Setting the record straight on LIGO/VIRGO O3 run.

Given the misinformation on LIGO’s O3 run exemplified by the lack of multi-messenger events (electromagnetic radiation associated with gravitational wave events), Christopher Berry from LIGO was kind enough answer a few of my questions to set the record straight.
Christopher Berry was one of the co-authors of the very first gravitational wave discovery paper.

Firstly are all SGRBs found assumed to be neutron star mergers or are there alternative mechanisms?

There have been alternative ideas for sGRBs, but at the moment it looks like we can explain them all with neutron star mergers. We'll need to observe a few more to sure that up.

Secondly during the O3 run I have used this database and found there have only been six definite SGRBs during this period.
Would the lack of a GW signal with these SGRBs be due to the limited detection range for neutron star mergers?
I haven’t seen any red shift data so I assume even the afterglow event is too faint (and distant) for measuring redshift.

sGRBs typically come from large distances. GW170817/GRB 170817A was luckily close. It's not surprising we can't find gravitational waves from all of them. We do dedicated searches to look for signals corresponding to gravitational waves. I expect those results from O3a will be out in a couple of months. Here are the results from O2 Search for gravitational-wave signals associated with gamma-ray....

Abstract;

We present the results of targeted searches for gravitational-wave transients associated with gamma-ray bursts during the second observing run of Advanced LIGO and Advanced Virgo, which took place from 2016 November to 2017 August. We have analyzed 98 gamma-ray bursts using an unmodeled search method that searches for generic transient gravitational waves and 42 with a modeled search method that targets compact-binary mergers as progenitors of short gamma-ray bursts. Both methods clearly detect the previously reported binary merger signal GW170817, with p-values of <9.38×10⁻⁶ (modeled) and 3.1×10⁻⁴ (unmodeled). We do not find any significant evidence for gravitational-wave signals associated with the other gamma-ray bursts analyzed, and therefore we report lower bounds on the distance to each of these, assuming various source types and signal morphologies. Using our final modeled search results, short gamma-ray burst observations, and assuming binary neutron star progenitors, we place bounds on the rate of short gamma-ray bursts as a function of redshift for z≤1. We estimate 0.07-1.80 joint detections with Fermi-GBM per year for the 2019-20 LIGO-Virgo observing run and 0.15-3.90 per year when current gravitational-wave detectors are operating at their design sensitivities.

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Thirdly there is a lot of nonsense on the Internet that black hole mergers should also be multi-messenger events.
Can you state for the record this is not true or is it possible if there is an accretion disk?
My understanding is that accretion disks around stellar mass black holes are rare anyway.

If there is an accretion disc, it is possible there is an electromagnetic counterpart. For stellar mass binaries (instead of supermassive ones found in the centres of galaxies), that would be quite faint, so it's unlikely we'd observe anything at the distances we typically see binary black hole mergers. Small discs could be quite common, if they formed from material ejected during supernova which didn't quite escape. We don't really know yet though. I think Electromagnetic signals following stellar-mass black hole mergers is the most reasonable paper to look at.

Abstract;
It is often assumed that gravitational wave (GW) events resulting from the merger of stellar-mass black holes are unlikely to produce electromagnetic (EM) counterparts. We point out that the progenitor binary has probably shed a mass ≳10M⊙ during its prior evolution. If a tiny fraction of this gas is retained until the merger, the recoil and sudden mass loss of the merged black hole shocks and heats it within hours of the GW event. Whether resulting EM emission is detectable is uncertain. The optical depth through the disk is likely to be high enough that the prompt emission consists only of photons from its optically thin skin, while the majority may take years to emerge. However, if some mechanism can release more photons in a time comparable to the few-hour energy production time, the peak luminosity of the EM signal could be detectable. For a disk retaining only ∼10⁻³ of the mass shed in the earlier binary evolution, medium-energy X-rays to infrared emission would be observable hours after the GW event for source distances ∼500Mpc. Events like this may already have been observed, but ascribed to unidentified active galactic nuclei. Improved sky-localization should eventually allow identification based on spatial coincidence. A detection would provide unique constraints on formation scenarios and potentially offer tests of strong-field general relativity. Accordingly we argue that the high scientific payoff of an EM detection fully justifies search campaigns.

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The current record for multi messenger astronomy is in fact 0 from 6 for the O3 run as black hole mergers even with accretion disks are too faint (assuming they do exist) whereas none of the 6 sGRBs (short gamma ray bursts) observed in this period are likely to have been outside the detection range for GWs.

