AstroNote 2024-100

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 nearby SN candidate in NGC 3621 at 7 Mpc. ATLAS24fsk (AT2024ggi) was discovered on MJD 60411.14 == 2024-04-11.14, at m_o = 18.92 +/- 0.08. There was no detection by ATLAS 6 days prior on MJD 60405.06. Forced photometry (attached plot) indicates a rapid intra-night rise of ~1 mag in a span of ~2.5 hrs, from an absolute magnitude of M_o = -10.3 to -11.3. Archival imaging from the Legacy survey suggests a well detected red source consistent with the transient position. In HST archival imaging, the source is resolved into two sources in F814W, indicating a red supergiant progenitor and this is most likely a young SN II caught shortly after explosion. The archival imges and a finder are attached, multi-wavelength 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.