AstroNote 2025-85

ATLAS is a quadruple 0.5m telescope system with two units in Hawaii (Haleakala and Mauna Loa), and one each in Chile (El Sauce) and South Africa (Sutherland), see  Tonry et al. 2018, (PASP,130:064505). With the installation of the two southern units, 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). 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, most likely a supernova in the galaxy ESO 443- G 016. We discovered ATLAS25cqi (AT2025efa) on MJD 60746.23 == 2025-03-12.23, at m_o = 18.19 +/- 0.16. There was no detection by ATLAS 3 days prior on MJD 60743.08. ATLAS25cqi is offset by 11.1 arcsec north, 6.0 arcsec west from ESO 443- G 016, which is at z = 0.016 or d = 70 Mpc (from NED), implying an absolute magnitude of M = -16.2 (assuming m-M = 34.24 and A_r = 0.16 and A_i = 0.12). Forced photometry indicates a young and rising transient, followup observations are encouraged. 

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. 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, and The Millennium Institute of Astrophysics (MAS), Chile.