Posted on by Tammy Plotner

Cygnus X – A Cosmic-ray Cocoon

Cygnus X hosts many young stellar groupings, including the OB2 and OB9 associations and the cluster NGC 6910. The combined outflows and ultraviolet radiation from the region's numerous massive stars have heated and pushed gas away from the clusters, producing cavities of hot, lower-density gas. In this 8-micron infrared image, ridges of denser gas mark the boundaries of the cavities. Bright spots within these ridges show where stars are forming today. Credit: NASA/IPAC/MSX

[/caption]

Situated about 4,500 light-years away in the constellation of Cygnus is a veritable star factory called Cygnus X… one estimated to have enough “raw materials” to create as many as two million suns. Caught in the womb are stellar clusters and OB associations. Of particular interest is one labeled Cygnus OB2 which is home to 65 of the hottest, largest and meanest O-type stars known – and close to 500 B members. The O boys blast out holes in the dust clouds in intense outflows, disrupting cosmic rays. Now, a study using data from NASA’s Fermi Gamma-ray Space Telescope is showing us this disturbance can be traced back to its source.

Discovered some 60 years ago in radio frequencies, the Cygnus X region has long been of interest, but dust-veiled at optical wavelengths. By employing NASA’s Fermi Gamma-ray Space Telescope, scientists are now able to peer behind the obscuration and take a look at the heart through gamma ray observations. In regions of star formation like Cygnus X, subatomic particles are produced and these cosmic rays shoot across our galaxy at light speed. When they collide with interstellar gas, they scatter – making it impossible to trace them to their point of origin. However, this same collision produces a gamma ray source… one that can be detected and pinpointed.

“The galaxy’s best candidate sites for cosmic-ray acceleration are the rapidly expanding shells of ionized gas and magnetic field associated with supernova explosions.” says the FERMI team. “For stars, mass is destiny, and the most massive ones — known as types O and B — live fast and die young.”

Because these star types aren’t very common, regions like Cygnus X become important star laboratories. Its intense outflows and huge amount of mass fills the prescription for study. Within its hollowed-out walls, stars reside in layers of thin, hot gas enveloped in ribbons of cool, dense gas. It is this specific area in which Fermi’s LAT instrumentation excels – detecting an incredible amount of gamma rays.

“We are seeing young cosmic rays, with energies comparable to those produced by the most powerful particle accelerators on Earth. They have just started their galactic voyage, zig-zagging away from their accelerator and producing gamma rays when striking gas or starlight in the cavities,” said co-author Luigi Tibaldo, a physicist at Padova University and the Italian National Institute of Nuclear Physics.

Clocked at up to 100 billion electron volts by the LAT, these highly accelerated particles are revealing the extreme origin of gamma-ray emission. For example, visible light is only two to three electron volts! But why is Cygnus X so special? It entangles its sources in complex magnetic fields and keeps the majority of them from escaping. All thanks to those high mass stars…

“These shockwaves stir the gas and twist and tangle the magnetic field in a cosmic-scale jacuzzi so the young cosmic rays, freshly ejected from their accelerators, remain trapped in this turmoil until they can leak into quieter interstellar regions, where they can stream more freely,” said co-author Isabelle Grenier, an astrophysicist at Paris Diderot University and the Atomic Energy Commission in Saclay, France.

However, there’s more to the story. The Gamma Cygni supernova remnant is also nearby and may impact the findings as well. At this point, the Fermi team considers it may have created the initial “cocoon” which holds the cosmic rays in place, but they also concede the accelerated particles may have originated through multiple interactions with stellar winds.

“Whether the particles further gain or lose energy inside this cocoon needs to be investigated, but its existence shows that cosmic-ray history is much more eventful than a random walk away from their sources,” Tibaldo added.

Original Story Source: NASA Fermi News.

Posted on by Steve Nerlich

Astronomy Without A Telescope – Oh-My-God Particles

Centaurus A - the closest galaxy with an active galactic nucleus - a mere 10-16 million light years away. Now I wonder where all those ultra high energy cosmic rays are coming from. Hmmm...

[/caption]

Cosmic rays are really sub-atomic particles, being mainly protons (hydrogen nuclei) and occasionally helium or heavier atomic nuclei and very occasionally electrons. Cosmic ray particles are very energetic as a result of them having a substantial velocity and hence a substantial momentum.

