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Tag Archives: highenergy
We now know why black hole jets make high-energy radiation
Enlarge / The jets of material ejected from around black holes can be enormous.
Active galactic nuclei, powered by the supermassive black holes they contain, are the brightest objects in the Universe. The light originates from jets of material hurled out at nearly the speed of light by the environment around the black hole. In most cases, these active galactic nuclei are called quasars. But, in rare instances where one of the jets is oriented directly toward Earth, they’re called a blazar and appear brighter.
While the general outline of how a blazar operates has been worked out, several details remain poorly understood, including how the fast-moving material generates so much light. Now, researchers have turned a new space-based observatory called the Imaging X-ray Polarimetry Explorer (IXPE) toward one of the brightest blazars in the sky. The data from it and other observations combined indicate that light is produced when the black hole jets slam into slower-moving materials.
Jets and light
The IXPE specializes in detecting the polarization of high-energy photons—the orientation of the wiggles in the light’s electric field. Polarization information can tell us something about the processes that created the photons. For example, photons that originate in a turbulent environment will have an essentially random polarization, while a more structured environment will tend to produce photons with a limited range of polarizations. Light that passes through material or magnetic fields can also have its polarization altered.
This turns out to be useful for studying blazars. The high-energy photons these objects emit are generated by charged particles in the jets. When these objects change course or decelerate, they have to give up energy in the form of photons. Since they’re moving at close to the speed of light, they have a lot of energy to give up, allowing blazars to emit across the entire spectrum from radio waves to gamma rays—some of the latter remaining at those energies despite billions of years of redshifting.
So, the question then becomes what causes these particles to decelerate. There are two leading ideas. One of those is that the environment in the jets is turbulent, with chaotic pile-ups of materials and magnetic fields. This decelerates the particles, and the messy environment would mean that the polarization becomes largely randomized.
The alternative idea involves a shockwave, where material from the jets slams into slower-moving material, and decelerates itself. This is a relatively orderly process, and it produces a polarization that’s relatively limited in range and gets more pronounced at higher energies.
Enter IXPE
The new set of observations is a coordinated campaign to record the blazar Markarian 501 using a variety of telescopes capturing polarization at longer wavelengths, with IXPE handling the highest energy photons. In addition, the researchers searched the archives of several observatories to obtain earlier observations of Markarian 501, allowing them to determine if the polarization is stable over time.
Overall, across the entire spectrum from radio waves to gamma rays, the measured polarizations were within a few degrees of each other. It was also stable over time, and its alignment increased at higher photon energies.
There’s still a bit of variation in the polarization, which suggests there’s some relatively minor disorder at the site of the collision, which isn’t really a surprise. But it’s far less disordered than you’d expect from a turbulent material with complicated magnetic fields.
While these results provide a better understanding of how black holes produce light, that process ultimately relies on the production of jets, which takes place much closer to the black hole. How these jets form is still not really understood, so people studying black hole astrophysics still have a reason to go back to work after the holiday weekend.
Nature, 2022. DOI: 10.1038/s41586-022-05338-0 (About DOIs).
Strange Long-Lasting Pulse of High-Energy Radiation Swept Over Earth
Astronomers think GRB 221009A represents the birth of a new black hole formed within the heart of a collapsing star. In this illustration, the black hole drives powerful jets of particles traveling near the speed of light. The jets pierce through the star, emitting X-rays and gamma rays as they stream into space. Credit: NASA/Swift/Cruz deWilde
NASA’s Swift and Fermi Missions Detect Exceptional Cosmic Blast
An unusually bright and long-lasting pulse of high-energy radiation swept over Earth Sunday, October 9, captivating astronomers around the world. The intense emission came from a gamma-ray burst (GRB) – the most powerful class of explosions in the universe – that ranks among the most luminous events known.
