Ghost (Particle) Hunting in Gamma-ray Bursts

Illustration of a jet of particles emitted when a massive star explodes.
These jets could be one cause of gamma-ray bursts.
Credit: NASA/Swift/Cruz deWilde

Title: Search for 10–1,000 GeV Neutrinos from Gamma-ray Bursts with IceCube
Authors: IceCube Collaboration
Status: Published in ApJ

Particle Accelerators, but in Space!

Gamma-ray bursts are some of the most powerful explosions in the universe, releasing a “fireball” of particle-filled plasma in a powerful jet that accelerates these particles through the universe. Gamma-ray bursts are understood to originate either from compact object (neutron star or stellar-mass black hole) mergers (these are called short gamma-ray bursts and last less than two seconds!) or from core-collapse supernovae (these are called long gamma-ray bursts but are just defined as anything longer than two seconds). Gamma-ray bursts are the most powerful particle accelerators in the universe and are really useful for looking for new particles and new particle interactions!

Today’s authors look at gamma-ray bursts as a possible source of ghost particles, i.e., neutrinos. Neutrinos rarely interact with other matter, which makes them really hard to detect — like a ghost! The IceCube Neutrino Observatory sees neutrinos all over the sky but can’t pinpoint where they’re coming from. Since gamma-ray bursts could produce neutrinos in their outbursts, the authors search through all of IceCube’s data to see if there are any bursts of high-energy neutrinos that came in at the same time as a gamma-ray bursts.

Figure 1: A: histogram of the initial gamma-ray burst emission (called prompt emission) duration for all 2,268 gamma-ray bursts used in this study. The time windows investigated in this article are shown as red arrows. [Adapted from IceCube Collaboration et al. 2024

How Many Gamma-ray Bursts Does It Take to Find a Neutrino?

Today’s authors search for coincident neutrinos in the time periods surrounding the 2,298 bursts that happened during the lifetime of IceCube-DeepCore (IceCube’s highest-energy neutrino detector). They do this by looking at each time window individually and by combining many time windows to add faint signals that might not be seen in individual windows, but together might show an association between neutrinos and gamma-ray bursts.

In the first search, the authors define search windows before and after each burst to look for neutrinos (see Figure 1). Since neutrinos don’t interact with matter very often, they can easily stream out of dusty environments from which photons struggle to escape, meaning that the neutrinos could actually be expected to arrive at Earth before gamma-ray (and other photon) emission. The authors search the entire sky for neutrinos in these windows and assess the probability that there is an excess of neutrinos coming from the source location compared to the neutrino background that we see all over the sky.

The second search looks at groups of gamma-ray bursts that are associated in location and time with neutrino events. The authors look at the combined probability of burst/neutrino association of all the events in this group. This makes it possible to correlate gamma-ray bursts with neutrinos even if the events don’t individually stand out. Using this method, the authors didn’t find any groups of neutrinos that are any more statistically significant than individual neutrinos that fall within gamma-ray burst time windows.

Trials Factors and Tribulations

The winning burst of the first search (i.e., the most significant neutrino–gamma-ray burst correlation) is GRB bn 140807500. (Since there are a lot of gamma-ray bursts recorded by burst-hunting instruments like the Fermi Gamma-ray Burst Monitor (Fermi-GBM) and the Swift Burst Alert Telescope (Swift-BAT), it’s too much of a hassle to give the bursts individual names. Instead, the bursts get “telephone numbers” corresponding to the date they were detected.) The corresponding neutrino falls within 100 seconds of GRB bn 140807500 and has a p-value of 4.6 x 10^-5, which is the probability that the correlation between the burst and the neutrino is just a lucky coincidence and not from actual correlation (i.e., small p-values mean a more likely detection of neutrinos from gamma-ray bursts!).

This probability seems really small, and at first glance it seems like the neutrino and the gamma-ray burst are most likely connected here! Unfortunately, this doesn’t take into account trials factors (also called the look-elsewhere effect), which quantify the statistical statement that if you look at enough gamma-ray bursts and neutrinos, there will be some events that line up with each other in space and time, just by chance. To account for this, the authors must correct for 2,268 trials, one for each burst. After correcting for trials, this leaves us with a much larger p-value of 0.097, meaning there’s a one in ten chance that the gamma-ray burst and the neutrino aren’t really connected. Generally particle (and astroparticle) physicists require a p-value of 3×10^-7 (about 1 in 3.5 million, corresponding to the [in]famous 5-sigma threshold!) to feel confident in saying that these events are actually correlated.