The Oh-My-God particle detected over Utah in 1991 was probably a proton traveling at 0.999 (and add another 20 x 9s after that) of the speed of light and it allegedly carried the same kinetic energy as a baseball traveling at 90 kilometers an hour.

Its kinetic energy was estimated at 3 x 1020 electron volts (eV) and it would have had the collision energy of 7.5 x 1014 eV when it hit an atmospheric particle – since it can’t give up all its kinetic energy in the collision. Fast moving debris carries some of it away and there’s some heat loss too. In any case, this is still about 50 times the collision energy we expect the Large Hadron Collider (LHC) will be able to generate at full power. So, this gives you a sound basis to scoff at doomsayers who are still convinced that the LHC will destroy the Earth.

Now, most cosmic ray particles are low energy, up to 1010 eV – and arise locally from solar flares. Another more energetic class, up to 1015 eV, are thought originate from elsewhere in the galaxy. It’s difficult to determine their exact source as the magnetic fields of the galaxy and the solar system alter their trajectories so that they end up having a uniform distribution in the sky – as though they come from everywhere.

But in reality, these galactic cosmic rays probably come from supernovae – quite possibly in a delayed release process as particles bounce back and forth in the persisting magnetic field of a supernova remnant, before being catapulted out into the wider galaxy.

And then there are extragalactic cosmic rays, which are of the Oh-My-God variety, with energy levels exceeding 1015 eV, even rarely exceeding 1020 eV – which are more formally titled ultra-high-energy cosmic rays. These particles travel very close to the speed of light and must have had a heck of kick to attain such speeds.

Left image: The energy spectrum of cosmic rays approaching Earth. Cosmic rays with low energies come in large numbers from solar flares (yellow range). Less common, but higher energy cosmic rays originating from elsewhere in the galaxy are in the blue range. The least common but most energetic extragalactic cosmic rays are in the purple range. Right image: The output of the active galactic nucleus of Centaurus A dominates the sky in radio light - this is its apparent size relative to the full Moon. It is likely that nearly all extragalactic cosmic rays that reach Earth originate from Centaurus A.

However, a perhaps over-exaggerated aura of mystery has traditionally surrounded the origin of extragalactic cosmic rays – as exemplified in the Oh-My-God title.

In reality, there are limits to just how far away an ultra-high-energy particle can originate from – since, if they don’t collide with anything else, they will eventually come up against the Greisen–Zatsepin–Kuzmin (GZK) limit. This represents the likelihood of a fast moving particle eventually colliding with a cosmic microwave background photon, losing momentum energy and velocity in the process. It works out that extragalactic cosmic rays retaining energies of over 1019 eV cannot have originated from a source further than 163 million light years from Earth – a distance known as the GZK horizon.

Recent observations by the Pierre Auger Observatory have found a strong correlation between extragalactic cosmic rays patterns and the distribution of nearby galaxies with active galactic nuclei. Biermann and Souza have now come up with an evidence-based model for the origin of galactic and extragalactic cosmic rays – which has a number of testable predictions.

They propose that extragalactic cosmic rays are spun up in supermassive black hole accretion disks, which are the basis of active galactic nuclei. Furthermore, they estimate that nearly all extragalactic cosmic rays that reach Earth come from Centaurus A. So, no huge mystery – indeed a rich area for further research. Particles from an active supermassive black hole accretion disk in another galaxy are being delivered to our doorstep.

Further reading: Biermann and Souza On a common origin of galactic and extragalactic cosmic rays.

Posted on by Nancy Atkinson

New Look Inside Tycho Supernova Remnant Hints at Cosmic Ray Origins

X-ray Image of Tycho's Supernova Remnant. (NASA/CXC/Rutgers/K.Eriksen et al.)

[/caption]

The Chandra X-Ray Observatory has taken a brand new, deep look inside the Tycho Supernova Remnant and found a pattern of X-ray “stripes.” The three-dimensional-like nature of this incredible image notwithstanding, nothing like these stripe-like features has ever been seen before inside the leftovers of an exploding star, but astronomers believe they could explain how some cosmic rays are created. Additionally, the stripes provide support for a theory about how magnetic fields can be dramatically amplified in supernova blast waves.