A week ago, on Sunday morning Eastern time, a wave of X-rays and gamma rays passed through the solar system. It triggered detectors aboard
Swift’s X-Ray Telescope captured the afterglow of GRB 221009A about an hour after it was first detected. The bright rings form as a result of X-rays scattered from otherwise unobservable dust layers within our galaxy that lie in the direction of the burst. Credit: Credit: NASA/Swift/A. Beardmore (University of Leicester)
Called GRB 221009A, the explosion provided an unexpectedly exciting start to the 10th Fermi Symposium, a gathering of gamma-ray astronomers now underway in Johannesburg, South Africa. “It’s safe to say this meeting really kicked off with a bang – everyone’s talking about this,” said Judy Racusin, a Fermi deputy project scientist at NASA’s Goddard Space Flight Center in Greenbelt, Maryland, who is attending the conference.
Astronomers think GRB 221009A represents the birth of a new
Originating from the direction of the constellation Sagitta, the signal traveled an estimated 1.9 billion years to reach Earth. Many astronomers believe it represents the birth cry of a new black hole, one that formed in the heart of a massive star collapsing under its own weight. In these circumstances, a developing black hole drives powerful jets of particles traveling near the speed of light. The energetic jets pierce through the star, emitting X-rays and gamma rays as they stream into space.
This sequence constructed from Fermi Large Area Telescope data reveals the sky in gamma rays centered on the location of GRB 221009A. Each frame shows gamma rays with energies greater than 100 million electron volts (MeV), where brighter colors indicate a stronger gamma-ray signal. In total, they represent more than 10 hours of observations. The glow from the midplane of our Milky Way galaxy appears as a wide diagonal band. The image is about 20 degrees across. Credit: NASA/DOE/Fermi LAT Collaboration
The burst also provided a long-awaited inaugural observing opportunity for a link between two experiments on the International Space Station (ISS) – NASA’s NICER X-ray telescope and a Japanese detector called the Monitor of All-sky X-ray Image (MAXI). Activated in April, the connection is dubbed the Orbiting High-energy Monitor Alert Network (OHMAN). It allows NICER to rapidly turn to outbursts detected by MAXI, actions that previously required intervention by scientists on the ground.
“OHMAN provided an automated alert that enabled NICER to follow up within three hours, as soon as the source became visible to the telescope,” said Zaven Arzoumanian, the NICER science lead at Goddard. “Future opportunities could result in response times of a few minutes.”
Images taken in visible light by Swift’s Ultraviolet/Optical Telescope show how the afterglow of GRB 221009A (circled) faded over the course of about 10 hours. The explosion appeared in the constellation Sagitta and occurred 1.9 billion years ago. The image is about 4 arcminutes across. Credit: NASA/Swift/B. Cenko
The light from this ancient explosion brings with it valuable new insights into stellar collapse, the birth of a black hole, the behavior and interaction of matter near the speed of light, the conditions in a distant galaxy – and much more. Astronomers may not detect another GRB this bright for decades.
Fermi’s Large Area Telescope (LAT) detected the burst for more than 10 hours, according to a preliminary analysis. One reason for the burst’s exceptional brightness and longevity is that, for a GRB, it lies relatively close to us.
“This burst is much closer than typical GRBs, which is exciting because it allows us to detect many details that otherwise would be too faint to see,” said Roberta Pillera, a Fermi LAT Collaboration member who led initial communications about the burst and a doctoral student at the Polytechnic University of Bari, Italy. “But it’s also among the most energetic and luminous bursts ever seen regardless of distance, making it doubly exciting.”
Enigmatic High-Energy X-Rays Have Been Spied Coming From Jupiter
Jupiter has finally been observed spitting out X-rays in high-energy wavelengths.
Emanating from the giant planet’s permanent auroras, and detected by NASA’s space-based X-ray telescope NuSTAR, the emissions are the most energetic light seen coming from any planet in the Solar System (aside from Earth).
The detection could shed light on the most powerful auroras in the Solar System, and solves a longstanding mystery: why the joint ESA-NASA Ulysses spacecraft didn’t detect any Jovian X-rays in its nearly three decades of operation between 1990 and 2009.
