GROWTH on S190425z: Searching Thousands of Square Degrees to Identify an Optical or Infrared Counterpart to a Binary Neutron Star Merger with the Zwicky Transient Facility and Palomar Gattini-IR
Authors: Coughlin Michael W. , Ahumada Tomás , Anand Shreya , De Kishalay , Hankins Matthew J. , Kasliwal Mansi M. , Singer Leo P. , Bellm Eric C. , Andreoni Igor , Cenko S. Bradley , Cooke Jeff , Copperwheat Christopher M. , Dugas Alison M. , Jencson Jacob E. , Perley Daniel A. , Yu Po-Chieh , Bhalerao Varun , Kumar Harsh , Bloom Joshua S. , Anupama G. C. , Ashley Michael C. B. , Bagdasaryan Ashot , Biswas Rahul , Buckley David A. H. , Burdge Kevin B. , Cook David O. , Cromer John , Cunningham Virginia , D’Aì Antonino , Dekany Richard G. , Delacroix Alexandre , Dichiara Simone , Duev Dmitry A. , Dutta Anirban , Feeney Michael , Frederick Sara , Gatkine Pradip , Ghosh Shaon , Goldstein Daniel A. , Golkhou V. Zach , Goobar Ariel , Graham Matthew J. , Hanayama Hidekazu , Horiuchi Takashi , Hung Tiara , Jha Saurabh W. , Kong Albert K. H. , Giomi Matteo , Kaplan David L. , Karambelkar V. R. , Kowalski Marek , Kulkarni Shrinivas R. , Kupfer Thomas , Masci Frank J. , Mazzali Paolo , Moore Anna M. , Mogotsi Moses , Neill James D. , Ngeow Chow-Choong , Martínez-Palomera Jorge , Parola Valentina La , Pavana M. , Ofek Eran O. , Patil Atharva Sunil , Riddle Reed , Rigault Mickael , Rusholme Ben , Serabyn Eugene , Shupe David L. , Sharma Yashvi , Singh Avinash , Sollerman Jesper , Soon Jamie , Staats Kai , Taggart Kirsty , Tan Hanjie , Travouillon Tony , Troja Eleonora , Waratkar Gaurav , Yatsu Yoichi 
Published: October 2019

Abstract
The third observing run by LVC has brought the discovery of many compact binary coalescences. Following the detection of the first binary neutron star merger in this run (LIGO/Virgo S190425z), we performed a dedicated follow-up campaign with the Zwicky Transient Facility (ZTF) and Palomar Gattini-IR telescopes. The initial skymap of this single-detector gravitational wave (GW) trigger spanned most of the sky observable from Palomar Observatory. Covering 8000 deg2 of the initial skymap over the next two nights, corresponding to 46% integrated probability, ZTF system achieved a depth of ≈21 m
AB in g- and r-bands. Palomar Gattini-IR covered 2200 square degrees in J-band to a depth of 15.5 mag, including 32% integrated probability based on the initial skymap. The revised skymap issued the following day reduced these numbers to 21% for the ZTF and 19% for Palomar Gattini-IR. We narrowed 338,646 ZTF transient “alerts” over the first two nights of observations to 15 candidate counterparts. Two candidates, ZTF19aarykkb and ZTF19aarzaod, were particularly compelling given that their location, distance, and age were consistent with the GW event, and their early optical light curves were photometrically consistent with that of kilonovae. These two candidates were spectroscopically classified as young core-collapse supernovae. The remaining candidates were ruled out as supernovae. Palomar Gattini-IR did not identify any viable candidates with multiple detections only after merger time. We demonstrate that even with single-detector GW events localized to thousands of square degrees, systematic kilonova discovery is feasible.