Tag Archives: #gammarays

Gamma Rays and Neutrinos from Mellow Supermassive Black Holes (Cosmology)

The Universe is filled with energetic particles, such as X rays, gamma rays, and neutrinos. However, most of the high-energy cosmic particles’ origins remain unexplained.

Now, an international research team has proposed a scenario that explains these; black holes with low activity act as major factories of high-energy cosmic particles.

Details of their research were published in the journal Nature Communications.

Gamma rays are high-energy photons that are many orders of magnitude more energetic than visible light. Space satellites have detected cosmic gamma rays with energies of megaelectron to gigaelectron volts.

Neutrinos are subatomic particles whose mass is nearly zero. They rarely interact with ordinary matter. Researchers at the IceCube Neutrino Observatory have also measured high-energy cosmic neutrinos.

Both gamma rays and neutrinos should be created by powerful cosmic-ray accelerators or surrounding environments in the Universe. However, their origins are still unknown. It is widely believed that active supermassive black holes (so-called active galactic nuclei), especially those with powerful jets, are the most promising emitters of high-energy gamma rays and neutrinos. However, recent studies have revealed that they do not explain the observed gamma rays and neutrinos, suggesting that other source classes are necessary.

The new model shows that not only active black holes but also non-active, “mellow” ones are important, acting as gamma-ray and neutrino factories.

A schematic picture of mellow supermassive black holes. Hot plasma is formed around a supermassive black hole. Electrons are heated up to ultrahigh temperature, which emits gamma-rays efficiently. Protons are accelerated to high energies, and they emit neutrinos. ©Shigeo S. Kimura

All galaxies are expected to contain supermassive black holes at their centers. When matter falls into a black hole, a huge amount of gravitational energy is released. This process heats the gas, forming high-temperature plasma. The temperature can reach as high as tens of billions of Celsius degrees for low-accreting black holes because of inefficient cooling, and the plasma can generate gamma rays in the megaelectron volt range.

Such mellow black holes are dim as individual objects, but they are numerous in the Universe. The research team found that the resulting gamma rays from low-accreting supermassive black holes may contribute significantly to the observed gamma rays in the megaelectron volt range.

In the plasma, protons can be accelerated to energies roughly 10,000 times higher than those achieved by the Large Hadron Collider — the largest human-made particle accelerator. The sped-up protons produce high-energy neutrinos through interactions with matter and radiation, which can account for the higher-energy part of the cosmic neutrino data. This picture can be applied to active black holes as demonstrated by previous research. The supermassive black holes including both active and non-active galactic nuclei can explain a large fraction of the observed IceCube neutrinos in a wide energy range.

Future multi-messenger observational programs are crucial to identify the origin of cosmic high-energy particles. The proposed scenario predicts gamma-ray counterparts in the megaelectron volt range to the neutrino sources. Most of the existing gamma-ray detectors are not tuned to detect them; but future gamma-ray experiments, together with next-generation neutrino experiments, will be able to detect the multi-messenger signals.

Publication Details:

Title: Soft gamma rays from low accreting supermassive black holes and connection to energetic neutrinos
Authors: : Shigeo S Kimura, Kohta Murase, Péter Mészáros
Journal: Nature Communications
DOI: 10.1038/s41467-021-25111-7


Provided by Tohoku University

How High-Velocity Winds Launch By Black Holes Produce Gamma-Rays? (Cosmology)

A team of international astronomers searched for the collective gamma-ray (γ-ray) emission from a sample of Ultra-fast outflows (UFOs) using a stacking technique. They found that, high-velocity winds launch by black holes can produce gamma-rays. Their study recently appeared in APS Physics.

Accreting super-massive black holes (SMBHs) at the centers of galaxies, often called active galactic nuclei (AGN), have been observed to launch and power winds. These winds, which are also called ultra-fast outflows (UFOs), are made of highly ionized gas and are likely launched from near the SMBH. Their wide solid angle, and fast velocity allow UFOs to transfer a significant amount of kinetic energy from the AGN to the host galaxy. Propagating through the galaxy, the wind should interact with the interstellar medium creating a strong shock, similar to those observed in supernovae explosions, which is able to accelerate cosmic rays (CRs).

“In order to search for the collective UFO emission, we adoped a stacking technique. Our sample consists of all radio quiet UFO, which gives 11 sources in total.”

