Spectral Analysis of the 2019 and 2022 Outbursts of SAX J1808.4-3658

TL;DR

This study analyzes the spectral evolution and reflection features during the 2019 and 2022 outbursts of the pulsar SAX J1808.4-3658 using NICER and NuSTAR data, revealing insights into its accretion and burst behavior.

Contribution

First combined broad-band spectral analysis of SAX J1808.4-3658 using NICER and NuSTAR data, including reflection modeling and inclination constraints.

Findings

Evidence of reflection in the spectra.

Inclination constrained to approximately 72 degrees.

Detection of a 1 keV feature during persistent emission.

Abstract

The accreting millisecond pulsar SAX J1808.4-3658 went into outburst from July to November in 2019 and August to October in 2022, which were observed by \textit{NICER} and \textit{NuSTAR}. In this paper, we first present the light curve for both outbursts using \textit{NICER} data. Several thermonuclear bursts occurred during these outbursts. We analyze the evolution of the spectra of two thermonuclear bursts that took place during the 2019 \textit{NuSTAR} observation. We proceed by analyzing the combined broad-band spectrum using \textit{NICER} and \textit{NuSTAR} for the first time for this source. We jointly modeled the combined quiescent spectra of both outbursts with a self-consistent reflection component. In our best-fit model, we find evidence of reflection, consistently constrain the inclination to 72\deg$^{+1\deg}_{-4\deg}$\, considering this reflection, and identify a 1 keV feature during persistent emission.