Gamma rays are the most energetic forms of light in the Universe. They’re generated by a variety of sources, from the heated material surrounding supermassive black holes, to the supernova explosions of dying stars. But some have theorized they might also be produced by dark matter. 

Dark matter doesn’t interact strongly with regular matter, nor does it interact strongly with light. But since efforts to detect dark matter directly have failed so far, we aren’t entirely sure what makes up dark matter. This has led theorists to develop lots of models about how dark matter might interact with itself. Some dark matter models propose that while dark matter doesn’t interact with regular matter, dark matter particles can collide and annihilate to produce gamma rays, similar to the way matter and antimatter can produce gamma rays through annihilation. Since dark matter is fairly spread out throughout our galaxy, this would produce a diffuse background of gamma rays.

Interestingly, the gamma ray background we observe is diffuse. About 80% of the gamma rays we observe don’t come from a specific source such as supermassive black holes. It’s possible that they come from distant sources we can’t pinpoint, but it could also come from diffuse dark matter interactions. At least that’s been one idea.

But a recent survey of the gamma ray background doesn’t support the dark matter models. Using 81 months of data from the Fermi telescope, the team was able to distinguish the energy levels of different gamma rays, and found that they tend to occur at two energies. The highest energy gamma rays seem to come from known sources such as black holes and supernovae, while lower energy gamma rays don’t have a clear source. However, the distribution and energy range of the lower energy gamma rays is inconsistent with dark matter models, so most of them can be ruled out as the source.

To be clear, this does not mean that dark matter doesn’t exist (as some popular articles have claimed). It does, however, mean that dark matter doesn’t emit gamma rays. So the dark matter enigma continues to evade a solution, and this new study simply adds to the mystery.

Paper: Mattia Fornasa, et al. Angular power spectrum of the diffuse gamma-ray emission as measured by the Fermi Large Area Telescope and constraints on its dark matter interpretation. Phys. Rev. D 94, 123005 (2016)  DOI: 10.1103/PhysRevD.94.123005

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Earlier this year I wrote about a diffuse band of gamma rays coming from regions above and below the Milky Way. The regions spanned about 25,000 light years above and below the galactic plane, and are thought to have formed from an active period of our galaxy’s supermassive black hole about 2 million years ago. While we could determine the size of these regions from their x-ray and gamma ray emissions, it has been difficult to determine their motion. But yesterday at the American Astronomical Society Meeting, new results from the Hubble telescope are measuring the motion of these regions using an interesting trick.

The team looked at ultraviolet light from a distant quasar. The light from this particular quasar passes through the gamma ray bubble before reaching us, and so interacts with the material there. By looking at the spectrum of this light, you can see dark lines where the intervening material absorbs light at particular frequencies. By comparing these absorption lines with ones from materials on Earth we can determine what the material is made of. But you can also determine the speed at which the material is moving due to its Doppler shift.

Normally when we talk about the Doppler effect, it is a shift in the wavelength of light toward the red or blue due to the motion of the light source. But in this case, the material in the region relative to the quasar, and the light it “sees” is redshifted or blueshifted due to its motion. This means if the material is moving toward us the absorption lines are blueshifted, and if they are moving away from us they’re redshifted. Since motion is relative, we can calculate the speed of the material by the shift of its absorption lines.

What the team found was that the material is streaming away from galactic center at about 800,000 m/s. That’s fast, but not quite active black hole fast. So it seems like these lobes of material may be the result of a burst of star production in the central region of the Milky Way. To know for sure we’ll need a more detailed analysis of material motion in this region. Fortunately there are about two dozen quasars that happen to have a line of sight through the region.

So more backlighting will likely illuminate this mystery.

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A supernova is a stellar explosion. They can occur when a large star exhausts its ability to fuse hydrogen into higher elements, and its core collapses. The resulting rebound rips apart the outer layers of the star, creating a supernova while the remains of the core collapses into a neutron star. Another type of supernova, known as a thermal runaway or type Ia, occurs when a white dwarf is a close companion with another star. As outer layers of the companion are captured by the white dwarf, it can trigger a runaway nuclear reaction that rips apart the white dwarf. This latter form always has about the same absolute brightness, which is why they are used to measure the distances of far galaxies.

A type Ia supernova, you might imagine is incredibly bright. The amount of energy released by such a supernova is more than the amount of energy the Sun will release in its entire lifetime. Now imagine a supernova ten times more powerful than that. Such a stellar explosion is known as a superluminous supernova, or hypernova.

Such a super-powerful stellar explosion would be extremely rare in modern galaxies, because they mark the final moments of the largest of stars. Those at the upper limit of mass possible for a star (about 150 -200 solar masses). Such stars are rare today, but it is thought that in the early universe they were more common. Thus, there has been an effort to find hypernovae in distant galaxies, since the travel time of light means we see such galaxies from a time when the universe was much younger.

In 2007, a hypernova known as SN2007bi. Not only was it extraordinarily bright, but it remained near its maximum brightness for nearly five months. This is unusual for a supernova, but such a slow decay is exactly what would be expected for a type of supernova known as a pair-instability supernova.

With a typical core-collapse supernova, the fusion of elements during the collapse of the core produces a great deal of energy in the form of neutrinos and gamma rays (high energy photons). As the gamma rays radiate out from the core, they collide with the outer layers of the star, preventing them from collapsing inward. This somewhat limits the size of the supernova explosion.

