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arXiv · 2610.10951

Flux Dependence of the 1\,keV Soft X-ray Feature in SAX J1808.4-3658 with \textit{NICER}

Katherine Bruce·Anne M. Lohfink·Jaiverdhan Chauhan·Suchismito Chattopadhyay·Sachiko Tsuruta·Andrew C. Liebmann
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Abstract

We present a systematic X-ray spectral analysis of the 2019 and 2022 outbursts of the accreting millisecond pulsar SAX J1808.4-3658 with extit{NICER} with a particular emphasis on the nature of the longstanding $\sim$1\,keV spectral feature. We report here the first clear evidence that the 0.9-1.0 keV feature varies systematically with source brightness, exhibiting strong correlations with both continuum and blackbody flux. This establishes that the feature is directly coupled to the accretion process. The scaling and energy distribution of the feature differ between outbursts, corresponding to a $\sim$2\,dex change in reprocessor density (log\,$n_e \approx 19.1$ in 2019 vs. $\sim$17.3 in 2022), which naturally explains the stronger, broader feature in 2019 and the weaker, more localized $\sim$1\,keV feature in 2022, in agreement with high-density reflection models. These results provide the strongest constraints to date on the origin of the $\sim$1\,keV feature in SAX J1808.4-3658.

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Katherine Bruce, Anne M. Lohfink, Jaiverdhan Chauhan, Suchismito Chattopadhyay, Sachiko Tsuruta, Andrew C. Liebmann. 2026-10-07. Flux Dependence of the 1\,keV Soft X-ray Feature in SAX J1808.4-3658 with \textit{NICER}. https://arxiv.org/abs/2610.10951

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