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Draft version July 19, 2019 Typeset using LATEX twocolumn style in AASTeX62 Searches After Gravitational-waves Using ARizona Observatories (SAGUARO): System Overview and First Results from Advanced LIGO/Virgo's Third Observing Run M. J. Lundquist,1 K. Paterson,2 W. Fong,2 D. J. Sand,1 J. E. Andrews,1 I. Shivaei,1, 3 P. N. Daly,1 S. Valenti,4 S. Yang,4, 5, 6 E. Christensen,7 A. R. Gibbs,7 F. Shelly,7 S. Wyatt,1 O. Kuhn,8 R. C. Amaro,1 I. Arcavi,9 P. Behroozi,1 N. Butler,10 L. Chomiuk,11 A. Corsi,12 M. R. Drout,13, 14 E. Egami,1 X. Fan,1 R. J. Foley,15 B. Frye,1 P. Gabor,16 E. M. Green,1 C. J. Grier,1 F. Guzman,17, 1 E. Hamden,1 D. A. Howell,18 B. T. Jannuzi,1 P. Kelly,19 P. Milne,1 M. Moe,1 A. Nugent,2 E. Olszewski,1 E. Palazzi,20 V. Paschalidis,1 D. Psaltis,1 D. Reichart,21 A. Rest,22, 23 A. Rossi,20 G. Schroeder,2 P. S. Smith,1 N. Smith,1 K. Spekkens,24 J. Strader,11 D. P. Stark,1 D. Trilling,25 C. Veillet,1, 8 M. Wagner,26, 8 B. Weiner,1, 27 J. C. Wheeler,28 G. G. Williams,1, 27 and A. Zabludoff1 1Steward Observatory, University of Arizona, 933 North Cherry Avenue, Tucson, AZ 85721-0065, USA 2Center for Interdisciplinary Exploration and Research in Astrophysics and Department of Physics and Astronomy, Northwestern University, 2145 Sheridan Road, Evanston, IL 60208-3112, USA 3Hubble Fellow 4Department of Physics, University of California, 1 Shields Avenue, Davis, CA 95616-5270, USA 5Department of Physics and Astronomy Galileo Galilei, University of Padova, Vicolo dell'Osservatorio, 3, I-35122 Padova, Italy 6INAF Osservatorio Astronomico di Padova, Vicolo dell'Osservatorio 5, I-35122 Padova, Italy 7Lunar and Planetary Lab, Department of Planetary Sciences, University of Arizona, Tucson, AZ 85721, USA 8Large Binocular Telescope Observatory, 933 North Cherry Avenue, Tucson, AZ, USA 9The School of Physics and Astronomy, Tel Aviv University, Tel Aviv 69978, Israel 10School of Earth and Space Exploration, Arizona State University, Tempe, AZ 85287, USA 11Center for Data Intensive and Time Domain Astronomy, Department of Physics and Astronomy, Michigan State University,East Lansing, MI 48824, USA 12Department of Physics and Astronomy, Texas Tech University, Box 1051, Lubbock, TX 79409-1051, USA 13Department of Astronomy and Astrophysics, University of Toronto, 50 St. George Street, Toronto, Ontario, M5S 3H4 Canada 14The Observatories of the Carnegie Institution for Science, 813 Santa Barbara St., Pasadena, CA 91101, USA 15Department of Astronomy and Astrophysics, University of California, Santa Cruz, CA 95064, USA 16Vatican Observatory, 00120 Citt`adel Vaticano, Vatican City State 17College of Optical Sciences, University of Arizona, 1630 E University Blvd, Tucson, AZ 85719, USA 18Las Cumbres Observatory, 6740 Cortona Drive, Suite 102, Goleta, CA 93117-5575, USA 19College of Science & Engineering, Minnesota Institute for Astrophysics, University of Minnesota, 115 Union St. SE, Minneapolis, MN 55455, USA 20INAF - Osservatorio di Astrofisica e Scienza dello Spazio - Via Piero Gobetti 93/3, I-40129 Bologna, Italy 21Department of Physics and Astronomy, University of North Carolina at Chapel Hill, Chapel Hill, NC 27599, USA 22Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218, USA 23The Johns Hopkins University, Baltimore, MD 21218, USA 24Department of Physics and Space Science Royal Military College of Canada P.O. Box 17000, Station Forces Kingston, ON K7K 7B4, Canada 25Department of Physics and Astronomy, Northern Arizona University, P.O. Box 6010, Flagstaff, AZ 86011, USA 26Department of Astronomy, The Ohio State University, 140 W. 18th Ave., Columbus, OH43210, USA 27MMT Observatory, PO Box 210065, University of Arizona, Tucson, AZ 85721-0065, USA 28Department of Astronomy, University of Texas at Austin, Austin, TX 78712, USA arXiv:1906.06345v2 [astro-ph.HE] 17 Jul 2019 ABSTRACT We present Searches After Gravitational-waves Using ARizona Observatories (SAGUARO), a com- prehensive effort dedicated to the discovery and characterization of optical counterparts to gravita- tional wave (GW) events. SAGUARO utilizes ground-based facilities ranging from 1.5m to 10m in diameter, located primarily in the Northern Hemisphere. We provide an overview of SAGUARO's telescopic resources, pipeline for transient detection, and database for candidate visualization. We describe SAGUARO's discovery component, which utilizes the 5 deg2 field-of-view optical imager on the Mt. Lemmon 1.5m telescope, reaching limits of ≈ 21:3 AB mag while rapidly tiling large areas. We also describe the follow-up component of SAGUARO, used for rapid vetting and monitoring of 2 optical candidates. With the onset of Advanced LIGO/Virgo's third observing run, we present results from the first three SAGUARO searches following the GW events S190408an, S190425z and S190426c, which serve as a valuable proof-of-concept of SAGUARO. We triggered and searched 15, 60 and 60 deg2 respectively, 17.6, 1.4 and 41.8 hrs after the initial GW alerts. We covered 7.8, 3.0 and 5.1% of the total probability within the GW event localizations, reaching 3σ limits of 19.8, 21.3 and 20.8 AB mag, respectively. Although no viable counterparts associated with these events were found, we recovered 6 known transients and ruled out 5 potential candidates. We also present Large Binocular Telescope spectroscopy of PS19eq/SN2019ebq, a promising kilonova candidate that was later determined to be a supernova. With the ability to tile large areas and conduct detailed follow-up, SAGUARO represents a significant addition to GW counterpart searches. 