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
Hi Gravity Spiers,
Today we announced the detection of the third confidently-observed gravitational wave event – GW170104. It was detected on January 4 2017, as the name suggests! It’s the most distant black hole merger detected to date.
We have a surprise waiting for you on the Gravity Spy project, try doing a few classifications and see if you can find the event for yourself…
Dear Gravity Spiers,
Gravity Spy has recently hit a new milestone – over one million classifications! Thank you for all your hard work so far – we are humbled by you continued enthusiasm. All these classifications have helped uncover new glitch classes in the data from LIGO’s first observing run, and have bolstered the labeled datasets that we use to train machine learning algorithms.
Now for some more good news! LIGO started its second observing run at the end of 2016 and is still collecting data and searching for more gravitational-wave events. We are happy to announce that we have added the data from this observing run (O2) and the preceding engineering run (ER10) to the Gravity Spy project. As the detector has evolved a great deal over the past year, we expect many new types of glitches in this dataset. Furthermore, we will now upload new glitches to the project every few days as LIGO records them, so there should be no shortage of data to look through! Your classifications and analysis will be a massive help to LIGO scientists, and help us to uncover even more of the gravitational-wave universe.
You may also notice that we’ve added two new categories to choose from – ‘1080 Line’ and ‘1400 Ripple’. As we find more prominent glitches in O2, we’ll add more glitch options to choose from.
All the best,
-Mike and the GSpy Team
Hi GSpy users,
At LIGO Hanford we recently discovered that some of the telephones in the large experimental areas of LIGO were accidentally left switched on during a a couple of weeks of O2 data collection up to December 13 when they were switched off. At least once these phones rang and the ringtone made loud sounds (glitches) show up in microphones but also in the gravitational-wave channel data. Attached is an O2-preview of what this glitch will look like in GravitySpy data. We am worried that there might be other times in the data set for O1 and O2 when a phone at one of the sites was on. We would be very interested to learn whether any of you have seen glitches similar to this in the GravitySpy data set so far?
For more information, see this LIGO Hanford logbook post, including wave files, https://alog.ligo-wa.caltech.edu/aLOG/index.php?callRep=32503.
LIGO’s second observing run (O2) started this morning – on November 30th 2016 at 8 am, pacific time. Get ready for some new glitches! We’ll have new data from the engineering run (ER10) and the first few days of O2 by the end of the week.
Who knows, maybe there’ll be a gravitational wave in there too!