Electromagnetic Counterparts to Black Hole Mergers?

The observations of the first gravitational wave by the Laser Interferometer Gravitational-Wave Observatory (LIGO) captured the attention of the world this February, confirming the existence of  gravitational waves as well as further confirming Einstein’s theory of general relativity.
The signature of merging blackholes resulted in a flurry of scientific articles reworking theories casting out models and creating new ones. Other observatories probed the sky looking for electromagnetic signatures across all wavelengths, but nothing was seen. Well almost nothing.

The Fermi Gamma-ray Burst Monitor (GBM) claimed to see an event occurring 0.4 seconds after the LIGO event. It was a very week fluctuation in the data lasting only 1 second. SPI Anti-Coincidence Shield on board The INTErnational Gamma-Ray Astrophysics Laboratory (INTEGRAL), with a similar observing range as GBM, could not confirm the signal even though it should have seen the same event. This lead to an interesting conundrum. If GBM really saw gamma-rays from a black hole merger, many existing theories would have to be changed as it is very difficult to have a scenario where such merger can produce gamma-rays.

gamma-rays coming from merging black holes
A cartoon of gamma-rays coming from merging black holes and hitting Fermi GBM. The green histogram shows the result of our analysis. The signal is consistent with background.
Image credit: Composite NASA Goddard Space Flight Center/SXS, the Simulating eXtreme Spacetimes (SXS) project

Therefore, we teamed up with part of the GBM and INTEGRAL teams to do a thorough investigation of the event. Instead of relying on existing analysis tools, we went back to basic statistics and designed new analysis schemes for the data. We applied the tools to the data from the GBM event and could not find a signal above the normal background in the instrument. When we checked with the normal analysis tools, we could see it. Therefore, data from other weak events that were also seen by INTEGRAL were checked with both sets of tools. This was very strange and we needed to find a way to see which analysis was right.

Luckily, INTEGRAL and GBM see a lot of the same gamma-ray events. For events that both INTEGRAL and GBM agree are real events, we can predict what INTEGRAL would see using parameters from GBM data analysis. Using our method and the standard method, we found again that you get two different predictions for what INTEGRAL would see. Our method always predicted correctly the INTEGRAL signal, but the standard analysis over-predicted the strength in INTEGRAL: just like the presumed gravitational-wave counterpart! Thus, we could explain why GBM results were in conflict with INTEGRAL for the gravitational-wave counterpart and prove that the counterpart was merely a background fluctuation in the GBM data.

There will be a lot of exciting science to come from LIGO! GBM is very likely to see a coincident signal when two neutron stars collide as these events should generate copious gamma-rays. When this day comes, we must have the statistical tools to analyze the data ready so that we get the next event right. Once we know we have a signal, we can explore all the exotic theories.

– J. Michael Burgess ( jamesb@kth.se )
Article preprint: http://arxiv.org/abs/1606.00314
GBM instrument: http://gammaray.nsstc.nasa.gov/gbm/

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