# Resolved, expanding jets in the Galactic black hole candidate XTE J1908+094

## TL;DR

This study presents high-resolution radio observations of the 2013 outburst of XTE J1908+094, revealing asymmetric, expanding jets that interact with a dense environment, providing insights into jet behavior post state transition.

## Contribution

First detailed high-angular resolution radio imaging of XTE J1908+094's jets, showing asymmetric, expanding knots and their interaction with the surrounding medium.

## Key findings

- Detected asymmetric, expanding jet knots post outburst.
- Observed a 90° magnetic field swing during jet evolution.
- Jets interact with a denser environment than typical microquasars.

## Abstract

Black hole X-ray binaries undergo occasional outbursts caused by changing inner accretion flows. Here we report high-angular resolution radio observations of the 2013 outburst of the black hole candidate X-ray binary system J1908+094, using data from the VLBA and EVN. We show that following a hard-to-soft state transition, we detect moving jet knots that appear asymmetric in morphology and brightness, and expand to become laterally resolved as they move away from the core, along an axis aligned approximately $-11$\degree\ east of north. We initially see only the southern component, whose evolution gives rise to a 15-mJy radio flare and generates the observed radio polarization. This fades and becomes resolved out after 4 days, after which a second component appears to the north, moving in the opposite direction. From the timing of the appearance of the knots relative to the X-ray state transition, a 90\degree\ swing of the inferred magnetic field orientation, the asymmetric appearance of the knots, their complex and evolving morphology, and their low speeds, we interpret the knots as working surfaces where the jets impact the surrounding medium. This would imply a substantially denser environment surrounding XTE J1908+094 than has been inferred to exist around the microquasar sources GRS 1915+105 and GRO J1655-40.