Jupiter’s auroras constitute an absolutely fascinating phenomenon. At both its poles, the planet is ringed by permanent auroras – invisible to our eyes, but glowing brilliantly in ultraviolet wavelengths. These regions have also been observed emitting low-energy, or ‘soft’ X-rays, by X-ray observatories Chandra and XMM-Newton.
Scientists thought that there should also be high-energy, or ‘hard’ X-rays X-rays, beyond what those instruments can detect. So they used NuSTAR to look for them.
“It’s quite challenging for planets to generate X-rays in the range that NuSTAR detects,” said astrophysicist Kaya Mori of Columbia University.
“But Jupiter has an enormous magnetic field, and it’s spinning very quickly. Those two characteristics mean that the planet’s magnetosphere acts like a giant particle accelerator, and that’s what makes these higher-energy emissions possible.”
Jupiter’s auroras are both like and unlike auroras here on Earth, where they are generated by particles blowing in from the Sun. They collide with Earth’s magnetic field, which sends charged particles like protons and electrons whizzing along the magnetic field lines towards the poles, where they rain down on Earth’s upper atmosphere and collide with atmospheric molecules. The resulting ionization of these molecules generates the stunning dancing lights.
On Jupiter, the basic mechanism is similar, but there are a few differences. The auroras are constant and permanent, as previously noted; that’s because the particles are not solar, but from the Jovian moon Io, the most volcanic world in the Solar System.
It’s constantly belching out sulfur dioxide, which is immediately stripped via a complex gravitational interaction with the planet, becoming ionized and forming a plasma torus around the gas giant. Particles from this torus get sent whizzing along magnetic field lines to the poles, and so on.
The emission detected by NuSTAR. (NASA/JPL-Caltech)
This process generates soft X-rays, as previously discovered. Now, hard X-rays have been found too. It wasn’t an easy detection to make, since the high-energy X-rays are actually quite faint, but that, the researchers said, doesn’t explain why Ulysses couldn’t detect them. The answer, they found, lies in the way the hard X-rays are generated.
When electrons are accelerated along Jupiter’s magnetic field lines, they end up entering the planet’s atmosphere at high speed. When these electrons enter the vicinity of atomic nuclei, and their electric fields, they are abruptly deflected and decelerated. However, their kinetic energy has to go somewhere, according to the law of the conservation of energy, so it gets converted into X-radiation.
This is called bremsstrahlung, or braking radiation. The soft X-rays are generated via a different mechanism called charge exchange, in which electrons are transferred to ions, the excitement of which generates a glow.
These mechanisms each produce a different light profile, the researchers said. At higher energies, bremsstrahlung X-rays should be fainter at higher energies, which would explain why Ulysses never found them.
The team modeled the data including the bremsstrahlung mechanism, and not only did it match the NuSTAR observations, it showed that the emission is outside Ulysses’ sensitivity range. So far so good, but we’ve only just begun to probe the phenomenon.
For example, while NuSTAR could detect hard X-rays in the general region of the Jovian auroras, it was unable to pinpoint a precise emission point.
“The discovery of these emissions does not close the case; it’s opening a new chapter,” said astronomer William Dunn of the University College London in the UK.
“We still have so many questions about these emissions and their sources. We know that rotating magnetic fields can accelerate particles, but we don’t fully understand how they reach such high speeds at Jupiter. What fundamental processes naturally produce such energetic particles?”
Future hard X-ray studies of Jupiter’s auroras could help shed more light on the physics at play.
The research has been published in Nature Astronomy.
Astronomers Locate the Source of High-Energy Cosmic Rays
Roughly a century ago, scientists began to realize that some of the radiation we detect in Earth’s atmosphere is not local in origin. This eventually gave rise to the discovery of cosmic rays, high-energy protons and atomic nuclei that have been stripped of their electrons and accelerated to relativistic speeds (close to the speed of light). However, there are still several mysteries surrounding this strange (and potentially lethal) phenomenon.
This includes questions about their origins and how the main component of cosmic rays (protons) are accelerated to such high velocity. Thanks to new research led by the University of Nagoya, scientists have quantified the amount of cosmic rays produced in a supernova remnant for the first time. This research has helped resolve a 100-year mystery and is a major step towards determining precisely where cosmic rays come from.
