TITLE: GCN CIRCULAR NUMBER: 33443 SUBJECT: GRB 230307A: ATLAS information on the ULTRACAM optical afterglow candidate DATE: 23/03/09 18:19:09 GMT FROM: Stephen Smartt at Queen's U/Belfast M. D. Fulton, K. W. Smith, S. Srivastav, D. R. Young, M. Nicholl, M. McCollum, T. Moore, J. Weston (QUB), S. J. Smartt (Oxford/QUB), L. Shingles (GSI/QUB), J. Sommer (LMU/QUB), L. Rhodes (Oxford), L. Denneau, J. Tonry, H. Weiland, A. Lawrence, R. Siverd (IfA, University of Hawaii), N. Erasmus, W. Koorts (South African Astronomical Observatory), A. Jordan, V. Suc (UAI, Obstech), A. Rest (STScI), T.-W. Chen (TUM/MPA), C. Stubbs (Harvard) We report optical observations in the field of the Fermi/GBM-discovered GRB 230307A (Dalessi et al. GCN Circ. 33407) in the context of the detection of a possible optical afterglow by Levan et al. (GCN Circ. 33439). ATLAS is a quadruple 0.5m telescope system with two units in Hawaii, one in Chile and one South Africa (see Tonry et al. 2018, PASP,130:064505), routinely surveying the visible sky on a daily basis. We promptly process all data with our transient science server (Smith et al. 2020, PASP, 132:085002). The ATLAS system observed the field of the GRB 230307A in normal survey mode before the GRB trigger date of MJD 60010.655 (=2023-03-07 15:44:06.67 UT). We forced the photometry at the proposed position of the optical afterglow RA=04:03:25.83, Dec=-75:22:42.7 (J2000) (Levan et al. GCN Circ. 33439; Source 2 from Evans et al. GCN Circ. 33429) thorough our ATLAS forced photometry server (Shingles et al. AstroNote #2021-7, Smith et al. 2020, PASP, 132:085002). We found no historical detections of any excess optical flux at this position. Forced photometry at those coordinates results in the following most recent non-detections in the o-band filter. These measurements are not corrected for any galactic dust extinction. The AB mag are quoted as 2 sigma. +-----------+------------+-----------------+-------------------+ | MJD | Flux (uJy) | Flux Err (duJy) | AB Mag Limit (2σ) | +-----------+------------+-----------------+-------------------+ | 60002.781 | 6.0 | 6.8 | >21.1 | +-----------+------------+-----------------+-------------------+ | 60004.034 | -8.8 | 8.6 | >20.8 | +-----------+------------+-----------------+-------------------+ | 60008.087 | 1.1 | 11.9 | >20.5 | +-----------+------------+-----------------+-------------------+ The candidate optical afterglow from Levan et al. (GCN Circ. 33439) was r=20.5 at MJD=60012.1. We find no sign of the object to similar flux levels before the GRB, which suggests the source is a new transient occurring after MJD 60008. This supports it being the candidate optical afterglow of GRB 230307A.