Figure 2: Estimated neutrino flux (number of neutrinos detected at a given energy per area) for 2,264 gamma-ray bursts combined (blue) compared with the BOAT gamma-ray burst (orange). (Four of the bursts used in this study were excluded from this analysis.) Credit: IceCube Collaboration et al. 2024

Don’t Forget About the BOAT

At the same time as this article was being prepared, the brightest of all time (BOAT) gamma-ray burst was detected. The authors didn’t include the BOAT directly in their dataset, but they made some predictions as to how it would measure up to the other gamma-ray bursts that were considered. The BOAT was so bright that the authors calculated the expected neutrino signal to be 6–8 times the combined expected signal of all 2,264 gamma-ray bursts used in this part of the study (see Figure 2)! This is because the BOAT is so much more energetic in gamma rays than other gamma-ray bursts, implying a large flux of high-energy neutrinos, which are localized to more precise regions in the sky than the lower-energy neutrinos expected to accompany other bursts. This means that we can more confidently associate any observed neutrinos with the location of the burst.

There’s still work to be done to see if there are any neutrino events that seem to come from the BOAT gamma-ray burst, but this leaves the idea open that neutrinos could come from gamma-ray bursts (or at least, that we can more confidently say that they don’t)! The universe sometimes throws surprises like the BOAT at us, allowing astronomers to study the high-energy universe a lot more easily. Luckily, we’ve entered into the era of time-domain astronomy where instruments like Fermi-GBM and Swift-BAT allow us to catch more bursts and explosions than ever before, giving us an increasingly large sample of gamma-ray bursts to study!

Original astrobite edited by Cole Meldorf.

Source: American Astronomical Society/AAS-Nova

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Editor’s Note: Astrobites is a graduate-student-run organization that digests astrophysical literature for undergraduate students. As part of the partnership between the AAS and astrobites, we occasionally repost astrobites content here at AAS Nova. We hope you enjoy this post from astrobites; the original can be viewed at astrobites.org.

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About the author, Samantha Wong:

I’m a graduate student at McGill University, where I study high-energy astrophysics. This includes studying all sorts of extreme environments in the universe like active galactic nuclei, pulsars, and supernova remnants with the VERITAS gamma-ray telescope.

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Solar activity likely to peak next year, new study suggests

Image from the Solar Dynamics Observatory mission of the solar disk with multiple sunspots, which appear dark compared with their surroundings.Credit: HMI/SDO/NASA

Licence type: Attribution-ShareAlike (CC BY-SA 4.0)

Researchers at the Center of Excellence in Space Sciences India at IISER Kolkata have discovered a new relationship between the Sun’s magnetic field and its sunspot cycle, that can help predict when the peak in solar activity will occur. Their work indicates that the maximum intensity of solar cycle 25, the ongoing sunspot cycle, is imminent and likely to occur within a year. The new research appears in Monthly Notices of the Royal Astronomical Society: Letters.

Our star, the Sun, is made up of hot ionized gas known as plasma. Huge plasma flows and convection conspire together to form magnetic fields inside the Sun which manifest on the surface as dark spots. These sunspots are comparable to the size of the Earth and are seats of intense magnetism, about 10,000 times stronger than the Earth’s magnetic field.

Sometimes the sunspot magnetic fields are disrupted in violent events which result in the birth of solar magnetic storms such as flares or coronal mass ejections. These storms release high energy radiation and hurl vast amounts of magnetized plasma in to outer space. The most intense of these storms can cause serious damage to orbiting satellites, electric power grids and telecommunications when Earth directed.

Centuries of observations starting from the early 1600s show that the number of sunspots observed on the Sun varies periodically. Approximately every 11 years the number of spots and the intensity of solar activity reach a peak when the most violent perturbations in planetary space environments – or space weather – are expected. However, predicting when this peak is going to occur has remained challenging.

