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GROWTH on S190510g: DECam Observation Planning and Follow-up of a Distant Binary Neutron Star Merger Candidate

Andreoni, Igor; Goldstein, Daniel A.; Anand, Shreya; Coughlin, Michael W.; Singer, Leo P.; Ahumada, Tomás; Medford, Michael; Kool, Erik C.; Webb, Sara; Bulla, Mattia; Bloom, Joshua S.; Kasliwal, Mansi M.; Nugent, Peter E.; Bagdasaryan, Ashot; Barnes, Jennifer; Cook, David O.; Cooke, Jeff; Duev, Dmitry A.; Fremling, U. Christoffer; Gatkine, Pradip Golkhou, V. Zach; Kong, Albert K. H.; Mahabal, Ashish; Martínez-Palomera, Jorge; Tao, Duo; Zhang, Keming

The first two months of the third Advanced LIGO and Virgo observing run (2019 April─May) showed that distant gravitational-wave (GW) events can now be readily detected. Three candidate mergers containing neutron stars (NS) were reported in a span of 15 days, all likely located more than 100 Mpc away. However, distant events such as the three new NS mergers are likely to be coarsely localized, which highlights the importance of facilities and scheduling systems that enable deep observations over hundreds to thousands of square degrees to detect the electromagnetic counterparts. On 2019 May 10 02:59:39.292 UT the GW candidate S190510g was discovered and initially classified as a binary neutron star (BNS) merger with 98% probability. The GW event was localized within an area of 3462 deg2, later refined to 1166 deg2 (90%) at a distance of 227 ± 92 Mpc. We triggered Target-of-Opportunity observations with the Dark Energy Camera (DECam), a wide-field optical imager mounted at the prime focus of the 4 m Blanco Telescope at Cerro Tololo Inter-American Observatory in Chile. This Letter describes our DECam observations and our real-time analysis results, focusing in particular on the design and implementation of the observing strategy. Within 24 hr of the merger time, we observed 65% of the total enclosed probability of the final skymap with an observing efficiency of 94%. We identified and publicly announced 13 candidate counterparts. S190510g was reclassified 1.7 days after the merger, after our observations were completed, with a “BNS merger” probability reduced from 98% to 42% in favor of a “terrestrial classification.


A strategy for LSST to unveil a population of kilonovae without gravitational-wave triggers

Andreoni, Igor; Anand, Shreya; Bianco, Federica B.; Cenko, Brad; Cowperthwaite, Philip; Coughlin, Michael W.; Drout, Maria; Golkhou, V. Zach; Kaplan, David; Mooley, Kunal P.; Pritchard, Tyler A.;

Singer, Leo P.; Webb, Sara


We present a cadence optimization strategy to unveil a large population of kilonovae using optical imaging alone. These transients are generated during binary neutron star and potentially neutron star-black hole mergers and are electromagnetic counterparts to gravitational-wave signals detectable in nearby events with Advanced LIGO, Advanced Virgo, and other interferometers that will come online in the near future. Discovering a large population of kilonovae will allow us to determine how heavy element production varies with the intrinsic parameters of the merger and across cosmic time. The rate of binary neutron star mergers is still uncertain, but only few (less than 15) events with associated kilonovae may be detectable per year within the horizon of next-generation ground-based interferometers. The rapid evolution (hours to days) at optical/infrared wavelengths, relatively low luminosity, and the low volumetric rate of kilonovae makes their discovery difficult, especially during blind surveys of the sky. We propose future large surveys to adopt a rolling cadence in which g-i observations are taken nightly for blocks of 10 consecutive nights. With the current baseline2018a cadence designed for the Large Synoptic Survey Telescope (LSST), less than 7.5 poorly-sampled kilonovae are expected to be detected in both the Wide Fast Deep (WFD) and Deep Drilling Fields (DDF) surveys per year, under optimistic assumptions on their rate, duration, and luminosity. We estimate the proposed strategy to return up to about 272 GW170817-like kilonovae throughout the LSST WFD survey, discovered independently from gravitational-wave triggers.
Figure: Light curve of the kilo nova associated with the gravitational-wave event GW170817 sampled as if it was observed with LSST using the proposed cadence strategy.
Upper figure: The refined skymap of the S190510g gravitational wave event, with the circles representing the DECam observations that we performed. 
Lower figure: Triangles indicate the observational constraints that we place on transient emission like GW170817.