We are excited to bring you new data from our Engineering Run 13! Data taken during Engineering Runs are meant to test not only some of our new upgrades to the detectors but some of our software (like Gravity Spy).
Dates: 10 am Central Time Dec 14 to 8 am Central Time Dec 18 (N.B. Due to some issues at the sites science ready data was not available until Saturday December 15)
What Detectors Are Running
Originally, we anticipated only having Hanford and Virgo due to critical repairs at Livingston. These repairs, however, completed yesterday and after a short delay Livingston has joined ER13. At first, we will be streaming in the data live for Hanford and Livingston over the weekend, and at a later date will add Virgo ER13 data to the Virgo only workflow.
The sensitivity of the LHO detector has increased its range to detect binary neutron stars from 80Mpc to 90Mpc, LLO has increased to 100Mpc and Virgo has nearly doubled its range from 25 to 43Mpc. A number of different glitch classes have arisen and the engineering run is a golden opportunity to identify and eliminate these so we can be rid of them for the year long O3 run which is anticipated to start in March 2019.
Some of the changes at both LHO and LLO that have led to this improvement include squeezed light and a new 70W laser amplifier that will improve LIGO’s quantum noise limit. In addition, Acoustic Mode Dampers will damp internal modes of the test masses to reduce parametric instability (light interacting with mirrors as positive feedback). Also, there was a change of several test masses to improve their coatings (especially for green light) and to remove a point absorber at LHO.
We look forward to your collections of interesting new glitches and for determining the cause of the new excess noise sources!
The Gravity Spy Team.
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!
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!