Properties of Faint X-ray Activity of XTE J1908+094 in 2019

TL;DR

This paper analyzes the faint X-ray activity of the black hole candidate XTE J1908+094 during its 2019 outburst, revealing spectral state, emission features, and black hole mass estimation through NuSTAR observations.

Contribution

It provides detailed spectral and temporal analysis of the 2019 outburst, including the first estimation of the black hole's mass and characterization of its spectral state during faint activity.

Findings

Source remained in soft-intermediate spectral state.

Detected broad Fe Kα emission line at 6.5 keV.

Estimated black hole mass as approximately 6.5 solar masses.

Abstract

We study the properties of the faint X-ray activity of Galactic transient black hole candidate XTE~J1908+094 during its 2019 outburst. Here, we report the results of detailed spectral and temporal analysis during this outburst using observations from {\it Nuclear Spectroscopic Telescope Array (NuSTAR)}. We have not observed any quasi-periodic-oscillations (QPOs) in the power density spectrum (PDS). The spectral study suggests that the source remained in the softer (more precisely in soft-intermediate) spectral state during this short period of the X-ray activity. We notice a faint but broad Fe K$\alpha$ emission line at around 6.5 keV. We also estimate the probable mass of the black hole to be $6.5^{+0.5}_{-0.7}~M_\odot$ with 90\% confidence.