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From: atel@astronomerstelegram.org
Subject: ATel  17551, 17552
Message-Id: <20251220071241.2FB0128807AE@server.astronomerstelegram.org>
Date: Sat, 20 Dec 2025 02:12:41 -0500 (EST)

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Posted: Within the last 24 hours
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ATEL #17551							     ATEL #17551

Title:	Post-perihelion Sunward Tail Detected on Interstellar Comet
		3I/ATLAS: A Geometric Projection of the Dust Tail
Author:	M. Serra-Ricart (Light Bridges, IAC, ULL), J. Licandro (IAC,
		ULL), M. R. Alarcon (Light Bridges, IAC, ULL)
Queries:	mserra@iac.es
Posted:	19 Dec 2025; 11:54 UT
Subjects:Comet

Interstellar comet 3I/ATLAS (C/2025 N1; hereafter 3I) has been extensively
monitored since 2025 November 29 using the imaging capabilities of the
Two-Meter Twin Telescope (TTT) and the Transient Survey Telescope (TST)
both located at the Observatorio del Teide (IAC, Tenerife, Spain).

We confirm (ATel #17490) the detection of a sunward-oriented tail-like
feature in deep TST imaging of 3I. The structure appears as a narrow elongation
extending sunward from the central condensation of the coma, with a projected
length that increases as the Sun-observer-target angle increases (see linked
figure). On 2025 December 19 (perigee), the projected length of the feature
is approximately 450000 km. A detailed geometric analysis based on images
obtained at different epochs (2025 November 29 and December 19), together
with the absence of a fan-shaped structure in the Laplacian-filtered images,
indicates that this feature is best explained as a geometric projection
of the curved dust tail, rather than as a physically sunward-directed outflow.

The measured position angles (PA) of the dust tail are 287.4 +/- 0.5 deg
on November 29 and 285.4 +/- 0.5 deg on December 19 (red lines in linked
figure). The observed change in the dust-tail PA, 2 +/- 1 deg, is consistent
with the corresponding change in the projected velocity-vector PA (from
289.94 deg to 288.63 deg, a difference of 1.3 deg), supporting a geometric
interpretation of the observed sunward feature. This feature is compatible
with older, large grains confined near the orbital plane in a simple synchrone/syndyne
analysis.

A faint ion tail is also detected on the December 19 image (yellow line
in linked figure). The measured PA angle of ion tail  293.8 +/- 0.5 deg
is coincident with the anti-solar PA direction 292.38 deg (yellow arrows
in linked figure).

Projection-induced sunward tails are well known in solar system comets,
but this detection demonstrates that interstellar comets can exhibit comparable
dust-tail geometries when observed under favorable viewing conditions.
Continued monitoring of 3I/ATLAS as the phase angle evolves will help to
further constrain the dust dynamics and grain properties of this object.
We encourage additional imaging and modeling efforts to better constrain
the dust size distribution and emission history responsible for the observed
morphology.

The linked figure presents the unprocessed images (right panels) and the
corresponding Laplacian-filtered images (left panels) of 3I. The observation
date (YYYY-MM-DD), start and end times (UTC), number of sidereal-tracking
exposures, and total integration time are indicated above each panel. The
projected velocity vector (red arrow) and the antisolar direction (yellow
arrow) are shown, together with the image scale and orientation. Red crosshairs
mark the comet optocenter. The position angles of the dust and ion tails
are indicated by thin red and yellow lines, respectively. Isophotal contours
of the original unfiltered image are overplotted on the filtered frame
using ten logarithmically spaced levels between the 80th and 95th percentiles
of the pixel-intensity distribution.

The TST, a 1 m telescope, is equipped with FERVOR-L, a 14304 x 10748 Sony
IMX411 BSI sCMOS detector providing a pixel scale of 0.60 arcsec per pixel
and a field of view of 2.4 x 1.8 degrees. For the study of fine coma and
tail structures, observations were also obtained with TTT3, a 2 m f/6 Ritchey-Chretien
telescope currently in its commissioning phase.

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).

Laplacian-filtered and original images of 3I obtained on November 29 and
December 19: https://drive.google.com/file/d/13QrhXpXQTjsiOerpojCESJb11RQ3lEzi/view?usp=sharing
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Password Certification: Miquel Serra-Ricart (mserra@iac.es)
https://www.astronomerstelegram.org/?read=17551
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ATEL #17552							     ATEL #17552

Title:	MAXI observation of an X-ray brightening from a black hole
		binary MAXI J1543-564 or a Be/X-ray binary XTE J1543-568
Author:	C. Kang (Ehime U.), M. Nakajima, H. Negoro (Nihon U.), T.
		Mihara (RIKEN), K. Takagi, H. Takahashi, H. Nishio (Nihon U.), 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,
		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:	19 Dec 2025; 15:18 UT
Subjects:X-ray, Binary, Black Hole, Neutron Star, Transient, Pulsar

The MAXI/GSC nova alert system triggered a bright X-ray transient source
on December 19, 2025.  Assuming that the source flux was constant over
the two scan transits, we derived the source position at (R.A., Dec) =
(236.034 deg, -56.510 deg) = (15 44 08, -56 30 35) (J2000) with a statistical
90% C.L. elliptical error region with long and short radii of 0.22 deg
and 0.21 deg, respectively.  The roll angle of long axis from the north
direction is 139.0 deg counterclockwise.  There is an additional systematic
uncertainty of 0.1 deg (90% containment radius).  The X-ray flux averaged
over the scan was 91 &plusmn; 28 mCrab (4.0-10.0keV, 1 sigma error).

Since the derived position is ~0.15 deg from MAXI J1543-564 (100 mCrab
transient in 2011, ATel#3330) and ~0.20 deg from XTE J1543-568 (50 mCrab
transient in 2000, in't Zand et al. 2001; smaller outburst in 2012, ATel#4008),
we cannot distinguish which source was active considering the positional
accuracy of MAXI/GSC.  In addition, the possibility of a new transient
source cannot be excluded, we therefore tentatively designate this transient
as MAXI J1544-565.  We encourage follow-up observations.

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Password Certification: Motoki Nakajima (nakajima.motoki@nihon-u.ac.jp)
https://www.astronomerstelegram.org/?read=17552
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