The solar cycle is produced by a dynamo mechanism driven by energy from plasma flows inside the Sun. This dynamo mechanism is understood to involve two primary components of the Sun’s magnetic field, one which manifests in the cycle of sunspots and another which manifests in a recycling of the large-scale dipole field of the Sun; the latter is much like the Earth’s magnetic field – stretching from one pole of the Sun to another. With the cycle of sunspots, the Sun’s dipole field is also observed to wax and wane in strength, the north and south magnetic poles swap places, also every 11 years.

In 1935, Swiss astronomer Max Waldmeier discovered that the faster the rate of rise of a sunspot cycle the stronger its strength, so stronger cycles take less time to rise to their peak intensity. This relationship has often been utilised to forecast the strength of a sunspot cycle based on observations of its early rising phase.

In a research manuscript appearing in the Monthly Notices of the Royal Astronomical Society Letters, Priyansh Jaswal, Chitradeep Saha and Dibyendu Nandy of IISER Kolkata report the discovery of a new relationship, namely, the rate of decrease in the Sun’s dipole magnetic field is also related to the rate of rise of the ongoing sunspot cycle.

This discovery, utilising decades-old data archives from multiple ground-based solar observatories around the world, complements the Waldmeier effect, connecting the two primary magnetic field components of the Sun and supporting the theory that the evolution of sunspots are integral to the functioning of the solar dynamo process rather than being a mere symptom of it.

The scientists demonstrate how observations of the rate of decrease of the Sun’s dipole magnetic field can be usefully combined with sunspot observations to predict when the ongoing cycle would peak.  Their analysis suggests that the maximum of solar cycle 25 is most likely to occur in early 2024 with an uncertainty in the estimate that ranges to September 2024.

With this discovery, a new window opens up for forecasting the timing of the peak of solar cycles - when the most intense activity and most frequent space weather disturbances are expected.

Source: Royal Astronomical Society (RAS)/News

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Science contacts:

Dibyendu Nandi
Professor of Physics and Head, Center of Excellence in Space Sciences India
Indian Institute of Science Education and Research Kolkata
dnandi@iiserkol.ac.in

Priyansh Jaswal
PhD Student, Center of Excellence in Space Sciences India
Indian Institute of Science Education and Research Kolkata
pj22rs046@iiserkol.ac.in
Phone: +91 8219647798

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Media contact:

Robert Massey
Royal Astronomical Society
Mob: +44 (0)7802 877699
press@ras.ac.uk

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Further information

Discovery of a relation between the decay rate of the Sun’s magnetic dipole and the growth rate of the following sunspot cycle: a new precursor for solar cycle prediction”, 2023, Jaswal, P., Saha, C., and Nandy, D., Monthly Notices of the Royal Astronomical Society: Letters

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Notes for editors:

The Royal Astronomical Society (RAS), founded in 1820, encourages and promotes the study of astronomy, solar-system science, geophysics and closely related branches of science. The RAS organises scientific meetings, publishes international research and review journals, recognises outstanding achievements by the award of medals and prizes, maintains an extensive library, supports education through grants and outreach activities and represents UK astronomy nationally and internationally. Its more than 4,000 members (Fellows), a third based overseas, include scientific researchers in universities, observatories and laboratories as well as historians of astronomy and others.

The RAS accepts papers for its journals based on the principle of peer review, in which fellow experts on the editorial boards accept the paper as worth considering. The Society issues press releases based on a similar principle, but the organisations and scientists concerned have overall responsibility for their content.

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The X-Rays That Shouldn't Be There

A plasma jet created in the lab.
Image credit: Paul Bellan et al., Caltech

For around 20 years, Caltech Professor of Applied Physics Paul Bellan and his group have been creating magnetically accelerated jets of plasma, an electrically conducting gas composed of ions and electrons, in a vacuum chamber big enough to hold a person. (Neon signs and lightning are everyday examples of plasma).

In that vacuum chamber, wisps of gas are ionized by several thousand volts. One hundred-thousand amps then flow through the plasma, producing strong magnetic fields that mold the plasma into a jet traveling around 10 miles per second. High-speed recordings show that the jet transitions through several distinct stages in a few tens of microseconds.

