IceCube analysis indicates there are many high-energy astrophysical neutrino sources

Posted on October 31, 2022

This story was originally published by WIPAC

Back in 2013, the IceCube Neutrino Observatory—a cubic-kilometer neutrino detector embedded in Antarctic ice—announced the first observation of high-energy (above 100 TeV) neutrinos originating from outside our solar system, spawning a new age in astronomy. Four years later, on September 22, 2017, a high-energy neutrino event was detected coincident with a gamma-ray flare from a cosmic particle accelerator, a blazar known as TXS 0506+056. The coincident observation provided the first evidence for an extragalactic source of high-energy neutrinos.

The identification of this source was possible thanks to IceCube’s real-time high-energy neutrino alert program, which notifies the community of directions and energies of individual neutrinos that are most likely to have come from astrophysical sources. These alerts trigger follow-up observations of electromagnetic waves from radio up to gamma-ray, aimed at pinpointing a possible astrophysical source of high-energy neutrinos. However, the sources of the vast majority of the measured diffuse flux of astrophysical neutrinos still remain a mystery, as do how many of those sources exist. Another mystery is whether the neutrino sources are steady or variable over time and, if variable, whether they vary over long or short time scales.

In a paper recently submitted to The Astrophysical Journal, the IceCube Collaboration presents a follow-up search that looked for additional, lower-energy events in the direction of the high-energy alert events. The analysis looked at low- and high-energy events from 2011-2020 and was conducted to search for the coincidence in different time scales from 1,000 seconds up to one decade. Although the researchers did not find an excess of low-energy events across the searched time scales, they were able to constrain the abundance of astrophysical neutrino sources in the universe.

a map of celestial coordinates with ovoid lines shown as a heatmap of locations where neutrino candidate events likely originated — Map of high-energy neutrino candidates (“alert events”) detected by IceCube. The map is in celestial coordinates, with the Galactic plane indicated by a line and the Galactic center by a dot. Two contours are shown for each event, for 50% and 90% confidence in the localization on the sky. The color scale shows the “signalness” of each event, which quantifies the likelihood that each event is an astrophysical neutrino rather than a background event from Earth’s atmosphere. Credit: IceCube Collaboration

This research also delves into the question of whether the astrophysical neutrino flux measured by IceCube is produced by a large number of weak sources or a small number of strong sources. To distinguish between the two possibilities, the researchers developed a statistical method that used two different sets of neutrinos: 1) alert events that have a high probability of being from an astrophysical source and 2) the gamma-ray follow-up (GFU) sample, where only about one to five out of 1,000 events per day are astrophysical.

“If there are a lot of GFU events in the direction of the alerts, that’s a sign that neutrino sources are producing a lot of detectable neutrinos, which would mean there are only a few, bright sources,” explained recent UW–Madison PhD student Alex Pizzuto, a lead on the analysis who is now a software engineer at Google. “If you don’t see a lot of GFU events in the direction of alerts, this is an indication of the opposite, that there are many, dim sources that are responsible for the flux of neutrinos that IceCube detects.”

a graph with power of each individual source on the y-axis and number density of astrophysical neutrino sources on the x-axis. there is a clear indirect relationship, with the lines starting in the upper left and moving toward the lower right of the graph. three "lines" are shown: an upper blue band that says "diffuse," a middle black lines that says "upper limit; this analysis" and a blue-green band that has +/-1 sigma sensitivity — Constraints on the luminosity (power) of each individual source as a function of the number density of astrophysical neutrino sources (horizontal axis). Previous IceCube measurements of the total astrophysical neutrino flux indicate that the true combination of the two quantities must lie within the diagonal band marked “diffuse.” The results of the new analysis are shown as an upper limit, compared to the sensitivity, which shows the range of results expected from background alone (no additional signal neutrinos associated with the directions of alert events). The upper limit is above the sensitivity because there is a statistical excess in the result (p = 0.018). Credit: IceCube Collaboration

They interpreted the results using a simulation tool called FIRESONG, which looks at populations of neutrino sources and calculates the flux from each of these sources. The simulation was then used to determine if the simulated sources might be responsible for producing a neutrino event.

“We did not find a clear excess of low-energy events associated with the high-energy alert events on any of the three time scales we analyzed,” said Justin Vandenbroucke, a physics professor at UW–Madison and colead of the analysis. “This implies that there are many astrophysical neutrino sources because, if there were few, we would detect additional events accompanying the high-energy alerts.”

Future analyses will take advantage of larger IceCube data sets and higher quality data from improved calibration methods. With the completion of the larger next-generation telescope, IceCube-Gen2, researchers will be able to detect even more dim neutrino sources. Even knowing the abundance of sources could provide important constraints on the identity of the sources.

