Broadband Timing and Spectral Study of Accreting Millisecond X-Ray Pulsar SAX J1808.4–3658 during Its 2022 Outburst

We report on our investigation of the NuSTAR and AstroSat observations along with simultaneous NICER observations of the accreting millisecond X-ray pulsar SAX J1808.4-3658, obtained during its tenth outburst from 2022. The NuSTAR observation captured the source near the outburst peak, while AstroSat observed it during the decay phase. Coherent pulsations at similar to 401 Hz were detected throughout the outburst, with the fundamental amplitude in the 3-30 keV range increasing from similar to 4% near the peak to similar to 6% during the decay. The pulsations display strong energy dependence and negative time lags of similar to 0.2-0.3 ms, with harder photons leading softer ones. The broadband spectra in both epochs are well described by a soft thermal component and Comptonized continuum, together with a prominent relativistic reflection component. As the outburst evolved, the continuum softened (Gamma increasing from similar to 1.88 to similar to 1.99) and the coronal electron temperature decreased (kTe from similar to 31 to similar to 18 keV), consistent with enhanced Compton cooling at lower accretion rates. The ionization parameter declined ( log xi from similar to 3.4 to similar to 1.8) while the reflection fraction increased, suggesting a changing accretion geometry with a more compact corona and a larger disk covering fraction during the decay phase. The X-ray luminosity decreased by a factor of similar to 3 between the two epochs. Our results suggest the coupled evolution of the corona, disk, and magnetosphere as the mass accretion rate declines.