A Gamma ray burst associated with LIGO GW event

[Jonathan Scott]

Quite unexpectedly, it seems that the Fermi Gamma-ray Burst Monitor spotted what appears to be a hard gamma-ray burst about 0.4s after the LIGO GW event, lasting about 1s: http://arxiv.org/abs/1602.03920

This is not expected from a black hole merger (and as a black hole sceptic, I find it very interesting).

There's also a new paper which suggests that perhaps this could be explained by the black hole merger occurring inside a star (which I find quite implausible, but which is generating a lot of news stories): http://arxiv.org/abs/1602.04735

 

[Chronos]

Loeb's paper is indeed very interesting. The collapse of a massive star just before it coallesced with a companion black hole strikes me as a very odd coincidence - in fact a bit too coincidental for my tastes. It could imply a hitherto unsuspected mechanism for stellar collapse or merely be a nearby event unrelated to GW15091. We have ample cause to be alert to the risk of making unwarranted associations between two unrelated astrophysical events

 

[Lord Crc]

Potentially very silly question, but could the magnetic fields of the black holes generate something like this when the black holes merge?

 

[Jonathan Scott]

Potentially very silly question, but could the magnetic fields of the black holes generate something like this when the black holes merge?

Black holes are not even supposed to be able to have magnetic fields (unlike neutron stars which can have extremely strong magnetic fields)!

 

[marcus]

Loeb's paper is indeed very interesting. The collapse of a massive star just before it coallesced with a companion black hole strikes me as a very odd coincidence - in fact a bit too coincidental for my tastes. It could imply a hitherto unsuspected mechanism for stellar collapse or merely be a nearby event unrelated to GW15091. We have ample cause to be alert to the risk of making unwarranted associations between two unrelated astrophysical events

Is that the sum total of what Loeb's considering? I got the impression that the core collapse of a single very rapidly rotating star could involve a brief stage during which there is an oblong or "dumb-bell" shape concentration of mass. NOT spherically symmetric. I didn't think coincidence needed to be involved. Let's look more closely at the loeb paper

 

[Greg Bernhardt]

John Baez wrote a bit on it
https://johncarlosbaez.wordpress.com/2016/02/25/gamma-ray-burst/

 

[Lord Crc]

Black holes are not even supposed to be able to have magnetic fields (unlike neutron stars which can have extremely strong magnetic fields)!

My bad, was certain I had read that rotating black holes would have a magnetic field, but I see now that they shed the magnetic field during the collapse.

 

[Keith_McClary]

There is also a paper on the non-detection of neutrinos with ~1400 authors.

"Within 500 s of the gravitational wave event, the number of neutrino candidates detected by IceCube and ANTARES were three and zero, respectively. This is consistent with the expected atmospheric background, and none of the neutrino candidates were directionally coincident with GW150914. We use this non-detection to constrain neutrino emission from the gravitational-wave event."​

http://arxiv.org/abs/1602.05411

 

[Dr. Courtney]

Loeb's paper is indeed very interesting. The collapse of a massive star just before it coallesced with a companion black hole strikes me as a very odd coincidence - in fact a bit too coincidental for my tastes. It could imply a hitherto unsuspected mechanism for stellar collapse or merely be a nearby event unrelated to GW15091. We have ample cause to be alert to the risk of making unwarranted associations between two unrelated astrophysical events

I think the timing warrants an association, and I don't see how that is problematic as long as the relationship is only treated as a hypothesis until more evidence of the same originating event is available.

I don't see the papers as anything more than "Hey, we saw this at (about) the same time" perhaps they are related. Exploring possible causal relationships between coincident events seems reasonable. Drawing conclusions is not yet reasonable.

 

[Jonathan Scott]

My reference to "hard gamma-ray burst" in my original post is slightly garbled as I hadn't noticed that the terminology is a bit confusing. It is correct to describe it as a "gamma-ray burst" and it is also described as a "short/hard burst", but I hadn't spotted that the "hardness" in the latter term is relative to the X-ray spectrum, not the gamma spectrum.

The Fermi team paper (section 2.5 and figures 5 and 6) actually suggests that a plausible value for the energy spectrum peak is around 3.5MeV, and the distribution seems to go up at least to 10MeV. Even the highest bin, 38MeV-50MeV, shows an excess, although with little statistical significance.

John Baez appeared to have initially read "above 50keV" as "about 50keV" but has now sorted that out.

