Probing the accreting millisecond X-ray pulsar SAX J1808.4-3658 using the evolution of its spectral and aperiodic timing properties

Aman Kaushik; Yash Bhargava; Sudip Bhattacharyya; Maurizio Falanga

doi:10.3847/1538-4357/ae0677

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arXiv · 2510.01207

Probing the accreting millisecond X-ray pulsar SAX J1808.4-3658 using the evolution of its spectral and aperiodic timing properties

Aman Kaushik·Yash Bhargava·Sudip Bhattacharyya·Maurizio Falanga

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Abstract

Understanding accretion components in neutron star (NS) low mass X-ray binary (LMXB) systems is important to probe fundamental aspects of accretion mechanism and evolution of the system constraining its physical properties. Here, we present spectral and aperiodic timing analyses of the NICER and AstroSat data from the accretion powered millisecond X-ray pulsar (AMXP) SAX J1808.4-3658 during its 2022 outburst. We find that emissions from a softer accretion disk and a harder, centrally located, compact, partially covering, Comptonizing corona explains the continuum spectra from the source throughout the outburst. The disk inner edge temperature, the coronal electron temperature and photon index are found to be around ~ 0.5-0.9 keV, a few keV and ~ 1.1-1.8, respectively, during the entire outburst. We also find an intrinsic atomic hydrogen medium in the system, which substantially and systematically evolved throughout the outburst. We detect two broadband aperiodic features (~ 0.004-2 Hz; ~ 10-100 Hz), with the former having a significant hard lag of ~ 11 ms between 1.5-10.0 keV and 0.5-1.5 keV photons. We conclude that both the disk photons and the photons up-scattered by the corona contributed to each aperiodic feature, with the disk and the corona contributing more to the low and high frequency ones, respectively.

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Aman Kaushik, Yash Bhargava, Sudip Bhattacharyya, Maurizio Falanga. 2025-09-16. Probing the accreting millisecond X-ray pulsar SAX J1808.4-3658 using the evolution of its spectral and aperiodic timing properties. https://doi.org/10.3847/1538-4357/ae0677

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