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 2023 April 13-15, for the first time since its disco v ery 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 (bb) = 1 . 61 +/- 0 . 04 keV, most likely arising from the NS surface and a Comptonization component with power-law index F = 1 . 79 +/- 0 . 02, arising from a hot corona at kT(e ) = 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, C P , to fit the reflection features. We find an upper limit to the inner disc radius of 6 R- ISCO and 9 R- ISCO from RELXILL and RELXILLCP C P model, respectively. The inclination of the system is estimated to be similar or equal to 40 degrees 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 x 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.