Electronic Telegram No. 5646
Central Bureau for Astronomical Telegrams
Mailing address:  Hoffman Lab 209; Harvard University;
 20 Oxford St.; Cambridge, MA  02138; U.S.A.
e-mail:  cbatiau@eps.harvard.edu (alternate cbat@iau.org)
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Prepared using the Tamkin Foundation Computer Network


COMET 3I/ATLAS
     C. M. Lisse, Applied Physical Laboratory, Johns Hopkins University;
S. H. C. Cabot, University of Cambridge; D. Z. Seligman, Michigan State
University; K. Dennerl, Max Planck Institute for Extraterrestrial Physics;
Q. D. Wang, University of Massachusetts; S. J. Wolk, Harvard-Smithsonian
Center for Astrophysics; and J. L. Miralles and P.M. Rodriguez, X-ray
Multi-Mirror Mission (XMM-Newton) Science Operations Centre, European Space
Astronomy Centre, Villafranca, report the first ever detailed x-ray imaging of
an interstellar object using the XMM satellite on Dec. 3 UT.  Comet 3I/2025 N1
(ATLAS) was observed for 75000 s.  Time variable imagery of charge exchange
emission of low-energy x-rays at 0.3-0.8 keV was obtained by the EPIC p-n
camera at high signal-to-noise versus the x-ray background.  Large amounts of
water gas surrounding the comet were found using imagery of bright OH radical
emission in a 22000-s exposure with the XMM OM camera in the UVW1 passband
(250-350 nm).  The spatial extent of the x-ray emission was at least 2'
across, similar to the width of the OH coma and the extent of gas-dominated
SDSS u-band imagery of the comet obtained with the 3.5-m reflector (+ ARCTIC
camera) at the Apache Point Observatory on Dec. 8.51 UT.

     E. Jehin, University of Liege, writes that images obtained on Dec. 7 UT
with the 0.6-m TRAPPIST-North telescope at Oukaimden, Morocco (cf. CBETs 4729,
4840) using broadband and cometary HB narrowband gas and dust continuum
filters show production rates of C_2 and CN that are typical of solar-system
comets, though the production rates (including of OH, NH, and C_3) have
dropped by more than 50 percent over the previous ten days.  Twenty-minute
exposures obtained on Nov. 27, 28, and 29 show that the radicals OH, CN, C_2,
and C_3 were easily detected, but NH was very weak; the gas comae were
spherical and several arcmin in size.  No tail was visible except for a
narrow ion tail in the CN filter (due presumably to CO+).

     K. Yoshimoto, Yamaguchi, Japan, reports that CMOS images obtained with a
0.3-m f/4 reflector on Nov. 17.84 UT show that comet 3I had total V magnitude
9.8 with a 4'.4 coma and a 27' tail in p.a. 292 degrees, with a 3' anti-tail
in p.a. 110 degrees.  Images from Nov. 21.85-21.87 show total V mag 10.0 with
a 4'.4 coma, a 15' tail in p.a. 298 deg, and a 3' anti-tail in p.a. 110 deg.
On Nov. 28.8, the total V mag was 9.7 with a 6'.5 coma, an 8' tail in p.a. 295
deg, and a 4' anti-tail in p.a. 105 deg (noting an increase in the brightness
of the anti-tail).  On Dec. 5.8, the comet showed a 3'.8 coma with a total
magnitude V = 11.7, a tail 8' long in p.a. 297 degrees, and an anti-tail 3'
long in p.a. 114 deg.
     Y. Nagai, Saitama, Japan, reports total mag 10.4 and coma diameter 1'.8
in CCD imagery obtained on Nov. 28.84 UT with a 200-mm-f.l. camera lens.
Hirohisa Sato (Sukagawa, Japan) reports that seventeen stacked 30-s CCD
exposures taken on Dec. 1.8 with a 0.25-m f/4.5 reflector show a total mag of
10.4 and a strongly condensed coma of size about 5'.1 with a 15'.5-long tail
in p.a. about 295 degrees and an anti-tail about 4'.2 long toward p.a. about
109 degrees.
     Older CCD observations obtained remotely on Sept. 12.4 UT with a 0.51-m
f/6.8 astrograph located at Siding Spring (NSW, Australia) and reported
belatedly by Hidetaka Sato of Tokyo show a magnitude of 14.5 in a circular
aperture of radius 9".7; the strongly condensed coma was 18" in diameter,
and there was a 1'.5 tail toward p.a. 108 degrees.  On Sept. 29.4, the
magnitude was 12.2 in a circular aperture of radius 59".4, with a strongly
condensed coma of size 55" and a tail 2'.5 long in p.a. 105 deg.

     Visual total-magnitude and coma-diameter estimates (cf. CBET 5640):
Nov. 20.81 UT, 9.3, 3'.5 (S. Yoshida, Ibaraki, Japan, 0.40-m f/4.5 reflector);
28.16, 10.2, 0'.8 (W. Hasubick, Buchloe, Germany, 0.44-m reflector); 28.83,
9.9, 4' (Yoshimoto, 20x800 binoculars); Dec. 3.14, 10.0, 4' (V. Gonano, Udine,
Italy, 20x80 binoculars).


NOTE: These 'Central Bureau Electronic Telegrams' are sometimes
      superseded by text appearing later in the printed IAU Circulars.

                         (C) Copyright 2025 CBAT
2025 December 12                 (CBET 5646)              Daniel W. E. Green