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

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 the Neutron Star Interior Composition Explorer (NICER) and Nuclear Spectroscopic Telescope Array (NuSTAR). In this paper, we first present the light curve for both outbursts using 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 NuSTAR observation. We proceed by analyzing the combined broadband spectrum using NICER and 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°° + ° 4 1 considering this reflection, and identify a 1 keV feature during persistent emission.