gamma rays – Close Read

The search for a powerful natural particle accelerator

Earth is almost constantly beset by a stream of particles from space called cosmic rays. These particles consist of protons, bundles of two protons and two neutrons each (alpha particles), a small number of heavier atomic nuclei and a smaller fraction of anti-electrons and anti-protons. Cosmic rays often have high energy – typically up to half of 1 GeV. One GeV is almost the amount of energy that a single proton has at rest. The Large Hadron Collider (LHC) itself can accelerate protons up to 7,000 GeV.

But this doesn’t mean cosmic rays are feeble: historically, some detectors have recorded high-energy and very-high-energy cosmic rays. The most energetic cosmic ray – dubbed the “oh my god” particle – was a proton recorded over Utah in 1991 with an energy of around 3 x 10¹² GeV, which is around three-billion-times higher than the energy to which the LHC can accelerate protons today. This proton was travelling at 99.9% the speed of light in vacuum. This is a phenomenal amount of energy – about as much kinetic energy as a baseball moving at 95 km/hr but concentrated into the volume of a proton, which has 10⁴²-times less space in which to hold that energy.

Detectors have also spotted some cosmic-ray events with energies exceeding 1,000,000 GeV – or 1 PeV. They’re uncommon compared to all cosmic-ray events but relatively more common than the likes of the “oh my god” particle. Physicists are interested in them because they indicate the presence of a natural particle accelerator somewhere in the universe that’s injecting protons with ginormous amounts of energy and sending them blasting off into space. One term for such natural accelerators seemingly capable of accelerating protons to 0.1-1 PeV is ‘PeVatron’. And the question is: where can we find a PeVatron?

There are three broad sources of cosmic rays: from the Sun, from somewhere in the Milky Way galaxy and from somewhere beyond the galaxy. Most of the cosmic rays we have detected have been from the latter two sources. In fact, there’s a curious feature called the ‘knee’ that physicists believe could distinguish between these sources. If you plot the number of cosmic rays on the y-axis and the energies of the cosmic rays on the x-axis, you’ll find yourself looking at the famous Swordy plot:

The Swordy plot of cosmic-rays flux versus energy. The yellow zone accounts for solar cosmic rays, the blue zone for galactic cosmic rays and the pink zone for extragalactic cosmic rays. Credit: Sven Lafebre/Wikimedia Commons, CC BY-SA 3.0

As you can see, the plot shows a peculiar bump, an almost imperceptible change in slope, when transitioning from the blue to the pink zones – this is the ‘knee’. Physicists have interpreted the cosmic rays above the knee to be from within the Milky Way and those below to be from outside the galaxy, although why this is so isn’t clear.

Before cosmic rays interact with other particles in their way, they’re called primary cosmic rays. After their interaction, such as the atoms and molecules in Earth’s upper atmosphere, they produce a shower of secondary particles; these are the secondary cosmic rays. Physicists can get a tighter fit on the potential source of primary cosmic rays by analysing the direction at which they strike the atmosphere, the composition of the secondary cosmic rays, and the energies of both the primary and the secondary rays. This is why we suspect supernovae are one source of within-the-galaxy cosmic rays, with some possible mechanisms of action.

One, for example, is shockfront acceleration: a proton could get trapped between two shockwaves from the same supernova. As the outer wave slows and the inner wave charges in, the proton could bounce rapidly between the two shockfronts and emerge greatly energised out of a gap. However, we don’t know what fraction of cosmic rays, at different energies, supernovae can account for.

Potential extragalactic sources include active galactic nuclei – the centres of galaxies, including the neighbourhood of supermassive black holes – and the extremely powerful gamma-ray bursts. Physicists have associated them with cataclysmic events like neutron-star mergers and the formative events of black-holes.

However, exercises to triangulate the sources of high-energy cosmic rays are complicated by galactic magnetic fields (which curve the paths of charged particles). A proton accelerated by the shockfront mechanism could also bump into some other particle as it emerges, producing a flash of gamma rays that physicists can look for – but only if they have a way to isolate it from other sources of gamma rays in a supernova’s vicinity. This is difficult work.

Researchers from the US recently analysed gamma-ray data collected by the Fermi Gamma-ray Space Telescope (FGST), in low-Earth orbit, of the supernova remnant G106.3+2.7. Astrophysicists have suspected that this object could be a PeVatron for more than a decade, and the US research team used FGST data to check if they the suspicion could be true. The difficult bit? The data spanned 12 years.

