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To: stefano@bicar.it
From: atel@astronomerstelegram.org
Subject: ATel  17581, 17582, 17583
Message-Id: <20260108071632.E877E2880912@server.astronomerstelegram.org>
Date: Thu,  8 Jan 2026 02:16:32 -0500 (EST)

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Posted: Within the last 24 hours
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ATEL #17581							     ATEL #17581

Title:	MAXI detection of X-ray brightening from the black hole X-ray
		binary V4641 Sgr
Author:	M. Nakajima, H. Negoro, K. Takagi, H. Takahashi, H. Nishio
		(Nihon U.), T. Mihara, T. Tamagawa, N. Kawai, M. Matsuoka (RIKEN),
		T. Sakamoto, M. Serino, S. Sugita, Y. Kawakubo, H. Hiramatsu, Y. Kondo,
		A. Yoshida (AGU), Y. Tsuboi, H. Sugai, N. Nagashima, Y. Ishihara (Chuo
		U.), M. Shidatsu, C. Kang, T. Nakamoto, M. Uenishi, T. Usuki, S. Yatsuzuka
		(Ehime U.), I. Takahashi, Y. Yatsu (Science Tokyo), S. Nakahira, S.
		Ueno, H. Tomida, M. Ishikawa, S. Ogawa, M. Kurihara (JAXA), Y. Ueda,
		K. Fujiwara, S. Kobayashi (Kyoto U.), M. Yamauchi, M. Nishio, C. Hiraizumi
		(Miyazaki U.), K. Yamaoka (Nagoya U.), M. Sugizaki (Kanazawa U.),
		W. Iwakiri (Chiba U.), T. Kawamuro (Osaka U.), S. Yamada (Tohoku U)
Queries:	nakajima.motoki@nihon-u.ac.jp
Posted:	7 Jan 2026; 10:49 UT
Subjects:X-ray, Binary, Black Hole, Quasar, Transient

The MAXI/GSC nova alert system triggered an X-ray brightening from the
black hole X-ray binary V4641 Sgr on January 6, 2026 (MJD 61046).  It is
a galactic microquasar discovered in 1999.  The last outburst detected
with MAXI was in September 2024 (ATel #16804, #16811, #16819, #16852, #16916).
During the 16 years of the MAXI/GSC monitoring, the source exhibited major
outbursts roughly once every two years.  The latest 2-4 and 4-10 keV fluxes
are 30 &plusmn; 4 and 19 &plusmn; 4 mCrab, respectively, on January 6,
2026.  Since the source region was in the vicinity of the Sun until January
3, 2026 (MJD 61043), the onset date of the outburst is unclear.  Compared
with the early phases of previous outbursts, the flux in the present outburst
is rising a few times more rapidly than previously observed ones with MAXI/GSC
(ATel #5803, #7858, #11931, #14968, #16804).  Follow-up observations are
encouraged.

Trigger Information about V4641 Sgr: https://maxi.riken.jp/alert/novae/1046526500/1046526500.htm
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Password Certification: Motoki Nakajima (nakajima.motoki@nihon-u.ac.jp)
https://www.astronomerstelegram.org/?read=17581
==============================================================================
ATEL #17582							     ATEL #17582

Title:	MeerKAT detects radio emission from black hole X-ray binary
		GS 1354-64
Author:	Chiara Salvaggio (INAF-OAB), Noa Grollimund (Universite Paris
		Cite &amp; CEA Saclay), Francesco Carotenuto (INAF-OAR), Stephane
		Corbel (Universite Paris Cite &amp; CEA Saclay), Rob Fender (Oxford
		University), Sara Motta (INAF-OAB), on behalf of the X-KAT collaboration
Queries:	chiara.salvaggio@inaf.it
Posted:	7 Jan 2026; 16:54 UT
Subjects:Radio, Binary, Black Hole, Transient

The black hole X-ray binary GS 1354-64 entered a new outburst in late December
2025 detected by MAXI (ATel #17563). Following the X-ray trigger, we initiated
follow-up radio observations with the MeerKAT radio telescope as part of
the X-KAT programme. Here, we report on the detection of radio emission
from GS 1354-64.

Radio observations were carried out 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. GS 1354-64 was
observed for 10 minutes on 2026-01-04 04:31:59 UTC (MJD 61044.19). We detect
a radio source at the known position of GS 1354-64. Fitting for a point
source in the image plane, we measure a preliminary flux density of 3.66
&plusmn 0.03 mJy (statistical uncertainties).

Multi-wavelength observations are encouraged to study the evolution of
the outburst. Further radio monitoring is planned, as well as weekly Swift
observations of the target.

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=17582
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ATEL #17583							     ATEL #17583

Title:	Swift Observations of the Stellar-mass Black Hole GS 1354-645
		in Outburst
Author:	Jon M. Miller (Univ. of Michigan)
Queries:	jonmm@umich.edu
Posted:	7 Jan 2026; 18:10 UT
Subjects:X-ray, Binary, Black Hole

GS 1354-645 is a transient X-ray binary system that harbors a 
stellar-mass black hole.  Following recent MAXI reports of renewed 
activity in this source (ATEL #17563), Swift rapidly observed the 
target to verify the outburst.  Observations started on January 7, 
2026 at 01:01:03 UTC.

The XRT observation was obtained in photon counting mode, and the 
image reveals a bright source at the known position of GS 1354-645. 
The source is bright enough to cause significant photon pile-up. 
Extracting events from a region that excludes the central 18 arc 
seconds of the event distribution still results in a sensitive 
spectrum with over 4000 counts in 704 s of exposure.

Fits to the XRT spectrum suggest a column density of NH = 1.4 +/- 0.2 
E+22 cm^-2, an inner disk temperature of kT = 0.18 +/- 0.02 keV, and a

flux of F = 2.9 E-9 erg/cm^2/s (0.5-10 keV) for an assumed power-law 
index of Gamma = 1.40.

MeerKAT has also detected radio emission from GS 1354-645 (ATEL 
#17582).  It is likely that the source is in a rising hard state.

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Password Certification: Jon Miller (jonmm@umich.edu)
https://www.astronomerstelegram.org/?read=17583
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