Near-infrared observations of GRS 1915+105 during its unprecedented low-luminosity state

ATel #17883; F. Coti Zelati (ICE-CSIC), M. C. Baglio, S. Motta, M. Ferro (INAF-OAB), A. Marino, N. Rea (ICE-CSIC), T. Munoz-Darias (IAC)
on 10 Jul 2026; 13:33 UT
Credential Certification: Francesco Coti Zelati (cotizelati@ice.csic.es)

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Following the recent reports of the unprecedented radio fading of GRS 1915+105 with MeerKAT (Motta et al. 2026; ATel #17865) and its non-detection in X-rays with Einstein Probe (Marino et al. 2026; ATel #17869), we obtained near-infrared imaging of the source with EMIR on the 10.4-m Gran Telescopio Canarias.

The observations were carried out on 2026 July 5 between 03:58:03 and 04:35:56 UTC. Dithered images were acquired in the J, H, and Ks filters, with total effective exposure times of 682, 280, and 252 s, respectively. The final stacked images had image quality of 0.64-0.67 arcsec full-width at half-maximum.

A source is clearly detected at the position of GRS 1915+105 in all three bands. Absolute photometry was calibrated using nearby stars with catalogued magnitudes from the UKIDSS Galactic Plane Survey (GPS; Lucas et al. 2008). The measured Vega magnitudes, uncorrected for Galactic extinction, are:

J = 18.58 +/- 0.07

H = 15.85 +/- 0.07

Ks = 14.37 +/- 0.06

The measured Ks magnitude is fainter than the lowest near-infrared brightness reported in recent years, including the GTC/EMIR observations obtained during a radio-quiet interval in 2022 (Sanchez-Sierras et al. 2023). It is also close to the K ~ 14.5-15.0 donor-star contribution inferred from VLT spectroscopy by Greiner et al. (2001), suggesting that the near-infrared emission is now close to the expected donor-star level.

We also extracted photometry from the individual dithered exposures to search for variability. No statistically significant short-timescale variability is detected over the ~20-min observing sequence. The measured fractional rms variability amplitude (Vaughan et al. 2003) is consistent with zero in H and is 7 +/- 3% in Ks.

We also monitored GRS 1915+105 in the Ks band with the 60-cm Rapid Eye Mount (REM) telescope at La Silla (Chile). Observations were carried out on 2026 July 5 between 07:19 and 08:01 UT, with each data point derived from a dithered sequence of five 15-s exposures. The photometry was calibrated against nearby 2MASS stars. Owing to blending within the REM point-spread function, we do not report an absolute mean magnitude; instead, we used the individual-epoch photometry to compute the fractional rms variability amplitude. The measured sample variance is consistent with the expected photometric noise: no significant variability is detected on timescales of ~4-42 min, with the rms variability amplitude remaining below ~10% as set by the photometric noise floor.

We thank the GTC and REM staff for their prompt support and execution of these observations.