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To: stefano@bicar.it
From: atel@astronomerstelegram.org
Subject: ATel  17637, 17638
Message-Id: <20260130071123.1406B2880830@server.astronomerstelegram.org>
Date: Fri, 30 Jan 2026 02:11:23 -0500 (EST)

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Posted: Within the last 24 hours
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ATEL #17637							     ATEL #17637

Title:	Decay of the Cygnus X-3 radio flare observed with the Nasu
		radio telescope array at 1.4 GHz
Author:	K. Tsubono (Univ. Tokyo), K. Asuma (Kawaguchi-Kita High School),
		K. Niinuma (Yamaguchi Univ.), K. Takefuji (JAXA) and T. Daishido (Waseda
		Univ.)
Queries:	tsubono@phys.s.u-tokyo.ac.jp
Posted:	29 Jan 2026; 07:28 UT
Subjects:Radio, Binary, Black Hole, Transient

We report continued radio monitoring of Cygnus X-3 with the Nasu radio
telescope array at 1.4 GHz, following the recent radio activity reported
in ATel #17600 and ATel #17605 and our subsequent detections.

The radio emission was first detected on 2026 January 16 at approximately
03:30 UT, after a prior non-detection. Since Cygnus X-3 is observed only
once per day near transit with the Nasu radio telescope array, the exact
onset time of the radio flare is not well constrained. The radio flux subsequently
increased to a peak flux density of about 6.5 Jy on the following day.

After reaching the peak, the radio flux exhibited a monotonic decay. Our
observations on 2026 January 28 indicate that the radio emission from Cygnus
X-3 has faded below our detection limit of approximately 200 mJy, with
no significant signal detected at the source position. This suggests that
the radio flare has substantially decayed and that the source is returning
to a low radio state.

The 1.4 GHz light curve obtained with the Nasu radio telescope array is
shown <a href="http://granite.phys.s.u-tokyo.ac.jp/tsubono/CygX-3/CygX-3_260128.png">here</a>.
Red triangles indicate non-detections (upper limits of approximately 200
mJy).

We plan to continue daily monitoring to confirm whether Cygnus X-3 remains
in this low-flux state.

The Nasu telescope array is a spatial fast Fourier transform (FFT) interferometer
consisting of eight linearly aligned antennas with 20 m spherical dishes.
This type of interferometer was developed to survey transient radio sources,
providing an angular resolution comparable to that of a 160 m dish and
a field of view equivalent to that of a 20 m dish.

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Password Certification: Kimio Tsubono (tsubono@phys.s.u-tokyo.ac.jp)
https://www.astronomerstelegram.org/?read=17637
==============================================================================
ATEL #17638							     ATEL #17638

Title:	The black hole X-ray binary GS 1354-64 in transition from the
		hard to the soft state 
Author:	Chiara Salvaggio, Sara E.  Motta, M. Cristina Baglio (INAF-OAB),
		Rob Fender (University of Oxford) on behalf of the X-KAT collaboration,
		Melania Del Santo Carlotta Miceli (INAF-IASF Palermo), Adam Ingram,
		Melissa Ewing (Newcastle University), Aru Beri (IIA Bangalore, University
		of Southampton)
Queries:	chiara.salvaggio@inaf.it
Posted:	29 Jan 2026; 14:23 UT
Subjects:Radio, Infra-Red, X-ray, Binary, Black Hole, Transient

The MAXI instrument detected a new outburst of the black hole X-ray binary
GS 1354-64 in December 2025 (ATel #17563). Following the X-ray trigger,
we initiated both X-ray and radio follow-up observations with the Swift
satellite and the MeerKAT radio telescope as part of the SwiftKAT and X-KAT
programmes. The latest MeerKAT observation was taken on 2026-01-26T03:28:52
UTC. GS 1354-64 was observed for 10 minutes at a central frequency of 1.28
GHz, with a total bandwidth of 856 MHz. We used J1939-6342 for flux and
bandpass calibration, and J1424-4913 for complex gain calibration.  
The source remains compact (ATel #17582) and is detected at a flux density
of (13 ± 1.3) mJy. Preliminary analysis shows that the radio spectrum
is consistent with being flat (alpha ~ -0.2).

We have been monitoring the current outburst of GS 1354-64 at near infrared
(NIR) frequencies with the Robotic Eye Mount (REM) 60-cm telescope in La
Silla (Cile), using the J (0.26 micron), H (0.29 micron), K (0.41 micron)
filters. The last observations took place on January 27th, 2026 (MJD 61067.2).
After calibrating with 2MASS stars in the field, we measure the following
magnitudes: J=14.45+/-0.23 mag, H=13.53 +/- 0.08 mag, K=13.22 +/- 0.22
mag. 
Once reddening is taken into account (Av~2.6 mag; <a href=”https://ui.adsabs.harvard.edu/abs/2016MNRAS.460..942K/abstract”>Koljonen
et al. 2016</a>), these magnitudes translate into the following de-reddened
flux densities: 5.09 +\- 1.09 mJy, 5.93 +/- 0.45 mJy, 4.22 +/- 0.88 mJy
in the J, H, K bands, respectively, consistent with a flat NIR spectrum.

The latest Swift observation was obtained in the Windowed Timing mode on
2026-01-28T08:57:57, for a total exposure of 1.5 ks. The X-ray spectrum
is dominated by powerlaw emission with photon index ~2.1, and the 0.3-10
keV unabsorbed flux is ~4 E-8 erg/cm^2/s for an nH of nH = 1.4 E+22 cm^-2
(Atel #17583).  
The X-ray spectrum is well described by a thermal disk component plus a
power-law continuum. The fit yields a photon index of ~1.5 and a disk temperature
of ~1.0 keV, for an absorbing column density of nH = 1.2 E+22 cm^-2. The
unabsorbed flux in the 1–10 keV band is ~1.9 E-8 erg/cm^2/s. 
The power density spectrum shows a clear Type-C QPO with a centroid frequency
of ~3.5 Hz, typical of the intermediate state.  
The MAXI and Swift/BAT lightcurves combined show that the source is currently
slowly softening. Compared to the previous (hard-only) outburst observed
from this source in 2006, the current outburst appears significantly brighter.

The above suggests that GS 1354-64 is likely crossing the hard-intermediate
state, and is approaching the hard-to-soft transition, where the launch
of relativistic ejecta may be expected.

The MeerKAT radio telescope will continue monitoring the target weekly
for the entire duration of the outburst. A Swift monitoring of the system
is planned. REM will continue to monitor the system daily during its outburst.

Multi-wavelength observations are encouraged to study the evolution of
the outburst.

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. The Swift follow-up of X-ray binaries
is largely performed as part of the SwiftKAT program, which provides quasi
simultaneous X-ray coverage of the X-KAT targets.

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Password Certification: Chiara  Salvaggio (chiara.salvaggio@inaf.it)
https://www.astronomerstelegram.org/?read=17638
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