Hey Gravity Spiers,
We are really excited to finally introduce a new detector, workflow structure, and tool this week. First, we present a new workflow containing glitches from the Virgo detector in Pisa, Italy. Second, we are changing the level structure to speed up our user training. Finally, we are bringing you an auxiliary web tool to help the search for unique and novel glitches.
Virgo differs from Hanford and Livingston in a few ways including the length of the arms, the apparatus holding up the test masses, and the suspensions. We anticipate there will be a number of interesting new glitches in Virgo. For some glitches, such as Scattered Light, they will appear different but have the same cause. For other glitches, such as the Violin Mode Harmonics, they will be the same source but at different frequencies due to the different suspension system. Below we demonstrate a few novel glitches you may find along the way while classifying Virgo glitches, including a new class we have called Fireball (bottom right).
New Workflow Structure
For those of you familiar with Gravity Spy’s training method, we intend to utilize pre-labelled images to help train new users in the classification task. In addition, in order to facilitate training, we introduce a different number of new families of glitches in different levels, culminating in Level 4 where all 22 classes are introduced. However, after some feedback, as well as looking at the data, we learned that getting from level 4 to level 5 was taking longer than anticipated. We decided that this was due to too many new classes being introduced between level 3 and 4. Specifically, the amount of pre-labelled images users were seeing was spread out across too many new classes causing the number of classifications a user must complete before seeing pre-labelled data to sky rocket. Therefore, we are adding another intermediate level that has 15 classes. We believe this will cause users to see pre-labelled images of the new classes faster and, in turn, move through the levels faster. In total, with the addition of Virgo, there are now 7 levels in Gravity Spy (see image above).
This restructuring of the workflows may cause some users to start on levels lower then they may expect. This can be due to a number of factors, and we encourage all users to simply charge ahead with classifying on whichever level they find themselves on. You should experience a fairly rapid promotion through the levels.
We want to thank all of our volunteers for their continued efforts on Gravity Spy and we appreciate all the feedback we have received. We look forward to seeing what novel Virgo glitches you are able to find! As always please reach out to me with all leveling issues. We hope this restructuring proves an effective method to boost training.
Gravity Spy Tools
With the introduction of the new Virgo workflow, we anticipate there being a number of novel glitches, some that will look like what you may have seen in Hanford and Livingston, and some very different. In an effort to help facilitate the generation of large collections of novel glitches, especially when we are not sure what to expect with Virgo, we are introducing a new supplementary tool for Gravity Spy, gravityspytools. For an idea of how to use this tool please watch the linked video. The goal of this tool is to maximize the impact of a new machine learning algorithm that the Gravity Spy team has developed called DIRECT. This algorithm utilizes transfer learning in order to learn what makes gravity spy images similar and dissimilar from each other. This allows every Gravity Spy image to be abstracted into a feature space containing 200 points. It is in this feature space that we calculate distances from one images to another. An interface to do this is provided on gravityspytools called the “Similarity Search.” It takes as input one sample from Gravity Spy and as output the closest samples in the feature space based on distance. An attempt to visualize in three dimensions what the set of known images (such as blip, whistle, etc) looks like in this 200 dimensional feature space is shown above.
We want to thank all of our volunteers for their continued efforts on Gravity Spy and we appreciate all the feedback we have received. Please let us know how you find using the gravityspytools! As always please reach out to me with features you would like to see!
Hello Gravity Spiers!
At long last and after much demand, we have added audio examples of what the Gravity Spy glitches sound like to our field guide! As many of you may know, the frequencies of gravitational waves (and frequencies of glitches) detectable by LIGO are similar to the frequencies of sound (i.e. the pitches) that humans can hear. Therefore, LIGO scientists oftentimes convert our signals to sound!* The MP3s are embedded directly into the field guide, so you should be able to play them straight from there.
Some glitch categories may be hard to distinguish above the background noise, whereas others you should hear quite distinctly. Either way, having a good set of headphones will help hear the subtle features of the glitches better.
Big thanks to our LIGO collaborator Derek Davis for putting together these glitch sounds! Head on over to the Gravity Spy field guide to take a listen to the sounds of glitches!
-Mike / the GSpy team
*There are a few changes done to the data that make the sounds easier to hear. Other than the standard whitening and band-passing, for the glitches in our field guide we also linearly shift the frequency up by 60Hz to make the sounds (especially the low frequency glitches) more in our audible range. Also, a filter that can be described as an “inverse A-weighting” filter is utilized. The basic idea of this filter is to account for the fact that our ears are less sensitive to particularly low and high frequencies. Since the drop off starts around 200 Hz, this affects a decent number of glitches. By increasing the loudness of these lower frequencies, we make it so that features of similar intensity in an omega scan are ideally heard equally loud, no matter their frequency.