 

[Jonathan Scott]

Note that the INTEGRAL team reckoned that they had ruled out any gamma ray activity around the time of the event down to a threshold which they considered would rule out the claim by the Fermi GBM team: http://arxiv.org/abs/1602.04180

The Fermi GBM team however (see section 2.6 of their paper) argue that there are plausible reasons why INTEGRAL might have missed it.

 

[Dr. Courtney]

Note that the INTEGRAL team reckoned that they had ruled out any gamma ray activity around the time of the event down to a threshold which they considered would rule out the claim by the Fermi GBM team: http://arxiv.org/abs/1602.04180

The Fermi GBM team however (see section 2.6 of their paper) argue that there are plausible reasons why INTEGRAL might have missed it.

One wonders if there are other events caught by the GBM team and missed by the INTEGRAL team.

When one observer "sees" and event and another observer fails to "see" an event, I tend to lean toward a genuine observation unless there is a documented history of spurious reports.

 

[Jonathan Scott]

One wonders if there are other events caught by the GBM team and missed by the INTEGRAL team.

See last paragraph of section 2.6 of the Fermi team paper:

Additionally, a search of the INTEGRAL-ACS data revealed a detection rate of only 55% of GBM-detected weak short GRBs (Briggs et al., in preparation). We do not consider, therefore, the non-detection of GW150914-GBM by INTEGRAL-ACS, a sufficient reason to reject our candidate.

 

[Dr. Courtney]

That seems like an important point. Thanks for the post.

 

[SWATHI N]

Do a fast acceleration of any celestial bodies( black holes) produce gravitational waves?

 

[Jonathan Scott]

Do a fast acceleration of any celestial bodies( black holes) produce gravitational waves?

Fast acceleration of a massive body cannot occur in isolation, as that would violate conservation laws. Gravitational waves are typically produced by changes in configuration of systems involving two masses, such as when they are orbiting rapidly around each other, and are only detectable in extreme cases, such as merging black holes. However, that is not the topic of this thread, so please take any further discussion elsewhere.

 

[Ken G]

I wonder about two things:
1) Why on Earth would anyone expect a neutrino event to be detected when the electromagnetic detection is so marginal? Don't we barely see neutrinos from nearby supernovae that can be seen optically with low grade telescopes?
2) The gravitational wave detection was a bit marginal-- a 5 sigma detection. The gamma-ray detection was also a bit marginal-- also 5 sigma! What kind of spectacular coincidence is that? Two totally different kinds of detectors, seeing different aspects of the same event, both registering 5 sigma detections. I'm pretty sure a Bayesian analysis of the relative probability of that, and a non-detection, would show that the non-detection of gamma rays is the more likely conclusion.

Basically, both my points are about how strange I find it that people are not analyzing the joint probabilities of these various detections, they almost seem to expect that marginal detections with one instrument should be associated with marginal detections with another. That is certainly not the normal state of affairs.

 

[Jonathan Scott]

If the original hypothesis about a black hole collision was correct, no detectable neutrinos or electromagnetic effects were expected. The observations were therefore to check for any surprises, and the Fermi GBM result was a surprise.

The gravitational wave detection was not at all marginal (confidence level of 99.99994%) and was far stronger statistically than the gamma-ray detection (for which they estimated a "false alarm" probability of about 0.22%), so I don't understand the rest of your post.

 

[Jeff Rosenbury]

My bad, was certain I had read that rotating black holes would have a magnetic field, but I see now that they shed the magnetic field during the collapse.

Is it possible the accretion disks would have electromagnetic fields which interacted (Two become one?) to spout gamma rays?

Some potential questions:

• How would "riding" the gravity waves have affected their timing?
  
• How would rapidly rotating (near C) atomic particles in two different orbits "collide"? It would seem they would leak synchrotron type radiation until they achieved a single stable orbit around the conglomerate black hole. But gravity waves might disrupt this, as might "ringing" deformations in the new hole.
  
• Why wouldn't there be neutrinos in the accretion disk merger?
  
• Why the 0.4 second delay?

My knowledge of the physics and the math are lacking, so I ask as a duffer.

I agree with Dr. Courtney that this is just a weak hypothesis lacking more data.

There were likely lots of energetic events going on nearby the collapse, but expecting any of them to be visible at this distance might be unreasonable.

 

[Ken G]

The gravitational wave detection was not at all marginal (confidence level of 99.99994%) and was far stronger statistically than the gamma-ray detection (for which they estimated a "false alarm" probability of about 0.22%), so I don't understand the rest of your post.