Cosmic rays are made up of electrons, positrons and atomic nuclei and they constantly bombard the Earth. In their near light-speed journey across the galaxy, the particles are deflected by magnetic fields, which scramble their paths and mask their origins. Supernova remnants have long been thought to be the source of cosmic rays, up to the “knee” of the cosmic ray spectrum at 10^15 eV, but so far, no specific sources have been located.

In 2010, the Fermi gamma ray telescope found evidence – also from supernova remnants – where radiation is emitted that is a billion times more energetic than visible light.

High Energy Stripes in the Tycho Supernova Remnant. Credit: NASA/CXC/Rutgers/K.Eriksen et al

But the stripes seen by Chandra, shown above in high-energy X-rays (blue), are thought to be regions where the turbulence is greater and the magnetic fields more tangled than surrounding areas. Electrons become trapped in these regions and emit X-rays as they spiral around the magnetic field lines. Regions with enhanced turbulence and magnetic fields were expected in supernova remnants, but the motion of the most energetic particles — mostly protons — was predicted to leave a messy network of holes and dense walls corresponding to weak and strong regions of magnetic fields, respectively.

Therefore, the detection of stripes was a surprise.

Schematic Illustration of the Tycho Stripes. Credit: NASA/CXC/M.Weiss.

The size of the holes was expected to correspond to the radius of the spiraling motion of the highest energy protons in the supernova remnant. These energies equal the highest energies of cosmic rays thought to be produced in our Galaxy. The spacing between the stripes corresponds to this size, providing evidence for the existence of these extremely energetic protons.

“We interpret the stripes as evidence for acceleration of particles to near the knee of the CR spectrum in regions of enhanced magnetic turbulence, while the observed highly ordered pattern of these features provides a new challenge to models of diffusive shock acceleration,” writes Kristoffer A. Eriksen and his team in their paper, “Evidence For Particle Acceleration to the Knee of the Cosmic Ray Spectrum in Tycho’s Supernova Remnant.”

Source: Chandra

Posted on by Nancy Atkinson

PAMELA Uncovers Cosmic Ray Surprise

PAMELA data show clear deviations from a single power law model between protons and helium nuclei. Credit: Adriani, et. al, Science.

[/caption]

High energy particles called cosmic rays are constantly bombarding Earth from all directions, and have been thought to come from the blast waves of supernova remnants. But new observations from the PAMELA cosmic ray detector show an unexpected difference in the speeds of protons and helium nuclei, the most abundant components of cosmic rays. The difference is extremely small, but if they were accelerated from the same event, the speeds should be the same.

The PAMELA instrument. Image courtesy of Piergiorgio Picozza

PAMELA, the Payload for Anti-Matter Exploration and light-Nuclei Astrophysics, is on board the Earth-orbiting Russian Resurs-DK1 satellite. It uses a permanent magnet spectrometer along with a variety of specialized detectors to measure the abundance and energy spectra of cosmic rays electrons, positrons, antiprotons and light nuclei over a very large range of energy from 50 MeV to hundreds of GeV.

Just as astronomers use light to view the Universe, scientists use galactic cosmic rays to learn more about the composition and structure of our galaxy, as well as to find out how things like how nuclei can accelerate to nearly the speed of light.

Oscar Adriani and his colleagues using the PAMELA instrument say their new findings are a challenge to our current understanding of how cosmic rays are accelerated and propagated. “We find that the spectral shapes of these two species are different and cannot be well described by a single power law,” the team writes in their paper. “These data challenge the current paradigm of cosmic-ray acceleration in supernova remnants followed by diffusive propagation in the Galaxy.”

Instead, the team concludes, the acceleration and propagation of cosmic rays may be controlled by now unknown and more complex processes.

Supernova remnants are expanding clouds of gas and magnetic fields and can last for thousands of years. Within this cloud, particles are accelerated by bouncing back and forth in the magnetic field of the remnant, and some of the particles gain energy, and eventually they build up enough speed that the remnant can no longer contain them, and they escape into the Galaxy as cosmic rays.

One key question that scientists hope to answer with PAMELA data is whether the cosmic rays are continuously accelerated over their entire lifetime, whether the acceleration occurs just once, or if there is any deceleration.

