The retirement party of GRS 1915+105: the radio fading to quiescence of the archetypal microquasar

Abstract

After more than 25 years of near-Eddington activity and over six years in a Compton-thick obscured phase, the black-hole X-ray binary GRS 1915+105 has begun to fade in the radio to a level lower than ever previously recorded, possibly marking the onset of its final descent towards quiescence. With the accretion flow hidden in the X-rays, the radio emission is currently the best tracer of the accretion activity. We aim to establish whether the fading marks the end of the outburst that began in 1992 or a temporary switch-off. We present the long-term MeerKAT monitoring of the source from the ThunderKAT and X-KAT programmes up to 2026 July, deep dual-band (L- and S-band) MeerKAT Director's Discretionary Time observations, and three Einstein Probe/FXT X-ray pointings bracketing them. Following a last Jy-level flare in early 2025, the 1.28 GHz flux density declined monotonically to $\sim$0.2-0.4 mJy by 2026 March, and after 2026 May 9 it was no longer formally detected in our 10-min monitoring observations. Deep MeerKAT observations in 2026 July detect the source at $\sim$150 $μ$Jy in L band and $\sim$90 $μ$Jy in S band, the lowest radio flux densities ever recorded from the system, corresponding to $L_{\rm R}\simeq2\times10^{28}$ erg s$^{-1}$ at 1.28 GHz, with a spectral index $α=-0.67\pm0.41$ across 1.28-2.62 GHz. The radio decline is the first direct evidence that the central engine itself is fading rather than merely receding further behind the absorber. The steep spectrum is consistent with either a combination of optically thick and thin emission, or an unusual, intrinsically steep core-jet spectrum. Should this mark the onset of quiescence, disc-instability models predict it to last of order $10^4$ yr. The switch-off of the central engine may shed light on the nature of the donor star and on the origin of the very-high-energy gamma-ray emission in this system.