By adopting a sensitive stacking analysis they were able to detect the average γ-ray emission from these galaxies. In order to confirm whether the γ-ray emission from these galaxies is truly related to the presence of UFOs or not, they also performed several tests and concluded that yes it is.

Finally, their observations of AGN winds have suggested that AGN transfer a small fraction (∼1–5 %) of their bolometric luminosity to the winds. Their analysis indicated that, a portion of this transferred luminosity in turn accelerates cosmic rays (CRs) and
produces gamma rays.

They found that AGN convert ≈ 3 × 10¯4 of their bolometric luminosity into
gamma (γ) rays. They also found that ≈ 4 × 10¯4 of the wind mechanical power is transferred to γ rays. For comparison, in the Milky Way galaxy, supernova explosions transfer ≈ 2 × 10¯4 of their mechanical energy to γ rays.

“This shows that AGN winds, if sustained for a few million years, can energize a large fraction of the CR population within a galaxy.”


For more: The Fermi-LAT Collaboration, “Gamma rays from Fast Black-Hole Winds”, APS Physics, 2021. Link to paper


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Ultra-high-energy Gamma Rays Originate from Pulsar Nebulae (Astronomy)

‘Significant evidence’ from a new study suggests these emissions are a universal feature found near pulsars

“We took advantage of the wide field-of-view and survey capabilities at the High Altitude Water Cherenkov Observatory (HAWC) to search around a collection of powerful pulsars.”

— Kelly Malone

The discovery that the nebulae surrounding the most powerful pulsars are pumping out ultra-high-energy gamma rays could rewrite the book about the rays’ galactic origins. Pulsars are rapidly rotating, highly magnetized collapsed stars surrounded by nebulae powered by winds generated inside the pulsars.

“We took advantage of the wide field-of-view and survey capabilities at the High Altitude Water Cherenkov Observatory (HAWC) to search around a collection of powerful pulsars. We found significant evidence that ultra-high-energy gamma-ray emission is a universal feature found near these objects,” said Kelly Malone, an astrophysicist in the Nuclear and Particle Physics and Applications group at Los Alamos National Laboratory and lead author of the HAWC Collaboration’s new study of gamma radiation from pulsars.

The HAWC Collaboration comprises more than 100 international researchers. Malone developed the gamma-ray energy estimation algorithm that is used in the paper and led the entire analysis.

Conventional wisdom had attributed ultra-high-energy (more than 56 teraelectronvolts) gamma ray emissions to sources of neutrinos and gamma rays called PeVatrons, which are associated with this acceleration of charged cosmic rays to petaelectronvolt energies. The energies are about five times higher than those produced by accelerators on Earth. This came into question when, in 2020, the HAWC Collaboration released a catalog of nine gamma-ray sources emitting above 56 teraelectronvolts. That’s the highest-energy catalog of astrophysical sources ever produced. The team was surprised to find that all of the detected sources were near the most powerful pulsars ever observed.

In the new paper, the team searched for ultra-high-energy gamma-ray signals near extremely powerful pulsars.

“We find that ultra-high-energy emission appears to be a generic feature for these objects. The gamma rays are likely created by interactions in the pulsar wind. This allows us to create a fuller picture of how the most energetic gamma rays ever detected are created in our galaxy,” Malone said. “Now that we know that pulsar wind nebulae emit at these energies, we can focus on detecting more of them with longer searches and more sensitive experiments.”

The HAWC Collaboration recently constructed an upgrade to the detector that is more sensitive to the highest energy gamma rays. Additionally, the planned Southern Wide-field Gamma-ray Observatory (SWGO) experiment will be extremely sensitive above 50 teraelectronvolts and will be able to perform detailed studies of emissions associated with pulsars.

The HAWC Observatory is composed of an array of water-filled tanks high on the flanks of the Sierra Negra volcano in Puebla, Mexico, where the thin atmosphere offers better conditions for observing gamma rays. When gamma rays strike molecules in the atmosphere they produce showers of energetic particles. When some particles in cosmic ray showers travel strike the water inside the HAWC detector tanks, they produce flashes of light called Cherenkov radiation. By studying these Cherenkov flashes, researchers reconstruct the sources of the particle showers to learn about the particles that caused them.

