AstroNote 2024-371

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 NGC 3521. We discovered ATLAS24rkq (AT2024aecx) on MJD 60660.56 == 2024-12-16.56, at m_o = 14.68 +/- 0.01. There was no detection by ATLAS on MJD 60659.35 == 2024-12-15.35 to a limit of about o ~ 19 (see attached forced photometry plot). ATLAS24rkq is offset by 3.7 arcsec north, 14.4 arcsec east from NGC 3521, which is at z = 0.003 or d = 11 Mpc (redshift independent, from NED), implying an absolute magnitude of M = -15.7 (assuming m-M = 30.25 and A_r = 0.13 and A_i = 0.10).  The source has risen by more than 4 mags in just over 24hrs and a young core-collapse SN is the most likely explanation. Given the distance of around 10 Mpc, rapid 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, The Millennium Institute of Astrophysics (MAS), Chile and the University of Oxford.