Both detections are reported as 5 sigma detections. Ponder the likelihood of that.

 

[Jonathan Scott]

Is it possible the accretion disks would have electromagnetic fields which interacted (Two become one?) to spout gamma rays?

I think the general view is that for any gamma rays at all to have reached us from such a distant event, the amount of energy (and the density of normal matter) involved in the gamma ray event had to be far more than could be accounted for by nearby free material such as accretion disks (even though the total gamma energy was probably less than 1/100,000 of the gravitational wave energy emitted in the last half second, if I've understood the estimates correctly).

 

[Jonathan Scott]

Both detections are reported as 5 sigma detections. Ponder the likelihood of that.

Can you provide a reference for the gamma detection being reported as 5 sigma? The paper gives at least a probability of 0.0022 that it could be a false alarm, which I'd say means the confidence must be somewhat less than 3 sigma.

 

[Jeff Rosenbury]

I think the general view is that for any gamma rays at all to have reached us from such a distant event, the amount of energy (and the density of normal matter) involved in the gamma ray event had to be far more than could be accounted for by nearby free material such as accretion disks (even though the total gamma energy was probably less than 1/100,000 of the gravitational wave energy emitted in the last half second, if I've understood the estimates correctly).

Wouldn't any planetary bodies in close stable orbits have been destablized, torn apart, accreted and partially absorbed, shedding energy as gamma rays? If the black holes had 0.1% of their mass as planets, etc., 1/100,000 sounds about right. One large gas giant would seem to do the trick.

The solar system has roughly 0.1% of it's mass as an accretion disk, broadly defined. Though that is one data point of a totally different kind of stellar object, so just speculative.

 

[epenguin]

Questions for the experts:

Can someone tell us please are gamma ray bursts observed every few seconds? If so, this might be coincidence though it might not. Or are they observed every minute or so on the average? Then the coincidence starts to be quite a coincidence. Or are they every hour on average, or roughly daily? If the experts would tell us we might all have an idea.

I ask them to help the rest of us answering: I remember reading in the report that from the difference of arrival time of the signal into the two detector locations they could say its origin to within a 20° angle. Er, that would be about 1% of the sky is that what what is meant? Anyway can you confirm that this latest burst is from within the stated area - I suppose we wouldn't be talking about it otherwise but it would be nice to be told.

The only rather wide localisation of sky area the GW the event is known to have been within it Is because there were only two detectors. Other detectors were down or not yet ready. Can you tell us how well such an event it would be localised with three or four detectors?

 

[Jonathan Scott]

Wouldn't any planetary bodies in close stable orbits have been destablized, torn apart, accreted and partially absorbed, shedding energy as gamma rays?

The observed gamma event was a short burst of gamma rays, lasting about a second or two and starting about 0.4 seconds after the GW event. I don't see how either the amount of energy needed or the time scale could have anything to do with tearing up objects in nearby orbits, even if a stable object were possible in the close vicinity of a pair of black holes.

 

[Jonathan Scott]

Can someone tell us please are gamma ray bursts observed every few seconds? If so, this might be coincidence though it might not.

The Fermi GBM paper (V Connaughton et al) mentioned in the original post discusses the statistics in considerable detail, and takes that into account in assessing the probability of a "false alarm" at 0.0022.

 

[Jonathan Scott]

Anyway can you confirm that this latest burst is from within the stated area - I suppose we wouldn't be talking about it otherwise but it would be nice to be told.

Again, if you read the paper, although the directions of the GW source and that of the gamma-ray burst cannot be exactly determined, the probable directions of both have a high degree of overlap.

 

[epenguin]

Ah thank you, yes I have now read the summary. I did not think I would be able to, and I can't get more than that. Seems people are being very cautious by talking about accidental coincidence.

Am I right in thinking this is 1% of the sky?

 

[Jonathan Scott]

The only rather wide localisation of sky area the GW the event is known to have been within it Is because there were only two detectors. Other detectors were down or not yet ready. Can you tell us how well such an event it would be localised with three or four detectors?

In most cases three detectors could reduce the probable direction from an arc to a small area, although there could be ambiguity in marginal cases, and four would help to avoid such ambiguity and provide additional accuracy. The accuracy of the localisation depends on how well the common signal can be resolved with respect to time. As the detectors are not exactly aligned in the same directions, I suspect that the phase of the signal may vary between them depending on the orientation of the wave, which may increase the difficulty of matching the timing exactly.

