GCN Circular 36371

Gregory S.H. Paek, Myungshin Im, Mankeun Jeong, Seo-Won Chang, Seong-Kook Lee (SNU/SNU ARC), Dong-Jin Kim and Chung-Uk Lee (KASI), on behalf of the GECKO team

We report transient candidates identified by the GECKO team using the three Korea Microlensing Telescope Network (KMTNet) 1.6-m telescopes (GCN 36343
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) in response to the LIGO/Virgo/KAGRA S240422ed trigger (GCN 36236
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).

Reference images for the observed fields were partially obtained from the KMTNet Synoptic Survey of the Southern Sky (KS4) project, and photometry was performed on subtracted images using HOTPANTS (Becker et al. 2015). Here are 10 transient candidates:

NAME     RA      DEC     FILTER  DATE-OBS            t-t0 MAG   MAGERR COMMENT
GECKO24a 122.320 -29.659 R       2024-04-23T09:50:58 0.51 20.54 0.08
GECKO24a 122.320 -29.659 R       2024-04-24T08:59:57 1.48 20.90 0.08
GECKO24b 129.629 -32.944 R       2024-04-23T12:35:42 0.63 16.50 0.00
GECKO24b 129.629 -32.944 R       2024-04-24T02:03:10 1.19 16.41 0.00
GECKO24b 129.629 -32.944 R       2024-04-24T18:46:05 1.88 16.57 0.00
GECKO24b 129.629 -32.944 R       2024-04-25T19:12:42 2.90 16.52 0.00
GECKO24c 130.456 -30.977 R       2024-04-24T18:46:05 1.88 19.44 0.04
GECKO24d 130.468 -31.666 R       2024-04-24T18:46:05 1.88 19.84 0.06
GECKO24e 124.768 -37.096 R       2024-04-23T12:23:27 0.62 18.78 0.05
GECKO24e 124.768 -37.096 R       2024-04-24T18:32:02 1.87 18.31 0.02
GECKO24f 127.124 -33.490 R       2024-04-23T18:54:32 0.89 18.69 0.02
GECKO24f 127.124 -33.490 R       2024-04-24T20:41:09 1.96 18.45 0.02
GECKO24g 121.753 -30.285 I       2024-04-26T18:18:43 3.86 18.81 0.02
GECKO24h 127.557 -34.603 R       2024-04-23T18:54:32 0.89 19.92 0.11
GECKO24h 127.557 -34.603 R       2024-04-24T02:03:10 1.19 20.32 0.04
GECKO24h 127.557 -34.603 R       2024-04-24T20:41:09 1.96 19.64 0.05   *
GECKO24i 122.275 -29.775 R       2024-04-23T09:50:58 0.51 21.00 0.06   AT 2024hea
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GECKO24i 122.275 -29.775 R       2024-04-24T08:59:57 1.48 20.14 0.07   AT 2024hea
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GECKO24j 123.527 -29.731 R       2024-04-23T09:50:58 0.51 20.90 0.10   AT 2024heb
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We have analyzed forced photometry results from the ATLAS data spanning from 2000 to the present (L. Shingles+21) and compared their light curves with AT2017gfo
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-like kilonova scaled to the luminosity distance of S240422ed.

GECKO24i and GECKO24j have already been reported in the Transient Name Server (TNS; Gal-Yam et al. 2021) as AT 2024hea
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 and AT 2024heb
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, respectively. GECKO24c and GECKO24d are suspected to be moving objects, as their movement was observed in individual frames. GECKO24b, GECKO24e, GECKO24f, and GECKO24g have been ruled out as kilonovae candidates due to their brightness and/or decay rates. GECKO24h exhibits a brightness similar to that of an AT2017gfo
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-like kilonova; however, the activity observed in the historical ATLAS data suggests otherwise. Note that the third epoch in the light curve stands out due to contamination of near galaxy residual induced by imperfect subtraction. All the candidates, except for GECKO24a, are unlikely to be associated with S240422ed. We have not excluded GECKO24a yet, due to comparable brightness and decay rate with AT2017gfo
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-like kilonova during early epochs. It is also in the vicinity of a galaxy association with ~1.7 arcsec separation.

Gravitational-wave EM Counterpart Korean Observatory (GECKO; Im et al. 2023, Proceedings of IAU Symp. Vol. 363, pp. 207.; Paek et al. 2024, The Astrophysical Journal, Vol. 960, Number 2, 113.) is a network of 0.5m to 1m class telescopes worldwide. KMTNet is operated by the Korea Astronomy and Space Science Institute. We thank the KMTNet operators for their support for the KMTNet observations.