The paper: “Evidence that Ultra-high-energy Gamma Rays Are a Universal Feature near Powerful Pulsars,” The Astrophysical Journal Letters. A. Albert, R Alfaro, C. Alvarez, et al.

The funding: The Laboratory Directed Research and Development (LDRD) program of Los Alamos National Laboratory; the US Department of Energy Office of High-Energy Physics; the National Science Foundation (NSF), Consejo Nacional de Ciencia y Tecnología (CONACyT), México; and a variety of international funders for members of this international collaboration.

Featured image: After discovering nine ultra-high-energy gamma-ray sources in 2020, researchers at the High Altitude Water Cherenkov Observatory (HAWC) took advantage of the facility’s wide field-of-view and survey capabilities to determine that the regions around powerful pulsars universally emit ultra-high-energy gamma rays. CREDIT: Instituto Nacional de Astrofísica, Óptica y Electrónica


Provided by LANL

Search For Axions From Nearby Star Betelgeuse Comes Up Empty (Planetary Science)

Results significantly narrow the range of possible places to find the hypothetical dark matter particles.

The elusive axion particle is many times lighter than an electron, with properties that barely make an impression on ordinary matter. As such, the ghost-like particle is a leading contender as a component of dark matter — a hypothetical, invisible type of matter that is thought to make up 85 percent of the mass in the universe.

Caption: An MIT-led search for axions from nearby star Betelgeuse (pictured here) came up empty, significantly narrowing the search for hypothetical dark matter particle. Credits: Image: Collage by MIT News. Betelgeuse image courtesy of ALMA (ESO/NAOJ/NRAO)/E. O’Gorman/P. Kervella

Axions have so far evaded detection. Physicists predict that if they do exist, they must be produced within extreme environments, such as the cores of stars at the precipice of a supernova. When these stars spew axions out into the universe, the particles, on encountering any surrounding magnetic fields, should briefly morph into photons and potentially reveal themselves.

Now, MIT physicists have searched for axions in Betelgeuse, a nearby star that is expected to burn out as a supernova soon, at least on astrophysical timescales. Given its imminent demise, Betelgeuse should be a natural factory of axions, constantly churning out the particles as the star burns away.

However, when the team looked for expected signatures of axions, in the form of photons in the X-ray band, their search came up empty. Their results rule out the existence of ultralight axions that can interact with photons over a wide range of energies. The findings set new constraints on the particle’s properties that are three times stronger than any previous laboratory-based axion-detecting experiments.

“What our results say is, if you want to look for these really light particles, which we looked for, they’re not going to talk very much to photons,” says Kerstin Perez, assistant professor of physics at MIT. “We’re basically making everyone’s lives harder because we’re saying, ‘you’re going to have to think of something else that would give you an axion signal.’”

Perez and her colleagues have published their results today in Physical Review Letters. Her MIT co-authors include lead author Mengjiao Xiao, Brandon Roach, and Melaina Nynka, along with Maurizio Giannotti of Barry University, Oscar Straniero of the Abruzzo Astronomical Observatory, Alessandro Mirizzi of the National Institute for Nuclear Physics in Italy, and Brian Grefenstette of Caltech.

A hunt for coupling

Many of the current experiments that search for axions are designed to look for them as a product of the Primakoff effect, a process that describes a theoretical “coupling” between axions and photons. Axions are not normally thought to interact with photons — hence their likelihood of being dark matter. However, the Primakoff effect predicts that, when photons are subjected to intense magnetic fields, such as in stellar cores, they could morph into axions. The center of many stars should therefore be natural axion factories.

When a star explodes in a supernova, it should churn the axions out into the universe. If the invisible particles run into a magnetic field, for instance between the star and Earth, they should turn back into photons, presumably with some detectable energy. Scientists are hunting for axions through this process, for instance from our own sun.

“But the sun also has flares and gives off X-rays all the time, and it’s hard to understand,” says Perez.

She and her colleagues instead looked for axions from Betelgeuse, a star that normally does not emit X-rays. The star is among those nearest to Earth that are expected to explode soon.

“Betelgeuse is at a temperature and lifestage where you don’t expect to see X-rays coming out of it, through standard stellar astrophysics,” Perez explains. “But if axions do exist, and are coming out, we might see an X-ray signature. So that’s why this star is a nice object: If you see X-rays, it’s a smoking gun signal that it’s got to be axions.”