Scientists say that determining the fluxes in the proton and helium nuclei will give information about the early Universe as well as the origin and evolution of material in our galaxy.

Adriani and his team hope to uncover more information with PAMELA to help better understand the origins of cosmic rays. They say possible contributions could be from additional galactic sources, such as pulsars or dark matter.

Abstract: PAMELA Measurements of Cosmic-Ray Proton and Helium Spectra

Source: Science

Posted on by Steve Nerlich

Astronomy Without A Telescope – A Universe Free Of Charge?

(Caption) When you weigh up all the positives and the negatives, does the universe still have a net charge of zero?

[/caption]

If there were equal amounts of matter and anti-matter in the universe, it would be easy to deduce that the universe has a net charge of zero, since a defining ‘opposite’ of matter and anti-matter is charge. So if a particle has charge, its anti-particle will have an equal but opposite charge. For example, protons have a positive charge – while anti-protons have a negative charge.

But it’s not apparent that there is a lot of anti-matter around as neither the cosmic microwave background, nor the more contemporary universe contain evidence of annihilation borders – where contact between regions of large scale matter and large scale anti-matter should produce bright outbursts of gamma rays.

So, since we do apparently live in a matter-dominated universe – the question of whether the universe has a net charge of zero is an open question.

It’s reasonable to assume that dark matter has either a net zero charge – or just no charge at all – simply because it is dark. Charged particles and larger objects like stars with dynamic mixtures of positive and negative charges, produce electromagnetic fields and electromagnetic radiation.

So, perhaps we can constrain the question of whether the universe has a net charge of zero to just asking whether the total sum of all non-dark matter has. We know that most cold, static matter – that is in an atomic, rather than a plasma, form – should have a net charge of zero, since atoms have equal numbers of positively charged protons and negatively charged electrons.

Stars composed of hot plasma might also be assumed to have a net charge of zero, since they are the product of accreted cold, atomic material which has been compressed and heated to create a plasma of dissociated nuclei (+ve) and electrons (-ve).

The principle of charge conservation (which is accredited to Benjamin Franklin) has it that the amount of charge in a system is always conserved, so that the amount flowing in will equal the amount flowing out.

Apollo 15's Lunar Surface Experiments Package (ALSEP). The Moon represents a good vantage point to measure the balance of incoming cosmic rays versus outgoing solar wind.

An experiment which has been suggested to enable measurement of the net charge of the universe, involves looking at the solar system as a charge-conserving system, where the amount flowing in is carried by charged particles in cosmic rays – while the amount flowing out is carried by charged particles in the Sun’s solar wind.

If we then look at a cool, solid object like the Moon, which has no magnetic field or atmosphere to deflect charged particles, it should be possible to estimate the net contribution of charge delivered by cosmic rays and by solar wind. And when the Moon is shadowed by the tail of the Earth’s magnetosphere, it should be possible to detect the flux attributable to just cosmic rays – which should represent the charge status of the wider universe.

Drawing on data collected from sources including Apollo surface experiments, the Solar and Heliospheric Observatory (SOHO), the WIND spacecraft and the Alpha Magnetic Spectrometer flown on a space shuttle (STS 91), the surprising finding is a net overbalance of positive charges arriving from deep space, implying that there is an overall charge imbalance in the cosmos.

Either that or a negative charge flux occurs at energy levels lower than the threshold of measurement that was achievable in this study. So perhaps this study is a bit inconclusive, but the question of whether the universe has a net charge of zero still remains an open question.

Further reading: Simon, M.J. and Ulbricht, J. (2010) Generating an electrical potential on the Moon by cosmic rays and solar wind?

Posted on by Nancy Atkinson

Sources of Cosmic Rays Found? Fermi Telescope Closes In

A multiwavelength look at Cas A. Credit: NASA/DOE/Fermi LAT Collaboration

The origin of cosmic rays has been a mystery since their discovery nearly a century ago. But new images from the Fermi Gamma-ray Space Telescope may bring astronomers a step closer to understanding the source of the Universe’s most energetic particles. The images show where supernova remnants emit radiation a billion times more energetic than visible light. “Fermi now allows us to compare emission from remnants of different ages and in different environments,” said Stefan Funk, an astrophysicist at the Kavli Institute for Particle Astrophysics and Cosmology (KIPAC).