 

[Jonathan Scott]

Am I right in thinking this is 1% of the sky?

The Fermi GBM paper says that if the two detections are indeed the same, the 90% confidence interval on sky location is reduced to 199 square degrees, which I think is less than 0.5% of the sky.

I'm puzzled as to why you can't read it yourself; I just followed the link to the arXiv abstract page and clicked "PDF" in the "Download" section on the right.

 

[Jeff Rosenbury]

Remember, if these have a co-origin there will possibly be some odd gravitational lensing.

0.002 error chance is small by everyday standards, but more than physics likes. Scientists might run billions of tests (colliding billions of atoms or whatnot), so 1 in a million occurrences pop up all the time and don't mean much.

Still it's suggestive and warrants further study.

 

[Jonathan Scott]

Remember, if these have a co-origin there will possibly be some odd gravitational lensing.

It is expected that gravitational waves and gamma rays from the same event will follow exactly the same path through space-time.

 

[Jonathan Scott]

0.002 error chance is small by everyday standards, but more than physics likes. Scientists might run billions of tests (colliding billions of atoms or whatnot), so 1 in a million occurrences pop up all the time and don't mean much.

Still it's suggestive and warrants further study.

This is a unique test, the very first associated with a GW event. The paper looks into the statistics quite thoroughly. I get the impression that in most areas of life, a result this strong would be treated as essentially a dead certainty. I get the impression that it is only because we don't have a satisfactory theory (at least not a mainstream one) to explain it that it is being called into question at all.

 

[Vanadium 50]

Both detections are reported as 5 sigma detections. Ponder the likelihood of that.

The probability of two independent 5 sigma events is 2.6 x 10^-12, or 6.9 sigma. However, the probability of what was reported, 5 sigma and 0.22% is 1.4 x 10^-8, or 5.5 sigma.

 

[marcus]

John Baez wrote a bit on it
https://johncarlosbaez.wordpress.com/2016/02/25/gamma-ray-burst/

==Baez excerpt==
Perhaps those expectations are wrong. Or maybe—just maybe—both the gravitational waves and X-rays were formed during the collapse of a single very large star! That’s what typically causes gamma ray bursts—we think. But it’s not at all typical—as far as we know—for a large star to form two black holes when it collapses! And that’s what we’d need to get that gravitational wave event: two black holes, which then spiral down and merge into one!

Here’s an analysis of the issue:

• Abraham Loeb, Electromagnetic counterparts to black hole mergers detected by LIGO.

As he notes, the collapsing star would need to have an insane amount of angular momentum to collapse into a dumb-bell shape and form twoblack holes, each roughly 30 times the mass of our Sun, which then quickly spiral down and collide.

Furthermore, as Tony Wells pointed to me, the lack of neutrinos argues against the idea that this event involved a large collapsing star:

• ANTARES collaboration, High-energy neutrino follow-up search of Gravitational wave event GW150914 with ANTARES and IceCube.

==endquote==
Interesting! Will take a while to sort out.

 

[Vanadium 50]

The paper looks into the statistics quite thoroughly.

True. But I don't find the statistics convincing: a common flaw of a posteriori significance calculations. What does the paper say? It says there were no events passing their requirement. They they changed this, and changed that, until they got a signal compatible with LIGO (and two dozen other signals). Now I don't want to say this procedure is generating a fake signal, but I am saying that once you go down this path it becomes pretty much impossible to calculate a p-value for whatever you find.

 

[Ken G]

My point is only stronger if the probability is 0.22% that the gamma-ray detection is real. The core of Bayesian thinking is the recognition that you only rule out unlikely things if likelier ones already exist. But if you already know that something unlikely has happened, then the least unlikely becomes the probable. So here, we have a 0.22% chance that noise could look like a signal, but we must compare that to the chance that a black-hole mergers have been going on all this time, and were not previously noticeable by Fermi, but suddenly we can see them if we know when to look. That is already quite unlikely, and we cannot assess the significance of a 0.22% chance of noise looking like a signal, until we can assess the relative likelihood of Fermi having all these detections just below our ability to notice.

 

[Jonathan Scott]

My point is only stronger if the probability is 0.22% that the gamma-ray detection is real. The core of Bayesian thinking is the recognition that you only rule out unlikely things if likelier ones already exist. But if you already know that something unlikely has happened, then the least unlikely becomes the probable. So here, we have a 0.22% chance that noise could look like a signal, but we must compare that to the chance that a black-hole mergers have been going on all this time, and were not previously noticeable by Fermi, but suddenly we can see them if we know when to look. That is already quite unlikely, and we cannot assess the significance of a 0.22% chance of noise looking like a signal, until we can assess the relative likelihood of Fermi having all these detections just below our ability to notice.

