MeerKAT observes the radio fading of GRS 1915+105 to its lowest luminosity level since discovery

ATel #17865; S. Motta (INAF-OAB), R. Fender (Uni. Oxford) for the X-KAT collaboration, A. Marino (USACH & ICE-CSIC), F. Carotenuto (INAF-OAR), M. C. Baglio (INAF-OAB), J. van den Eijnden (Uni. of Amsterdam), P. Atri (ASTRON), D. Williams-Baldwin (Uni of. Manchester)
on 2 Jul 2026; 09:49 UT
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Credential Certification: Sara Elisa Motta (sara.motta@physics.ox.ac.uk)

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We have been monitoring the black hole X-ray binary GRS 1915+105 since 2018 with the MeerKAT radio telescope. After the obscuration phase started in 2019 (Motta et al. 2021, MNRAS, 503, 152), we have been observing the source every one or two weeks.

As part of our regular monitoring, the field of GRS 1915+105 was observed for 10 minutes in S-band on June 7th starting at 21:34:52 UT, and in L-band for 10 min on June 11th starting at 19:22:37 UT. Observations were carried out at a central frequency of 2.625 GHz with a bandwidth of 854 MHz, and at a central frequency of 1.28 GHz with a total bandwidth of 856 MHz, respectively. J1939-6342 was used for flux and bandpass calibration, while J2011-0644 was used for complex gain calibration.

No radio source was detected at the position of GRS 1915+105 in L-band. Given the RMS within this region of ~60 μJy/beam, we report a 3-sigma upper limit of ~180 μJy for the flux density of the source at 1.28 GHz. A marginal detection was obtained in S-band, of ~0.1 mJy, with an rms noise of ~30 μJy/beam.

In order to confirm the radio dimming we observed GRS 1915+105 in L-band for 30 min on June 27th, starting at 22:13:38 UT. The same calibration set-up as in the previous observations was adopted. The resulting radio image returns a non-detection of the source, with an rms noise of 30 μJy/beam.

This latest non-detection corresponds to a 3-sigma upper limit of ~90 μJy, or an upper limit on the radio luminosity of ~4.8 x 10²⁸ erg s⁻¹ at a distance of 9.4 kpc. Given that the target has been in an obscured state since 2019, the radio luminosity is currently the best tracer of its accretion activity. This is the first time that GRS 1915+105 shows such a low radio flux since its discovery, sampling a low-luminosity state never probed before. More radio observations are planned.

X-KAT is a large MeerKAT open-time programme to observe X-ray binaries in the radio band, performing weekly monitoring of bright, active systems, with capacity for higher cadence observations, and in coordination with large X-ray and optical monitoring programmes. For further information on this programme contact Rob Fender. We thank the MeerKAT operations team for promptly scheduling the observation reported.