AstroNote 2024-101

As part of routine all-sky observations, The Gravitational-wave Optical Transient Observer (GOTO; Steeghs et al. 2022) observed the field containing AT2024ggi/ATLAS24fsk, reported by Srivastav et al. (Astronote 2024-100), with the GOTO-S node (GOTO-4) located at Siding Spring Observatory.

We detect AT2024ggi at a magnitude of L =16.83 +/- 0.02 at MJD 60411.45 (2024-04-11T10:51:22), markedly brighter than reported in AstroNote 2024-100, confirming the rapidly rising (implied ~7 mag/day), infant transient reported. The absolute magnitude is L = -12.25 at time of GOTO observations (assuming m - M = 29.08, Freedman et al. 2001). Further corroborating the extremely young nature of this transient, we do not detect AT2024ggi in images taken MJD 60410.46 (2024-04-10T10:56:25) down to a 5-sigma limiting magnitude of L = 19.46, just 16 hours prior to the ATLAS discovery detection. This strongly constrains the explosion epoch. Taking the mid-point between GOTO non-detection and ATLAS discovery, this implies an approximate explosion epoch of MJD 60410.8 (2024-04-10T19:00).

Magnitudes were calibrated using ATLAS-REFCAT2 (Tonry et al. 2018) and are not corrected for Galactic extinction.

Prompt spectroscopic classification, multi-wavelength observations, and intensive follow-up are strongly encouraged.

GOTO (https://goto-observatory.org) is a network of telescopes that is principally funded by the STFC and operated at the Roque de los Muchachos Observatory on La Palma, Spain, and Siding Spring Observatory in NSW, Australia, on behalf of a consortium including the University of Warwick, Monash University, Armagh Observatory & Planetarium, the University of Leicester, the University of Sheffield, the National Astronomical Research Institute of Thailand (NARIT), the University of Turku, the University of Portsmouth, the University of Manchester and the Instituto de Astrofisica de Canarias (IAC).