GROWTH on S190426c: Real-time Search for a Counterpart to the Probable Neutron Star–Black Hole Merger using an Automated Difference Imaging Pipeline for DECam

Abstract

The discovery of a transient kilonova following the gravitational-wave event GW 170817 highlighted the critical need for coordinated rapid and wide-field observations, inference, and follow-up across the electromagnetic spectrum. In the Southern hemisphere, the Dark Energy Camera (DECam) on the Blanco 4-m telescope is wellsuited to this task, as it is able to cover wide-fields quickly while still achieving the depths required to find kilonovae like the one accompanying GW170817 to ∼500 Mpc, the binary neutron star horizon distance for current generation of LIGO/Virgo collaboration (LVC) interferometers. Here, as part of the multi-facility followup by the Global Relay of Observatories Watching Transients Happen (GROWTH) collaboration, we describe the observations and automated data movement, data reduction, candidate discovery, and vetting pipeline of our target-of-opportunity DECam observations of S190426c, the first possible neutron star-black hole merger detected via gravitational waves. Starting 7.5hr after S190426c, over 11.28 hr of observations, we imaged an area of 525 deg 2 (r-band) and 437 deg 2 (z-band); this was 16.3% of the total original localization probability and nearly all of the probability density visible from the Southern hemisphere. The machine-learning based pipeline was optimized for fast turnaround, delivering transient candidates for human vetting within 17 minutes, on average, of shutter closure. We reported nine promising counterpart candidates 2.5 hours before the end of our observations. Our observations yielded no detection of a bona fide coun-arXiv:1905.06980v1 [astro-ph.HE] 16 May 2019 Goldstein, Andreoni, et al. terpart to m z = 22.5 and m r = 22.9 at the 5σ level of significance, consistent with the refined LVC positioning. We view these observations and rapid inferencing as an important real-world test for this novel end-to-end wide-field pipeline. * Hubble Fellow

An ASKAP Search for a Radio Counterpart to the First High-significance Neutron Star–Black Hole Merger LIGO/Virgo S190814bv

“…2019B-0372 (PI: Soares-Santos). Images including the location of AT2019osy were taken in i and z bands nightly from 2019-08-15 to 2019-08-18 and on 2019-08-21 (UT) and reduced in real-time (Goldstein et al 2019). A detailed offline analysis of the subtraction images zooming in on the location around AT2019osy, reveals no robust point source at this location to a depth of i > 21.2mag and z > 20.0mag on UT 2019-08-15 (the night of the merger) increasing linearly in limiting magnitude to i > 23.5mag and z > 23.5mag on UT 2019-08-21 (consistent with independent analysis by Herner et al 2019).…”

Searching for electromagnetic counterparts to gravitational-wave merger events with the prototype Gravitational-Wave Optical Transient Observer (GOTO-4)

“…Numerous wide-field follow-up missions have tiled GW error boxes searching for transients, including the All-Sky Automated Survery for Supernovae (ASASSN; Shappee et al 2014), the Asteroid Terrestrial impact Last Alert System (ATLAS; Tonry et al 2018), the Deca-Degree Optical Transient Imager (DDOTI; Watson et al 2016), the Dark Energy Survey (DES; Dark Energy Survey Collaboration et al 2016), the Global Rapid Advanced Network Devoted to the Multimessenger Addicts (GRANDMA; Antier et al 2020b), KMT-Net (Kim et al 2016), the Mobile Astronomical System of TElescope Robots (MASTER; Lipunov et al 2010), MeerLICHT (Bloemen et al 2016), PanSTARRS (Kaiser et al 2010), Searches After Gravitational waves Using ARizona Observatories (SAGUARO; Lundquist et al 2019), the Télescope à Action Rapide pour les Objets Transitoires (TAROT; Boër 2001), the Visible and Infrared Survey Telescope for Astronomy (VISTA;Sutherland et al 2015), the VLT Survey Telescope (VST; Capaccioli & Schipani 2011) and the Zwicky Transient Facility (ZTF; Bellm et al 2019). No associated transients were identified (Anand et al 2020;Antier et al 2020b,a;Coughlin et al 2020;Sagués Carracedo et al 2020), but constraining limits were placed on a number of milestone events, including S190814bv, the first NSBH merger candidate identified in GW (Dobie et al 2019;Gomez et al 2019;LIGO Scientific Collaboration & Virgo Collaboration 2019;Ackley et al 2020;Andreoni et al 2020;Vieira et al 2020;Watson et al 2020), and several candidate BNS systems (Goldstein et al 2019;Hosseinzadeh et al 2019;Lundquist et al 2019), including the unusually massive GW190425 (Coughlin et al 2019;Hosseinzadeh et al 2019;Lundquist et al 2019;Abbott et al 2020b).…”

Dynamic scheduling: target of opportunity observations of gravitational wave events

“…It is important to promptly process images during the transientfiltering stage so that we can narrow down the hundreds of thousands of sources of variability to a select few candidates; highperformance image subtraction pipelines have been developed for this purpose (e.g. Kessler et al 2015;Goldstein et al 2019). In order to rule out moving objects such as near-Earth asteroids, the candidate must have a minimum of two detections separated by at least 30 min (Bellm et al 2019).…”

“…The dedicated follow-up of BNS and NSBH merger candidates undertaken by the GROWTH (e.g. Andreoni et al 2019bGoldstein et al 2019) and GRANDMA (e.g. Antier et al 2019Antier et al , 2020 networks throughout O3 led to the realization early on that more scheduling features would need to be implemented in order to facilitate this process, prompting Figure 3.…”

“…Due to the association of BNS and NSBH mergers with potentially detectable electromagnetic counterparts (Metzger & Berger 2012;, substantial efforts have been invested into optimizing follow-up observations of such candidates (e.g. Coughlin et al 2019e;Goldstein et al 2019;Gomez et al 2019;Andreoni et al 2020). These counterparts may come in the form of short gamma-ray bursts (sGRBs) accompanied by optical and near-infrared transients ('kilonovae', or KNe) powered by the decay of r-process nuclei that are synthesized in the merger ejecta, as well as prolonged radio emission resulting from the interaction of the sub-relativistic ejecta with the surrounding medium (e.g.…”