AstroNote 2025-321

ATLAS operates with 4 x 0.5m telescopes in Hawaii (Haleakala and Mauna Loa), Chile (El Sauce) and South Africa (Sutherland), see  Tonry et al. 2018, (PASP,130:064505)and a fifth unit in Tenerife, Canary Islands (ATLAS-TDO: 16 x 28 cm aperture RASA telescopes. We are robotically surveying the whole sky with a cadence of 1 day between -50 and +50 and 2 days in the polar regions, weather permitting. Two filters are used, cyan and orange (denoted c and o; all mags quoted are in the AB system) and the ATLAS-TDO is unfiltered with   QHY600 CMOS cameras (Licandro et al. 2022 https://doi.org/10.5194/epsc2022-634). While carrying out the primary mission for Near-Earth Objects, we search for and publicly report stationary transients to the IAU Transient Name Server.  Automated data processing is described in Smith et al. (2020, PASP, 132:08500), which combines spatial cross-matching with astronomical catalogues (Young 2023) and a machine learning model that performs real-bogus classification on the images (Weston et al. 2024, RASTI 3, 385). Finally, the ATLAS Virtual Research Assistant prioritises alerts using multi-modal information present in the stream (Stevance 2025). More information is on the ATLAS homepage. We are submitting AstroNotes for transients that are either within 100 Mpc, or have some other interesting feature to bring to the community's attention, such as bright nuclear transients, slowly rising or rapidly fading objects.

We report a new transient source, a Type IIP supernova in the galaxy IC1722. We discovered ATLAS25nsq (SN2025acfm) on MJD 60981.36 == 2025-11-02.36, at m_o = 18.40 +/- 0.16. There was no detection by ATLAS on MJD 60978.10 == 2025-10-30.10, and recent forced photometry indicates the transient was rising and has now entered a plateau phase (see attached forced photometry plot). ATLAS25nsq is offset by 19.4 arcsec south, 29.2 arcsec west from IC1722, which is at  z = 0.014 and d = 54 Mpc (from NED), implying an absolute magnitude of M = -15.3 (assuming m-M = 33.66 and A_g =  0.07 and A_r = 0.05 ). Followup observations were carried out by the BlackGEM group on 2025-11-11, which classified the object as a Type IIP supernova.

The Asteroid Terrestrial-impact Last Alert System [ATLAS] project is primarily funded to search for Near-Earth asteroids through NASA grants NN12AR55G, 80NSSC18K0284, and 80NSSC18K1575; byproducts of the NEO search include images and catalogs from the survey area. This work was partially funded by Kepler/K2 grant J1944/80NSSC19K0112 and HST GO-15889,  STFC grants ST/Y001605/1, ST/X001253/1, the Royal Society and Schmidt Sciences. ATLAS-Teide is an IAC instrument included in the present “Strategic plan of the Canarian Observatories”, funded by the European Union  NextGenerationEU  EQC2021-007122-P and ICT2022-007828 projects. The ATLAS science products have been made possible through the contributions of the University of Hawaii Institute for Astronomy,  Queen's University Belfast, University of Oxford, the Space Telescope Science Institute, the South African Astronomical Observatory, The Millennium Institute of Astrophysics (MAS), Chile, and the Instituto de Astrofisica De Canarias.