“Data are data”

The researchers looked for X-ray signatures of axions from Betelgeuse, using data taken by NuSTAR, NASA’s space-based telescope that focuses high-energy X-rays from astrophysical sources. The team obtained 50 kiloseconds of data from NuSTAR during the time the telescope was trained on Betelgeuse.

The researchers then modeled a range of X-ray emissions that they might see from Betelgeuse if the star was spewing out axions. They considered a range of masses that an axion might be, as well as a range of likelihoods that the axions would “couple” to and reconvert into a photon, depending on the magnetic field strength between the star and Earth.

“Out of all that modeling, you get a range of what your X-ray signal of axions could possibly look like,” Perez says.

When they searched for these signals in NuSTAR’s data, however, they found nothing above their expected background or outside of any ordinary astrophysical sources of X-rays.

“Betelgeuse is probably in the late stages of evolution and in that case should have a big probability of converting into axions,” Xiao says. “But data are data.”

Given the range of conditions they considered, the team’s null result rules out a large space of possibilities and sets an upper limit that is three times stronger than previous limits, from laboratory-based searches, for what an axion must be. In essence, this means that if axions are ultralight in mass, the team’s results show that the particles must be at least three times less likely to couple to photons and emit any detectable X-rays.

“If axions have ultralight masses, we can definitely tell you their coupling has to be very small, otherwise we would have seen it,” Perez says.

Ultimately, this means that scientists may have to look to other, less detectable energy bands for axion signals. However, Perez says the search for axions from Betelgeuse is not over.

“What would be exciting would be if we see a supernova, which would ignite a huge amount of axions that wouldn’t be in X-rays, but in gamma rays,” Perez says. “If a star explodes and we don’t see axions, then we’ll get really stringent constraints on an axion’s coupling to photons. So everyone’s crossing their fingers for Betelgeuse to go off.”

This research was supported, in part, by NASA.

Reference: Mengjiao Xiao, Kerstin M. Perez, Maurizio Giannotti, Oscar Straniero, Alessandro Mirizzi, Brian W. Grefenstette, Brandon M. Roach, and Melania Nynka, “Constraints on Axionlike Particles from a Hard X-Ray Observation of Betelgeuse”, Phys. Rev. Lett. 126, 031101 – Published 21 January 2021. https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.126.031101 https://doi.org/10.1103/PhysRevLett.126.031101

Provided by MIT

“Game Changer” Perovskite Can Detect Gamma Rays (Material Science)

Scientists at EPFL have developed a game-changing perovskite material that can be used as a cheaper and highly efficient alternative to gamma-ray detectors.

Perovskites are materials made up of organic compounds bound to a metal. Propelled into the forefront of materials’ research because of their structure and properties, perovskites are earmarked for a wide range of applications, including in solar cells, LED lights, lasers, and photodetectors.

Scientists at EPFL have developed a game-changing perovskite material that can be used as a cheaper and highly efficient alternative to gamma-ray detectors. © EPFL

That last application, photo – or light – detection, is of particular interest to scientists at EPFL’s School of Basic Sciences who have developed a perovskite that can detect gamma rays. Led by the labs of Professors László Forró and Andreas Pautz, the researchers have published their work in Advanced Science.

“This photovoltaic perovskite crystal, grown in this kilogram size, is a game changer,” says Forró. “You can slice it into wafers, like silicon, for optoelectronic applications, and, in this paper, we demonstrate its utility in gamma-ray detection.”

Monitoring gamma rays

Gamma-rays are a kind of penetrating electromagnetic radiation that is produced from the radioactive decay of atomic nuclei, e.g., in nuclear or even supernovae explosions. Gamma-rays are on the shortest end of the electromagnetic spectrum, which means that they have the highest frequency and the highest energy. Because of this, they can penetrate almost any material, and are used widely in homeland security, astronomy, industry, nuclear power plants, environmental monitoring, research, and even medicine, for detecting and monitoringtumors and osteoporosis.

But exactly because gamma rays can affect biological tissue, we have to be able to keep an eye on them. To do this, we need simple, reliable, and cheap gamma-ray detectors. The perovskite that the EPFL scientists developed is based on crystals of methylammonium lead tribromide (MAPbBr3) and seems to be an ideal candidate, meeting all these requirements.

