Outflows and spectral evolution in the eclipsing AMXP SWIFT J1749.4-2807 with NICER, XMM-Newton and NuSTAR

TL;DR

This study provides a detailed spectral analysis of the eclipsing AMXP SWIFT J1749.4-2807 during its 2021 outburst, revealing accretion disk properties, disc winds, and neutron star surface changes, enhancing understanding of such systems.

Contribution

First comprehensive spectral characterization of SWIFT J1749.4-2807 during outburst, including detection of disc winds and analysis of spectral evolution during decay.

Findings

Detection of a hot spot on the neutron star surface.

Observation of weakly relativistic disc winds.

Evidence of a high truncation radius of the accretion disc.

Abstract

The neutron star low-mass X-ray binary SWIFT J1749.4-2807 is the only known eclipsing accreting millisecond X-ray pulsar. In this manuscript we perform a spectral characterization of the system throughout its 2021, two-week-long outburst, analyzing 11 NICER observations and quasi-simultaneous XMM-Newton and NuSTAR single observations at the outburst peak. The broadband spectrum is well-modeled with a black body component with a temperature of $\sim$0.6 keV, most likely consistent with a hot spot on the neutron star surface, and a Comptonisation spectrum with power-law index $\Gamma \sim 1.9$, arising from a hot corona at $\sim$12 keV. No direct emission from the disc was found, possibly due to it being too cool. A high truncation radius for the disc, i.e., at $\sim$20--30 R$_{G}$ , was obtained from the analysis of the broadened profile of the Fe line in the reflection component. The significant detection of a blue-shifted Fe XXVI absorption line at $\sim$7 keV indicates weakly relativistic X-ray disc winds, which are typically absent in the hard state of X-ray binaries. By comparing the low flux observed during the outburst and the one expected in a conservative mass-transfer, we conclude that mass-transfer in the system is highly non-conservative, as also suggested by the wind detection. Finally, using the Nicer spectra alone, we followed the system while it was fading to quiescence. During the outburst decay, as the spectral shape hardened, the hot spot on the neutron star surface cooled down and shrank, a trend which could be consistent with the pure power-law spectrum observed during quiescence.