While scientists theorize that cosmic rays originate from many sources – our Sun, supernovae, gamma-ray bursts (GRBs), and Active Galactic Nuclei (aka. quasars) – their exact origin has been a mystery since they were first discovered in 1912. Similarly, astronomers have theorized that supernova remnants (the after-effects of supernova explosions) are responsible for accelerating them to nearly the speed of light.
As they travel through our galaxy, cosmic rays play a role in the chemical evolution of the interstellar medium (ISM). As such, understanding their origin is critical to understanding how galaxies evolve. In recent years, improved observations have led some scientists to speculate that supernova remnants give rise to cosmic rays because the protons they accelerate interact with protons in the ISM to create very high-energy (VHE) gamma rays.
However, gamma-rays are also produced by electrons that interact with photons in the ISM, which can be in the form of infrared photons or radiation from the Cosmic Microwave Background (CMB). Therefore, determining which source is greater is paramount to determining the origin of cosmic rays. Hoping to shed light on this, the research team – which included members from Nagoya University, the National Astronomical Observatory of Japan (NAOJ), and the University of Adelaide, Australia – observed the supernova remnant RX J1713.7?3946 (RX J1713).
The key to their research was the novel approach they developed to quantify the source of gamma-rays in interstellar space. Past observations have shown that the intensity of VHE gamma-rays caused by protons colliding with other protons in the ISM is proportional to the interstellar gas density, which is discernible using radio-line imaging. On the other hand, gamma-rays caused by the interaction of electrons with photons in the ISM are also expected to be proportional to the intensity of nonthermal X-rays from electrons.
For the sake of their study, the team relied on data obtained by the High Energy Stereoscopic System (HESS), a VHE gamma-ray observatory located in Namibia (and operated by the Max Planck Institute for Nuclear Physics). They then combined this with X-ray data obtained by the ESA’s X-ray Multi-Mirror Mission (XMM-Newton) observatory and data on the distribution of gas in the interstellar medium.
They then combined all three data sets and determined that protons account for 67 ± 8% of cosmic rays while cosmic-ray electrons account for 33 ± 8% – roughly a 70/30 split. These findings are groundbreaking since they are the first time that the possible origins of cosmic rays have been quantified. They also constitute the most definitive evidence to date that supernova remnants are the source of cosmic rays.
These results also demonstrate that gamma-rays from protons are more common in gas-rich interstellar regions, whereas those caused by electrons are enhanced in the gas-poor regions. This supports what many researchers have predicted, which is that the two mechanisms work together to influence the evolution of the ISM. Said Emeritus Professor Yasuo Fukui, who was the study’s lead author:
“This novel method could not have been accomplished without international collaborations. [It] will be applied to more supernova remnants using the next-generation gamma-ray telescope CTA (Cherenkov Telescope Array) in addition to the existing observatories, which will greatly advance the study of the origin of cosmic rays.”
In addition to leading this project, Fukui has been working to quantify interstellar gas distribution since 2003 using the NANTEN radio telescope at the Las Campanas Observatory in Chile and the Australia Telescope Compact Array. Thanks to Professor Gavin Rowell and Dr. Sabrina Einecke of the University of Adelaide (co-authors on the study) and the H.E.S.S. team, the spatial resolution and sensitivity of gamma-ray observatories has finally reached the point where it is possible to draw comparisons between the two.
Meanwhile, co-author Dr. Hidetoshi Sano of the NAOJ led the analysis of archival datasets from the XMM-Newton observatory. In this respect, this study also shows how international collaborations and data-sharing are enabling all kinds of cutting-edge research. Along with improved instruments, improved methods and greater opportunities for cooperation are leading to an age where astronomical breakthroughs are becoming a regular occurrence!