Again, I'm fairly sure this is addressed in the Fermi GBM paper by V Connaughton et al. Their normal trigger for a checking for an event is an unusually high count. However, in this case the number of counts in the relevant interval was not very significantly far above average for anyone energy bin, but the fact that a whole range of energy bins all gave somewhat above-average counts at the same time, specifically for the second after the event, is very interesting. Applying similar filtering to an extended period, they got a few other occasional matches, and they used that to determine the probability of a random timing match. I don't fully understand the details, but they are explained in the paper.

 

[Ken G]

They do not include any Bayesian analysis that involves the prior unlikeliness that exactly the kind of signal that flies under their radar, but is accessible after the fact, is what is sent out by exactly the kind of event that is the first type detectable by LIGO.

 

[Jonathan Scott]

They do not include any Bayesian analysis that involves the prior unlikeliness that exactly the kind of signal that flies under their radar, but is accessible after the fact, is what is sent out by exactly the kind of event that is the first type detectable by LIGO.

The "kind of signal" is merely a weak one, below their normal trigger threshold, that would normally be of no interest. It is only the timing relative to the LIGO event that is highly suggestive.

I'd be interested to know how you think the statistics should be modified. Are you saying that you'd assign some prior probability that there would be no gamma ray burst, based on some theoretical expectations about this situation which has never been experimentally observed before?

 

[myuncle]

Jonathan, as a layman, I wonder how is it possible to assume that the gravitational waves are coming from black holes, considering that we have zero convincing proof that black holes exist. Wouldn't make more sense to, first proove the black holes existence, and then, try to detect their gravitational waves?

 

[Ken G]

The "kind of signal" is merely a weak one, below their normal trigger threshold, that would normally be of no interest. It is only the timing relative to the LIGO event that is highly suggestive.

Which is entirely my point. Whenever one is asking the question "how likely is this to be random chance", one always has to know what one is comparing the likelihood to. It's a complicated situation when the only reason the signal is regarded as interesting is when it happened. It means that something unexpected occurred, but it is no simple matter to decide which is the more likely unexpected result: that black hole mergers produce just enough gamma rays to make a signal that is only detectable by virtue of its timing (when it could have made no signal at all, or a signal that is easy to detect without timing), or that an unlucky noise event just happened to occur at that time. Fortunately, the issue will quickly be decided, as this merger event is not regarded as unique, and correlations between multiple observations should easily resolve the issue. I'm just saying I'm not convinced the 0.22% number means anything at all, until the careful comparison I'm talking about has been made in some more formal Bayesian sense.

I'd be interested to know how you think the statistics should be modified.

The Bayesian approach would be to assess some kind of prior expectation that a black hole merger would produce a gamma ray signal that is only detectable if you include its timing. The fact that this prior expectation is not easy to assess is merely the evidence that whether or not the result can be viewed as significant may depend on initial assumptions. It is certainly a logical fallacy to conclude that because something non-generic happened, implies that a detection has occurred, rather than simply an unlikely noise event. The issue is the difference between absolute probabilities and relative probabilities.

Are you saying that you'd assign some prior probability that there would be no gamma ray burst, based on some theoretical expectations about this situation which has never been experimentally observed before?

It is essential to include theoretical expectations of that nature. This is clear, you only have to ask yourself what if that exact same paper had appeared, except that the timing was not the black hole merger, but rather the event of the death of some famous world figure. No one would take it seriously that the death of a world figure can produce gamma rays, so the 0.22% would have no importance posed against our extreme skepticism of its plausibility. So it's all Bayesian analysis, it's merely a question of whether it is formally acknowledged, or simply implied informally. It certainly is reason to look for correlations of a similar type for all other gravitational wave events, but it will be a long time before there can be any confidence in this detection.

 

[Jonathan Scott]

Jonathan, as a layman, I wonder how is it possible to assume that the gravitational waves are coming from black holes, considering that we have zero convincing proof that black holes exist. Wouldn't make more sense to, first proove the black holes existence, and then, try to detect their gravitational waves?

Gravitational waves depend on the mass and motion of the bodies, and are not affected by whether they are black holes. The point at which the amplitude of the wave peaked and then tapered off is presumably the point at which the bodies merged, so it gives some clues as to their sizes, which must clearly have been extremely dense, compatible with GR predictions of black holes.