Cosmic rays are made up of electrons, positrons and atomic nuclei and they constantly bombard the Earth. In their near-speed-of-light journey across the galaxy, the particles are deflected by magnetic fields, which scramble their paths and mask their origins. When cosmic rays collide with interstellar gas, they produce gamma rays. While instruments can infer the presence of cosmic rays by looking for the glow of gamma ray emissions, so far, no specific sources have been located. .

“Understanding the sources of cosmic rays is one of Fermi’s key goals,” said said Funk, who presented the new images and findings at the American Physical Society meeting in Washington, D.C. on Monday.

[/caption]

Fermi’s Large Area Telescope (LAT) mapped billion-electron-volt (GeV) gamma rays from three middle-aged supernova remnants — known as W51C, W44, and IC 443 — that were never before resolved at these energies. (The energy of visible light is between 2 and 3 electron volts.) Each remnant is the expanding debris of a massive star that blew up between 4,000 and 30,000 years ago.

In addition, Fermi’s LAT also spied GeV gamma rays from Cassiopeia A (Cas A), a supernova remnant only 330 years old. Ground-based observatories, which detect gamma rays thousands of times more
energetic than the LAT was designed to see, have previously detected Cas A.

“Older remnants are extremely bright in GeV gamma rays, but relatively faint at higher energies. Younger remnants show a different behavior,” explained Yasunobu Uchiyama, a Panofsky Fellow at SLAC. “Perhaps the highest-energy cosmic rays have left older remnants, and Fermi sees emission from trapped particles at lower energies.”

Fermi mapped GeV-gamma-ray emission regions (magenta) in the W44 supernova remnant. The features clearly align with filaments detectable in other wavelengths. This composite merges X-ray data (blue) from the Germany/U.S./UK ROSAT mission, infrared (red) from NASA’s Spitzer Space Telescope, and radio (orange) from the Very Large Array near Socorro, N.M. Credit: NASA/DOE/Fermi LAT Collaboration, NASA/ROSAT, NASA/JPL-Caltech, and NRAO/AUI

In 1949, physicist Enrico Fermi — for whom the Fermi telescope was named –suggested that the highest-energy cosmic rays were accelerated in the magnetic fields of gas clouds. In the decades that followed,
astronomers showed that supernova remnants are the galaxy’s best candidate sites for this process.

Young supernova remnants seem to possess both stronger magnetic fields and the highest-energy cosmic rays. Stronger fields can keep the highest-energy particles in the remnant’s shock wave long enough to speed them to the energies observed.

The Fermi observations show GeV gamma rays coming from places where the remnants are known to be interacting with cold, dense gas clouds.

“We think that protons accelerated in the remnant are colliding with gas atoms, causing the gamma-ray emission,” Funk said. An alternative explanation is that fast-moving electrons emit gamma rays as they fly past the nuclei of gas atoms. “For now, we can’t distinguish between these possibilities, but we expect that further observations with Fermi will help us to do so,” he added.

Either way, these observations validate the notion that supernova remnants act as enormous accelerators for cosmic particles.

“How fitting it is that Fermi seems to be confirming the bold idea advanced over 60 years ago by the scientist after whom it was named,” noted Roger Blandford, director of KIPAC.

Source: Fermi/Sonoma State University

Posted on by Nicholos Wethington

An Exotic Source for Cosmic Rays: ‘Baby’ Black Holes

Cosmic rays – particles that have been accelerated to near the speed of light – stream out from our Sun all of the time, though they are positively sluggish compared to what are called Ultra-High-Energy Cosmic Rays (UHECRs). These types of cosmic rays originate from sources outside of the Solar System, and are much more energetic than those from our Sun, though also much rarer. The merger between a white dwarf and neutron star or black hole may be one source of these rays, and such mergers may occur often enough to be the most significant source of these energetic particles.

The Sloan White dwArf Radial velocity data Mining Survey (SWARMS) – which is part of the Sloan Digital Sky Survey – recently uncovered a binary system of exotic objects only 50 parsecs away from the Solar System. This system, named SDSS 1257+5428, appears to be a white dwarf star that is orbiting a neutron star or low-mass black hole. Details about the system and its initial discovery can be found in a paper by Carles Badenes, et al. here.

