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GCN Circular 33443

GRB 230307A: ATLAS information on the ULTRACAM optical afterglow candidate
2023-03-09T18:19:09Z (a year ago)
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 (2��) |
| 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.
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