A Gamma ray burst associated with LIGO GW event

[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.

 

[Jeff Rosenbury]

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?

Science doesn't prove anything exists. Proofs are for mathematics. What science does is give evidence for models. There is evidence for black holes, though perhaps not conclusive evidence. (IMO, the evidence is pretty conclusive.)

The detection of these gravity waves is another solid piece of that evidence. The gamma burst both confirms the broad model (blackholes exist) and undermines the specifics (black holes "look" like what we think they look like) of that model. So the data holds exciting possibilities of a closer matching of the model with reality (whatever that is).

But we need to remember science is about the observable and the repeatable. So one data point has no meaning on its own. Ideally we will detect a lot more gravity waves with or without gamma bursts. Then we will know more.

 

[looking4sophia]

The Model I found most feasible to generate a GWave and GRBurst
was the Merger of Two Large Stars.

As Yen and Yang merged, their combined Gravitational Field caused
each to collapse into 2 counter rotating Black Holes
which then formed a rotating QuadroPole which generated the GWave
seems the infall of the 2 star's remaining fuel supply then created a GRBlast a fraction of a Second later?

A portion of this GRBurst could have occurred Inside the Event Horizon
thus weakening the strength of the signal received on Earth, imo.

 

[Drakkith]

A number of off-topic posts have been removed. Please stick to the facts of the topic and not to personal opinions about science in general.

 

[ohannuks]

Both of the detections were also poorly localized. I'm curious, would it be expected that IceCube would detect anything?

 

[Jonathan Scott]

Neutrinos are produced by nuclear fusion, so none would be expected for a black hole merger. Even for a hypothetical neutron star merger, most of the material would already be fused into neutronium, so very little neutrino emissions would be expected. Huge amounts of neutrinos are however produced by stellar collapse to a neutron star as in a supernova.

The Wikipedia article First observation of gravitational waves has brought together a lot of useful information. It mentions that Icecube only saw three neutrinos around that time, which is compatible with background levels, and that none of them was in a direction which matched the likely direction of the gravitational wave or gamma ray detection.

In John Baez's blog entry mentioned by Greg earlier in this thread, he mentions that Tony Wells pointed out to him that the lack of neutrinos is evidence against the event having taken place inside a large collapsing star, as suggested by Loeb. However, neutrinos are of course very difficult to detect, so the absence of detection only places weak constraints on possible theories.

 

[greswd]

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

Maybe this burst came from another astronomical event?

 

[Jonathan Scott]

Maybe this burst came from another astronomical event?

Yes, that is of course possible, but it occurred within a second of the LIGO event and probably within the same less than 1% of the sky. Based on the average rate of such weak events, the Fermi GBM team estimated what they called the "false alarm" probability as 0.0022. Read the paper for more details of what that means.

 

[Ken G]

Yes, the "false alarm" probability applies to ruling out other astrophysical events. My point is that it does not apply to ruling out a non-event, because that would require comparison of the small probability of a noise event to the small probability that a black hole merger makes a signal that cannot be identified as such without coincident gravitational wave detections. Both of those likelihoods are quite small, so the question that is not addressed by the false alarm probability is the question of which of those likelihoods is the smaller. That depends on assumptions about the source, as per my analogy with the death of a famous world figure.

 

[looking4sophia]

. 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..

Apparently we can expect this experimental Result to be repeated about Once every year ?

Gravitational Waves are the Result of the rapid Acceleration of Mass that is distributed unevenly, ie nonSpherical ?
apparently this Configuration of Matter is predicted by GR to be a QuadruPole

As I understand it, Two Masses in rapid orbit will Not generate a significant GW
until they rapidly Accelerate towards each other
when their Combined Gravity overcomes the Centrifugal Forces pulling them apart.

It is hard to imagine all the dynamics involved in these 2 Masses merging
given we have no exact Values of the Parameters in this Experiment nature has provided in such a timely manner.

Monte Carlo modeling of possible combinations of Mass, Density, Elements Involved, Orbit Velocities,
Rotational Directions and Velocities, Etc... can only suggest what actually happened
and many Models might explain what we can barely observe;
due to the exxxxxxxxxtremely small measurements required to detect a GWave originating thousands of LightYears away..

So, What ShapeS would the Surface of 2 Black Holes form during collapse/merger into a single Singularity ??

First theSimplest Example: With No Orbital Velocities and No Rotational Velocities .

Seems the 2 Spheres would form a Dumbell or Hour Glass Shape very briefly
if there are no Centrifugal Forces to overcome ?
Also brief because the Radius of Black Holes is relatively small considering their MASS.

Adding Orbital Velocities and Rotational Spins near Light Speed to the above
could complicate the mathematical models a bit .

 

[rootone]

Two highly confident observations of an event within microseconds of each other.
Yet one of them supports what GR predicts, and the other is unexpected.
I'd say this one should run for a while.