Co-author Todd Thompson, assistant professor in the Department of Astronomy at Ohio State University, argues in a recent letter to The Astrophysical Journal Letters that this type of system, and subsequent merger of these exotic remnants of stars, may be commonplace, and could account for the amount of UHECRs that are currently observed. The merger between the white dwarf and neutron star or black hole may also create a black hole of low mass, a so-called “baby” black hole.

Thompson wrote in an email interview:

“White dwarf/neutron star or black hole binaries are thought to be quite rare, although there is a huge range in the number per Milky Way-like galaxy in the literature.  SWARMS was the first to detect such a system using the “radial velocity” technique, and the first to find such an object so nearby, only 50 parsecs away (about 170 light years). For this reason, it was very surprising, and its relative proximity is what allowed us to make the argument that these systems must be quite common compared to most previous expectations.  SWARMS would have had to be very lucky to see something so rare so near by.”

Thompson, et al. argue that this type of merger may be the most significant source of UHECRs in the Milky Way galaxy, and that one should merge in the galaxy about every 2,000 years. These types of mergers may be slightly less common than Type Ia supernovae, which originate in binary systems of white dwarfs.

A white dwarf merging with a neutron star would also create a low-mass black hole of about 3 times the mass of the Sun. Thompson said, “In fact, this scenario is likely since we think that neutron stars cannot exist above 2-3 times the mass of the Sun. The idea is that the WD would be disrupted and accrete onto the neutron star and then the neutron star would collapse to a black hole.  In this case, we might see the signal of BH formation in gravity waves.”

The gravity waves produced in such a merger would be above the detectable range by the Laser Interferometer Gravitational-Wave Observatory (LIGO), an instrument that uses lasers to detect gravity waves (of which none have been detected…yet), and even possibly a spaced base gravitational wave observatory, NASA’s Laser Interferometer Space Antenna, LISA.

Common cosmic rays that come from our Sun have an energy on the scale of 10^7 to 10^10 electron-volts. Ultra-high-energy cosmic rays are a rare phenomenon, but they exceed 10^20 electron-volts. How do systems like SDSS 1257+5428 produce cosmic rays of such high energy? Thompson explained that there are two equally fascinating possibilities.

In the first, the formation of a black hole and subsequent accretion disk from the merger would generate a jet somewhat like those seen at the center of galaxies, the telltale sign of a quasar. Though these jets would be much, much smaller, the shockwaves at the front of the jet would accelerate particles to the necessary energies to create UHECRs, Thompson said.

In the second scenario, the neutron star steals matter off of the white dwarf companion, and this accretion starts it rotating rapidly. The magnetic stresses that build at the surface of the neutron star, or “magnetar”, would be able to accelerate any particles that interact with the intense magnetic field to ultra-high energies.

The creation of these ultra-high-energy cosmic rays by such systems is highly theoretical, and just how common they may be in our galaxy is only an estimate. It remains unclear so soon after the discovery of SDSS 1257+5428 whether the companion object of the white dwarf is a black hole or neutron star. But the fact that SWARMS made such a discovery so early in the survey is encouraging for the discovery of further exotic binary systems.

“It is not likely that SWARMS will see 10 or 100 more such systems. If it did, the rate of such mergers would be very (implausibly) high.  That said, we’ve been surprised many times before. However, given the total area of the sky surveyed, if our estimate of the rate of such mergers is correct, SWARMS should see only about 1 more such system, and they may see none. A similar survey in the southern sky (there is nothing at present comparable to the Sloan Digital Sky Survey, on which SWARMS is based) should turn up approximately 1 such system,” Thompson said.

Observations of SDSS 1257+5428 have already been made using the Swift X-ray observatory, and some measurements have been taken in the radio spectrum. No source of gamma-rays was to be found in the location of the system using the Fermi telescope.

Thompson said, “Probably the most important forthcoming observation of the system is to get a true distance via parallax. Right now, the distance is based on the properties of the observed white dwarf.  In principle,
it should be relatively easy to watch the system over the next year and get a parallax distance, which will alleviate many of the uncertainties surrounding the physical properties of the white dwarf.”

