Identify the broad near-infrared emission line

Determine whether the broad near-infrared emission feature in VT J1906+0849 is blueshifted Brγ and establish whether its emitting region traces a stable wind geometry, orbital motion, or a more complex outflow using multi-epoch, high-resolution near-infrared spectroscopy.

Background

The paper detects a persistent broad near-infrared emission feature near 2.146 μm, with a nearly stable centroid but variable width and luminosity. The authors regard blueshifted Brγ as the most natural interpretation, implying an asymmetric outflow with a characteristic velocity of approximately 2,000–2,600 km s−1, but they emphasize that this identification is not secure because alternative atomic or molecular transitions remain possible and no additional hydrogen or helium lines are detected.

The proposed jet–wind interpretation depends in part on the line tracing a persistent asymmetric disk wind. Multi-epoch, higher-resolution spectroscopy could distinguish among a stable wind geometry, orbital motion, and a more complex outflow, thereby testing the physical interpretation of the line and its connection to the compact radio source.

References

Several open questions remain. The identification of the near-infrared line as blueshifted Brγ is not secure. The absence of additional hydrogen and helium lines is also notable, particularly given the rich emission and absorption line spectrum of SS 433 (D. R. Gies et al. 2002; Y. Fuchs et al. 2002; S. Fabrika 2004; E. L. Robinson et al. 2017). The absence of, e.g., moving lines associated with the baryonic jets in SS 433 is expected in VT J1906+0849 if the jets are leptonic. Multi-epoch, high-resolution (R ≳ 10, 000) near-infrared spectroscopy would help determine whether the broad line traces a stable wind geometry, orbital motion, or a more complex outflow.

— VLASS Discovery of a Luminous Galactic Radio Transient Evolving on Decade Timescales  (2608.21320 - Miller et al., 21 Aug 2026) in Section 4.1.2, near the end of the discussion of the jet–wind interaction