Thanks to the hard work by GravitySpy Citizen Scientists, we now have more than 53,000 retired images, images that have had enough consistent classifications by citizens that we are quite sure they are correctly categorized. New images are retired every day, so this data set is always growing. Using this set of retired images, along with information from machine learning image analysis, has allowed us to get a clearer picture of which glitches appear often and rarely in LIGO Hanford (H1) and LIGO Livingston (L1).
Since image retirement relies on classifications by citizens, the images that get retired the fastest and most often are those that are the clearest, the ones that contain only one sort of glitch, and look like the example images. Thus using only retired images may not be a good measure of the total number of glitches. Because of this caveat, we also looked at glitches that the machine learning algorithm identified as belonging to a given category with a confidence of 90% or above.
Looking at the summary information for categorization done by these two methods for LIGO’s two Observing Runs, O1 and O2, we found that there are a handful of glitch categories that are never or almost never found in H1 and L1.
Above is a summary of all the retired glitch categories at Hanford in O1 that had at least one glitch in them. As one can see, this is not every glitch category that we have; 1080 Line, 1400 Ripple, Chirp, Helix, None of the Above, Tomte, Violin Mode Harmonic, and Whistle don’t show up at all! However, None of the Above likely does not show up because it is such a diverse category that it is challenging to have enough consistency to retire such glitches or reach high machine learning confidence. Also, chirps are found infrequently, but when they are, they are very interesting! Paired Doves and Wandering Line show up only twice each out of the 4,448 retired glitches. Here are the pair of Paired Doves:
For Hanford, we have determined that Paired Doves and Helix happen infrequently. Similarly for Livingston, 1080 Line, 1400 Ripple, Air Compressor, Helix, Light Modulation, Paired Doves, Tomte, and Wandering Line are very rarely used.
For the interested reader, below are links to screenshots of the full summary of retired and machine learning categorizations.
ML Confidence 0.9
Again, thank you all for your continued hard work in classifying these glitches. Without you all, this wouldn’t have been possible!
The GravitySpy team
There is some debate on whether Koi fish and blip glitches are part of the same morphology distinguished mainly by loudness or amplitude of the signal. Andy Lundgren has shown that removing calibration lines as part of “data cleaning” makes them look much more similar. A possible explanation of this is that for loud enough glitches, the calibration lines may be causing the Q-transform’s whitening filter to ring at those frequencies, creating the fins on the koi fish. Whitening is a process that removes stationary differences in the loudness of Individual frequencies in the signal.
Fig 1 – Q-transform of typical Koi fish glitch
Fig 2 – Q-transform plot of the same data as above, but with calibration lines removed prior to calculating the Q-transform.
Beverly Berger has suggested that the amplitude of the glitches may be an effective way of discriminating between Koi fish and blips, with Koi fish being louder. To dig into that a little further we plotted the frequency and amplitude of the two glitch types during the 02 run.
Fig 3 – Frequency distribution of blip glitches during 02
Fig 4 – Frequency distribution of Koi fish glitches during 02
The plots of frequency distributions show significant overlap between the two glitch types. Koi fish have a lower peak frequency due to the shape but not enough of a difference to help in classification.
Fig 5 – Amplitude distribution of blip glitches during 02
Fig 6 – Amplitude distribution of Koi fish blitzes during 02
When we look at the amplitude differences between Koi fish and blips we see a pretty sharp dividing line around 10-21, especially during the first 21 weeks. The gap starting at week 23 is a time when there was significant commissioning. There was also a significant increase in range at Livingston around this time.
It is still unclear whether Koi fish are the same as blip glitches, only louder. We also have not been able to identify what exactly causes either glitch. This is just an interesting observation that we thought we would share with our colleagues at Gravity Spy.
Hey Gravity Spy Team!
On August 17, 2017, ripples traveling along the fabric of spacetime passed through a small planet after more than a 100 million year journey, gently stretching and squeezing the pale blue dot by fraction of an atom. Moments later, a split-second burst of high-energy gamma rays finished their journey to our little speck of dust in the Milky Way, with a rainbow of light across the electromagnetic spectrum in its wake. This flurry of information was the long-sought-after holy grail of multi-messenger astronomy.
The ripples in space, or gravitational waves (GWs), came from two objects called neutron stars — the remnant cores of long-dead stars as dense as an atomic nuclei, with masses comparable to our Sun packed into the size of a city. The LIGO/Virgo network of three gravitational-wave interferometers witnessed the last 100 seconds of the final inspiraling dance and collision, after the two objects lived and evolved together for possibly billions of years. This event was subsequently named GW170817. Figure 1 shows the final 30 seconds of the inspiraling dance, growing in frequency and in amplitude as the neutron star orbit shrinks and emitted gravitational waves become stronger.
