The Astronomer's Telegram: Top ATels

For reporting and commenting upon new observations of astronomical transients by the professional astrophysics community.

ATel 17543: Independent Discovery of an Apparent Nova in M31, AT 2025agwf

We report the independent discovery of an apparent nova found during our survey of the central region of M31. The object was discovered on a co-added 360-s R-band CCD frame taken on 2025 Dec. ...
https://www.astronomerstelegram.org/?read=17543
date: 2025-12-15T19:16:52-00:00
creator: K. Hornoch, H. Kucakova, A. W. Shafter, R. Fidrich

ATel 17542: Photometry of M31 Nova AT 2025agmm

We report multi-color photometry of the M31 nova AT 2025agmm (= PNV J00421476+4117284, Vend78) discovered by R. Fidrich on 2025 Dec 11.776 UT. Our follow-up observations of the nova, which is designated by us as M31N 2025-12b, were taken with the Mount Laguna Observatory 40-in reflector and the Danish 1.54-m telescope at La Silla, and are given below. ...
https://www.astronomerstelegram.org/?read=17542
date: 2025-12-14T21:25:38-00:00
creator: C. Gobat, K. J. Casciotti, J. C. Horst, R. M. Quimby, A. W. Shafter, K. Hornoch, H. Kucakova, R. Fidrich, T. Tordai

ATel 17541: Spectroscopic Classification of Astrophysical Transients with the Lick Shane Telescope

We report the classifications of astrophysical transients from spectroscopic observations with the Kast spectrograph on the Shane telescope obtained on 2025 December 13 UT. ...
https://www.astronomerstelegram.org/?read=17541
date: 2025-12-14T01:32:54-00:00
creator: K. W. Davis, R. J. Foley

ATel 17540: The CSS 3C 138: secondary R band maximum after 40 days

The compact steep-spectrum (CSS) quasar 3C 138 demonstrate enhanced activity in optical range (R band) since 2025 September. As we reported in ATel #17539 its brightness achieved 2 mJy level (R~15.45 +/- 0.01 mag) (October 31) and faded after with a slope -0.03 mJy/day. ...
https://www.astronomerstelegram.org/?read=17540
date: 2025-12-11T21:32:06-00:00
creator: V. V. Vlasyuk, O. I. Spiridonova

ATel 17539: Continued broadband radio and optical brightening and record-high 22 GHz and R-band flux density of 3C 138

We report continued long-term broadband radio and optical brightening of the compact steep-spectrum (CSS) quasar 3C 138, with the variability amplitude increasing toward higher frequencies. ...
https://www.astronomerstelegram.org/?read=17539
date: 2025-12-11T07:25:00-00:00
creator: Yu. V. Sotnikova, T. V. Mufakharov, V. V. Vlasyuk, Tao An, O. I. Spiridonova, A. K. Erkenov, R. Yu. Udovitskij, T. A. Semenova,, Yu. A. Kovalev, Yu. Y. Kovalev, A. V. Popkov, M. A. Kharinov, I. A. Rahimov, T. S. Andreeva

ATel 17538: TRAPPIST bright comets production rates: 3I/ATLAS, 24P/Schaumasse, C/2025 T1 (ATLAS), C/2025 K1 (ATLAS) and C/2025 R2 (SWAN)

The author report that they obtained, using the TRAPPIST-North (Z53) robotic telescope, recent observations of the following bright comets including 3I/ATLAS. ...
https://www.astronomerstelegram.org/?read=17538
date: 2025-12-10T22:50:04-00:00
creator: E. Jehin, S. Hmiddouch, K. Aravind, J. Manfroid, Z. Benkhaldoun, and A. Jabiri .

ATel 17537: Follow-up Imaging Observations of Comet 240P and its fragment 240P-B

Between 30 October and 3 December 2025, we conducted follow-up imaging observations of comet 240P and its fragment 240P-B (MPEC2025-T124) in seven observation epochs, using the Schmidt-Teleskop-Kamera (STK), which is operated in the Schmidt focus of the 90 cm telescope at the University Observatory Jena. ...
https://www.astronomerstelegram.org/?read=17537
date: 2025-12-10T16:45:39-00:00
creator: M. Mugrauer, M. Belger, Z. Jamil, K.-U. Michel, B. Dincel, S. Aruna Ravi, M. Jory, S. Mahagaon, G. Payli, C. Soltmann

ATel 17536: XRISM/Xtend Transient Search (XTS) detected an X-ray flare possibly from EXO 092202-6244.9

XRISM/Xtend Transient Search (XTS) detected an X-ray flare from an X-ray source XRISM J0923-6258 on 2025-12-08 TT. The source position is determined to be (R.A., Dec.) = (140.824,-62.964), with a systematic error of & sim; 40 arcsec. ...
https://www.astronomerstelegram.org/?read=17536
date: 2025-12-10T12:41:06-00:00
creator: M. Yoshimoto, H. Takahashi, K. Fukushima, Y. Kanemaru, S. Ogawa, Y. Uchida, T. Yoneyama, M. Audard, E. Behar, T. Hakamata, S. Inoue, Y. Ishihara, C. Kang, T. Kiyomoto, T. Kohmura, H. Kuramoto, J. Kurashima, Y. Maeda, H. Matsumoto, T. Matsushima, A. Miyamoto, M. Mizumoto, K. Mori, Y. Motogami, N. Nagashima, T. Narita, M. Nobukawa, H. Noda, Y. Sakamoto, M. Shidatsu, H. Sugai, T. Takagi, S. Takatsuka, R. Takemoto, Y. Terada, Y. Terashima, Y. Tsuboi, H. Uchida, A. Yoshimoto

ATel 17535: LHAASO detection of Markarian 421 in a TeV-active state

Based on the LHAASO-WCDA real-time alert system, Markarian 421 has been detected in a very active state since September 2025. The LHAASO-WCDA real-time alert system has reported several significant flares: |--------------------+--------------------------+ | Time [MJD] | Flux( > 1TeV)[Crab Units] | |--------------------+--------------------------+ | 61016.97-61017.14 | 1.4 | | 61010.82-61011.15 | 0.8 | | 60947.99-60957.30 | 1.1 | |--------------------+--------------------------+ We strongly encourage multi-wavelength observations. ...
https://www.astronomerstelegram.org/?read=17535
date: 2025-12-10T07:47:17-00:00
creator: Guangman Xiang, Min Zha, Zhiguo Yao, Jianeng Zhou and Yi Xing report on behalf of the LHAASO Collaboration

ATel 17534: Fermi-LAT detection of enhanced gamma-ray activity from FSRQ S5 1027+74

The Large Area Telescope (LAT), one of the two instruments on the Fermi Gamma-ray Space Telescope, has observed enhanced gamma-ray activity from a source positionally consistent with the flat spectrum radio quasar (FSRQ) S5 1027+74, also known as 4FGL J1031.1+7442 (Abdollahi et al. ...
https://www.astronomerstelegram.org/?read=17534
date: 2025-12-09T16:01:53-00:00
creator: P. van Zyl, C. Bartolini, G. La Mura, F. Casaburo, on behalf of the Fermi Large Area Telescope Collaboration