SelfSim said:

So just attempting to summarise all this:

- overall: there are still many uncertainties which depend on the actual merger scenario and no-one knows that scenario before the reception of a GW signal (measurement and model uncertainties still shape the detection conclusions);
- distances to the actual events limit possible co-incident sGRB detections;
- the interferometer sensitivity of the various detectors at the time of a GW observation run, can still limit GW detections;
- false X-ray to IR emissions may have already been detected, but may have been attributed to AGN emissions (false negative EMs may still be present) .. and thus sky-localisation improvements for GW detections need to continue.

If that's a reasonable summary, then there are already quite a few known reasons for potential detection mismatches, whereas actually expecting a matchup, would demonstrate a denial of these perfectly valid explanations(?)

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The bottom line is whether a GW is associated with EM radiation gets down to the odds on whether the source is close enough for multi-messenger astronomy.
The only thing we can be fairly certain of as Christopher Berry alluded to that all sGRBs are neutron star mergers.
So far during the O3 run there have been 6 sGRB discoveries none of which are bright enough for the redshift to be measured let alone a GW being detected.

The only thing in “favour” for a GW over EM is that the amplitude or brightness of GW falls off as a 1/r law instead of 1/r² law where r is the distance.
The minus and it is a very big minus is GWs interact only very weakly with matter which is why gravitational wave detectors only detect very weak GW signals.

The interesting fact that emerges from the accretion disk black hole merger model is the EM distance range is only 500 million parsecs.
This is nothing compared to monsters such quasars, AGNs or even supernovae where the range is in billions of parsecs.

So it appear astronomers have been quite fortunate for GW170817 (=GRB 170817A) was close enough for multi-messenger astronomy.

Michael said:

Actually, since I'm not insisting/claiming that BBH events are *necessarily* "dark" events, and quite some time has passed since that quote of mine that you just cited, I would say that LIGO is now 0 for 75+ in multimessenger astronomy in 03.

I simply used *your* logic (excluding BBH events) when coming up the other figures mentioned. You're making up your own strawmen again. I didn't "contradict" myself. I simply looked at the problem in several different ways.



I'm not ignoring anything, I'm simply pointing out that 03 has resulted in exactly *no* multimessenger events whatsoever. You're welcome to rationalize away that fact anyway you wish. It's a bit amusing however that your own cited paper on BBH events didn't agree that BBH events were all necessarily "dark" events.



So essentially holding or expressing any "doubt" is intolerable now? Sheesh. With that attitude its no mystery why astronomers got stuck in a Ptolemy rut for 18 centuries after Aristarchus explained that the sun was the center of our solar system to them. "Burn the heretic....."

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Total utter confusion so it's now its 0 from 75+ instead, 0 from 19 or was that 0 from 11.
I'm still waiting on your explanation how distance plays no role................

LIGO and VIRGO make their GW data available where you can do perform your own data analysis.
It's the best way of gaining an understanding of data processing when you can do it yourself.
Fourier transform – Christopher Berry

Here is a video explaining how sGRBs are formed from merging neutron stars which also emit gravitational waves.

On the subject of GRBs is this interesting article where gamma ray pulses appear to go backwards in time.
This effect is possibly related to Cherenkov radiation where charged particles in a medium can travel faster than the speed of light in that medium and emit radiation.
Faster-Than-Light Speeds Could Be Why Gamma-Ray Bursts Seem to Go Backwards in Time

Abstract from Astrophysical Journal.

We introduce a simple model to explain the time-reversed and stretched residuals in gamma-ray burst (GRB) pulse light curves. In this model an impactor wave in an expanding GRB jet accelerates from subluminal to superluminal velocities, or decelerates from superluminal to subluminal velocities. The impactor wave interacts with the surrounding medium to produce Cerenkov and/or other collisional radiation when traveling faster than the speed of light in this medium, and other mechanisms (such as thermalized Compton or synchrotron shock radiation) when traveling slower than the speed of light. These transitions create both a time-forward and a time-reversed set of light-curve features through the process of relativistic image doubling. The model can account for a variety of unexplained yet observed GRB pulse behaviors, including the amount of stretching observed in time-reversed GRB pulse residuals and the relationship between stretching factor and pulse asymmetry. The model is applicable to all GRB classes since similar pulse behaviors are observed in long/intermediate GRBs, short GRBs, and X-ray flares. The free model parameters are the impactor's Lorentz factor when moving subluminally, its Lorentz factor when moving superluminally, and the speed of light in the impacted medium.

