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To: stefano@bicar.it
From: atel@astronomerstelegram.org
Subject: ATel  17883
Message-Id: <20260711070323.957352880349@server.astronomerstelegram.org>
Date: Sat, 11 Jul 2026 03:03:23 -0400 (EDT)

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Posted: Within the last 24 hours
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ATEL #17883							     ATEL #17883

Title:	Near-infrared observations of GRS 1915+105 during its unprecedented
		low-luminosity state
Author:	F. Coti Zelati (ICE-CSIC), M. C. Baglio, S. Motta, M. Ferro
		(INAF-OAB), A. Marino, N. Rea (ICE-CSIC), T. Munoz-Darias (IAC)
Queries:	cotizelati@ice.csic.es
Posted:	10 Jul 2026; 13:33 UT
Subjects:Infra-Red, Binary, Black Hole, Transient

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.

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Password Certification: Francesco Coti Zelati (cotizelati@ice.csic.es)
https://www.astronomerstelegram.org/?read=17883
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