AstroNote 2024-330

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, intrinsically faint, transient source in the galaxy Sextans B. We discovered ATLAS24qcl (AT2024aawe) on MJD 60624.56 == 2024-11-10.56, at m_o = 18.55 +/- 0.13. There was no detection by ATLAS on MJD 60623.35 == 2024-11-09.35 (see attached forced photometry plot). ATLAS24qcl is offset by 50.5 arcsec north, 89.6 arcsec west from Sextans B, which is at a distance of about  D = 1 Mpc (from NED), implying an absolute magnitude of M = -7.3 (assuming m-M = 25.78 and A_r = 0.07 and A_i = 0.05).

We think the source is most likely a nova, but given the rapid rise (~1.5 mag in 24hrs) further followup to trace the evolution is encouraged. There is no stellar counterpart visible in the Pan-STARRS or DESI Legacy Surveys archive images, indicating no massive stellar counterpart to about M >~ -2 mags. Sextans B is a very metal poor galaxy, roughly a tenth solar (Kniazev et al. 2005, AJ 130, 1558), giving the opportinity to study a nova, or other faint transient, from a low metallicity stellar progenitor system. Followup photometric and spectroscopic 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.