TITLE:   GCN CIRCULAR
NUMBER:  33829
SUBJECT: LIGO/Virgo/KAGRA S230518h: Swope Search and Candidate Counterpart Identification
DATE:    23/05/19 17:08:20 GMT
FROM:    David Coulter at UCSC <dcoulter@ucsc.edu>

D. A. Coulter (UCSC), C. D. Kilpatrick (Northwestern), C. Rojas-Bravo (UCSC), R. J. Foley (UCSC), Y. Kong (LCO), B. O’Connor (GWU), A. L. Piro (Carnegie), and A. Rest (STScI), report on behalf of the 1M2H collaboration:

We conducted an optical search for candidate counterparts in the localization region of LVK gravitational wave event S230518h (GCN 33813) with the Swope 1-m telescope using the 29.8’ x 29.7’ field-of-view Direct camera.  The tiling was optimally determined using the latest localization map from the LVK and galaxy catalogs in Teglon (Coulter et al., in prep).  In total, Swope observed 9 fields within the 90th percentile localization region of S230518h at:

MJD RA Dec Band Exptime Maglim(AB)
60082.97419 07:22:15.898 -49:05:26.261 i 300.000 21.7621
60082.98236 07:20:39.617 -84:47:34.465 i 285.000 21.3325
60082.98727 07:20:03.166 -85:47:04.692 i 300.000 21.3605
60082.99201 07:23:52.430 -84:17:49.351 i 285.000 21.5397
60082.99720 07:42:22.186 -54:32:42.515 i 300.000 21.8507
60083.00245 07:43:16.761 -70:24:46.161 i 300.000 21.1865
60083.00728 07:50:34.461 -67:26:15.477 i 285.000 21.3315
60083.01301 07:53:15.693 -75:52:02.414 i 315.000 21.1845
60083.01791 07:54:53.802 -78:50:33.098 i 330.000 20.4372

These observations have been reported to the TreasureMap (Wyatt et al., 2020).

Subtracting DECam and SkyMapper template images from the Swope images using photpipe (Rest et al., 2005), we identified the following candidate counterparts in our imaging:

Name MJD RA(deg) Dec(deg) Filter Mag Magerr 2D_Percentile
SSS23a 60082.98236 108.74882083333331 -84.62690277777777 i 20.9 0.2 0.2479
SSS23b 60082.98727 112.276725 -85.9142638888889 i 21.1 0.2 0.2435
SSS23c 60083.01791 117.93889999999998 -78.77603055555555 i 20.1 0.2 0.1924

We have checked the Minor Planet Center and Transient Name Server, finding no previously reported solar system object or transient. We encourage follow-up observations of these sources.