However, this is only the beginning of the story.
In 2015, LIGO made the first observations of gravitational waves from the inspiral and merger of two black holes, designated GW150914, and since has confidently detected 3 more systems of binary black holes (with the help of Virgo on the most recent discovery — GW170814).
However, the detection of neutron stars using gravitational waves remained elusive until the present. Neutron stars provided the first observational confirmation that gravitational waves exist by observing the orbital evolution of the Hulse-Taylor binary. This binary was discovered in the 1970s, and its decreasing period first hinted at the existence of gravitational waves which Einstein predicted sixty years earlier. (It’s discoverers Hulse and Taylor later won the Nobel Prize!). Now, fifty years (and another Nobel Prize) later, we have finally found direct evidence for gravitational waves from a binary neutron star (BNS) system.
Based on the gravitational-wave signal, we can gain a great deal of information about the BNS, such as the masses of the two neutron stars, how fast they are spinning, and how far away they merge. The best-measured property of the system that can be gleaned from a GW inspiral is a combination of the two masses known as the chirp mass. This quantity is the primary driver of the “chirp” signal that can be seen in Figure 1. However, other parameters of the system which have higher-order contributions to the signal can also be gleaned from the data. The masses of the neutron stars were found to be 1.17–1.6 times the mass of the Sun, consistent with binary neutron star systems we have found in our own Milky Way. But what object was created when they merged? Turns out, we don’t know! It could either be one of the most massive neutron stars ever observed, or the lightest black hole ever observed!
The neutron stars merged about 130 Million light-years away — meaning they collided when the dinosaurs were still roaming the Earth, and have been traveling towards our planet every since. Though this seems far, this is in fact very close for an event to be detected with gravitational waves (about 11 times closed than GW150914). Furthermore, the much lower masses of the neutron stars compared to their black hole counterparts means that they spend far longer in LIGO’s sensitive band, completing about 1500 orbits in band (compared to the ~10 orbits in the case of first GW150914). The combination of its close distance, the long time in band, the increased sensitivity of the detectors, and the addition of Virgo into the interferometer network made GW170817 the loudest GW signal yet!
Possibly most important to the story, GW170817 was localized to a much smaller region of the sky than previous GW events — about 30 sq degrees (great for GW localization, but still fairly large, as 30 sq degrees in the sky could comfortable fit 150 full moons!) As neutron stars are made of matter (unlike black holes), they are expected release large amounts of light across the electromagnetic spectrum when they merge.
Things Get a Little Glitch-y
As this is Gravity Spy, we would be in trouble if we did not mention the big glitch that occurred in the middle of the signal in Livingston! The Electrostatic Drive (ESD) system, which controls the test mass/reaction mass that the mirrors are on, discharges when too much signal is sent to the system. This glitch is called an ESD overflow and has occurred many times! See if you cannot find them all (I know some have) in Gravity Spy! Below is a figure from the discovery paper showing the glitch and how the signal power right through it. Also, here is a link to the ESD Overflow As Found On Gravity Spy. You may wonder why the images look different, and it has everything to do with the Q Transform function used to make the spectrograms you see every day. More on this and the glitch to come later!
Thanks and happy spying!
The Gravity Spy Team
N.B. The first part of this post was written by team member Michael Zevin for AstroBites Please feel free to read the rest of the linked post which describes the Electromagnetic part of the discovery!
Hello Gravity Spy team,
As you may have heard, the LIGO and Virgo detector network has found its first triple-coincident signal! This signal, GW170814, is a binary black hole inspiral, with the masses of the two colliding black holes at about 25 and 30 times the mass of the Sun. Virgo’s observation of this event, and the relative time delay of receiving the signals between the 3 detectors, GREATLY improves our ability to pinpoint the location of the event on the sky, which will help our electromagnetic partners to possibly find a coincident signal with their telescopes. You can find the full paper of this discovery here.
Take a look at the spectrogram from the Virgo detector below? See the signal? If you’re looking at the bright thing just right of center, you’re actually looking at a glitch that occurred right after the signal! Soon, we will be adding data from the Virgo interferometer to the Gravity Spy project, so we can help to better the sensitivity of this instrument, just like what Gravity Spy has been doing for the two LIGO instruments.
Also, we have finally figured out and updated our statistics page with the correct retirement information! Take a gander at our progress here.
-Mike & the Gravity Spy team