TITLE:   GCN CIRCULAR
NUMBER:  33848
SUBJECT: # LIGO/Virgo/KAGRA S230521k: Zwicky Transient Facility observations
DATE:    23/05/22 04:35:25 GMT
FROM:    Tomas Ahumada at U. of Maryland <tahumada@astro.umd.edu>

Tomas Ahumada (CIT), Viraj Karambelkar (CIT), Robert Stein (CIT), Gaurav
Waratkar (IITB), Anirudh Salgundi (IITB), Theophile du Laz (CIT), Igor
Andreoni (UMD), Michael Coughlin (UMN), Mansi Kasliwal (CIT), Varun
Bhalerao (IITB), Simeon Reusch (DESY), Jannis Necker (DESY), Shreya Anand
(CIT), report on behalf of the ZTF and GROWTH collaborations:


We serendipitously observed the localization region of LVC trigger S230521k
as part of routine Zwicky Transient Facility (ZTF; Graham et al., 2019;
Bellm et al., 2019) survey operations. We obtained a total of 100 images
covering g, r, and i bands for a total of 3810 seconds. These serendipitous
observations covered 1238.8 square degrees beginning at 2023-05-21T05:32:51
(2 minutes after the burst trigger time) corresponding to ~9% of the
probability enclosed in the localization region.

We queried the ZTF alert stream using Kowalski (Duev et al. 2019) through
Fritz (Coughlin et al. 2023) and emgwcave (Karambelkar et al. in prep), and
AMPEL (Nordin et al. 2019). We required at least 2 detections separated by
at least 15 minutes to select against moving objects. Furthermore, we
cross-match our candidates with the Minor Planet Center to flag known
asteroids, reject stellar sources (Tachibana and Miller 2018), and apply
machine learning algorithms (Mahabal et al. 2019). We require that no
spatially coincident ZTF alerts were issued before the detection time of
the LVC trigger.

Two sources passed our criteria and were inside the 95% error region:

|-----------------------------------------------------------------------------------------------------
| id           | alias                |       ra |     dec |        mjd  |
mag±err (ab)   | filter   |
|-----------------------------------------------------------------------------------------------------
| ZTF23aaladzz | AT2023jcp            | 275.9841 | 24.9580 | 60085.36780 |
20.18±0.17     | r        |
| ZTF23aaladoy | AT2016bqh/AT 2023jbu | 280.1828 | 27.0236 | 60085.36780 |
15.56±0.03     | g        |
|-----------------------------------------------------------------------------------------------------


Forced photometry at the location of AT2023jcp detections 4 days prior to
the GW event merger time. Thus, we rule out this source as a candidate
counterpart. A source 0.1 arcsec from ZTF23aaladoy was previously reported
to TNS in 2016, thus we rule it out as a viable candidate.

Further follow-up of this localization region will continue as part of
regular survey operations.


ZTF and GROWTH are worldwide collaborations comprising Caltech, USA; IPAC,
USA, WIS, Israel; OKC, Sweden; JSI/UMd, USA; U Washington, USA; DESY,
Germany; MOST, Taiwan; UW Milwaukee, USA; LANL USA; Tokyo Tech, Japan;
IITB, India; IIA, India; LJMU, UK; TTU, USA; SDSU, USA and USyd, Australia.
ZTF acknowledges the generous support of the NSF under AST MSIP Grant No
1440341. GROWTH acknowledges generous support of the NSF under PIRE Grant
No 1545949. Alert distribution service provided by DIRAC@UW (Patterson et
al. 2019). Alert database searches are done by AMPEL (Nordin et al. 2019)
and Kowalski (Duev et al. 2019). GROWTH India telescope is located at the
Indian Astronomical Observatory (Hanle), operated by the Indian Institute
of Astrophysics (IIA). GROWTH-India project is supported by SERB and
administered by IUSSTF, under grant number IUSSTF/PIRE
Program/GROWTH/2015-16 and IUCAA.