AstroNote 2023-223

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 report the ATLAS discovery of the fast and blue optical transient ATLAS23qty (AT 2023pcw). The transient event was initially spotted on MJD = 60166.14 == 2023-08-10.14 at m_o = 19.30 +/- 0.20. There was no detection by ATLAS on MJD 60162.10 == 2023-08-06.10. Subsequent survey observations unveiled its evolving light curve: a second epoch on MJD = 60167.25 recorded m_c = 18.59 +/- 0.09, followed by a third epoch on MJD = 60168.03 with m_o = 19.11 +/- 0.18. ATLAS23qty either has a very blue colour (c-o = -0.6) or a rapid rise and fall. There is no history of any flux excess in forced photometry over the history of the ATLAS survey.

The underlying source is a blue starforming galaxy that is clearly detected in both Dark Energy Survey (DR1) and Legacy Surveys (DR10) images, with an apparent magnitude r=20.61 and a photometric redshift of z = 0.116 +/- 0.036, as derived from the Legacy Surveys data. By adopting a distance modulus of m-M = 38.5 (and A_g = 0.04 and A_r = 0.03), ATLAS23qty exhibits an absolute magnitude of M_c = -20. Given the luminosity, possible rapid evolution or extreme blue colour, and a star-forming host, ATLAS23qty shows similarities with Fast Blue Optical Transients (FBOTs) and objects resembling AT 2018cow (Prentice et al. 2018; Perley et al. 2019). To facilitate comprehensive investigations, we provide accompanying finder and forced photometry plots. Follow-up multi-wavelength observations are strongly encouraged to unravel the underlying nature of this transient event.

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.