 

[nikkkom]

Black holes are not even supposed to be able to have magnetic fields (unlike neutron stars which can have extremely strong magnetic fields)!

A *stationary* black hole can have only electric field, not magnetic, yes.

But a *moving* electric change (any change, including a charged black hole) is seen as generating magnetic field.

This is just a consequence of the fact that electromagnetic potential is a four-vector, and if it has only temporal non-zero component in one coordinate system, it will have non-zero spatial components too in other coordinate systems after Lorentz transform.

 

[Jonathan Scott]

A *stationary* black hole can have only electric field, not magnetic, yes.

But a *moving* electric change (any change, including a charged black hole) is seen as generating magnetic field.

True. To be more accurate I should have said that a black hole doesn't have any intrinsic magnetic field.

However, that's not really very relevant. Note that black holes are not expected to be able to pick up much electric charge anyway (since that would generally repel particles with the same charge more strongly than gravity would attract it).

 

[nikkkom]

Do they really need to have "much" electric charge?
A "slightly" charged pair of 30 Msun objects, orbiting each other with nearly light-speed velocities, may end up having a substantial magnetic field. Whet it all decays, where all its energy goes? I say don't discount electromagnetics yet for this case.

 

[Jonathan Scott]

Do they really need to have "much" electric charge?
A "slightly" charged pair of 30 Msun objects, orbiting each other with nearly light-speed velocities, may end up having a substantial magnetic field. Whet it all decays, where all its energy goes? I say don't discount electromagnetics yet for this case.

You're welcome to try a calculation, but I'd guess that's many orders of magnitude too small to be relevant.

 

[myuncle]

Gravitational waves depend on the mass and motion of the bodies, and are not affected by whether they are black holes. The point at which the amplitude of the wave peaked and then tapered off is presumably the point at which the bodies merged, so it gives some clues as to their sizes, which must clearly have been extremely dense, compatible with GR predictions of black holes.

Extremely dense, ok, but is it enought as a proof of black holes existence? Considering that something like a black hole is incredibly big to claim, don't you think that an incredibly strong proof should be need to support this claim?

 

[Jonathan Scott]

Extremely dense, ok, but is it enought as a proof of black holes existence? Considering that something like a black hole is incredibly big to claim, don't you think that an incredibly strong proof should be need to support this claim?

No-one has said that this proves black holes exist. The gravitational wave observation gave results which indicated two massive objects spiralling together and merging, and various parameters used to fit the results show that the objects had masses sufficiently large and radii sufficiently small that GR says they have to be black holes, if it is still correct in this extreme case, which is what is generally assumed. It was a really beautiful experimental result which neatly confirmed theoretical predictions.

However, the apparent gamma ray burst raises a challenge. If it is real and really associated with the GW event, then one potential "simple" explanation would be that GR isn't quite right and what collided was actually something like a pair of neutron stars or quark stars, so the gamma ray burst was energy being given off from that collision. Given that GR is the best theory of gravity that we have, that would be a nasty surprise to have to face, so even though the evidence for an associated gamma ray event seems fairly clear at first glance, it is a case where extraordinary claims require extraordinary proof. My guess is that any explanation that appears to be consistent with current GR, however contrived, will be considered far more plausible that the idea that black holes may not exist, although I'm personally very interested in that possibility.

 

[myuncle]

No-one has said that this proves black holes exist. The gravitational wave observation gave results which indicated two massive objects spiralling together and merging, and various parameters used to fit the results show that the objects had masses sufficiently large and radii sufficiently small that GR says they have to be black holes, if it is still correct in this extreme case, which is what is generally assumed. It was a really beautiful experimental result which neatly confirmed theoretical predictions.

However, the apparent gamma ray burst raises a challenge. If it is real and really associated with the GW event, then one potential "simple" explanation would be that GR isn't quite right and what collided was actually something like a pair of neutron stars or quark stars, so the gamma ray burst was energy being given off from that collision. Given that GR is the best theory of gravity that we have, that would be a nasty surprise to have to face, so even though the evidence for an associated gamma ray event seems fairly clear at first glance, it is a case where extraordinary claims require extraordinary proof. My guess is that any explanation that appears to be consistent with current GR, however contrived, will be considered far more plausible that the idea that black holes may not exist, although I'm personally very interested in that possibility.

Thanks Jonathan.