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 ∼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 ∼150 μJy in L band and ∼90 μJy in S band, the lowest radio flux densities ever recorded from the system, corresponding to LR≃2×1028 erg s−1 at 1.28 GHz, with a spectral index α=−0.67±0.41 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 104 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.
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BibTeX
@misc{motta2026retirement,
title = {{The retirement party of GRS 1915+105: the radio fading to quiescence of the archetypal microquasar}},
author = {S. E. Motta and R. P. Fender and A. Marino and P. Atri and F. Carotenuto and D. Williams-Baldwin and M. C. Baglio and Jun-Ping Chen and Wei Chen and F. Coti Zelati and F. J. Cowie and L. De Regt and N. Grollimund and A. K. Hughes and Yuan Liu and I. Mariani and N. Rea and L. Rhodes and P. Saikia and C. Salvaggio and K. Solomons and J. van den Eijnden and Kong-Jun Zhang and Dan Zhu},
howpublished = {arXiv},
year = {2026},
month = nov,
doi = {10.48550/arxiv.2611.07039},
eprint = {2611.07039},
archivePrefix = {arXiv},
url = {https://arxiv.org/abs/2611.07039},
}
RIS
TY - UNPB
TI - The retirement party of GRS 1915+105: the radio fading to quiescence of the archetypal microquasar
AU - S. E. Motta
AU - R. P. Fender
AU - A. Marino
AU - P. Atri
AU - F. Carotenuto
AU - D. Williams-Baldwin
AU - M. C. Baglio
AU - Jun-Ping Chen
AU - Wei Chen
AU - F. Coti Zelati
AU - F. J. Cowie
AU - L. De Regt
AU - N. Grollimund
AU - A. K. Hughes
AU - Yuan Liu
AU - I. Mariani
AU - N. Rea
AU - L. Rhodes
AU - P. Saikia
AU - C. Salvaggio
AU - K. Solomons
AU - J. van den Eijnden
AU - Kong-Jun Zhang
AU - Dan Zhu
PY - 2026
DA - 2026/11/04
PB - arXiv
DO - 10.48550/arxiv.2611.07039
UR - https://arxiv.org/abs/2611.07039
AB - 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 ∼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 ∼150 μJy in L band and ∼90 μJy in S band, the lowest radio flux densities ever recorded from the system, corresponding to L_R≃2×10^28 erg s^−1 at 1.28 GHz, with a spectral index α=−0.67±0.41 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.
ER -