here, we report on the first follow-up NuSTAR observation of the source, performed on April 23, 2023, around ten days after the peak of the outburst. The NuSTAR spectrum of the persistent emission (3 − 60 keV band) is well described by an absorbed blackbody with a temperature of kTbb = 1.61 ± 0.04 keV, most likely arising from the NS surface and a Comptonization component with power-law index Γ = 1.79 ± 0.02, arising from a hot corona at kTe = 16 ± 2 keV. The X-ray spectrum of the source shows robust reflection features which have not been observed before. We use a couple of self-consistent reflection models, relxill and relxillCp, to fit the reflection features. We find an upper limit to the inner disc radius of 6 RISCO and 9 RISCO from relxill and relxillCp model, respectively. The inclination of the system is estimated to be ≃ 40○ from both reflection models. Assuming magnetic truncation of the accretion disc, the upper limit of magnetic field strength at the pole of the NS is found to be B ≲ 1.8 × 108 G. Furthermore, the NuSTAR observation revealed two type I X-ray bursts and the burst spectroscopy confirms the thermonuclear nature of the burst. The blackbody temperature reaches nearly 2.2 keV at the peak of the burst.