Bellan says the plasma jet looks like an umbrella growing in length. Once the length reaches one or two feet, the jet undergoes an instability that causes it to transform into a rapidly expanding corkscrew. This rapid expansion triggers a different, faster instability that creates ripples.

"The ripples choke the jet's 100-kiloamp electric current, much like putting your thumb over a water hose restricts the flow and creates a pressure gradient that accelerates water," Bellan says. "Choking the jet current creates an electric field strong enough to accelerate electrons to high energy."

Those high-energy electrons were previously identified in the jet experiment by the X-rays they generate, and Bellan says their presence was a surprise. That's because conventional understanding says the jet plasma was too cold for electrons to be accelerated to high energy. Note that "cold" is a relative term: Although this plasma had a temperature of about 20,000 Kelvin (35,500 degrees Fahrenheit)—far hotter than anything humans normally encounter—it is nowhere near the temperature of the Sun's corona, which is over a million Kelvin (1.8 million degrees F.)

"So, the question is, 'Why are we seeing X-rays?'" he says.

Cold plasmas were thought to be incapable of generating high-energy electrons because they are too "collisional," meaning an electron cannot travel very far before colliding with another particle. It is like a driver trying to drag race through freeway gridlock. The driver might hit the accelerator but would travel only a few feet before smashing into another car. In the case of a cold plasma, an electron would accelerate only about one micron before colliding and slowing down.

The Bellan group's first attempt at explaining this phenomenon was a model suggesting that some fraction of the electrons manages to avoid colliding with other particles during the first micron of travel. According to the theory, that allowed the electrons to accelerate to slightly higher velocity, and once going faster, they could travel just a little bit farther before encountering another particle with which they might collide. Some fraction of those now-faster electrons would again avoid a collision for a time, allowing them to attain an even higher speed, which would allow them to travel even farther, creating a positive feedback loop that would allow a few lucky electrons to go farther and faster, attaining high speeds and high energies.

But while compelling, the theory was wrong, Bellan says.

"It was realized that this argument has a flaw," he says, "because electrons don't really collide in the sense of hitting something or not hitting something. They are all actually deflecting a little bit all the time. So, there's no such thing as an electron that's colliding or not colliding."

Yet, high-energy electrons do appear in the cold plasma of the jet experiment. To find out why, Bellan developed a computer code that calculated the actions of 5,000 electrons and 5,000 ions continuously deflecting off each other in an electric field. To suss out how a few electrons were managing to reach high energies, he tweaked the parameters and watched how the electrons' behavior changed.

As electrons accelerate in the electric field, they pass near ions but never actually touch them. Occasionally, an electron whizzes so closely past an ion that it transfers energy to an electron attached to the ion and slows down, with the now "excited" ion radiating visible light. Because electrons only occasionally pass so closely, they usually just deflect slightly from the ion without exciting it. This occasional energy leakage occurs in most electrons, which means they never attain high energies.

When Bellan tweaked his simulation, a few high-energy electrons capable of creating X-rays appeared. "The lucky few that never come close enough to an ion to excite it never lose energy," he adds. "These electrons are continuously accelerated in the electric field and ultimately attain sufficient energy to produce the X-rays."

Bellan says that if this behavior occurs in the plasma jet in his Caltech lab, it probably happens in solar flares and astrophysical situations as well. This may also explain why unexpectedly high-energy X-rays are sometimes seen during fusion-energy experiments.

"There's a long history of people seeing things that they thought were useful fusion," he says. "It turns out it was fusion, but it wasn't really useful. It was intense transient electric fields produced by instabilities accelerating a few particles to extremely high energy. This might be explaining what was going on. That's not what people want, but it is probably what happens."

The paper describing the work, "Energetic electron tail production from binary encounters of discrete electrons and ions in a sub-Dreicer electric field," appeared in the October 20 issue of Physics of Plasmas and was presented on November 3 at the 65th Annual Meeting of the American Physical Society Division of Plasma Physics in Denver, Colorado.

Funding for the research was provided by the National Science Foundation and the Air Force Office of Scientific Research.

Written by
Emily Velasco

Source: Caltech/News

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