Further Reading: Nagoya University, The Astrophysical Journal
NASA’s Fermi Spots a Weird Pulse of High-Energy Radiation Racing Toward Earth
When the core of massive star collapses, it can form a black hole. Some of the surrounding matter escapes in the form of powerful jets that rush outward at almost the speed of light in opposite directions, as illustrated here. Normally jets from collapsing stars produce gamma rays for many seconds to minutes. Astronomers think the jets from GRB 200826A were shut down quickly, producing the shortest gamma-ray burst (magenta) from a collapsing star ever seen. Credit: NASA’s Goddard Space Flight Center/Chris Smith (KBRwyle)
On August 26, 2020, NASA’s Fermi Gamma-ray Space Telescope detected a pulse of high-energy radiation that had been racing toward Earth for nearly half the present age of the universe. Lasting only about a second, it turned out to be one for the record books – the shortest gamma-ray burst (GRB) caused by the death of a massive star ever seen.
GRBs are the most powerful events in the universe, detectable across billions of light-years. Astronomers classify them as long or short based on whether the event lasts for more or less than two seconds. They observe long bursts in association with the demise of massive stars, while short bursts have been linked to a different scenario.
Astronomers combined data from NASA’s Fermi Gamma-ray Space Telescope, other space missions, and ground-based observatories to reveal the origin of GRB 200826A, a brief but powerful burst of radiation. It’s the shortest burst known to be powered by a collapsing star – and almost didn’t happen at all. Credit: NASA’s Goddard Space Flight Center
“We already knew some GRBs from massive stars could register as short GRBs, but we thought this was due to instrumental limitations,” said Bin-bin Zhang at Nanjing University in China and the University of Nevada, Las Vegas. “This burst is special because it is definitely a short-duration GRB, but its other properties point to its origin from a collapsing star. Now we know dying stars can produce short bursts, too.”
Named GRB 200826A, after the date it occurred, the burst is the subject of two papers published in Nature Astronomy on Monday, July 26. The first, led by Zhang, explores the gamma-ray data. The second, led by Tomás Ahumada, a doctoral student at the University of Maryland, College Park and NASA’s Goddard Space Flight Center in Greenbelt, Maryland, describes the GRB’s fading multiwavelength afterglow and the emerging light of the supernova explosion that followed.
“We think this event was effectively a fizzle, one that was close to not happening at all,” Ahumada said. “Even so, the burst emitted 14 million times the energy released by the entire Milky Way galaxy over the same amount of time, making it one of the most energetic short-duration GRBs ever seen.”
When a star much more massive than the Sun runs out of fuel, its core suddenly collapses and forms a black hole. As matter swirls toward the black hole, some of it escapes in the form of two powerful jets that rush outward at almost the speed of light in opposite directions. Astronomers only detect a GRB when one of these jets happens to point almost directly toward Earth.
Each jet drills through the star, producing a pulse of gamma rays – the highest-energy form of light – that can last up to minutes. Following the burst, the disrupted star then rapidly expands as a supernova.
Short GRBs, on the other hand, form when pairs of compact objects – such as neutron stars, which also form during stellar collapse – spiral inward over billions of years and collide. Fermi observations recently helped show that, in nearby galaxies, giant flares from isolated, supermagnetized neutron stars also masquerade as short GRBs.
GRB 200826A was a sharp blast of high-energy emission lasting just 0.65 second. After traveling for eons through the expanding universe, the signal had stretched out to about one second long when it was detected by Fermi’s Gamma-ray Burst Monitor. The event also appeared in instruments aboard NASA’s Wind mission, which orbits a point between Earth and the Sun located about 930,000 miles (1.5 million kilometers) away, and Mars Odyssey, which has been orbiting the Red Planet since 2001. ESA’s (the European Space Agency’s) INTEGRAL satellite observed the blast as well.
All of these missions participate in a GRB-locating system called the InterPlanetary Network (IPN), for which the Fermi project provides all U.S. funding. Because the burst reaches each detector at slightly different times, any pair of them can be used to help narrow down where in the sky it occurred. About 17 hours after the GRB, the IPN narrowed its location to a relatively small patch of the sky in the constellation Andromeda.
