TITLE: GCN CIRCULAR NUMBER: 25616 SUBJECT: LIGO/Virgo S190901ap: Candidates from the Zwicky Transient Facility DATE: 19/09/02 14:32:28 GMT FROM: Mansi M. Kasliwal at Caltech/Carnegie Erik Kool (OKC), Robert Stein (DESY), Yashvi Sharma (Caltech), Viraj Karambelkar (Caltech), Mansi Kasliwal (Caltech), Daniel Perley (LJMU), Valery Brinnel (HU Berlin), Jakob Nordin (HU Berlin), Shreya Anand (Caltech), Michael Coughlin (Caltech), Leo P. Singer (NASA GSFC), Igor Andreoni (Caltech), Gaurav Waratkar (IITB), Harsh Kumar (IITB), Maitreya Khandagale (IITB), Kunal Deshmukh (IITB), Varun Bhalerao (IITB), G. C. Anupama (IIA), Dougal Dobie (USyd/CSIRO), Brad Cenko (NASA GSFC), Tomas Ahmuda (UMD), Eric Bellm (UW), Albert Kong (NTHU), Anna Franckowiak (DESY), Pradip Gatkine (UMD) 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 S190901ap (LVC et al. GCN 25606, GCN 25614) with the Palomar 48-inch telescope equipped with the 47 square degree ZTF camera (Bellm et al. 2019, Graham et al. 2019). The tiling was 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 beginning at UT 2019-09-02 03:08 UT. We covered 44% of the enclosed probability based on the new lalinference map (38% of the enclosed probability based on the initial bayestar map) in 6500 sq deg mapped before we had to close due to clouds. Each exposure was 30s with a typical depth of 20.7 mag. The images were processed in real-time through the ZTF reduction and image subtraction pipelines at IPAC to search for potential counterparts (Masci et al. 2019). AMPEL (Nordin et al. 2019) was used to search the alerts database for candidates. After rejecting stellar sources (Tachibana and Miller 2018) and moving objects and applying machine learning algorithms (Mahabal et al. 2019), and after removing candidates with history of variability prior to the merger time, the following high-significance transient candidates were identified by our pipeline in the 95% localization of the new lalinference map (LVC et al. GCN 25614). ------------------------------------------------------------------------ ZTF Name | RA (deg) | DEC (deg) | Filter | Mag | Magerr ------------------------------------------------------------------------ ZTF19abvizsw | 279.472820 | +61.497984 | r | 19.45 | 0.11 ZTF19abvixoy | 279.552972 | +27.420935 | r | 19.22 | 0.10 ZTF19abvjnsm | 267.202697 | +44.693203 | r | 20.23 | 0.20 ZTF19abvionh | 253.750924 | +14.051330 | g | 20.73 | 0.31 ------------------------------------------------------------------------ ZTF19abvizsw has a red color (g-r ~ 0.5 mag), no underlying host in the reference image and is on the outer periphery of the new LVC sky localization. ZTF has observed this field every night for the past month as part of routine survey operations and the first detections of this transient are only after the binary neutron star merger time. ZTF19abvixoy has an upper limit from Aug 30 UT and possibly a faint counterpart in PS1. ZTF19abvjnsm has an upper limit from Sep 1 UT but its host galaxy has too high a phot-z estimate from SDSS of 0.51 +/- 0.11. The host galaxy of ZTF19abvionh has a consistent SDSS phot-z (0.064 +/- 0.016) but the two detections last night are separated by only a short baseline of 7 minutes (a moving object in our solar system cannot be ruled out for this candidate). We encourage spectroscopic and photometric follow-up to discern the nature of these transients. 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; 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). Alert filtering and follow-up co-ordination is being undertaken by the GROWTH marshal system (Kasliwal et al. 2019).