In 2008, physicists recorded very high energy (100-100,000 GeV) gamma rays from G106.3+2.7, located around 800 parsec (2,600 lightyears) away. The US research team figured that they could have been produced in two ways. Let’s call them Mechanism A and Mechanism B. Physicists already know Mechanism A is associated with cosmic rays while Mechanism B is not. The US team members used 12 years of data to characterise gamma-ray, X-ray and radio emissions around the remnant so they could determine which mechanism could have been responsible for all of them the way they have been observed, with the gamma rays as secondary cosmic rays.

The team’s analysis found that the theory of Mechanism A almost exactly accounted for the energies of the gamma rays from the remnant while also accommodating the other radiation – whereas the theory of Mechanism B couldn’t explain the gamma rays and the remnant’s X-ray emissions together. In effect, the team had a way to justify the idea that G106.3+2.7 could be a PeVatron.

Mechanism B is inverse Compton scattering by relativistic electrons. Inverse Compton scattering is when high-energy electrons collide with low-energy photons and the photons gain energy (in regular Compton scattering, the electrons gain energy). When this model couldn’t account for the gamma-ray emissions, the team invoked a modified version involving two sets of electrons, with each set accelerated to different energies by different mechanisms. But the team found that the FGST data continued to disfavour the involvement of leptons, and instead preferred the involvement of hadrons. Leptons – like electrons – are particles that don’t interact with other particles through the strong nuclear force. Hadrons, on the other hand, do, and they were implicated in Mechanism A: the decay of neutral pions.

Pions are the lightest known hadrons and come in three types: π⁺, π⁰ and π^–. Neutral pions are π⁰. They have a very short lifetime, around 85 attoseconds – that’s 0.000000000000000085 seconds. And when they decay, they decay into gamma rays, i.e. high-energy photons.

Some 380,000 years after the Big Bang, a series of events in the universe left behind some radiation that survives to this day. This relic radiation is called the cosmic microwave background, a sea of photons in the microwave frequency pervading the cosmos. When a cosmic-ray proton collides with one of these photons, a delta-plus baryon is formed that then decays into a proton and a neutral pion. The neutral pion then decays to gamma rays, which are detectable as secondary cosmic rays.

Source: Wikipedia/’Greisen–Zatsepin–Kuzmin limit’

Knowing the energy of the gamma rays allows physicists to work back to the energy of the cosmic ray. And according to the team’s calculations, the 2009 gamma-ray emission indicates G106.3+2.7 could be a PeVatron. As the team’s preprint paper concluded,

“… only a handful, out of hundreds of radio-emitting supernova remnants, have been observed to emit very high energy radiation with a hard spectrum. The scarcity of PeVatron candidates and the rareness of remnants with very high energy emission make … G106.3+2.7 a unique source. Our study provides strong evidence for proton acceleration in this nearby remnant, and by extension, supports a potential role for G106.3+2.7-like supernova remnants in meeting the challenge of accounting for the observed cosmic-ray knee using galactic sources”.

Featured image: An artist’s impression of supernova 1993J. Credit: NASA, ESA and G. Bacon (STScI).

HESS telescopes discover new source of gamma rays called a superbubble

Optical image of the Milky Way and a multi-wavelength (optical, Hα) zoom into the Large Magellanic Cloud with superimposed H.E.S.S. sky maps. (Milky Way image: © H.E.S.S. Collaboration, optical: SkyView, A. Mellinger; LMC image: © H.E.S.S. Collaboration, Hα: R. Kennicutt, J.E. Gaustad et al. (2001), optical (B-band): G. Bothun

Astronomers using the HESS telescopes have discovered a new source of high-energy gamma rays. Dubbed a superbubble, it appears to be a massive shell of gas and dust 270 light-years in diameter being blown outward by the radiation from multiple stars and supernovas. HESS also discovered two other gamma-ray sources, each a giant of its kind. One is a powerful supernova remnant and the other a pulsar wind nebula. All three objects are located in the Large Magellanic Cloud, a small satellite galaxy orbiting the Milky Way at a distance of 170,000 ly. As a result, these objects are not only the most luminous gamma-ray sources discovered to date but also the first sources discovered outside the Milky Way.

Gamma-rays are emitted when very energetic charged particles collide with other particles, such as in a cloud of gas. Therefore, gamma radiation in the sky is often used as a proxy for high-energy phenomena. And astronomers have for long known that the Large Magellanic Cloud houses many such clusters of frenzied activity: weight for weight of their stars, the Cloud’s supernova rate is five times that of the Milky Way. It also hosts the Tarantula Nebula, which is the most active star-forming region in the Local Group of galaxies (which includes the Milky Way, Andromeda, the Cloud and more than 50 others).