Using the National Science Foundation-funded Zwicky Transient Facility (ZTF) at Palomar Observatory, the team scanned the sky for changes in visible light that could be linked to the GRB’s fading afterglow.
Discovery image of the fading afterglow (center) of GRB 200826A. Credit: ZTF and T. Ahumada et al., 2021
“Conducting this search is akin to trying to find a needle in a haystack, but the IPN helps shrink the haystack,” said Shreya Anand, a graduate student at Caltech and a co-author on the afterglow paper. “Out of more than 28,000 ZTF alerts the first night, only one met all of our search criteria and also appeared within the sky region defined by the IPN.”
Within a day of the burst, NASA’s Neil Gehrels Swift Observatory discovered fading X-ray emission from this same location. A couple of days later, variable radio emission was detected by the National Radio Astronomy Observatory’s Karl Jansky Very Large Array in New Mexico. The team then began observing the afterglow with a variety of ground-based facilities.
Observing the faint galaxy associated with the burst using the Gran Telescopio Canarias, a 10.4-meter telescope at the Roque de los Muchachos Observatory on La Palma in Spain’s Canary Islands, the team showed that its light takes 6.6 billion years to reach us. That’s 48% of the universe’s current age of 13.8 billion years.
But to prove this short burst came from a collapsing star, the researchers also needed to catch the emerging supernova.
“If the burst was caused by a collapsing star, then once the afterglow fades away it should brighten again because of the underlying supernova explosion,” said Leo Singer, a Goddard astrophysicist and Ahumada’s research advisor. “But at these distances, you need a very big and very sensitive telescope to pick out the pinpoint of light from the supernova from the background glare of its host galaxy.”
To conduct the search, Singer was granted time on the 8.1-meter Gemini North telescope in Hawaii and the use of a sensitive instrument called the Gemini Multi-Object Spectrograph. The astronomers imaged the host galaxy in red and infrared light starting 28 days after the burst, repeating the search 45 and 80 days after the event. They detected a near-infrared source – the supernova – in the first set of observations that could not be seen in later ones.
The researchers suspect that this burst was powered by jets that barely emerged from the star before they shut down, instead of the more typical case where long-lasting jets break out of the star and travel considerable distances from it. If the black hole had fired off weaker jets, or if the star was much larger when it began its collapse, there might not have been a GRB at all.
The discovery helps resolve a long-standing puzzle. While long GRBs must be coupled to supernovae, astronomers detect far greater numbers of supernovae than they do long GRBs. This discrepancy persists even after accounting for the fact that GRB jets must tip nearly into our line of sight for astronomers to detect them at all.
The researchers conclude that collapsing stars producing short GRBs must be marginal cases whose light-speed jets teeter on the brink of success or failure, a conclusion consistent with the notion that most massive stars die without producing jets and GRBs at all. More broadly, this result clearly demonstrates that a burst’s duration alone does not uniquely indicate its origin.
References:
“A peculiarly short-duration gamma-ray burst from massive star core collapse” by B.-B. Zhang, Z.-K. Liu, Z.-K. Peng, Y. Li, H.-J. Lü, J. Yang, Y.-S. Yang, Y.-H. Yang, Y.-Z. Meng, J.-H. Zou, H.-Y. Ye, X.-G. Wang, J.-R. Mao, X.-H. Zhao, J.-M. Bai, A. J. Castro-Tirado, Y.-D. Hu, Z.-G. Dai, E.-W. Liang and B. Zhang, 26 July 2021, Nature Astronomy.
