AstroNote 2024-60

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, near the galaxy ESO103-G035 at z = 0.013 or d = 58 Mpc. While the host is ambiguous, with a fainter candidate host (with unknown redshift) at a smaller projected offset, the magnitude of the transient, m_o = 17.12 +/- 0.07, likely implies a distance within ~ 100 Mpc consistent with the distance of ESO103-G035. We discovered ATLAS24czw (AT2024diq) on MJD 60368.34 == 2024-02-28.34, at m_o = 17.12 +/- 0.07. There are no recent non-detections to constrain the explosion date, as the source has recently emerged from solar conjunction. ATLAS24czw is offset by 37.6 arcsec northwest from ESO103-G035, and offset by 2.3 arcsec from the fainter galaxy, which may be a companion of ESO103-G035 or a background galaxy. At d = 58 Mpc the implied absolute magnitude is M = -16.8 (assuming m-M = 33.80 and A_r = 0.17 and A_i = 0.13). 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.