Source: Arxiv, email interview with Todd Thompson

Posted on by Nancy Atkinson

Solving the Mystery of Cosmic Rays’ Origins

What accelerates cosmic rays to nearly the speed of light? Astronomer have pondered that question for nearly 100 years, and now new evidence supports a theory held for two decades that cosmic rays likely are powered by exploding stars and stellar winds. “This discovery has been predicted for almost 20 years, but until now no instrument was sensitive enough to see it,” said Wystan Benbow, an astrophysicist at the Smithsonian Astrophysical Observatory who coordinated this project for the Very Energetic Radiation Imaging Telescope Array System (VERITAS) collaboration.

Nearly 100 years ago, scientists detected the first signs of cosmic rays, which are actually not rays or beams but subatomic particles (mostly protons) that zip through space at nearly the speed of light. The most energetic cosmic rays hit with the punch of a 98-mph fastball, even though they are smaller than an atom. Astronomers questioned what natural force could accelerate particles to such a speed.

The rarest cosmic rays carry over 100 billion times as much energy as generated by any particle accelerator on Earth. Astronomers have devised ingenious methods for detecting cosmic rays that hit Earth’s atmosphere. However, detecting cosmic rays from a distance requires much more effort.

This representative-color figure shows the very-high-energy gamma-ray emission observed by VERITAS coming from the Cigar Galaxy, also known as Messier 82. The black star is the location of the active starburst region. The emission from M82 is effectively point-like for VERITAS, and the white circle indicates the size of a simulated point source. The entire galaxy would be contained within the circle. Credit: CfA/V.A. Acciari
This representative-color figure shows the very-high-energy gamma-ray emission observed by VERITAS coming from the Cigar Galaxy, also known as Messier 82. The black star is the location of the active starburst region. The emission from M82 is effectively point-like for VERITAS, and the white circle indicates the size of a simulated point source. The entire galaxy would be contained within the circle. Credit: CfA/V.A. Acciari

VERITAS has found new evidence for cosmic rays in the “Cigar Galaxy,” also known as Messier 82 (M82), which is located 12 million light-years from Earth in the direction of the constellation Ursa Major, which strongly support the long-held theory that supernovae and stellar winds from massive stars are the dominant accelerators of cosmic-ray particles.

Galaxies with high levels of star formation like M82, also known as “starburst” galaxies, have large numbers of supernovae and massive stars. If the theory holds, then starburst galaxies should contain more cosmic rays than normal galaxies. The VERITAS discovery confirms that expectation, indicating that the cosmic-ray density in M82 is approximately 500 times the average density in our Galaxy, the Milky Way.

“This discovery provides fundamental insight into the origin of cosmic rays,” said Rene Ong, a professor of physics at the University of California, Los Angeles, and the spokesperson for the VERITAS collaboration.

Using gamma rays to infer cosmic rays

VERITAS could not detect M82’s cosmic rays directly because they are trapped within the Cigar Galaxy. Instead, VERITAS looked for clues to the presence of cosmic rays: gamma rays. Gamma rays are the most energetic form of light, far more powerful than ultraviolet light or even X-rays. When cosmic rays interact with interstellar gas and radiation within M82, they produce gamma rays, which can then escape their home galaxy and reach Earthbound detectors.

It took two years of dedicated data collection to tease out the faint signal coming from M82.

“We knew that the detection of M82 would have important scientific implications. As a result, we scheduled an exceptionally long exposure immediately after the experiment became fully operational” said Benbow. “The data needed to be meticulously analyzed to extract the gamma-ray signal, which is over a million times smaller than the background noise. Although the signal is only a tiny fraction of the data, we made many checks for possible bias and we are confident that the signal is genuine.”

“The detection of M82 indicates that the universe is full of natural particle accelerators, and as ground-based gamma-ray observatories continue to improve, further discoveries are inevitable.” said Martin Pohl, a professor of physics at Iowa State University who helped lead the study. A next-generation VHE gamma-ray observatory, the Advanced Gamma-ray Imaging System (AGIS), is already under development.

VERITAS is operated by a collaboration of more than 100 scientists from 22 different institutions in the United States, Ireland, England and Canada. Click here for more information on VERITAS.

Lead image caption: A composite of multi-wavelength images of the active galaxy M82 from Hubble, Chandra, and Spitzer. Credit: NASA, ESA, CXC, and JPL-Caltech

Source: Harvard Smithsonian Center for Astrophysics