Crystal-clear advantages

Perovskites are first “grown” as crystals, and the quality and clarity of the crystals determines the efficiency of the material when it is turned into thin films that can be used in devices like solar panels.

The perovskite crystals that the EPFL scientists made show high clarity with very low impurities. When they tested gamma-rays on the crystals, they found that they generated photo-carriers with a high “mobility-lifetime product”, which is a measurement of the quality of radiation detectors. In short, the perovskite can efficiently detect gamma rays at room temperatures, simply by resistivity measurement.

Cheaper and scalable synthesis

The MAPbBr3 part of the “metal halide” family of perovskites, meaning that, unlike market-leading crystals, its crystals can be grown from abundant and low-cost raw materials. The synthesis takes place in solutions close to room temperature without needing expensive equipment.

Of course, this is not the first perovskite made for gamma ray-detection. But the volume of most lab-grown metal halide perovskites used for this is limited to about 1.2 ml, which is hardly scalable to commercial levels. However, the team at EPFL also developed a unique method called ‘oriented crystal-crystal intergrowth’ that allowed them to make a whole liter of crystals weighing 3.8 kg in total.

“Personally, I enjoyed very much to work at the common frontiers of condensed matter physics, chemistry and reactor physics, and to see that this collaboration could lead to important application to our society,” says Pavao Andričević, the lead-author.

References: Pavao Andričević, Pavel Frajtag, Vincent Pierre Lamirand, Andreas Pautz, Márton Kollár, Bálint Náfrádi, Andrzej Sienkiewicz, Tonko Garma, László Forró, Endre Horváth. Kilogram-scale crystallogenesis of halide perovskites for gamma-rays dose rate measurements. Advanced Science 07 December 2020. DOI: 10.1002/advs.2020018828 https://onlinelibrary.wiley.com/doi/full/10.1002/advs.202001882

Provided by EPFL

Improved Model Shows Gamma Rays And Gold At Merging Neutron Stars (Astromomy)

An international team of astrophysicists under Dutch leadership has demonstrated with an improved model that colliding neutron stars can emit gamma rays. Old models did not predict this and faltered since the merging of two neutron stars in 2017 that released gamma rays. The researchers publish their findings in the The Astrophysical Journal.

A snapshot of the simulation of two merged neutron stars. Gamma radiation is created in the grey strand running through the red ring. In the blue hourglass form, gold may be formed. Credit: Philipp Mösta et al.

The researchers, led by Philipp Mösta (University of Amsterdam), provided their model of colliding neutron stars with more variables than ever before. They considered, among other things, the theory of relativity, gas laws, magnetic fields, nuclear physics and the effects of neutrinos. The researchers ran their simulations on the Blue Waters supercomputer at the University of Illinois in Urbana-Champaign (United States) and on the Frontera supercomputer at the University of Texas, Austin (United States).

In the simulation, a ring is created around the merged neutron stars from which a thin strand of gamma radiation shoots up and down. This radiation then finds its way out like a whirlwind along the magnetic field lines of the merged stars. Furthermore, an hourglass-like cone moves up and down from the ring. This is where heavier elements such as gold possibly form. Gold is, like gamma rays, observed in the merging neutron stars in 2017 where a kilonova was formed.

Philipp Mösta (University of Amsterdam) led the new simulations: “The gamma radiation is really new for these kind of simulations. That radiation had not appeared in the old simulations. The production of heavy elements, such as gold, had already been simulated. However, our simulation shows that these heavy elements move much faster than previously predicted. Our simulation is therefore more in line with what astronomers observed in the merging neutron stars in 2017”.

The simulations are not only meant to explain the observed phenomena around merging neutron stars. They also serve to predict new phenomena. For example, the researchers want to further refine and expand their model so that it can also deal with large stars that explode as supernova at the end of their lives and with a collision of a neutron star with a black hole.

Movie of the simulation of two merged. Credit: Philipp Mösta et al.

References: Philipp Mösta et al. A Magnetar Engine for Short GRBs and Kilonovae, The Astrophysical Journal (2020). DOI: 10.3847/2041-8213/abb6ef

Provided by Netherlands Research School for Astronomy