The Rapid Reddening and Featureless Optical Spectra of the Optical Counterpart of GW170817, AT 2017gfo, during the First Four Days

Abstract

We present the spectroscopic evolution of AT 2017gfo, the optical counterpart of the first binary neutron star (BNS) merger detected by LIGO and Virgo, GW170817. While models have long predicted that a BNS merger could produce a kilonova (KN), we have not been able to definitively test these models until now. From one day to four days after the merger, we took five spectra of AT 2017gfo before it faded away, which was possible because it was at a distance of only 39.5 Mpc in the galaxy NGC 4993. The spectra evolve from blue (∼ 6400K) to red (∼ 3500K) over the three days we observed. The spectra are relatively featureless -some weak features exist in our latest spectrum, but they are likely due to the host galaxy. However, a simple blackbody is not sufficient to explain our data: another source of luminosity or opacity is necessary. Predictions from simulations of KNe qualitatively match the observed spectroscopic evolution after two days past the merger, but underpredict the blue flux in our earliest spectrum. From our best-fit models, we infer that AT 2017gfo had an ejecta mass of 0.03M , high ejecta velocities of 0.3c, and a low mass fraction ∼ 10 −4 of high-opacity lanthanides and actinides. One possible explanation for the early excess of blue flux is that the outer ejecta is lanthanide-poor, while the inner ejecta has a higher abundance of high-opacity material. With the discovery and follow-up of this unique transient, combining gravitational-wave and electromagnetic astronomy, we have arrived in the multi-messenger era. arXiv:1710.05853v1 [astro-ph.HE]

Modeling GW170817 based on numerical relativity and its implications

“…Note that the observed spectrum is consistent broadly with the blackbody one with decreasing temperature (but see Ref. [83] for detailed comparisons), and hence, the evolution of the luminosity is consistent with the macronova/kilonova model.…”

Evaluating radiation transport errors in merger simulations using a Monte Carlo algorithm

“…This event also shows the current limits of our ability to reliably extract information about merging compact objects using EM observations. For example, GW170817 was followed by a bright kilonova [3][4][5][6][7][8][9][10][11][12][13][14][15], an optical/infrared transient powered by radioactive decays in the neutron-rich ejecta produced by the merger [16][17][18][19][20]. EM observations of that kilonova have been used to infer plausible properties of the ejecta, and the outcome of r-process nucleosynthesis in the outflows.…”

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“…The reduced spectra appears to have a relatively blue, featureless continua as seen in Figure 7. The data are also presented and interpreted in McCully et al (2017) and Buckley et al (2017).…”

“…Four exposures were analysed, revealing an E/S0-like galaxy spectrum (Figure 8) with a weak red flux enhancement (Andreoni et al 2017b). The source was isolated by subtracting the host galaxy using the SALT host galaxy spectrum (McCully et al 2017) extracted from the region of the galaxy near the source. The SALT spectrum was cleaned over chip gaps and telluric line regions using the average value on either edge of each feature.…”