TITLE: GCN CIRCULAR NUMBER: 24191 SUBJECT: LIGO/Virgo S190425z: Candidates from the Zwicky Transient Facility DATE: 19/04/25 17:46:37 GMT FROM: Mansi M. Kasliwal at Caltech/Carnegie Mansi M. Kasliwal (Caltech), Michael W. Coughlin (Caltech), Eric C. Bellm (UW), Leo P. Singer (NASA GSFC), K. De (Caltech), Igor Andreoni (Caltech), Dmitry Duev (Caltech), Shreya Anand (Caltech), Tomas Ahumada (UMD), S. Bradley Cenko (NASA GSFC), D. Goldstein (Caltech), A. Ho (Caltech), D. A. Perley (LJMU), V. Bhalerao (IITB), H. Kumar (IITB), Y. Sharma (IITB), C. Copperwheat (LJMU), Virginia Cunningham (UMD), Shaon Ghosh (UWM), Ariel Goobar (OKC), David Kaplan (UWM), Jesper Sollerman (OKC), Joshua S. Bloom (UCB), M. Bulla (OKC), Nobuyuki Kawai (Tokyo Tech), Yoichi Yatsu (Tokyo Tech), Katsuhiro Murata (Tokyo Tech), Hidekazu Hanayama (IAO), Takashi Horiuchi (IAO), G. C. Anupama (IIA), M. Rigault (CNRS/IN2P3), C. Barbarino(OKC), R. Biswas (OKC), D. Cook (Caltech), G. Helou (IPAC) 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 S190425z (GCN 24168) with the Palomar 48-inch telescope equipped with the 47 square degree ZTF camera (Bellm et al. 2019, Graham et al. 2019). We were already observing the GW localization region as part of routine survey operations. 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 2019-04-25 09:19:07.161. A total of 4327 square degrees covering 41% of the enclosed probability were observed before 12-deg twilight and analyzed in real-time. Each exposure was 30s with a typical depth of 20.4 mag. 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. Our most promising candidates thus far are: ------------------------------------------------------------------------ ZTF Name | RA (deg) | DEC (deg) | Filter | Mag | Magerr --------------+-------------+------------+--------+-------+------------- ZTF19aarykkb | 258.341454 | -9.964466 | r | 18.63 | 0.10 ZTF19aarzaod | 262.791487 | -8.450722 | r | 20.11 | 0.18 --------------+-------------+------------+--------+-------+------------- ZTF19aarykkb is near 2MASX J17132113-0957536 at z=0.024. ZTF19aarzaod is near 2MASX J17311017-0827103 at z=0.028. Both have no ZTF detections prior to the reported binary neutron star merger time (although our most recent upper limits are only from 2019 April 19). Both host galaxies are in the Census of the Local Universe (CLU) galaxy catalog (Cook et al. 2017). Thus, with the data in-hand, the position, redshift of putative host galaxy , age, color and luminosity of both events are consistent with the GW trigger. The positions are also consistent with being in the region that Fermi-GBM could not constrain (GCN 24185). Both candidates have not yet been reported to the Transient Name Server. We caution that we cannot rule out these are young supernovae and spectroscopic follow-up is urgently needed. Additional analysis and follow-up is ongoing. Also, just as a reference, we also provide a list of relatively young candidates whose first ZTF detection was within the past three days below, including a magnitude measurement from tonight. We emphasize that all the events listed below have recent previous detections with ZTF prior to the gravitational wave event trigger time and are unlikely to be related to S190425z (e.g., background supernovae, active galactic nuclei, foreground CV). ------------------------------------------------------------------------ ZTF Name | RA (deg) | DEC (deg) | Filter | Mag | Magerr --------------+-------------+------------+--------+-------+------------- ZTF19aarzfoz | 235.600249 | 19.5876421 | g | 19.90 | 0.21 ZTF19aarpptk | 254.2036677 | 9.6049366 | g | 18.10 | 0.05 ZTF19aarywve | 250.9039956 | 17.5801296 | r | 20.45 | 0.19 ZTF19aarpfjs | 252.0464742 | 1.0668639 | r | 18.93 | 0.10 ZTF19aarkmsl | 242.401716 | 25.7497917 | r | 19.40 | 0.18 ZTF19aarppnl | 258.9096792 | 15.7457655 | g | 19.80 | 0.11 ZTF19aarpplo | 262.9779752 | 18.7380718 | g | 19.74 | 0.17 ZTF19aarypbv | 248.5874816 | 30.5050049 | g | 20.48 | 0.29 ZTF19aartzok | 238.9634555 | 36.3591878 | g | 20.27 | 0.20 ZTF19aarxvub | 265.2782205 | 8.470448 | r | 19.53 | 0.13 ZTF19aaryvld | 250.075357 | 9.1071601 | r | 20.01 | 0.18 ZTF19aarpzuh | 252.1452493 | 15.7402238 | r | 20.19 | 0.16 ZTF19aarprog | 248.5427404 | 19.6347848 | r | 19.00 | 0.11 ZTF19aarycpi | 245.0910206 | 18.348845 | r | 19.20 | 0.10 ZTF19aailtzw | 256.934132 | 2.948056 | r | 18.50 | 0.08 ------------------------------------------------------------------------ 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 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).