AstroNote 2024-374

ATLAS24rkq (AT 2024aecx) was discovered in NGC 3521 by ATLAS on MJD 60660.56 == 2024-12-16.56, at m_o = 14.68 +/- 0.01 (Tonry et al., TNS Astronomical Transient Report No. 236140 and Stevance et al., TNS AstroNote 2024-371) and it was not detected by ATLAS on MJD 60659.35 == 2024-12-15.35 to a limit of about o ~ 19, with a corresponding rise of more than 4 mags in just over a day. Chen et al. (TNS AstroNote 2024-372) have reported Kinder follow-up observations that confirm this transient is very blue and is starting to fade. The transient has been classified as a type IIb SN by Hinkle (SCAT, TNS Classification Report No. 18921). We have used the ATLAS forced photometry server (Shingles et al. 2021, TNS AstroNote 2021-7) to check for additional ATLAS detections. The ATLAS photometry of the first two epochs show that SN 2024aecx is fading at a rate of about 0.42 magnitudes per day (see the attached file with the ATLAS light curve). These are the ATLAS forced photometry magnitudes of the individual observations, all in the ATLAS orange filter: # MJD m dm 60660.557222 14.677 0.012 60660.559031 14.760 0.011 60660.563125 14.700 0.009 60660.573150 14.703 0.009 60661.034696 14.965 0.014 60661.056519 14.907 0.016 60661.059680 14.911 0.019 Multi-wavelength followup observations are encouraged. This work has made use of data from the Asteroid Terrestrial-impact Last Alert System (ATLAS) project. 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.