Probing burst-disc interaction and disc reflection in SAX J1808.4–3658 with NICER, XMM-Newton , and NuSTAR

We performed a comprehensive study of thermonuclear bursts from the millisecond X-ray pulsar SAX J1808.4–3658 with XMM-Newton and NICER. We report the results from the analysis of an intense burst with NICER using a self-consistent and physically motivated disc reflection modeling approach and investigate the burst-disc interaction. The dynamic evolution of the spectral parameters suggested evidence of photospheric radius expansion (PRE) of the neutron star using the disc reflection modeling approach, which indicates a maximum expansion of the photosphere up to 14.8 ± 0.7 km. The corresponding blackbody temperature drops to a minimum of 1.9 keV. In addition, an emission line at 1 keV is observed, possibly originating from the Ne or Fe L-band transition as a result of the reprocessing of burst photons by cold gas in the accretion disc. The 1 keV emission line flux is found to be strongly correlated with the flux of the reflection component. We also investigated a thermonuclear burst observed with XMM-Newton EPIC-PN from SAX J1808.4–3658using the variable persistent emission method and the disc reflection modeling approach. The X-ray reflection feature is also investigated in persistent emission using a NuSTAR observation. The best-fitting results provide an inner disc radius of 14-5.9+9.7 Rg and an inclination of 38○ − 60○ during the NuSTAR observation. The magnetic field is estimated to be ≃3.7 × 108 G at the poles of the neutron star.