TITLE:   GCN CIRCULAR
NUMBER:  34096
SUBJECT: LIGO/Virgo/KAGRA S230627c: ATLAS information on the ZTF candidates
DATE:    23/06/27 20:11:36 GMT
FROM:    Michael Fulton at Queen's U, Belfast <mfulton07@qub.ac.uk>

M. D. Fulton (QUB), S. J. Smartt (Oxford/QUB), K. W. Smith, S. Srivastav, D. R. Young, M. Nicholl, M. McCollum, T. Moore, J. Weston, X. Sheng (QUB), L. Shingles (GSI/QUB), J. Sommer (LMU/QUB), 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), A. Jordan, V. Suc (UAI, Obstech), A. Rest (STScI), T.-W. Chen (TUM), C. Stubbs (Harvard)

Here we report the ATLAS forced photometry (Shingles et al. AstroNote #2021-7; Smith et al., 2020, PASP, 132:085002) at the coordinates of the ZTF candidates highlighted in Anumarlapudi et al. (GCN Circ. 34089). We rule out AT2023lxu and disfavour AT2023lxt as optical counterparts to the compact binary merger event S230627c (LIGO-Virgo-KAGRA Scientific Collaboration GCN Circ. 34086). 

We measure multiple 3σ detections at the location of AT2023lxu >6 days before the LVK trigger time, implying AT2023lxu is most likely a faint SN. Two 3σ detections of AT2023lxt were made prior to the LVK trigger, suggesting this is also not a viable candidate. We note that the AT2023lxt detections are marginal, being at the observing limit of ATLAS, and we are uncertain whether or not these detections represent subtraction residuals due to the close proximity to the underlying galaxy core. We present these 3σ measurements below, which represent nightly flux stacks and have not been corrected for foreground extinction. Phase is with respect to the LVK trigger time of MJD 60122.079 (2023-06-27 01:53:37 UTC).

+----------------------+--------------+------------------+--------+---------------------+
| Candidate Name | MJD            | Phase (Days)  | Filter  | Mag +/- MagErr  |
+----------------------+--------------+------------------+--------+---------------------+
|                                    | 60106.266 | -15.812             | c          | >20.84                    |
+                                  +--------------+------------------+--------+---------------------+
|                                    | 60108.262 | -13.817             | c          | 20.37 +/- 0.32      |
+                                  +--------------+------------------+--------+---------------------+
|      AT2023lxu        | 60110.265 | -11.814             | c           | 19.97 +/- 0.16      |
+                                  +--------------+------------------+--------+---------------------+
|                                    | 60112.263 | -9.816               | c           | 19.97 +/- 0.18     |
+                                  +--------------+------------------+--------+---------------------+
|                                    | 60114.264 | -7.815               | c           | >19.98                   |
+                                  +--------------+------------------+--------+---------------------+
|                                    | 60116.280 | -5.799               | c           | 20.27 +/- 0.20     |
+----------------------+--------------+------------------+--------+---------------------+
|                                    | 60112.262 | -9.817               | c           | >20.41                   |
+                                  +--------------+------------------+--------+---------------------+
|      AT2023lxt          | 60114.277 | -7.802               | c           | 20.50 +/- 0.25     |
+                                  +--------------+------------------+--------+---------------------+
|                                    | 60116.283 | -5.796               | c           | 20.60 +/ 0.25       |
+                                  +--------------+------------------+--------+---------------------+
|                                    | 60118.262 | -3.817               | o          | >20.30                    |
+----------------------+--------------+------------------+--------+---------------------+

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, and STFC grants ST/T000198/1 and ST/S006109/1. The ATLAS science products have been made possible through the contributions of the University of Hawaii Institute for Astronomy, the Queen's University Belfast, the Space Telescope Science Institute, the South African Astronomical Observatory, and The Millennium Institute of Astrophysics (MAS), Chile.