Electronic Telegram No. 5661
Central Bureau for Astronomical Telegrams
Mailing address:  Hoffman Lab 209; Harvard University;
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Prepared using the Tamkin Foundation Computer Network


COMET C/2026 B2 (SUN-GAO)
     M. S. P. Kelley and Q. Ye, University of Maryland, College Park, on
behalf of the "Zwicky Transient Facility" (ZTF) Solar System Working Group,
report an apparent outburst of comet C/2026 B2 in ZTF images taken with the
Oschin 1.2-m Schmidt telescope at Palomar.  The comet was observed in ZTF's
Twilight Survey starting 2026 Jan 25.  At first, the comet's apparent
brightness trend was near constant with time, measuring magnitude r = 17.17
(+/- 0.08) on Jan. 25.578 and 17.06 (+/- 0.08) on Feb. 1.577 UT (all
photometry is measured in a 5"-radius aperture and reported in the PS1
photometric system).  The comet brightened by -1.08 (+/- 0.09) mag to r =
15.98 (+/- 0.03) in the average of three images centered at Feb. 3.573.  A
comparison of the pre- and post-brightening images show a new compact source
centered in the coma.  Continued ZTF imaging of the comet showed evidence for
another brightening event of relative strength -0.90 (+/- 0.03) mag between
Feb. 9 and 12.  The following r-band photometry was measured (PS1 photometric
system) using a 5"-radius aperture, with the number of 30-s exposures in
parentheses:  Feb. 4.57, 16.07 +/- 0.03 (3); Feb. 5.57, 16.13 +/- 0.11 (2);
Feb. 9.57, 15.99 +/- 0.03 (3); Feb. 12.57, 15.09 +/- 0.02 (3).  Based on a
difference between the Feb. 9 and 12 ZTF images, the morphology of the new
material on Feb. 12 was extended approximately 18" in radius from the nuclear
condensation.  In addition, an 30-s image taken with the 4.3-m Lowell
Discovery Telescope (LDT) at Happy Jack, AZ, with additional observers L.
Feaga and Q. Zhang, shows that the comet had already brightened by Feb. 10.55,
measuring magnitude r = 15.1 +/- 0.2 (5"-radius aperture).  Further analysis
or data will be required to better understand this event (e.g., if the LDT
data were taken during the brightening phase of an outburst, or if the comet's
activity had increased to a new established level.
     J.-r. Xiong, Xingyuan Observatory, Ocean University of China, reports
that a 60-s unfiltered exposure taken with Y. Lu, C.-y. Guo, B.-y. Ai, T.
Zhang, and W.-t. Yue using a 0.25-m f/4.8 reflector indicate a significant
brightening to magnitude G = 14.2 (Gaia EDR3 comparison stars; 15" aperture)
on Feb. 2.95 UT, compared to mag 15.2 on Jan. 26.96 with the same exposure
and photometric specifications.


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

                         (C) Copyright 2026 CBAT
2026 February 15                 (CBET 5661)              Daniel W. E. Green