The Electromagnetic Counterpart of the Binary Neutron Star Merger LIGO/Virgo GW170817. III. Optical and UV Spectra of a Blue Kilonova from Fast Polar Ejecta

By M. Nicholl, E. Berger, D. Kasen, B. Metzger, J. Elias, C. Briceño, K. Alexander, P. Blanchard, Ryan Chornock, P. Cowperthwaite and 24 more. 2017

337 citations recorded by the source corpus.

We present optical and ultraviolet spectra of the first electromagnetic counterpart to a gravitational-wave (GW) source, the binary neutron star merger GW170817. Spectra were obtained nightly between 1.5 and 9.5 days post-merger, using the Southern Astrophysical Research and Magellan telescopes; the UV spectrum was obtained with the Hubble Space Telescope at 5.5 days. Our data reveal a rapidly fading blue component ( T ≈ 5500 K at 1.5 days) that quickly reddens; spectra later than ≳ 4.5 days peak beyond the optical regime. The spectra are mostly featureless, although we identify a possible weak emission line at ∼7900 Å at t ≲ 4.5 days. The colors, rapid evolution, and featureless spectrum are consistent with a “blue” kilonova from polar ejecta comprised mainly of light r-process nuclei with atomic mass number A ≲ 140 . This indicates a sightline within θ obs ≲ 45 ° of the orbital axis. Comparison to models suggests ∼0.03 M⊙ of blue ejecta, with a velocity of ∼ 0.3 c . The required lanthanide fraction is ∼ 10 − 4 , but this drops to < 10 − 5 in the outermost ejecta. The large velocities point to a dynamical origin, rather than a disk wind, for this blue component, suggesting that both binary constituents are neutron stars (as opposed to a binary consisting of a neutron star and a black hole). For dynamical ejecta, the high mass favors a small neutron star radius of ≲ 12 km. This mass also supports the idea that neutron star mergers are a major contributor to r-process nucleosynthesis.

Subfields: Astronomy and Astrophysics

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In Astronomy and Astrophysics

1. B. Metzger
2. E. Berger
3. Bing Zhang
4. E. Quataert
5. Michela Mapelli
6. David Radice
7. N. Giacobbo
8. D. Kasen

Citations in the corpus

• The Combined Ultraviolet, Optical, and Near-infrared Light Curves of the Kilonova Associated with the Binary Neutron Star Merger GW170817: Unified Data Set, Analytic Models, and Physical Implications (2017, methodology): “02 ) is indicative of a small neutron star radius of 12 km when compared to the results of numerical simulations (Bauswein et al. 2013; Hotokezaka et al. 2013; see also Nicholl et al. 2017).”
• Constraints on the neutron star equation of state from AT2017gfo using radiative transfer simulations (2018, background): “…2017; Drout et al. 2017; Evans et al. 2017; Haggard et al. 2017; Hallinan et al. 2017; Kasliwal et al. 2017; Kilpatrick et al. 2017; Margutti et al. 2017; McCully et al. 2017; Nicholl et al. 2017; Pian et al. 2017a; Shappee et al. 2017; Smartt et al. 2017; Troja et al. 2017; Utsumi et al. 2017).”
• The Electromagnetic Counterpart of the Binary Neutron Star Merger LIGO/Virgo GW170817. IV. Detection of Near-infrared Signatures of r-process Nucleosynthesis with Gemini-South (2017, background): “A pure blue kilonova model with a higher ejecta mass (as found by Nicholl et al. 2017) would probably be a better fit by itself.”
• A Magnetar Origin for the Kilonova Ejecta in GW170817 (2018, background): “For a wind duration equal to the magnetar lifetime trem, producing the observed quantity of ejecta M 2blue » ´ M10 2- (e.g., Cowperthwaite et al. 2017; Nicholl et al. 2017; Perego et al. 2017; Villar et al. 2017) requires an average wind mass-loss rate M t M s2 10 1 s .”
• The evolution of the X-ray afterglow emission of GW 170817 / GRB 170817A in XMM-Newton observations (2018, background): “…Li & Paczyński 1998), providing the first compelling observational evidence for the existence of such elusive transient sources (Cowperthwaite et al. 2017; Drout et al. 2017; Evans et al. 2017; Kasliwal et al. 2017; Nicholl et al. 2017; Pian et al. 2017; Smartt et al. 2017; Villar et al. 2017).”
• The Electromagnetic Counterpart of the Binary Neutron Star Merger LIGO/Virgo GW170817. VI. Radio Constraints on a Relativistic Jet and Predictions for Late-time Emission from the Kilonova Ejecta (2017, methodology): “For the first time, we can make specific predictions for the KN radio emission using the parameters inferred from modeling of the UV/optical/NIR emission (Chornock et al. 2017; Cowperthwaite et al. 2017; Nicholl et al. 2017).”
• Merging black holes in young star clusters (2019, background): “…et al. 2017d; Abbott et al. 2017e; Goldstein et al. 2017; Savchenko et al. 2017; Margutti et al. 2017; Coulter et al. 2017; Soares-Santos et al. 2017; Chornock et al. 2017; Cowperthwaite et al. 2017; Nicholl et al. 2017; Pian et al. 2017; Alexander et al. 2017). c© 0000 The Authors ar X iv :1 90 1.”
• AT 2017gfo: An Anisotropic and Three-component Kilonova Counterpart of GW170817 (2017, background): “It peaked in less than one day after the merger in the optical and ultraviolet bands, before fading out rapidly (Nicholl et al. 2017).”
• The progenitors of compact-object binaries: impact of metallicity, common envelope and natal kicks (2018, background): “…et al. 2017e; Goldstein et al. 2017; Savchenko et al. 2017; Margutti et al. 2017; Coulter et al. 2017; Soares-Santos et al. 2017; Chornock et al. 2017; Cowperthwaite et al. 2017; Nicholl et al. 2017; Pian et al. 2017; Alexander et al. 2017), marking the beginning of multi-messenger astronomy.”