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From: atel@astronomerstelegram.org
Subject: ATel 17468, 17469, 17470
Message-Id: <20251103071701.01F6A288085B@server.astronomerstelegram.org>
Date: Mon, 3 Nov 2025 02:17:01 -0500 (EST)

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ATEL #17468							     ATEL #17468

Title:	Spectroscopic Follow-up Observation of WNTR23bzdiq = TCP J00384865+4046079
		= AT 2025abao: a Luminous Red Nova in M31?
Author:	Kenta Taguchi, Keiichi Maeda (Kyoto University)
Queries:	kentagch@kusastro.kyoto-u.ac.jp
Posted:	2 Nov 2025; 14:42 UT
Subjects:Optical, Binary, Cataclysmic Variable, Nova, Transient, Variables

We report the spectroscopic follow-up observation of <a href="https://iopscience.iop.org/article/10.3847/1538-4357/ae00bc"
target="_blank">WNTR23bzdiq</a> = <a href="http://www.cbat.eps.harvard.edu/unconf/followups/J00384865+4046079.html"
target="_blank">TCP J00384865+4046079</a> = <a href="https://www.wis-tns.org/object/2025abao"
target="_blank">AT 2025abao</a>. 
We obtained its spectra by the fiber-fed integral field spectrograph
(<a href="http://www.o.kwasan.kyoto-u.ac.jp/inst/p-kools/" target="_blank">KOOLS-IFU</a>;
Matsubayashi et al. <a href="https://ui.adsabs.harvard.edu/abs/2025PASJ...77.1065M/abstract"
target="_blank">2025</a>, <a href="https://ui.adsabs.harvard.edu/abs/2019PASJ...71..102M/abstract"
target="_blank">2019</a>) mounted on the 3.8 m Seimei telescope (<a href="https://ui.adsabs.harvard.edu/abs/2020PASJ...72...48K/abstract"
target="_blank">Kurita et al. 2020</a>) at Okayama Observatory of Kyoto
University on 2025-10-29.61 and 2025-10-31.72 UT using the VPH-blue grism
(covering 4100 - 8500 &Aring;, resolution &sim; 500).

As presented by <a href="https://iopscience.iop.org/article/10.3847/1538-4357/ae00bc"
target="_blank">Karambelkar et al. (2025)</a>, this transient was discovered
by <a href="http://winter.caltech.edu/" target="_blank">WINTER</a> in the
J band on 2023-12-17. 
As reported to <a href="http://www.cbat.eps.harvard.edu/unconf/followups/J00384865+4046079.html"
target="_blank">CBAT</a>, this object has been brightening in the optical,
discovered at 17.8 mag (unfiltered) on 2025-10-17.4798 UT by Koichi Itagaki
(also independently discovered by <a href="http://www.cbat.eps.harvard.edu/unconf/followups/J00384865+4046079.html"
target="_blank">Robert Fidrich</a> on 2025-10-18.8112 and <a href="https://www.wis-tns.org/object/2025abao"
target="_blank">MASTER</a> on 2025-10-19.8196 UT). 
<a href="https://www.kusastro.kyoto-u.ac.jp/~kentagch/atel/2025abao.pdf"
target="_blank">Our spectra taken by Seimei/KOOLS-IFU</a> are dominated
by a blue continuum. 
The emission line of H&alpha; is prominently seen in both epochs
(with Lorentzian FWHM &sim; 630 and 700 km/s on 2025-10-29.61 and 2025-10-31.72,
respectively), while H&beta; is only prominent on 2025-10-29.61 but has
significantly weakened on 2025-10-31.72. 
The absorption line of NaI D is clearly seen in both epochs,
suggesting that this object suffers from extinction. 
The evolution is similar to that of the luminous red nova <a href="https://www.wis-tns.org/object/2019zhd"
target="_blank">AT 2019zhd</a>, reported by <a href="https://ui.adsabs.harvard.edu/abs/2021A%26A...646A.119P/abstract"
target="_blank">Pastorello et al. (2021)</a>. 
From this similarity and the luminosity of the optical event, we
propose that this event is likely a luminous red nova in M31, consistent
with the common-envelope transient scenario of WNTR23bzdiq proposed by
<a href="https://iopscience.iop.org/article/10.3847/1538-4357/ae00bc" target="_blank">Karambelkar
et al. (2025)</a>. 
Further follow-up observation of this event is strongly encouraged.

The spectra: https://www.kusastro.kyoto-u.ac.jp/~kentagch/atel/2025abao.pdf
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Password Certification: Keisuke Isogai (isogai@kusastro.kyoto-u.ac.jp)
https://www.astronomerstelegram.org/?read=17468
==============================================================================
ATEL #17469							     ATEL #17469

Title:	C/2025 R2 (SWAN): Possible Fragmentation Observed
Author:	M. Serra-Ricart (Light Bridges, IAC, ULL), J. Licandro (IAC,
		ULL), M. R. Alarcon (Light Bridges, IAC, ULL)
Queries:	mserra@iac.es
Posted:	2 Nov 2025; 22:54 UT
Subjects:Optical, Comet

Recent images of comet C/2025 R2 (SWAN) obtained with the 2.0-m TTT3 telescope
suggest that the nucleus has undergone fragmentation. Although full confirmation
is pending, the data indicate that the nucleus may be splitting into two
components or releasing a substantial fragment.