Super-luminous sources

It is in this environment that the superbubble – designated 30 Dor C – thrives. According to the HESS team’s notice, it “appears to have been created by several supernovae and strong stellar winds”. In the data, it is visible as a strong source of gamma-rays because it is filled by highly energetic particles. The notice adds that this freak of nature

“represents a new class of sources in the very high-energy regime.”

The other two super-luminous sources are familiar to astronomers. Pulsars, especially, are the extremely dense remnants of stars that have run out of hydrogen to fuse and imploded, resulting in a rapidly spinning core composed of neutrons and wound by fierce magnetic fields. They emit a jet of energetic particles from polar points on their surface that form nebulaic clouds. One such cloud is N 157B, emitted by PSR J0537 – 6910. According to the HESS team, N 157B outshines the Crab Nebula in gamma-rays. The Crab Nebula is Milky Way’s most famous and most powerful source of gamma-rays.

The third is a supernova remnant: the rapidly expanding shell of gas that a once-heavy dying star blows away as its core collapses. The shell can be expelled at more than thousand times the speed of sound, resulting in a shockwave that can accelerate nearby particles and heat up upstream gas clouds to millions of kelvin. The resulting glow can last for thousands of years – but the one HESS has seen in the Cloud seems to going strong for 2,500-6,000 years, much longer than astronomers thought possible. It’s called N132D.

“Obviously, the high star formation rate of the LMC causes it to breed very extreme objects,” said Chia Chun Lu, a student at the Max Planck Institute for Astronomy in Heidelberg who analyzed the data for her thesis.

Imaging Cherenkov radiation

Detecting gamma-rays is no easy task because it requires the imaging of Cherenkov radiation. Just as when a jet flies through air at faster than the speed of sound and results in a sonic boom, a charged particle traveling at faster than the speed of light in that medium results in a shockwave of energy called Cherenkov radiation. This typically lasts a few billionths of a second and requires extremely sensitive cameras to capture.

When high-energy particles collide with the upper strata of Earth’s atmosphere, they percolate through while triggering the release of Cherenkov radiation. The five ground-based HESS telescopes – whose name stands for High Energy Stereoscopic System – quickly capture their bluish flashes before they disappear, and reconstruct their sources’ energy based on theirs. So, while gamma-rays can be a proxy for high-energy phenomena in the distant reaches of the cosmos, Cherenkov radiation in the upper atmosphere is a proxy for the gamma radiation itself.

Very-high-energy gamma-rays, of the order emitted by the Crab pulsar at the center of its nebula, are often the result of events that have made astronomers redefine what they consider anomalous. A good example is of GRB 080916C, a gamma-ray burst spotted in 2009 at about 12 billion ly from Earth. It was the result of a star collapsing into a black hole, with consequent ‘burp’ of energy lasting for a whopping 23 minutes. Valerie Connaughton, of the University of Alabama, Huntsville, and one of the members of the team studying the burst, said of its energy: “… it would be equivalent to 4.9 times the mass of the sun being converted to gamma rays in a matter of minutes”.

Natural particle accelerators

Such profuse emissions can behave like natural particle accelerators, often reaching energies the Large Hadron Collider can only dream of. They give scientists the opportunity to study particles as well as the vacuum of space in conditions closer to that prevalent at the time of the Big Bang, in effect rendering the telescopes that study them as probes of fundamental physics. In the case of GRB 080916C, for example, low-energy gamma-rays dominated the first five seconds of emissions, following by the high-energy gamma-rays for the next twenty minutes. As astronomy-blogger Paul Gilster interpreted this,

They might also give us a read on theories of quantum gravity that suggest empty space is actually a froth of quantum foam, one that would allow lighter, lower-energy gamma rays to move more quickly than their higher-energy cousins. Future observations to study unusual time lags like these should help us pin down a plausible explanation.

The Fermi orbiting telescope that spotted the burst is also used to look for dark matter. When certain hypothetical particles of dark matter annihilate or decay, they yield high-energy antielectrons that could then annihilate upon colliding with electrons and yield gamma-rays. These are measured by Fermi. Then, astronomers use preexisting data as a filter to extrude anomalous observations and use it inform their theories of dark matter.

