TITLE: GCN CIRCULAR NUMBER: 24283 SUBJECT: LIGO/Virgo S190426c: Optical Wide-field Search with the Zwicky Transient Facility DATE: 19/04/27 14:30:40 GMT FROM: Mansi M. Kasliwal at Caltech/Carnegie Michael W. Coughlin (Caltech), Mansi M. Kasliwal (Caltech), Daniel A. Perley (LJMU), Ariel Goobar (OKC), Leo P. Singer (NASA GSFC), Shreya Anand (Caltech), Tomas Ahumada (UMD), Igor Andreoni (Caltech), Eric C. Bellm (UW), K. De (Caltech), R. Biswas (OKC), S. Nissanke (UvA), Dmitry Duev (Caltech), S. Bradley Cenko (NASA GSFC), D. Goldstein (Caltech), A. Ho (Caltech), V. Bhalerao (IITB), H. Kumar (IITB), V. Karambelkar (IITB), K. Deshmukh (IITB), D. Saraogi (IITB), G. C. Anupama (IIA), C. Copperwheat (LJMU), Virginia Cunningham (UMD), Shaon Ghosh (UWM), David Kaplan (UWM), Jesper Sollerman (OKC), Joshua S. Bloom (UCB), M. Bulla (OKC), Matthew Graham (Caltech), L. Yan (Caltech), C. Fremling (Caltech), Pradip Gatkine (UMD), A. Miller (Northwestern) On behalf of the Zwicky Transient Facility (ZTF) and Global Relay of Observatories Watching Transients Happen (GROWTH) collaborations We observed the localization region of the gravitational wave trigger S190426c (GCN 24237) with the Palomar 48-inch telescope equipped with the 47 square degree ZTF camera (Bellm et al. 2019, Graham et al. 2019). A new tiling was automatically optimally determined and triggered using the GROWTH Target of Opportunity marshal (Coughlin et al. 2019a, Kasliwal et al. 2019b). We started obtaining target-of-opportunity observations in the g-band and r-band filters beginning at UT 2019-04-27 05:45. The projected enclosed probability with the original sky map was 75%. However, with the new sky map (GCN 24277, GCN 24279) and taking account into chip gaps and processing, a total of 4340 square degrees covering 55% of the enclosed probability were observed before 12-deg twilight and analyzed in real-time. Exposure length varied between 120s, 180s and 300s. We note that the area around the north celestial pole covered by our partner GROWTH-India telescope covers an additional 6% of the updated probability map, complementary to ZTF (see GCN 24258). The images were processed through the ZTF reduction and image subtraction pipelines at IPAC to search for potential counterparts (Masci et al. 2019). After rejecting stellar sources (Tachibana and Miller 2018) and moving objects and applying machine learning algorithms (Mahabal et al. 2019), several high-significance transient candidates were identified by our pipeline in the area observed. Thanks to the overlap in sky maps between the two GW triggers S190426c and S190425z, we have very good constraints on past history of variability in the last few days. The only candidate with the first detection after the merger time is: ----------------------------------------------------------------------------------------------- ZTF Name | RA (deg) | DEC (deg) | Filter | Mag | Magerr | Filter| Mag | Magerr --------------+-------------+-------------+--------+-------+---------+-------+-------+--------- ZTF19aaslzfk | 308.968271 | 72.3536353 | r | 20.91 | 0.17 | g | 21.38 | 0.18 ----------------------------------------------------------------------------------------------- We caution that our upper limits in the last few days for ZTF19aaslzfk are shallower than the detection. So we cannot rule out an old, unrelated transient. The line-of-sight extinction is Ar of 1.4 mag (Schlafly et al. 2011). We note that the source is detected in all four WISE filters in the AllWISE catalog (Wright et al. 2010). Its W1-W3 colors are intermediate between galaxies and AGN relative to the color loci of Assef et al. (2018), but the clear W4 detection suggests contribution from an active galactic nucleus. Additional analysis and continued follow-up is in progress. 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; IIT-B, India; IIA, India; LJMU, UK; TTU, 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 filtering and follow-up co-ordination is being undertaken by the GROWTH marshal system (Kasliwal et al. 2019).