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

ABSTRACT The accreting millisecond X-ray pulsar IGR J17498−2921 went into X-ray outburst on 2023 April 13–15, for the first time since its discovery on 2011 August 11. Here, we report on the first follow-up NuSTAR observation of the source, performed on 2023 April 23, around 10 d 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 $kT_{\mathrm{ bb}}=1.61\pm 0.04$ keV, most likely arising from the NS surface and a Comptonization component with power-law index $\Gamma =1.79\pm 0.02$, arising from a hot corona at $kT_{e}=16\pm 2$ 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 $6\: R_{\mathrm{ ISCO}}$ and $9\: R_{\mathrm{ ISCO}}$ from relxill and relxillCp model, respectively. The inclination of the system is estimated to be $\simeq 40^{\circ }$ 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 $B\lesssim 1.8\times 10^{8}$ 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.