DOI: 10.1038/s41550-021-01395-z
“Discovery and confirmation of the shortest gamma-ray burst from a collapsar” by Tomás Ahumada, Leo P. Singer, Shreya Anand, Michael W. Coughlin, Mansi M. Kasliwal, Geoffrey Ryan, Igor Andreoni, S. Bradley Cenko, Christoffer Fremling, Harsh Kumar, Peter T. H. Pang, Eric Burns, Virginia Cunningham, Simone Dichiara, Tim Dietrich, Dmitry S. Svinkin, Mouza Almualla, Alberto J. Castro-Tirado, Kishalay De, Rachel Dunwoody, Pradip Gatkine, Erica Hammerstein, Shabnam Iyyani, Joseph Mangan, Dan Perley, Sonalika Purkayastha, Eric Bellm, Varun Bhalerao, Bryce Bolin, Mattia Bulla, Christopher Cannella, Poonam Chandra, Dmitry A. Duev, Dmitry Frederiks, Avishay Gal-Yam, Matthew Graham, Anna Y. Q. Ho, Kevin Hurley, Viraj Karambelkar, Erik C. Kool, S. R. Kulkarni, Ashish Mahabal, Frank Masci, Sheila McBreen, Shashi B. Pandey, Simeon Reusch, Anna Ridnaia, Philippe Rosnet, Benjamin Rusholme, Ana Sagués Carracedo, Roger Smith, Maayane Soumagnac, Robert Stein, Eleonora Troja, Anastasia Tsvetkova, Richard Walters and Azamat F. Valeev, 26 July 2021, Nature Astronomy.
DOI: 10.1038/s41550-021-01428-7
The Fermi Gamma-ray Space Telescope is an astrophysics and particle physics partnership managed by NASA’s Goddard Space Flight Center in Greenbelt, Maryland. Fermi was developed in collaboration with the U.S. Department of Energy, with important contributions from academic institutions and partners in France, Germany, Italy, Japan, Sweden, and the United States.
Lone high-energy neutrino likely came from shredded star in distant galaxy
Enlarge / The remains of a shredded star formed an accretion disk around the black hole whose powerful tidal forces ripped it apart. This created a cosmic particle accelerator spewing out fast subatomic particles.
Roughly 700 million years ago, a tiny subatomic particle was born in a galaxy far, far away and began its journey across the vast expanses of our universe. That neutrino finally reached the Earth’s South Pole last October, setting off detectors buried deep beneath the Antarctic ice. A few months earlier, a telescope in California had recorded a bright glow emanating from the friction of that same distant galaxy—evidence of a so-called “tidal disruption event” (TDE), most likely the result of a star being shredded by a supermassive black hole.
According to two new papers (here and here) published in the journal Nature Astronomy, that lone neutrino was likely born from the TDE, which serves as a cosmic-scale particle accelerator near the center of the distant galaxy, spewing out high-energy subatomic particles as the star’s matter is consumed by the black hole. This finding also sheds light on the origin of ultrahigh-energy cosmic rays, a question that has puzzled astronomers for decades.
“The origin of cosmic high-energy neutrinos is unknown, primarily because they are notoriously hard to pin down,” said co-author Sjoert van Velzen, a postdoc at New York University at the time of the discovery. “This result would be only the second time high-energy neutrinos have been traced back to their source.”
Neutrinos travel very near the speed of light. John Updike’s 1959 poem, “Cosmic Gall,” pays tribute to the two most defining features of neutrinos: they have no charge and, for decades, physicists believed they had no mass (they actually have a teeny bit of mass). Neutrinos are the most abundant subatomic particle in the universe, but they very rarely interact with any type of matter. We are constantly being bombarded every second by millions of these tiny particles, yet they pass right through us without our even noticing. That’s why Isaac Asimov dubbed them “ghost particles.”
That low rate of interaction makes neutrinos extremely difficult to detect, but because they are so light, they can escape unimpeded (and thus largely unchanged) by collisions with other particles of matter. This means they can provide valuable clues to astronomers about distant systems, further augmented by what can be learned with telescopes across the electromagnetic spectrum, as well as gravitational waves. Together, these difference sources of information have been dubbed “multi-messenger” astronomy.
The majority of neutrinos that reach the Earth come from our own Sun, but every now and then, neutrino detectors pick up the rare neutrino that hails from further afield. Such is the case with this latest detection: a neutrino that began its journey in a faraway, as yet-unnamed-galaxy in the constellation Delphinus, born from the death throes of a shredded star.
Enlarge / A view of the accretion disc around the supermassive black hole, with jet-like structures flowing away from the disc. The extreme mass of the black hole bends spacetime, allowing the far side of the accretion disc to be seen as an image above and below the black hole.