“The future is very exciting as this analysis shows that planned improvements might reveal more astrophysical sources and populations,” said Abhishek Desai, postdoctoral fellow at UW–Madison and co-lead of the analysis. “This will be due to better event localization, which is already being studied and should be optimized in the near future.”

+ info “Constraints on populations of neutrino sources from searches in the directions of IceCube neutrino alerts,” The IceCube Collaboration: R. Abbasi et al. Submitted to The Astrophysical Journal. arxiv.org/abs/2210.04930.

Alex Levchenko, Mark Rzchowski elected Fellows of the American Physical Society

Posted on October 19, 2022

Congratulations to Profs. Alex Levchenko and Mark Rzchowski, who were elected 2022 Fellows of the American Physical Society!

Levchenko was elected for “broad contributions to the theory of quantum transport in mesoscopic, topological, and superconducting systems.” He was nominated by the Division of Condensed Matter Physics.

Rzchowski was elected for “pioneering discoveries and understanding of physical principles governing correlated complex materials and interfaces, including superconductors, correlated oxide systems multiferroic systems, and spin currents in noncollinear antiferromagnets.” He was nominated by the Division of Materials Physics.

APS Fellowship is a distinct honor signifying recognition by one’s professional peers for outstanding contributions to physics. Each year, no more than one half of one percent of the Society’s membership is recognized by this honor.

See the full list of 2022 honorees at the APS Fellows archive.

Decades of work at UW–Madison underpin discovery of corona protecting Milky Way’s neighboring galaxies

Posted on October 6, 2022

a domed observatory with the night sky as a backdrop. the long exposure makes the stars look like they're rotating, with long blurry tails

This story was originally posted by UW–Madison News

Two dwarf galaxies circling our Milky Way, the Large and Small Magellanic Clouds, are losing a trail of gaseous debris called the Magellanic Stream. New research shows that a shield of warm gas is protecting the Magellanic Clouds from losing even more debris — a conclusion that caps decades of investigation, theorizing and meticulous data-hunting by astronomers working and training at the University of Wisconsin–Madison.

The findings, published recently in the journal Nature, come courtesy of quasars at the center of 28 distant galaxies. These extremely bright parts of galaxies shine through the gas that forms a buffer, or corona, that protects the Magellanic Clouds from the pull of the Milky Way’s gravity.

“We use a quasar as a light bulb,” says Bart Wakker, senior scientist in UW–Madison’s Astronomy Department. “If there is gas at a certain place between us and the quasar, the gas will produce an absorption line that tells us the composition of the clouds, their velocity and the amount of material in the clouds. And, by looking at different ions, we can study the temperature and density of the clouds.”

The temperature, location and composition — silicon, carbon and oxygen — of the gases that shadow the passing light of the quasars are consistent with the gaseous corona theorized in another study published in 2020 by UW–Madison physics graduate student Scott Lucchini, Astronomy professors Elena D’Onghia and Ellen Zweibel and UW–Madison alumni Andrew Fox and Chad Bustard, among others.

That work explained the expected properties of the Magellanic Stream by including the effects of dark matter: “The existing models of the formation of the Magellanic Stream are outdated because they can’t account for its mass,” Lucchini said in 2020.

“Our first Nature paper showed the theoretical developments, predicting the size, location and movement of the corona’s gases,” says Fox, now an astronomer at the Space Telescope Science Institute and, with Lucchini, a co-author of both studies.

The new discovery is a collaboration with a team that includes its own stream of former UW–Madison researchers pulled out into the world through the 1990s and 2000s — former graduate students Dhanesh Krishnarao, who is leading the work and is now a professor at Colorado College, David French, now scientist at the Space Telescope Science Institute, and Christopher Howk, now a professor at the University of Notre Dame — and former UW–Madison postdoctoral researcher Nicolas Lehner, also a Notre Dame professor.

UW–Madison research leading to the new discovery dates back at least to an inkling of hot gases seen in a study of stars in the Magellanic Cloud published in 1980 by the late astronomy professor Blair Savage and his then-postdoc Klaas de Boer.

“All that fell into place to allow us to look for data from the Hubble Space Telescope and a satellite called the Far Ultraviolet Spectroscopic Explorer, FUSE — which UW also played an important role in developing,” Wakker says. “We could reinterpret that old data, collected for many different reasons, in a new way to find what we needed to confirm the existence of a warm corona around the Magellanic Clouds.”

“We solved the big questions. There are always details to work out, and people to convince,” D’Onghia says. “But this is a real Wisconsin achievement. There aren’t many times where you can work together to predict something new and then also have the ability to spot it, to collect the compelling evidence that it exists.”

Read more about the research on NASA’s website.