AstroNote 2023-202

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.  Data processing is carried out at Queen's University which combines automated source parameter filtering, machine learning image recognition, and spatial cross-matching with astronomical catalogues (Smith et al. 2020, PASP, 132:085002). 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 WISEA J124150.77-071009.9. We discovered ATLAS23nuv (AT2023mtq) on MJD 60136.71 == 2023-07-11.71, at m_c = 17.54 +/- 0.05. There was no detection by ATLAS on MJD 60132.05 == 2023-07-07.05, and recent forced photometry indicates the transient is rising (see attached forced photometry plot). ATLAS23nuv is offset by 6.8 arcsec South, 6.2 arcsec West from WISEA J124150.77-071009.9, which is at z = 0.025 or d = 108 Mpc (from NED), implying an absolute magnitude of M = -17.7 (assuming m-M = 35.17 and A_g = 0.10 and A_r = 0.07). 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, and STFC grants ST/T000198/1 and ST/S006109/1. The ATLAS science products have been made possible through the contributions of the University of Hawaii Institute for Astronomy, the Queen's University Belfast, the Space Telescope Science Institute, the South African Astronomical Observatory, and The Millennium Institute of Astrophysics (MAS), Chile.