How does the Gamma Ray Burst that occurred when 2 black holes merged compare to other GRB's?

Question

A Gamma Ray Burst was detected 0.4 seconds after the gravitational wave event, GW150914, caused by a black hole merger, and it was in the same part of the sky. It is uncertain whether that Gamma Ray Burst was associated with the black hole merger. The odds of a GRB being coincident (or just background noise) is 0.22%. That implies a 99.78% chance the black hole merger was related to the GRB. Later analysis suggests the GRB was just a background event that occurred in the same place in the sky at just 0.4 seconds after the black hole merger, and therefore not related.

While a GRB may have relativistic jets beaming out from it in opposite directions, research "excludes the possibility that the event is associated with substantial gamma-ray radiation, directed towards the observer." I interpret that to mean this GRB did not have relativistic jets, but was omnidirectional. (Not sure how astronomers came to that conclusion.)

Anyhow, whether it was coincident or not, I am asking about how the energy output of that GRB compares to other GRB's. The Wikipedia article in the link says the "energy emission in gamma-rays and hard X-rays from the event was less than one millionth of the energy emitted as gravitational waves."

How much energy does a typical GRB emit? What was the spectrum spread of a typical GRB? Over what amount of time does a typical GRB emit that energy? (How many seconds?)

How do the energy levels, spectrums, and time durations of other GRB's compare to the GRB associated with the black hole merger?

If it is similar to other GRB's, that would support the hypothesis this GRB was just a coincident event at nearly the same time and location in the sky.

If this GRB has different energy emissions and time duration than other GRB's, that would support the hypothesis it is truly associated with the black hole merger.

When you answer, please provide data, citations, or quotes from original research. I am not looking for unsupported speculation, but real analysis supported by real data.

Answer 1

The interpretation you suggest in the second paragraph is incorrect. It is understandable, since there is a debate in the literature - different papers come to potentially contradicting conclusions.

"Excluding a possibility that the event is associated with substantial gamma-ray radiation, directed towards the observer" simply means that no observable GRB is found, hinting that may be the original detection was caused by instrumental background: each detector has it's own instrumental backgrounds, only real events should be seen by all sufficiently sensitive instruments.

In principle, it is always possible that the merger generated beamed emission, directed somewhere else. No instrument should have seen it directly, and there is no simple way to know if it happened.

This upper limit is derived from the observation of another satellite (INTEGRAL/SPI-ACS, Savchenko et al 2016) than the original detection (Fermi/GBM, Connaughton et al 2016). Also an alternative analysis of the Fermi/GBM data (Greiner et al 2016) suggested that no event can be found in the GBM data - their opinion is that it was a background fluctuation of some kind.

Right now, the teams who reported these conflicting results are working together, trying to come up with a consistent picture, which might be, in principle, a GRB with this or that properties associated with the GW150914, an unrelated GRB with some properties, or no detection whatsoever. This work is centered on cross-calibrating and comparing instruments, and is also useful to avoid these kind of uncertainties in the future.

One could try to characterize the spectral properties of this event, following the approach of the original Fermi/GBM team. But unfortunately, the measurement appeared to be in very unlucky conditions for Fermi/GBM (in bad direction). Which is why the signal was very weak (below what would be usually reported for a real GRB, though recently the attempts were made to decrease these thresholds, see Goldstein et al 2017 ), and spectral characterization is loose. You can look for some details to Veres et al 2016. With these large uncertainties the spectrum is compatible to that of known short GRBs.

The luminosity estimate depends on the spectrum, but is seems to be at the lower end of the short GRB sample (see e.g. Wanderman et al 2015)
But because the uncertainties are large, the event, if real, might be unusual as well.

The INTEGRAL observation, non-detection, would imply much softer (perhaps, unusual for a short GRB) or/and a weaker burst, possibly incompatible even with the highly uncertain Fermi/GBM data.

Duration of this possible GRB possibly associated with GW event is the easy part and is about 1 s long, typical for a short GRB (Kouveliotou et al 1993).