The Electromagnetic Counterpart of the Binary Neutron Star Merger LIGO/Virgo GW170817. II. UV, Optical, and Near-infrared Light Curves and Comparison to Kilonova Models

Abstract

We present UV, optical, and near-infrared (NIR) photometry of the first electromagnetic counterpart to a gravitational wave source from Advanced Laser Interferometer Gravitational-wave Observatory (LIGO)/Virgo, the binary neutron star merger GW170817. Our data set extends from the discovery of the optical counterpart at 0.47-18.5 days post-merger, and includes observations with the Dark Energy Camera (DECam), Gemini-South/ FLAMINGOS-2 (GS/F2), and the Hubble Space Telescope (HST). The spectral energy distribution (SED) inferred from this photometry at 0.6 days is well described by a blackbody model with » T 8300 K, a radius of »Ŕ 4.5 10 14 cm (corresponding to an expansion velocity of » v c 0.3 ), and a bolometric luminosity of »Ĺ 5 10 bol 41 erg s −1 . At 1.5 days we find a multi-component SED across the optical and NIR, and subsequently we observe rapid fading in the UV and blue optical bands and significant reddening of the optical/ NIR colors. Modeling the entire data set, we find that models with heating from radioactive decay of 56 Ni, or those with only a single component of opacity from r-process elements, fail to capture the rapid optical decline and red optical/NIR colors. Instead, models with two components consistent with lanthanide-poor and lanthanide-rich ejecta provide a good fit to the data; the resulting "blue" component has » . These ejecta masses are broadly consistent with the estimated r-process production rate required to explain the Milky Way r-process abundances, providing the first evidence that binary neutron star (BNS) mergers can be a dominant site of r-process enrichment.

The Electromagnetic Counterpart of the Binary Neutron Star Merger LIGO/Virgo GW170817. I. Discovery of the Optical Counterpart Using the Dark Energy Camera

“…Data taken on days 5.5, 6.5, and 11.5 were obtained in poor weather conditions, hence the broader uncertainty bands. The photometry has been checked against a difference imaging reduction using Pan-STARRS PS1 templates (Cowperthwaite et al 2017) and a galaxy fit and subtract PSF source magnitude reduction; these measurements are consistent with our results. The dashed lines show the initial decay of the source: µ a -m t , where a = ( ) 2.0, 1.4, 0.7, 0.5, 0.5, 0.4 in u, g, r, i, z, Y bands, respectively.…”

“…Results agree within uncertainties and calibration differences. The photometry used in the next paper in this series (Cowperthwaite et al 2017), measured using a difference image reduction using Pan-STARRS PS1 templates, also agrees within uncertainties.…”

“…Our last deep image of the source is on 2017 August 31, 14.5 days post-merger. These follow-up observations are discussed in detail in Cowperthwaite et al (2017).…”

Localization of binary neutron star mergers with second and third generation gravitational-wave detectors

“…This multimessenger data provided convincing answers to many outstanding questions. For instance, the detection of a short gamma ray burst (GRB) 1.7 seconds after GW170817 [24][25][26], and subsequent kilonova [27][28][29][30][31][36][37][38][39][40][41][42][43][44], confirmed that BNS mergers are a progenitor of these events. Lanthanide signatures in the kilonova light curves also showed BNS mergers to be a major site for nucleosynthesis of elements heavier than iron [40,44,47,48].…”

“…Taking our fiducial kilonova model, the current generation of wide-field telescopes, such as DECam [29], Pan-STARRS [39] and VISTA [40], which have limiting magnitudes around 22 would be able to observe kilonova emission to z ≈ 0.1 or a luminosity distance of 400 Mpc. The results in Figs.…”