DESY, Science Communication Lab
As we’ve reported previously, it’s a popular misconception that black holes behave like cosmic vacuum cleaners, ravenously sucking up any matter in their surroundings. In reality, only stuff that passes beyond the event horizon—including light—is swallowed up and can’t escape, although black holes are also messy eaters. That means that part of an object’s matter is actually ejected out in a powerful jet. If that object is a star, the process of being shredded (or “spaghettified”) by the powerful gravitational forces of a black hole occurs outside the event horizon, and part of the star’s original mass is ejected violently outward. This in turn can form a rotating ring of matter (aka an accretion disk) around the black hole that emits powerful X-rays and visible light.
Tidal disruption describes the large forces created when a small body passes very close to a much larger one, like a star that strays too close to a supermassive black hole. “The force of gravity gets stronger and stronger the closer you get to something. That means the black hole’s gravity pulls the star’s near side more strongly than the star’s far side, leading to a stretching effect,” said co-author Robert Stein of DESY in Germany. “As the star gets closer, this stretching becomes more extreme. Eventually it rips the star apart, and then we call it a tidal disruption event. It’s the same process that leads to ocean tides on Earth, but luckily for us, the moon doesn’t pull hard enough to shred the Earth.”
TDEs are likely quite common in our universe, even though only a few have been detected to date. For instance, in 2018, astronomers announced the first direct image of the aftermath of a star being shredded by a black hole 20 million times more massive than our Sun, in a pair of colliding galaxies called Arp 299 about 150 million light years from Earth. And last fall, astronomers recorded the final death throes of a star being shredded by a supermassive black hole, publishing the discovery in Nature Astronomy.
The glow from this most recent TDE was first detected on April 9, 2019 by the Zwicky Transient Facility (ZTF) at California’s Mount Palomar observatory, which has spotted more than 30 such events since it came online 2018. Nearly five months later, on October 1, 2019, the IceCube neutrino observatory at the South Pole recorded the signal from a highly energetic neutrino originating from the same direction as the TDE. Just how energetic was it? Co-author Anna Franckowiak of DESY pegged the energy at over 100 teraelectronvolts (TEV), 10 times the maximum energy for subatomic particles that can be produced by the Large Hadron Collider.
Enlarge / Artistic rendering of the IceCube lab at the South Pole. A distant source emits neutrinos that are then detected below the ice by IceCube sensors.
Ice Cube/NSF
The likelihood of detecting this solitary high-energy neutrino was just 1 in 500. “This is the first neutrino linked to a tidal disruption event, and it brings us valuable evidence,” said Stein. “Tidal disruption events are not well understood. The detection of the neutrino points to the existence of a central, powerful engine near the accretion disc, spewing out fast particles. And the combined analysis of data from radio, optical and ultraviolet telescopes gives us additional evidence that the TDE acts as a gigantic particle accelerator.”
It’s yet one more example of all the new knowledge to be gained by combining multiple data sources to get different perspectives on the same celestial event. “The combined observations demonstrate the power of multi-messenger astronomy,” said co-author Marek Kowalski of DESY and Humboldt University in Berlin. “Without the detection of the tidal disruption event, the neutrino would be just one of many. And without the neutrino, the observation of the tidal disruption event would be just one of many. Only through the combination could we find the accelerator and learn something new about the processes inside.”
As for the future, “We might only be seeing the tip of the iceberg here. In the future, we expect to find many more associations between high-energy neutrinos and their sources,” said Francis Halzen of the University of Wisconsin-Madison, who was not directly involved in the study. “There is a new generation of telescopes being built that will provide greater sensitivity to TDEs and other prospective neutrino sources. Even more essential is the planned extension of the IceCube neutrino detector that would increase the number of cosmic neutrino detections at least tenfold.”
DOI: Nature Astronomy, 2021. 10.1038/s41550-020-01295-8
DOI: Nature Astronomy, 2021. 10.1038/s41550-021-01305-3 (About DOIs).