AstroNote 2025-144

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 fast evolving blue transient source in the galaxy WISEA J092321.66-202025.8. We discovered ATLAS25evh (AT2025jzi) on MJD 60802.07 == 2025-05-07.07, at m_o = 18.50 +/- 0.17.  Forced photometry indicates there was no detection by ATLAS on MJD = 60798.82 == 2025-05-03.82, and a 2-3-day rise between 60799.82 and 60802.09. AT2025jzi is offset by 5.79 arcsec south, 2.01 arcsec east from its likely host galaxy WISEA J092321.66-202025.8 . There is no spectroscopic redshift available. The transient has declined after peak at an estimated rate of approximately 0.2 mag/day indicating a rapid evolution. There is one ZTF detection (in the Lasair broker : https://lasair-ztf.lsst.ac.uk/objects/ZTF25aapqkyd/) and this g = 18.15 measurement indicates a blue transient.  A finder and forced photometry plot are attached, followup observations of this rapidly evolving blue transient 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.