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

Abstract: We present UV, optical, and NIR photometry of the first electromagnetic counterpart to a gravitational wave source from Advanced LIGO/Virgo, the binary neutron star merger GW170817. Our data set extends from the discovery of the optical counterpart at $0.47$ days to $18.5$ days post-merger, and includes observations with the Dark Energy Camera (DECam), Gemini-South/FLAMINGOS-2 (GS/F2), and the {\it Hubble Space Telescope} ({\it HST}). The spectral energy distribution (SED) inferred from this photometry at $0.6$ days is well described by a blackbody model with $T\approx 8300$ K, a radius of $R\approx 4.5\times 10^{14}$ cm (corresponding to an expansion velocity of $v\approx 0.3c$), and a bolometric luminosity of $L_{\rm bol}\approx 5\times10^{41}$ 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 $M_\mathrm{ej}^\mathrm{blue}\approx 0.01$ M$\odot$ and $v\mathrm{ej}^\mathrm{blue}\approx 0.3$c, and the "red" component has $M_\mathrm{ej}^\mathrm{red}\approx 0.04$ M$\odot$ and $v\mathrm{ej}^\mathrm{red}\approx 0.1$c. 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 BNS mergers can be a dominant site of $r$-process enrichment.

• The paper demonstrates a comprehensive analysis of UV, optical, and NIR light curves to trace the evolution of GW170817's EM counterpart.
• The study applies a multi-instrument approach to construct detailed spectral energy distributions and capture rapid light curve changes.
• The results reveal that a dual-component kilonova model best explains the observations, supporting BNS mergers as key sites for r-process nucleosynthesis.

Analysis of the Electromagnetic Counterpart of Binary Neutron Star Merger GW170817 in UV, Optical, and Near-IR Light Curves

The investigation detailed in this paper provides an in-depth analysis of the electromagnetic (EM) counterpart of the binary neutron star (BNS) merger event GW170817. This study, conducted by Cowperthwaite et al., focuses on the ultraviolet (UV), optical, and near-infrared (NIR) light curves observed post-merger, presenting a meticulous comparison to kilonova models.

Observational Campaign

The data set spans observations from the initial detection of the optical counterpart at 0.47 days post-merger and extends to 18.5 days, utilizing varied instruments including DECam, Gemini-South/FLAMINGOS-2, and the HST. This comprehensive observational effort allowed for constructing a detailed spectral energy distribution (SED) and understanding the evolution of the EM counterpart.

Key Findings

1. Initial Conditions: At approximately 0.6 days, the SED is effectively described by a blackbody model, with a temperature of around 8300 K and an expansion velocity close to 0.3c, signaling rapid expansion.
2. Temporal Evolution: Significant change is observed at 1.5 days, with the SED requiring a multi-component description across optical and NIR wavelengths. Rapid fading in the UV and optical blue bands highlights dynamic processes affecting the ejecta.
3. Kilonova Model Comparison: The paper rigorously examines different kilonova models. Models incorporating radioactive decay of $^{56}$Ni or a singular opacity profile from $r$-process elements fail to capture the observational data's nuances, particularly the rapid decline in optical band brightness and the notable reddening of optical/NIR colors.
4. Dual-Component Model Alignment: Introducing a two-component kilonova model proved effective. Lanthanide-poor and lanthanide-rich ejecta components together account for observed properties. The blue ejecta component indicated high velocity and low mass, whereas the red component, though slower, contained greater mass.
5. Astrophysical Implications: These findings align kilonovae as significant contributors to $r$-process nucleosynthesis in the universe. The suggested mass ejections corroborate theoretical expectations for BNS mergers to play a decisive role in enriching galaxies with heavy elements.

Implications for Future Research

This detailed study provides a robust framework for interpreting similar events and contributes to the broader understanding of astrophysical nucleosynthesis. Future gravitational wave events detected by enhanced LIGO/VIRGO facilities will benefit from this study by offering comparative benchmarks to improve model accuracy and predictive capabilities for the characteristics of EM counterparts.

The scope for future investigations will likely focus on refining opacity values and ejecta compositions, and dynamics to better match diverse observations and extend the application of these models to various merger environments. Moreover, as multi-messenger astronomy progresses, this research exemplifies the integration of gravitational wave and electromagnetic signals to explore and understand cosmic phenomena comprehensively.