AstroNote 2025-282

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, most likely a supernova in the galaxy SDSS J074931.57+492934.3. We discovered ATLAS25kxz (AT2025wsz) on MJD 60920.60 == 2025-09-02.60, at m_o = 17.91 +/- 0.07. There was no detection by ATLAS on MJD 60918.63 == 2025-08-31.63 (the first since solar conjunction), but being shallower than the first detection does not provide an indication of how the transient is evolving (see attached forced photometry plot). ATLAS25kxz is offset by 0.3 arcsec north, 7.0 arcsec west from SDSS J074931.57+492934.3, which is at z = 0.021 or d = 95 Mpc (from NED), implying an absolute magnitude of M = -17.1 (assuming m-M = 34.89 and A_r = 0.18 and A_i = 0.14). 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. 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.