Radio polarimetry as a probe of unresolved jets: the 2013 outburst of XTE J1908+094

TL;DR

This study uses multi-frequency radio and X-ray observations, including polarimetry, to analyze the 2013 outburst of XTE J1908+094, revealing jet behavior and polarization changes that inform about jet structure and evolution.

Contribution

It provides the first detailed radio polarimetry analysis of XTE J1908+094 during an outburst, linking polarization angle changes to jet structure and ejecta dynamics.

Findings

Jet orientation consistent with resolved observations.

Gradual polarization angle drift linked to ejecta structure.

Unexpected 90-degree polarization rotation observed.

Abstract

XTE J1908+094 is an X-ray transient black hole candidate in the Galactic plane that was observed in outburst in 2002 and 2013. Here we present multi-frequency radio and X-ray data, including radio polarimetry, spanning the entire period of the 2013 outburst. We find that the X-ray behaviour of XTE J1908+094 traces the standard black hole hardness-intensity path, evolving from a hard state, through a soft state, before returning to a hard state and quiescence. Its radio behaviour is typical of a compact jet that becomes quenched before discrete ejecta are launched during the late stages of X-ray softening. The radio and X-ray fluxes, as well as the light curve morphologies, are consistent with those observed during the 2002 outburst of this source. The polarisation angle during the rise of the outburst infers a jet orientation in agreement with resolved observations but also displays a gradual drift, which we associate with observed changes in the structure of the discrete ejecta. We also observe an unexpected 90deg rotation of the polarisation angle associated with a second component.