1. INTRODUCTION In this paper, we describe a telescope network brought The onset of the advanced era of gravitational wave online in LVC's O3 dedicated to optical counterpart (GW) detectors has heralded a new era of discovery. discovery and follow-up of GW events: Searches Af- The Advanced Laser Interferometer Gravitational-wave ter Gravitational-waves Using ARizona Observatories Observatory (Abbott et al. 2009) and Advanced Virgo (SAGUARO). In Section2 we provide an overview of (Acernese et al. 2015; hereafter termed \LVC") have SAGUARO's scope and telescopic resources. In Sec- discovered a total of 11 GW events in their first two tion3 we describe our wide-field photometric counter- observing runs (O1-O2; The LIGO Scientific Collab- part search, automated pipeline for transient discovery, oration et al. 2018), including ten binary black holes database for candidate visualization, and current sta- (BBH) mergers as well as the first binary neutron star tus. In Section4 we describe our resources and methods (BNS) merger, GW170817 (Abbott et al. 2017a). How- for spectroscopic classification of candidates and con- ever, identifying an electromagnetic (EM) counterpart centrated follow-up of true EM counterparts. In Sec- presents an observational challenge, as GW events thus tion5 we present results from the first three SAGUARO far have been localized to ≈ 16 − 1650 deg2 (90% con- searches following the GW events S190408an, S190425z fidence; The LIGO Scientific Collaboration et al. 2018), and S190426c as proof-of-concept studies. Finally, in often requiring wide-field imagers to cover a meaning- Section6 we summarize and discuss future prospects. ful fraction of the localization regions for photometric Unless otherwise stated, all magnitudes reported here discovery. are in Gaia G-band and are converted to the AB system Two primary approaches have been taken to identify via mAB = mGaia + 0:125 (Ma´ız Apell´aniz& Weiler optical counterparts to GW events: \galaxy-targeted" 2018). searches which focus on plausible galaxies within the GW localization regions (e.g., Gehrels et al. 2016), and 2. SAGUARO OVERVIEW AND SCIENTIFIC wide-field searches which cover more substantial areas SCOPE on the sky and are relatively agnostic to the distribution The SAGUARO GW follow-up program has two dis- of stellar mass within a GW localization. GW170817 tinct but intertwined components: 1) a wide-field op- was localized to ≈30 deg2 from the GW signal alone tical search for EM counterparts utilizing the Steward (Abbott et al. 2017a) at the time of the initial counter- Observatory 1.5m Mt. Lemmon telescope and its 5 deg2 part searches, and both wide-field and galaxy-targeted imager; 2) a comprehensive optical and near-infrared strategies proved fruitful for the discovery of the op- (NIR) follow-up program composed largely of Steward tical counterpart (e.g., Arcavi et al. 2017a; Coulter Observatory telescopes, but also including a few pro- et al. 2017; Lipunov et al. 2017; Tanvir et al. 2017; grams outside of Arizona. We detail these components Soares-Santos et al. 2017; Valenti et al. 2017). Look- in Sections3 and4, respectively. ing forward, the median localization of BNS mergers The search component of SAGUARO utilizes the ex- for the third LVC observing run (\O3") is predicted to isting infrastructure and personnel of the Catalina Sky be ≈ 120 − 180 deg2 for events detected by all three Survey (CSS; Christensen et al. 2018) to respond in real LIGO/Virgo detectors (Abbott et al. 2016a). Moreover, time to GW events of interest. Given the 5 deg2 field most BNS mergers are expected to be detected at dis- of view of the imager, we employ a wide-field search tances of & 100 Mpc, where galaxy catalogs are incom- strategy that directly tiles the GW localization region, plete (White et al. 2011; D´alya et al. 2018), motivating similar to that of other groups with access to wide-field dedicated wide-field searches to discover optical coun- facilities (e.g. Smartt et al. 2016; Kasliwal et al. 2016; terparts. Abbott et al. 2016b; Soares-Santos et al. 2017; Goldstein et al. 2019), rather than a galaxy-targeted approach (e.g. 3 Gehrels et al.

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