{
  "bibcode": "2023GCN.33443....1F",
  "body": "M. D. Fulton, K. W. Smith, S. Srivastav, D. R. Young, M. Nicholl, M.\nMcCollum, T. Moore, J. Weston (QUB), S. J. Smartt (Oxford/QUB), L.\nShingles (GSI/QUB), J. Sommer (LMU/QUB), L. Rhodes (Oxford), L.\nDenneau, J. Tonry, H. Weiland, A. Lawrence, R. Siverd (IfA, University\nof Hawaii), N. Erasmus, W. Koorts (South African Astronomical\nObservatory), A. Jordan, V. Suc (UAI, Obstech), A. Rest (STScI), T.-W.\nChen (TUM/MPA), C. Stubbs (Harvard)\n\nWe report optical observations in the field of the\nFermi/GBM-discovered GRB 230307A (Dalessi et al. GCN Circ. 33407) in\nthe context of the detection of a possible optical afterglow by Levan\net al. (GCN Circ. 33439).\n\nATLAS is a quadruple 0.5m telescope system with two units in Hawaii,\none in Chile and one South Africa (see Tonry et al. 2018,\nPASP,130:064505), routinely surveying the visible sky on a daily\nbasis. We promptly process all data with our transient science server\n(Smith et al. 2020, PASP, 132:085002). The ATLAS system observed the\nfield of the GRB 230307A in normal survey mode before the GRB trigger\ndate of MJD 60010.655 (=2023-03-07 15:44:06.67 UT). We forced the\nphotometry at the proposed position of the optical afterglow\nRA=04:03:25.83, Dec=-75:22:42.7 (J2000) (Levan et al. GCN Circ. 33439;\nSource 2 from Evans et al. GCN Circ. 33429) thorough our ATLAS forced\nphotometry server (Shingles et al. AstroNote #2021-7, Smith et al.\n2020, PASP, 132:085002). We found no historical detections of any\nexcess optical flux at this position.\n\nForced photometry at those coordinates results in the following most\nrecent non-detections in the o-band filter. These measurements are not\ncorrected for any galactic dust extinction. The AB mag are quoted as\n2 sigma. \n\n+-----------+------------+-----------------+-------------------+\n|    MJD    | Flux (uJy) | Flux Err (duJy) | AB Mag Limit (2��) |\n+-----------+------------+-----------------+-------------------+\n| 60002.781 |     6.0    |       6.8       |       >21.1       |\n+-----------+------------+-----------------+-------------------+\n| 60004.034 |    -8.8    |       8.6       |       >20.8       |\n+-----------+------------+-----------------+-------------------+\n| 60008.087 |     1.1    |       11.9      |       >20.5       |\n+-----------+------------+-----------------+-------------------+\n\nThe candidate optical afterglow from Levan et al. (GCN Circ. 33439)\nwas r=20.5 at MJD=60012.1. We find no sign of the object to similar\nflux levels before the GRB, which suggests the source is a new\ntransient occurring after MJD 60008. This supports it being the\ncandidate optical afterglow of GRB 230307A.",
  "circularId": 33443,
  "createdOn": 1678385949000,
  "email": "s.smartt@qub.ac.uk",
  "subject": "GRB 230307A: ATLAS information on the ULTRACAM optical afterglow candidate",
  "submitter": "Stephen Smartt at Queen's U/Belfast  <s.smartt@qub.ac.uk>",
  "eventId": "GRB 230307A"
}