In this sense, the HESS telescopes are important observers of the universe. They comprise five telescopes, of which four, each 12 meters in diameter, are situated on the corners of a square of side 120 m. At the center is the fifth telescope of diameter 28 m. The array, fixed up with computers to work as one big telescope, is located in Namibia, and is capable of observing gamma-ray fluxes in the range 30 GeV to 100 TeV. In 2015, in fact, construction for the more-impressive $268-million Cherenkov Telescope Array will start. Upon completion, it will be able to study gamma-ray fluxes of 100 TeV but with a wider angle of observation and much larger collecting area.

Whether or not the CTA can pinpoint the existence of dark matter, it will likely allow astronomers to discover more superbubbles, pulsar wind nebulae, supernova remnants and gamma-ray bursts, each more revealing than the last about the universe’s deepest secrets.

Type 1a supernova spotted in M82

(A version of this piece appeared on The Hindu, Chennai, website on January 22 as written by me.)

A Type 1a supernova was spotted a few hours ago by stargazers in the starburst galaxy M82, which is only 11.4 million light-years away from Earth (here’s an interactive map and a helpful sky-chart). This is the closest such supernova that has been detected since 1972, and is poised to give astronomers and cosmologists some invaluable insight into how such stellar explosions pan out, and what we can learn about neutrinos, gamma rays and dark energy from them.

See the bright, blinking spot of light on the galaxy's 'lower' half? That's your SN1a. — See the bright, blinking spot of light on the galaxy’s ‘lower’ half? That’s your SN1a. Animation by E. Guido, N. Howes, M. Nicolini

The supernova is a Type 1a supernova (SN1a), which means it’s not the explosion that happens when a star runs out of fuel and blows itself apart. Instead, it’s what happens when a white dwarf pulls in too much material from a nearby star and blows itself apart—having bitten off more than it could chew.

That M82 is a starburst galaxy means it’s rapidly producing stars. This also means it has a lot of old stars, many of which are continuously dying. They could either be dying as Type 2 supernovae—which is the run-out-fuel kind—or Type 1. The SN1a that’s gone off now (i.e. so many millions of years ago) has chosen to go off as Type 1a, and that’s a good thing because we haven’t spotted a Type 1a since 1972 that’s so close.

When the explosion releases light, it doesn’t immediately start its journey and head straight for Earth. Instead, the light gets trapped in the explosion behind lots of matter, and is delayed. In fact, the ‘ghost particles’ that can pass through matter almost undetected, neutrinos, get a headstart. They reach us before light from the explosion does.

However, a Type 1a supernova produces far fewer neutrinos than does a Type 2, so while the neutrinos flying our way will still be valuable, they might not be valuable enough to study a supernova with. On the other hand, the M82-SN1a could be our big chance to study SN-origin gamma rays in the best detail for the first time in more than four decades.

However, since we haven’t had our detectors trained for neutrinos from M82 particularly, how do we know when that white dwarf in M82 blew up? We measure how its brightness varies over time. Using that information, we know the thing blew up 11.4 million years ago. Because a 1a’s variation of brightness over time consistently follows a well-established pattern, white dwarfs across the universe can be used as cosmic candlesticks: astronomers use them to judge the relative distances of nearby objects.

In fact, white dwarfs did play an important role in astronomers discovering that the universe was expanding at an accelerating rate due to dark energy. Paraphrasing astronomer Katharine Mack’s tweet: “With a better estimate of the distance [as judged from their brightness], we get a better link between the distance and the universe’s expansion.”

M82’s relative closeness is useful because it provides a lot more information to work with before it could get (more) adulterated through the distance of space. In fact, according to astronomer Daniel Fischer, the supernova’s been going on for a full week now, and was missed by the bigger budget telescopes because it was, and I quote, ‘too bright’. As Brad Tucker, an astronomer from Berkeley, tweeted,

[tweet https://twitter.com/btucker22/status/425989187188187137 hide_thread=’true’]

So, hadn’t it been for amateur astronomers, who’ve made this remarkable observation, too, we wouldn’t have spotted this beauty. Already, according to Skymania’s Paul Sutherland, astronomers believe they’ve caught this supernova early in its act and think it could brighten even further.

This particular find was made by Russian amateur astronomers on January 22, and later confirmed by multiple sources. In fact, M82-SN1a seems to have appeared in the photographs taken by noted Japanese amateur astronomer Koichi Itagaki on January 14 itself (beating Patrick Wiggins by a day). And if you’re interested in reporting such discoveries, check this page out. If you want to keep up with the social media conversation over M82, follow @astrokatie. She’s going nuts (in a good way).