The attached figure shows the evolution of the cometary morphology from
October 19.83 to November 2.85. On that date, the comet displayed a well-defined,
symmetric central condensation. In subsequent nights, the central region
became progressively elongated, culminating in the apparent detachment
of a distinct secondary condensation. 
All the images were bias, dark and flat-field corrected in the standard
way and aligned-and-staked from the cometary optocenter. Stacked images,
shown in figure, include the projected velocity vector of the comet (blue
arrow), the antisolar direction (yellow arrow), as well as the image orientation
and distance scale.

Comet C/2025 R2 (SWAN) has been extensively monitored from 2025 October
19 using the imaging capabilities of both the Two-Meter Twin Telescope
(TTT, ttt.iac.es) and the Transient Survey Telescope (TST, tst.iac.es).

TTT3 is a 2-meter f/6 Ritchey–Chretien telescope, currently in its commissioning
phase. Images were obtained with COLORS, a 2k camera mounted at the Nasmyth
2 focus, equipped with a back-illuminated BEX2-DD CCD sensor, resulting
in a field of view of 7.85 x 7.85 arcsec and a plate scale of 0.23 arcsec/pixel.

TTT and TST are operated by Light Bridges in Tenerife, Canary Islands (Spain).
The observation time rights (DTO) used for this research were consumed
in the PEI PLANETIX25. This research used storage and computing capacity
in ASTRO POCs EDGE computing center at Tenerife under the form of Indefeasible
Computer Rights (ICR).

Evolution of coma morphology of C/2025 R2: https://drive.google.com/file/d/1lE_BpMe1gRg4B2-PAwux2y01KZxnAcUS/view?usp=drive_link
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Password Certification: Miquel Serra-Ricart (mserra@iac.es)
https://www.astronomerstelegram.org/?read=17469
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ATEL #17470							     ATEL #17470

Title:	 XRISM/Xtend Transient Search (XTS) detected an X-ray flare
		possibly from an M-type star Gaia DR3 1706200845241330432
Author:	Haruki Kuramoto (UOsaka), Yoshitomo Maeda (ISAS),  K. Fukushima,
		Y. Kanemaru, S. Ogawa, Y. Uchida, T. Yoneyama (JAXA), M. Audard (U.
		de Geneve), E. Behar (Technion), T. Hakamata (UOsaka), S. Inoue (Kyoto
		U.), Y. Ishihara (Chuo U.), C. Kang (Ehime U.), T. Kiyomoto (Saitama
		U.), T. Kohmura (TUS), J. Kurashima (U. of Miyazaki),   H. Matsumoto
		(UOsaka), T. Matsushima (U. of Miyazaki), A. Miyamoto (UOsaka), M.
		Mizumoto (UTEF), K. Mori (U. of Miyazaki), Y. Motogami (Saitama U.),
		N. Nagashima (Chuo U.), T. Narita (Kyoto U.), M. Nobukawa (NUE), H.
		Noda (Tohoku U.), Y. Sakamoto (Tohoku U.), M. Shidatsu (Ehime U.),
		H. Sugai (Chuo U.), H. Takahashi (Hiroshima U.), T. Takagi (Ehime
		U.),  S. Takatuska (UOsaka), R. Takemoto (U. of Miyazaki), Y. Terada
		(Saitama U., JAXA), Y. Terashima (Ehime U.), Y. Tsuboi (Chuo U.),
		H. Uchida (Kyoto U.), M. Yoshimoto (Ehime U.), I. Zhuravleva (U. of
		Chicago)
Queries:	tyoneyama263@g.chuo-u.ac.jp
Posted:	3 Nov 2025; 00:30 UT
Subjects:X-ray, Star, Transient

XRISM/Xtend Transient Search (XTS) detected an X-ray flare from an X-ray
source XRISM J1703+7849 on 2025-11-01 TT. The source position is determined
to be (R.A., Dec.) = (255.836, +78.820), with a systematic error of &sim;
40 arcsec. The plausible counterpart is Gaia DR3 1706200845241330432, likely
an M-type star, located &sim; 1 arcsec apart from the position of XRISM
J1703+7849. A previously cataloged X-ray source, 4XMM J170321.0+784912,
is located &sim; 1 arcsec from both XRISM J1703+7849 and Gaia DR3 1706200845241330432.

The flare reached its peak on 2025-11-01 at &sim; 10:37. The flare exponentially
decayed in 7 &times; 103 sec. The peak flux is calculated as
&sim; 3 &times; 10-12 erg s-1 cm-2 (0.4
&ndash; 10.0 keV). The corresponding luminosity is 8 &times; 1031
erg s-1, assuming a distance to XRISM J1703+7849 of 150 pc,
based on Gaia DR3 1706200845241330432.

We derived the above systematic error for the flux by comparing our derived
values for the sources detected with XTS in several observations with those
for the corresponding X-ray counterparts. We estimated the systematic error
for the source position from the separations between the detected sources
with the corresponding counterparts in the same field of view.

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Password Certification: Tomokage Yoneyama (tyoneyama263@g.chuo-u.ac.jp)
https://www.astronomerstelegram.org/?read=17470
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