NuSTAR and Swift observations of the black hole candidate XTE J1908+094 during its 2013 outburst

TL;DR

This study reports on NuSTAR and Swift observations of the black hole candidate XTE J1908+094 during its 2013 outburst, revealing spectral features, variability, and flare activity, but not constraining the black hole spin.

Contribution

First detailed spectral and timing analysis of XTE J1908+094 during its 2013 outburst using NuSTAR and Swift data, highlighting flare behavior and spectral modeling.

Findings

Detection of a broad relativistic iron line.

Observation of a ~40 ks flare with spectral variation.

Variability driven by changes in the corona or jet activity.

Abstract

The black hole candidate XTE J1908+094 went into outburst for the first time since 2003 in October 2013. We report on an observation with the Nuclear Spectroscopic Telescope Array (NuSTAR) and monitoring observations with Swift during the outburst. NuSTAR caught the source in the soft state: the spectra show a broad relativistic iron line, and the light curves reveal a ~40 ks flare with the count rate peaking about 40% above the non-flare level and with significant spectral variation. A model combining a multi-temperature thermal component, a power-law, and a reflection component with an iron line provides a good description of the NuSTAR spectrum. Although relativistic broadening of the iron line is observed, it is not possible to constrain the black hole spin with these data. The variability of the power-law component, which can also be modeled as a Comptonization component, is responsible for the flux and spectral change during the flare, suggesting that changes in the corona (or possibly continued jet activity) are the likely cause of the flare.