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To: stefano@bicar.it
From: atel@astronomerstelegram.org
Subject: ATel 17523, 17524
Message-Id: <20251204071110.7FB932880819@server.astronomerstelegram.org>
Date: Thu, 4 Dec 2025 02:11:10 -0500 (EST)

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Posted: Within the last 24 hours
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ATEL #17523							     ATEL #17523

Title:	X-ray observation of the cometary interloper C/2025 N1 (3I/ATLAS)
		by XRISM/Xtend
Author:	D. Ishi, Y. Kanemaru, K. Fukushima, S. Ogawa, Y. Uchida, T.
		Yoneyama, R. Iizuka (JAXA/ISAS), Chris Baluta (NASA/GSFC), Y. Terada
		(Saitama U.), E. Costantini (SRON), M. Guainazzi (ESA/ESTEC), R. Kelley
		(NASA/GSFC), K. Matsushita (TUS), R. Petre, B. Williams (NASA/GSFC),
		H. Yamaguchi, S. Watanabe (JAXA/ISAS), M. Tashiro (Saitama U.)
Queries:	ishi.daiki@jaxa.jp
Posted:	3 Dec 2025; 08:13 UT
Subjects:X-ray, Comet

The follow-up XRISM observation has been performed for the interstellar
comet C/2025 N1, also known as 3I/ATLAS, discovered by the ATLAS survey
telescope on July 01, 2025. This comet was followed by many instruments
in various wavebands, such as NOT in optical, IRTF in infrared, and MeerKAT
in radio (ATels #17263, #17283, and #17473) following the discovery. An
unanticipated ToO observation of this interloper was requested, and it
began at 23:20 UT on November 26, 2025, and ended at 20:38 UT on November
28, 2025, with a 60-ks exposure using the Director's Discretionary Time
allocation for the XRISM observing cycle 2. In order to track the apparent
motion along the sky plane of the comet, we conducted 14 slew maneuvers
during the observation, keeping the comet near the on-axis position on
the detector plane.

After applying noisy-pixel screening, instrument background subtraction,
vignetting correction, point-source removal, and apparent-motion correction
to the Xtend data, we constructed a comet-centered image in the 0.3&ndash;1.0
keV band, which reveals plausible extended X-ray emission over approximately
5 arcmin of radius. Because the X-ray Mirror Assembly (XMA) for Xtend has
a half-power diameter of 1.4 arcmin, the emission appears broader than
its point-spread function. It is important to note that the vignetting
effect caused by the XMA intrinsically produces such a brightness distribution
for a flat emission, whose profile has a broader radial distribution (FWHM
&sim;17&ndash;18 arcmin) than the extent of the diffuse emission. Nonetheless,
it may be premature to conclude that this diffuse emission is attributable
to the atmospheric coma associated with the comet at this time. Further
analysis using fully processed data is required.

A preliminary spectral analysis on the Xtend data shows that the emission
from the 5 arcmin region centered on the comet is globally reproduced by
the empirical background model, comprising Milky Way Halo (MWH, kT = 0.2
keV), Local Hot Bubble (LHB, kT = 0.1 keV), Cosmic X-ray Background (CXB,
&Gamma; = 1.4), and neutral Oxygen line emission (OI, E = 0.525
keV). The MWH and CXB components are subject to the Galactic absorption
with a column density of NH = 1.77 &times; 1020 cm-2.
The emission measures and brightness for these components are consistent
with previous studies, as EMMWH = 0.002 cm-6 pc,
EMLHB = 0.014 cm-6 pc, and SCXB = 8.4
cm-2 s-1 keV-1 sr-1. However,
this model fails to explain the soft band with Gaussian-like residual structures
at around 0.4 and 0.6 keV. The energies suggest that these deficits likely
correspond to the CVI, NVI, and OVII lines.
If these excess components come from the comet, the most plausible origin
is charge-exchange emission between comet volatiles and the solar wind.

Follow-up monitors are encouraged.

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Password Certification: Yoshiaki Kanemaru (kanemaru.yoshiaki@jaxa.jp)
https://www.astronomerstelegram.org/?read=17523
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ATEL #17524							     ATEL #17524

Title:	Optical imaging of M31N 2008-12a during its 2025 eruption from
		IAO
Author:	Judhajeet Basu (IIA), Shatakshi Chamoli (IIA), Aditya Pawan
		Saikia (IIT Bombay), Vishwajeet Swain (IIT Bombay), Sudhanshu Barway
		(IIA), G. C. Anupama (IIA), Varun Bhalerao (IIT Bombay)
Queries:	judhajeet.basu@iiap.res.in
Posted:	3 Dec 2025; 09:22 UT
Subjects:Optical, Cataclysmic Variable, Nova, Transient

We report the optical imaging observations of M31N 2008-12a using the GROWTH-India
Telescope (GIT) and the Himalayan Chandra Telescope (HCT) located at IAO,
Hanle. The 2025 eruption was discovered by K. Itagaki et al. (ATel #17511)
on 2025 Nov 30.3634 UT at a magnitude of 18.7 in clear filter. F. Diaz-Segado
et al. (ATel #17514) reported detection in an image taken 6 hours prior
to discovery on 2025 Nov 30.1130 UT at g&prime; = 19.39 &plusmn; 0.22.
The spectroscopic confirmations were reported by S. Chamoli et al (ATel
#17520) and D.P.K. Banerjee et al (ATel #17521).

GIT daily monitoring of M31 did not detect the source on 2025 Nov 29.6393
UT up to a 3 sigma limiting mag of 20.367 in g&prime; filter. This constrains
the eruption time to be within 2025 Nov 29.8761 &plusmn; 0.2368 UT

We triggered ToO observations with HCT on 30th Nov, when it was detected
in g&prime; (&sim;18.5), r&prime; (&sim;18.4) and i&prime; (&sim;18.7)
filters, approximately 17 hrs after the first detection. Follow-up observations
with GIT on 1st December revealed that the nova has faded by approximately
0.5 mags in g&prime; and r&prime; filters and by 0.2 mags in i&prime; filter
within 24 hrs

We are continuing to monitor this object. Follow-up photometric and spectroscopic
observations are highly encouraged.

GIT is a 70 cm telescope with a 0.7 degree field of view, set up by the
Indian Institute of Astrophysics (IIA) and the Indian Institute of Technology
Bombay (IITB) with funding from DST-SERB and IUSSTF. It is located at the
Indian Astronomical Observatory (Hanle), operated by IIA. We acknowledge
funding by the IITB alumni batch of 1994, which partially supports the
operations of the telescope. Telescope technical details are available
at <a href="https://sites.google.com/view/growthindia/">https://sites.google.com/view/growthindia/</a>.
We thank the HCT staff for their support during the observations. We thank
the HCT observers for giving us time to carry out the ToO observations.
The Indian Astronomical Observatory is operated by the Indian Institute
of Astrophysics, Bengaluru, India.

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Password Certification: Judhajeet Basu (judhajeet.basu@iiap.res.in)
https://www.astronomerstelegram.org/?read=17524
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