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Two Gamma Ray Bursts

A gamma ray burst  (GRB) detected 2 years before a LIGO event matched the 2018 GRB but in 2016 no LIGO detections were reported. 2015 had 3 LIGO events; 2017 had 8.

The claim is both GRB were the same type of kilo-nova event even though the 2016 GRB had no LIGO event.

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A team re-examined data from a gamma-ray burst spotted in August 2016 and found new evidence for a kilonova that went unnoticed during the initial observations.

NASA's Neil Gehrels Swift Observatory began tracking the 2016 event, named GRB160821B, minutes after it was detected. The early catch enabled the research team to gather new insights that were missing from the kilonova observations of the LIGO event, which did not begin until nearly 12 hours after the initial collision. Troja and her colleagues reported these new findings  on August 27, 2019.

"The 2016 event was very exciting at first. It was nearby and visible with every major telescope, including NASA's Hubble Space Telescope. But it didn't match our predictions—we expected to see the infrared emission become brighter and brighter over several weeks," said Troja. "Ten days after the event, barely any signal remained. We were all so disappointed. Then, a year later, the LIGO event happened. We looked at our old data with new eyes and realized we had indeed caught a kilonova in 2016. It was a nearly perfect match. The infrared data for both events have similar luminosities and exactly the same time scale."
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 "Ten days after the event, barely any signal remained. We were all so disappointed. Then, a year later, the LIGO event happened. We looked at our old data with new eyes and realized we had indeed caught a kilonova in 2016. It was a nearly perfect match. The infrared data for both events have similar luminosities and exactly the same time scale."

The similarities between the two events suggest that the 2016 kilonova also resulted from the merger of two neutron stars. Kilonovae may also result from the merger of a black hole and neutron star, but it is unknown whether such an event would yield a different signature in X-ray, infrared, radio and optical light observations.
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The key quote:
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it is unknown whether such an event BH-NS] would yield a different signature in X-ray, infrared, radio and optical light observations.
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Another:
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As we observe more of these events, we may learn that there are many different types of kilonovae all in the same family, as is the case with the many different types of supernovae. It's so exciting to be shaping our knowledge in real time."
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Conclusion:
The 2016 GRB event is 'exactly' like the 2018 GRB event but only the one in 2018 had a LIGO event.

it is unknown whether events for two possible scenarios would yield different observations.

Cosmologists are clearly trying to understand a gamma ray burst and are considering different scenarios.

These statements  could justify doubt in the certainty that the 2018 event was the inspiral merger exactly as claimed by LIGO.


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