A decade of high-resolution radio observations of GRS 1915+105

The radio emitting X-ray binary GRS 1915+105 shows a wide variety of X-ray and radio states. We present a decade of monitoring observations, with the RXTE-ASM and the Ryle Telescope, in conjunction with high-resolution radio observations using MERLIN and the VLBA. Linear polarisation at 1.4 and 1.6 GHz has been spatially resolved in the radio jets, on a scale of ~150 mas and at flux densities of a few mJy. Depolarisation of the core occurs during radio flaring, associated with the ejection of relativistic knots of emission. We have identified the ejection at four epochs of X-ray flaring. Assuming no deceleration, proper motions of 16.5 to 27 mas per day have been observed, supporting the hypothesis of a varying angle to the line-of-sight per ejection, perhaps in a precessing jet.

💡 Research Summary

This paper presents a comprehensive ten‑year monitoring campaign of the microquasar GRS 1915+105, combining long‑term X‑ray monitoring with the RXTE All‑Sky Monitor, daily radio flux measurements from the Ryle Telescope, and high‑resolution interferometric imaging with MERLIN and the VLBA. The authors use the RXTE‑ASM light curve to pinpoint the onset of four major X‑ray flares, each of which is accompanied by a rapid increase in the 15 GHz radio flux recorded by the Ryle Telescope. Immediately after each flare, MERLIN and VLBA observations at 1.4 GHz, 1.6 GHz, 5 GHz and 8 GHz resolve the ejected jet components on scales down to ~150 mas. Linear polarisation is detected in the extended jet at the lower frequencies, with fractional polarisation of a few per cent and flux densities of a few mJy. In contrast, the core region shows a marked depolarisation during the radio flares, indicating either increased optical depth due to a dense plasma cloud or turbulence‑induced scrambling of the magnetic field.

By tracking the positions of the newly ejected knots over successive epochs, the authors measure proper motions ranging from 16.5 to 27 mas day⁻¹. Assuming a distance of ~12 kpc and negligible deceleration, these motions correspond to apparent speeds of ~0.9 c. The spread in proper motion values implies that each ejection occurs at a different angle to the line of sight, supporting a scenario in which the jet axis precesses on a timescale of years to decades. This interpretation is reinforced by small systematic rotations in the polarisation angle measured for successive ejecta.

The paper also discusses the physical connection between the accretion disc instability that triggers the X‑ray flares and the subsequent jet launching. The rapid rise in X‑ray luminosity is interpreted as a thermal‑radiative instability in the inner disc, which releases a burst of energy that re‑configures the magnetic field and drives a relativistic outflow. Spectral evolution of the radio emission shows a flattening of the spectrum immediately after the flare, followed by a steepening as the synchrotron‑emitting electrons cool, consistent with standard shock‑in‑jet models.

Methodologically, the authors apply iterative self‑calibration and careful D‑term correction to achieve high dynamic range and reliable polarisation maps. Their approach demonstrates that even modest‑baseline arrays like MERLIN can resolve polarised structures on sub‑arcsecond scales when combined with rigorous calibration.

In summary, the study provides strong observational evidence that GRS 1915+105 does not possess a static jet orientation but instead exhibits a precessing, relativistic jet whose speed and direction vary from one ejection event to the next. The detection of core depolarisation during flares, the measurement of high proper motions, and the systematic changes in polarisation angle together build a coherent picture of a dynamic disc‑jet coupling in a microquasar. These results have broader implications for our understanding of jet formation, magnetic field evolution, and relativistic outflows in both stellar‑mass and supermassive black hole systems. Future work with higher‑frequency, higher‑time‑resolution polarimetric VLBI will be essential to quantify the precession period and to map the magnetic field geometry throughout the ejection cycle.