{
  "bibcode": "2019GCN.24283....1C",
  "body": "Michael W. Coughlin (Caltech), Mansi M. Kasliwal (Caltech), Daniel A.\nPerley (LJMU), Ariel Goobar (OKC), Leo P. Singer (NASA GSFC), Shreya Anand\n(Caltech), Tomas Ahumada (UMD), Igor Andreoni (Caltech), Eric C. Bellm\n(UW), K. De (Caltech), R. Biswas (OKC), S. Nissanke (UvA), Dmitry Duev\n(Caltech), S. Bradley Cenko (NASA GSFC), D. Goldstein (Caltech), A. Ho\n(Caltech), V. Bhalerao (IITB), H. Kumar (IITB), V. Karambelkar (IITB), K.\nDeshmukh (IITB), D. Saraogi (IITB), G. C. Anupama (IIA), C. Copperwheat\n(LJMU), Virginia Cunningham (UMD), Shaon Ghosh (UWM), David Kaplan (UWM),\nJesper Sollerman (OKC), Joshua S. Bloom (UCB), M. Bulla (OKC), Matthew\nGraham (Caltech), L. Yan (Caltech), C. Fremling (Caltech), Pradip Gatkine\n(UMD), A. Miller (Northwestern)\n\nOn behalf of the Zwicky Transient Facility (ZTF) and Global Relay of\nObservatories Watching Transients Happen (GROWTH) collaborations\n\nWe observed the localization region of the gravitational wave trigger\nS190426c (GCN 24237) with the Palomar 48-inch telescope equipped with the\n47 square degree ZTF camera (Bellm et al. 2019, Graham et al. 2019). A new\ntiling was automatically optimally determined and triggered using the\nGROWTH Target of Opportunity marshal (Coughlin et al. 2019a, Kasliwal et\nal. 2019b). We started obtaining target-of-opportunity observations in the\ng-band and r-band filters beginning at UT 2019-04-27 05:45. The projected\nenclosed probability with the original sky map was 75%. However, with the\nnew sky map (GCN 24277, GCN 24279) and taking account into chip gaps and\nprocessing, a total of 4340 square degrees covering 55% of the enclosed\nprobability were observed before 12-deg twilight and analyzed in real-time.\nExposure length varied between 120s, 180s and 300s. We note that the area\naround the north celestial pole covered by our partner GROWTH-India\ntelescope covers an additional 6% of the updated probability map,\ncomplementary to ZTF (see GCN 24258).\n\nThe images were processed through the ZTF reduction and image subtraction\npipelines at IPAC to search for potential counterparts (Masci et al. 2019).\nAfter rejecting stellar sources (Tachibana and Miller 2018) and moving\nobjects and applying machine learning algorithms (Mahabal et al. 2019),\nseveral high-significance transient candidates were identified by our\npipeline in the area observed. Thanks to the overlap in sky maps between\nthe two GW triggers S190426c and S190425z, we have very good constraints on\npast history of variability in the last few days.\n\nThe only candidate with the first detection after the merger time is:\n\n-----------------------------------------------------------------------------------------------\n ZTF Name     | RA (deg)    | DEC (deg)   | Filter | Mag   |  Magerr |\nFilter| Mag   | Magerr\n--------------+-------------+-------------+--------+-------+---------+-------+-------+---------\nZTF19aaslzfk  | 308.968271  | 72.3536353  | r      | 20.91 |  0.17   | g\n  | 21.38 |  0.18\n-----------------------------------------------------------------------------------------------\n\nWe caution that our upper limits in the last few days for ZTF19aaslzfk are\nshallower than the detection. So we cannot rule out an old, unrelated\ntransient. The line-of-sight extinction is Ar of 1.4 mag (Schlafly et al.\n2011). We note that the source is detected in all four WISE filters in the\nAllWISE catalog (Wright et al. 2010).  Its W1-W3 colors are intermediate\nbetween galaxies and AGN relative to the color loci of Assef et al. (2018),\nbut the clear W4 detection suggests contribution from an active galactic\nnucleus.\n\nAdditional analysis and continued follow-up is in progress.\n\nZTF and GROWTH are worldwide collaborations comprising Caltech, USA; IPAC,\nUSA, WIS, Israel; OKC, Sweden; JSI/UMd, USA; U Washington, USA; DESY,\nGermany; MOST, Taiwan; UW Milwaukee, USA; LANL USA; Tokyo Tech, Japan;\nIIT-B, India; IIA, India; LJMU, UK; TTU, USA; and USyd, Australia. ZTF\nacknowledges the generous support of the NSF under AST MSIP Grant No\n1440341. GROWTH acknowledges generous support of the NSF under PIRE Grant\nNo 1545949. Alert distribution service provided by DIRAC@UW (Patterson et\nal. 2019). Alert filtering and follow-up co-ordination is being undertaken\nby the GROWTH marshal system (Kasliwal et al. 2019).",
  "circularId": 24283,
  "createdOn": 1556375440000,
  "email": "mansikasliwal@gmail.com",
  "subject": "LIGO/Virgo S190426c: Optical Wide-field Search with the Zwicky Transient Facility",
  "submitter": "Mansi M. Kasliwal at Caltech/Carnegie  <mansikasliwal@gmail.com>",
  "eventId": "LIGO/Virgo S190426c"
}