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FrumiousBandersnatch said:

I also remember an article about illusory FTL pulses caused by the laser-pointer 'sweeping beam' effect or 'photonic boom' - see Superluminal Spot Pair Events.

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Looks like a practical application of the Lighthouse Paradox.

I asked Christopher Berry from LIGO a few more questions this time about GraceDB which is the Gravitational Wave Candidate Event Database .

I’ve noticed in the GraceDB for the O3 run even though the bulk of detection events are BBH mergers only a small number are retracted when compared to the combined BNS, NSBH and Mass Gap events.
Is is this due to the amplitudes for BBH mergers being larger and therefore have a larger SNR compared to the others or am I missing something?

The true alarm rate is higher for BBHs, as we can detect them from further away, whereas we'd expect roughly equal numbers of retractions for all the different points in our template bank (which is denser at lower masses), which is why the retraction rate looks the way it does.

Expanding on Chris’ answer a template bank is based on the following.


The templates act as noise filters and if the SNR of the filtered data is greater than 4, the false alarm rate is calculated, otherwise the data is rejected.
Without going into details the higher chirp mass of BH binary systems shows significant increases in SNR after filtering when compared to the lower chirp masses for NS or BHNS binaries which is why the false alarm rate for BH mergers is higher and retractions lower.

What constitutes the term “terrestial”?
Does this cover Poisson instrument noise and extraneous noise such as vibrations?

Yes. Anything which isn't a real gravitational wave.

There are some detection events such as S190923y and S190910h which have a high probability of being terrestrial but are still open despite being detected in September.
Is this once again an issue of a lower SNR for NSBH and BNS events assuming this is correct?

Yes. With events on the boundary we may never be certain.
There should be an updated analysis in papers in the next few months.

SelfSim said:

Hmm .. that's terrific feedback .. (thanks for asking the question, sjastro).
Sounds like a fairly standard approach to signal processing ..
Interestingly that Wiki says:I wonder whether (or not) these 'blip' transients may also be thought of as perhaps being possible artefacts showing up when trying to grapple with non-linear processes occurring at the source?

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You have hit the nail on the head about signal processing.
When a noisy electromagnetic signal is Fourier transformed into the frequency domain it can be resolved into frequency components;


Furthermore the false alarm rate (FAR) is also applied to radar.

When a Fourier transform was applied to Hanford (H1) and Livingston (L1) strain data
near GW 150914;


The strong spectral lines in the frequency domain are all instrument related some of which are classified as "blip transients".
They are not processing artefacts but the real thing.
The GW signal is in the plot but relatively weak and less than a second long.
The plot averages over 32 seconds of data and is entirely dominated by instrumental noise.
It knocks on the head the notion that blip transients can be mistaken for GWs.

From my previous post, in the frequency domain there are spectral lines corresponding to instrument noise such as blip transients along with a very faint GW signal.

To extract the GW signal which is in the 20-300 Hz range, the spectral lines need to be removed and is accomplished through whitening and high-pass band filtering.
Whitening removes the lower frequency lines and suppressing lower frequency noise by dividing the data by the noise amplitude spectrum in the Fourier domain.
The higher frequency lines are removed by high-pass filtering based on the Butterworth signal processing filter.

Whitening and high-pass filtering lead to the following.
Included is the NR matched waveform which is the theoretical waveform using Numerical Relativity.


Thanks to Christopher Berry from LIGO who clarified the use of template banks.
The GW signal which is now obvious tells us nothing about the nature of the merger whether it is BH or NS merger, the masses of the binary system or even the sigma level which is the probability the GW signal is nothing more than random noise that looks like a signal.
The signal is matched to the template banks which helps to "optimally" separate signals from instrumental noise, and to infer the parameters of the source (masses, spins, sky location, orbit orientation, etc) from the best match templates.

Examples using GW150914 are;

The black hole masses of GW150904 is circled.


The sigma level is >5.1 which means there is approximately a 1 in 3.5 million chance that GW150914 is noise that resembles a real signal.

The O3 run has been suspended due to the coronavirus.
News | LIGO Suspends Third Observing Run (O3) | LIGO Lab | Caltech

Ironically the O3 run has shown an "exponential increase" in detections.