Probing burst–disc interaction and disc reflection in SAX J1808.4−3658 with <i>NICER, XMM–Newton</i> , and <i>NuSTAR</i>

ABSTRACT We performed a comprehensive study of thermonuclear bursts from the millisecond X-ray pulsar SAX J1808.4−3658 with XMM–Newton and NICER (Neutron star Interior Composition ExploreR). We report the results from the analysis of an intense burst with NICER using a self-consistent and physically motivated disc reflection modelling 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 modelling approach, which indicates a maximum expansion of the photosphere up to $14.8\pm 0.7$ 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 J1808.4−3658 using the variable persistent emission method and the disc reflection modelling approach. The X-ray reflection feature is also investigated in persistent emission using a NuSTAR (Nuclear Spectroscopic Telescope Array) observation. The best-fitting results provide an inner disc radius of $14_{-5.9}^{+9.7}$ $R_g$ and an inclination of $38^\circ -60^\circ$ during the NuSTAR observation. The magnetic field is estimated to be $\simeq$3.7 $\times 10^8$ G at the poles of the neutron star..