Hello everyone!

Pizzicato (originally known as High Frequency Burst 500 Hz) has been added as a new glitch category option in Level 5 and Level 7!

Pizzicato was brought to our attention when it was proposed as a new glitch class by @EcceruElme. She, along with the help of other users, is ultimately the reason why Pizzicato has been added as a new category. 

Pizzicato glitches typically resemble a UFO and show up within the frequency ranges of First Order (~500 Hz) and/or Second Order(~1000 Hz) Violin Modes. Pizzicato first showed up during Engineering Run 14 (ER14), and continued to the end of O3. About 80% of these glitches are found at the Livingston detector, with the other 20% occurring at Hanford. They showed up in both detectors within months of each other; however, since we have seen possible previous incarnations of Pizzicato (see Church), we cannot be certain that whatever is causing these glitches is something completely new to the detectors.

This glitch is so interesting to us primarily because it so clearly seems related to the violin modes of the test mass suspensions, but at the same time we haven’t yet been able to pinpoint an exact cause.

Violin modes occur when the glass fibers that hold up the test masses (the mirrors that we bounce lasers between to measure gravitational waves) in the detectors experience excitations from changes in temperature. These excitations manifest in the fibers at around 500 Hz and, depending on how strong these excitations are, can cause vibrations at higher frequencies, similar to how plucking a violin string can cause reverberations at multiple frequencies. Excitations around 500 Hz are First Order Violin Modes, 1000 Hz are Second Order, and 1500 Hz are Third Order Violin Modes.

Comparing Spectra

Since these glitches are present in the same frequency ranges that Violin Mode Harmonics glitches take place in, the first thing we checked was if Pizzicato glitches were directly correlated to the violin modes being excited. We did this by first finding a time where a Pizzicato glitch was seen, and then picking a nearby time where there were no nearby glitches (Fig. 1 below). We then plotted data from these times as a spectrum (Fig. 2 below), which shows us the Amplitude Spectral Density (ASD) versus frequency. The ASD shows us the average amount of noise in the detectors at any given time.

The blue in Figure 2 above gives us the ASD at the time of the glitch, and orange is a nearby time that didn’t contain any glitches in this frequency range. Both spectra line up perfectly, which tells us that the violin modes were present in this exact capacity during both times, but Pizzicato wasn’t. This tells us that Pizzicato could not have been caused by a change or gain in the amount of violin modes present in the detector. After comparing several Pizzicato glitch times to nearby times, it seemed like the biggest takeaway was that when the Pizzicato glitch was centered around 500 Hz, the spectra during the lock time showed the ASD as being higher around 500 Hz than 1000 Hz, and vice versa for glitches centered at 1000 Hz. 

Comparing to Violin Mode Changes in Amplitude

Below (Fig. 3) is a day where Pizzicato glitches were cropping up in the Second Violin Mode frequency range, between 999 Hz and 1038 Hz. There are multiple lines of glitches (dots) laid over the amplitude of the Second Order Violin Modes relative to the median amplitude. The confirmed Second Order Violin Mode frequencies that Pizzicato glitches occur at are 999 Hz, 1005 Hz, 1018 Hz, 1023 Hz, 1028 Hz, and 1038 Hz. We can see lines of glitches at all these frequencies. This tells us that the glitches in these lines are very likely Pizzicato.

You will also notice in Figure 3 that there seems to be a blue band between 1009 Hz and 1023 Hz and a red band between 1013 Hz to 1018 Hz where the amplitude of the violin modes seems to be heightened. We are not sure if that has anything to do with Pizzicato, but I thought I would note it anyway since we know that 1018 Hz and 1023 Hz are proven to be the peak frequencies of some Pizzicato glitches.

Looking at the spectrum in Figure 4 below that gives us the ASD of the Second Violin Modes for this date, we see that the range where these higher frequency Pizzicato glitches occur also happens to encompass the majority of the range where Second Order Violin Mode Harmonics are seen ringing up. 

Below (Fig. 5) is another day where we were seeing Pizzicato, but this time the glitches are seen between 492 Hz and 521 Hz, which corresponds to the First Order Violin Mode frequency range. We determined that these lower frequency Pizzicato glitches can occur at 492 Hz, 496 Hz, 510 Hz, 514 Hz, and 521 Hz. Again, these frequencies line up with multiple lines of glitches on the glitchgram, although in this example we also see lines of unknown glitches at frequencies Pizzicato is not currently known to occur at.

Like the previous example, we also have a frequency band, between 505 Hz and 515 Hz,  where the First Violin Mode amplitude is changing relative to its median, however, we do not see any similarities between the frequencies for the changes in amplitude here and the frequencies Pizzicato manifests at.

Figure 6 below, similar to Figure 4, shows a spectrum that tells us that the frequencies that these lower frequency Pizzicato glitches occur at spans the entirety of the frequency band for the First Order Violin Modes. Because of this, it’s hard to determine whether Pizzicato is or is not a direct outcome of the violin mode harmonics.

So then what causes Pizzicato? Why do they occur at these particular frequencies? The evidence doesn’t lead us in a particular direction, so we can only speculate for now. 

A theory for what may be causing Pizzicato are the damping mechanisms for the violin modes. Since violin modes generate a lot of noise in the detector, LIGO has multiple ways to dampen the fibers, and it’s very possible that one of the many systems put in place to dampen them is causing a glitch to appear in the main channel. This theory is supported by the fact that Pizzicato glitches around 500 Hz seem to be correlated with lock stretches of higher First Order Violin Modes, and glitches around 1000 Hz appear when Second Order Violin Modes are particularly high.

On the other hand, it is also possible that Pizzicato glitches are indeed directly caused by violin modes. If they are a direct result, it would be very interesting to compare them to the Violin Mode Harmonics glitches (Fig. 7) that we already keep track of in Gravity Spy, especially since they have similarities in frequency but are otherwise extremely different in terms of morphology, duration, variation over time, and in how they appear in the detectors – Violin Mode Harmonics like to string together whereas Pizzicato glitches don’t.

We will be continuing to explore possibilities and other avenues of looking through the data and hope we can update you all soon!

We want to thank everyone who interacts on Talk, proposes categories, and creates collections. It really helps the gravitational wave community! If you haven’t already, check out our recent announcement of the discovery of two separate Neutron Star – Black Hole Mergers! Discoveries like these would not be happening nearly as often without the dedication of citizen scientists like you!

Oli Patane and the Gravity Spy Team