A comparison between SALT/SAAO observations and kilonova models for AT 2017gfo: the first electromagnetic counterpart of a gravitational wave transient - GW170817

TL;DR

This study presents optical spectroscopy and photometry of the kilonova AT 2017gfo, the first electromagnetic counterpart to a gravitational wave event, and compares observations with kilonova models to understand ejecta composition and properties.

Contribution

It provides the first SALT spectrum of AT 2017gfo and compares observational data with kilonova simulations to assess model accuracy and ejecta characteristics.

Findings

Spectral evolution shows a transition from blue to red over 10 days.

Spectra are consistent with models of neutron star merger ejecta with specific proton ratios.

Discrepancies suggest the need for improved atomic data and refined system parameters.

Abstract

We report on SALT low resolution optical spectroscopy and optical/IR photometry undertaken with other SAAO telescopes (MASTER-SAAO and IRSF) of the kilonova AT 2017gfo (aka SSS17a) in the galaxy NGC4993 during the first 10 days of discovery. This event has been identified as the first ever electromagnetic counterpart of a gravitational wave event, namely GW170817, which was detected by the LIGO and Virgo gravitational wave observatories. The event is likely due to a merger of two neutron stars, resulting in a kilonova explosion. SALT was the third telescope to obtain spectroscopy of AT 2017gfo and the first spectrum, 1.2 d after the merger, is quite blue and shows some broad features, but no identifiable spectral lines and becomes redder over time. We compare the spectral and photometric evolution with recent kilonova simulations and conclude that they are in qualitative agreement for post-merger wind models with proton: nucleon ratios of $Y_e$ = 0.25$-$0.30. The blue colour of the first spectrum is consistent with the lower opacity of the Lathanide-free r-process elements in the ejecta. Differences between the models and observations are likely due to the choice of system parameters combined with the absence of atomic data for more elements in the ejecta models.