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

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.

arXiv comment5 pages, 4 figures, 1 table

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• Late-time Evolution of Afterglows from Off-Axis Neutron-Star Mergers
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• The Rise and Fall of ASASSN-18pg: Following a TDE from Early To Late Times
• ASASSN-18tb: A Most Unusual Type Ia Supernova Observed by TESS and SALT
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• Radioactive $\gamma$-Ray Emissions from Neutron Star Mergers
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• Synchrotron Polarization Radiative Transfer: Relativistic Thermal Electrons Contribution
• Line Expansion Opacity in Relativistically Expanding Media
• Kilonova evolution -- the rapid emergence of spectral features
• Helium features are inconsistent with the spectral evolution of the kilonova AT2017gfo
• Prompt emission from the counter jet of a short gamma-ray burst
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• Serendipitous discovery of the magnetic cataclysmic variable SRGE J075818-612027
• A multiwavelength study of the flat spectrum radio-quasar NVSS J141922-083830 covering four flaring episodes
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• A new Bowen Fluorescence Flare and Extreme Coronal Line Emitter discovered by SRG/eROSITA
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• Southern African Large Telescope Spectroscopy of BL Lacs for the CTA project
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• Mini-Tracker concepts for the SALT transient follow-up program