TITLE: GCN CIRCULAR NUMBER: 41451 SUBJECT: LIGO/Virgo/KAGRA S250818k: ATLAS pre-discovery limits for AT2025ulz DATE: 25/08/20 15:30:45 GMT FROM: S. Srivastav at Oxford shubhamsrivastav@gmail.com

S. Srivastav (Oxford), K. W. Smith, S. J. Smartt (Oxford/QUB), D. R. Young, M. Nicholl, M. D. Fulton, T. Moore, A. Aamer, C. R. Angus, M. McCollum, S. Sim, J. Weston, X. Sheng (QUB), L. Shingles (GSI/QUB), J. Sommer (LMU), J. Gillanders, H. Stevance, L. Rhodes, A. Andersson (Oxford), L. Denneau, J. Tonry, H. Weiland, A. Lawrence, R. Siverd (IfA, University of Hawaii), N. Erasmus, W. Koorts (South African Astronomical Observatory), J. Anderson (ESO), A. Jordan, V. Suc (UAI, Obstech) A. Rest (STScI), T.-W. Chen (NCU), C. Stubbs (Harvard):

We report pre-discovery upper limits from ATLAS observations of AT2025ulz/ZTF25abjmnps reported by Stein et al. (GCN 41414), possibly associated with the sub-threshold gravitational wave event S250818k (GCN 41437).

ATLAS observed the field most recently before the GW event on MJD 60903.32, 1.74 days prior to the trigger. The observations were made as part of regular survey operations in the o-band. During the August 2025 lunation, we have been experimenting with a different cadence and longer exposures. The data on MJD 60903.32 were taken as 4 x 110 sec exposures (compared to 30 sec exposures in normal operations). These longer exposures provide a 3-sigma upper limit of o > 21.6 mag (AB) after stacking the 4 individual measurements. The discovery r-band mag of r = 21.29 +/- 0.13 (Stein et al., GCN 41414) implies a significant 3-sigma rise in 1.74 days, or a new transient appearing on that timescale.

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, STFC grants ST/Y001605/1, ST/X001253/1, the Royal Society and Schmidt Sciences. ATLAS-Teide is an IAC instrument included in the present “Strategic plan of the Canarian Observatories”, funded by the European Union NextGenerationEU EQC2021-007122-P and ICT2022-007828 projects. The ATLAS science products have been made possible through the contributions of the University of Hawaii Institute for Astronomy, Queen's University Belfast, University of Oxford, the Space Telescope Science Institute, the South African Astronomical Observatory, The Millennium Institute of Astrophysics (MAS), Chile, and the Instituto de Astrofisica De Canarias.

View this GCN Circular online at https://gcn.nasa.gov/circulars/41451. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0LW...