Itagaki's photos of M82 — Itagaki’s photos of M82

Ambitious gamma-ray telescope takes shape

I wrote a shortened version of this piece for The Hindu on July 4, 2013. This is the longer version, with some more details thrown in.

—

Scientists and engineers from 27 countries including India are pitching for a next-generation gamma-ray telescope that could transform the future of high-energy astrophysics.

Called the Cherenkov Telescope Array (CTA), the proposed project is a large array of telescopes to complement existing observatories, the most potent of which are in orbit around Earth. By building it on land, scientists feel the CTA could be much more sophisticated than orbiting observatories, which are limited by logistical constraints.

Werner Hofmann, CTA spokesperson of the Max Planck Institute for Nuclear Physics, Germany, told Nature, a comparable orbiting telescope would have to be “the size of a football stadium”.

The CTA’s preliminary designs reveal that it boasts of greater angular resolution, and 10 times more sensitivity and energy-coverage, than existing telescopes. The collaboration will finalise the locations for setting up the CTA, which will consist of two networked arrays in the northern and southern hemispheres, by end-2013. Construction is slated for 2015 at a cost of $268 million.

One proposed northern hemisphere location is in Ladakh, Jammu and Kashmir.

Indian CTA collaboration

Dr. Pratik Majumdar, Saha Institute of Nuclear Physics (SINP), Kolkata, said via email, “A survey was undertaken in the late 1980s. Hanle, in Ladakh, was a good site fulfilling most of our needs: very clear and dark skies throughout the year, with a large number of photometric and spectroscopic nights at par with other similar places in the world, like La Palma in Canary Islands and Arizona desert, USA.”

However, it serves to note that the Indian government does not permit foreign nationals to visit Hanle. “I do think India needs to be more proactive about opening up to people from abroad, especially in science and technology, in order to benefit from international collaboration – unfortunately this is not happening,”said Dr. Subir Sarkar, Rudolf Peierls Centre for Theoretical Physics, Oxford University, via email. Dr. Sarkar is a member of the collaboration.

Each network will consist of four 23-metre telescopes to image weaker gamma-ray signals, and dozens of 12-metre and 2-4-metre telescopes to image the really strong ones. Altogether, they will cover an area of 10 sq. km on ground.

Scientists from SINP are also conducting simulations to better understand the performance of CTA.

Led by it, the Indian collaboration comprises Indian Institute of Astrophysics, Bhabha Atomic Research Centre, and Tata Institute of Fundamental Research (TIFR). They will be responsible for building the calibration system with the Max Planck Institute, and developing structural sub-systems of various telescopes to be fabricated in India.

Dr. B.S. Acharya, TIFR, believes the CTA can add great value to existing telescopes in India, especially the HAGAR gamma-ray telescope array in Hanle. “It is a natural extension of our work on ground-based gamma-ray astronomy in India, since 1969,” he said in an email to this Correspondent.

Larger, more powerful

While existing telescopes, like MAGIC (Canary Islands) and VERITAS (Arizona), and the orbiting Fermi-LAT and Swift, are efficient up to the 100-GeV energy mark, the CTA will be able to reach up to 100,000 GeV with the same efficiency.

Gamma rays originate from sources like dark matter annihilation, dying stars and supermassive black holes, whose physics has been barely understood. Such sources accelerate protons and electrons to huge energies and these interact with ambient matter, radiation and magnetic fields to generate gamma rays, which then travel through space.

When such a high-energy gamma-ray hits atoms in Earth’s upper atmosphere, a shower of particles are produced that cascade downward. Individual telescopes pick these up for analysis, but a network of telescopes spread over a large area would collect greater amounts, tracking them back better to their sources.

Here, CTA’s large collection area will come to play.

“No telescope based at one point on Earth can see the whole sky. The proposed CTA southern observatory will be able to study the centre of the galaxy, while the northern observatory will focus on extragalactic sources,” said Dr. Sarkar.

Gamma-ray astronomy has seen global interest since the early 1950s, when astronomers began to believe some cosmic phenomena ought to emit the radiation. After developing the telescopes in the 1960s to analyse it, some 150 sources have been mapped. The CTA is expected to chart a 1,000 more.

The HESS II gamma-ray telescope in the Khoma Highland, Namibia, is currently the world's largest telescope for gamma-ray astrophysics, possessing a 28-meter wide mirror. — The HESS II gamma-ray telescope in the Khoma Highland, Namibia, is currently the world’s largest telescope for gamma-ray astrophysics, possessing a 28-meter wide mirror.