With very massive stars, the gamma rays have so much energy that they produce pairs of electrons and positrons. You might recall that we use this effect in gamma ray telescopes. Since the gamma rays produce particle pairs, they don’t work to push back the outer layers of the star. As a result, even more of the star collapses inward, further heating the core and producing more intense nuclear reactions, until eventually the star is completely ripped apart by the hypernova explosion.

With SN2007bi, it now seems that pair-instability supernova exist. Since such supernovae completely destroy themselves, they would typically release massive amounts of heavier elements into the cosmos. Such elements were crucial to the formation of stars such as ours and our solar system. It is very likely that a similar explosion produced the elements such as carbon, iron and oxygen that exist in your body today.

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Yesterday I talked about the Fermi gamma ray telescope, and how it allowed us to make much more precise observations of gamma rays in the universe. Part of the purpose of the Fermi telescope is to observe gamma ray bursts, but its broader purpose is to make a sky survey of gamma ray sources in the universe. Already it has found something quite interesting.

The image above shows the plane of the Milky Way with x-rays indicated in blue and gamma rays indicated in purple. The x-ray regions had been observed earlier by the ROSAT satellite, but it took FGST (Fermi Gamma-ray Space Telescope) to observe the gamma rays. Neither the x-ray nor gamma ray sources are particularly bright. Instead they come from a diffuse region producing these rays.

What’s particularly striking about these “bubbles” is that that are quite large. The regions span 25,000 light years in either direction of the central region of our galaxy. It is so large that it spans more than half the sky. It is caused by a process known as inverse Compton scattering. Electrons moving at speeds near the speed of light collide with low energy (radio or infrared) photons, giving them a massive energy boost and making them gamma rays.

The large size of these bubbles means that they were caused by a large process. They also seem to have a clearly defined boundary. Given that they are caused by electrons moving at nearly the speed of light, and the fact that they extend about 25,000 light years out from galactic center, a likely cause would seem to be from jets emanating from the supermassive black hole in the center of our galaxy. Currently the black hole doesn’t seem to be producing jets, but the bubbles could be evidence of past activity.

Last year new evidence hinted at even more interesting effects. A study published in the Journal for Cosmology and Astroparticle Physics has proposed that some of the gamma rays in the bubbles could be produced by the decay of dark matter in the galaxy. The paper looks at the energy distribution of gamma rays near the galactic plane, and seems to find a spike of gamma rays at an energy of 130 GeV. Such a spike could be evidence of dark matter, but the research looked at such a narrow energy band that the data set consists of only 50 gamma ray photons. Even the author of the paper says this result should be considered very tentative.

Gamma ray astrophysics is still a relatively new field of study, but it is quickly providing us with new results and new mysteries to ponder.

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Yesterday I talked about the detection of gamma ray bursts, intense blasts of gamma rays that occasionally appear in distant galaxies. Gamma ray bursts were only detected when gamma ray satellites were put into orbit in the 1960s. This is because gamma rays are absorbed by our atmosphere. Even then, the detectors were relatively primitive and couldn’t determine the direction of the bursts. Instead, multiple satellites were used to triangulate the location of these bursts. Since then, gamma ray astronomy has gotten much more sophisticated.

Gamma ray photons are so energetic that observing them requires tools from particle physics. As we’ve gotten better at particle physics detectors, we’ve gotten better gamma ray telescopes. You can see this in the Fermi Gamma-ray Space Telescope, launched in 2008.

You may remember that x-ray telescopes require long focal lengths, and therefore are typically the size of a bus. Since gamma ray photons have even more energy and even shorter wavelengths than x-rays, you might think gamma ray telescopes are huge. But in fact, Fermi is only about 3 meters (10 feet) long, not counting the solar panels. This is because the Fermi telescope is not a telescope in the traditional sense. It doesn’t focus gamma ray light with mirrors or lenses. While it is referred to as a telescope, it is really a particle detector.

Gamma ray photons have so much energy that when they collide with material they produce a pair of particles, an electron and positron The positron is the antimatter partner of the electron. It is actually this particle pair that is observed by Fermi, as seen in the image. As the gamma ray enters the telescope, it collides with layers of tungsten (known as conversion foils) which causes the gamma ray to produce an electron-positron pair. Layered between the conversion foils are silicon detectors which allow us to determine the direction of original gamma ray. Finally the particle pair strikes a calorimeter, which measures the energy of the particles (and hence the energy of the original gamma ray).

The real challenge is distinguishing gamma rays from high energy cosmic rays. Cosmic rays have an energy similar to gamma rays, but are actually charged particles such as high energy protons or electrons. They too can be observed by the silicon detectors and calorimeter. So to distinguish them, the entire detector is surrounded by another detector known as an anti-coincidence shield. This shield only detects the passing of a charged particle. Since gamma rays are photons (and therefore have no charge) they don’t trigger the shield. Cosmic rays do trigger the shield. So when an event triggers the Fermi detector and the shield you know it is a cosmic ray. When it just triggers the Fermi detector you know it is a gamma ray. In this way you can filter out the signal from the noise.

The real advantage of the Fermi telescope is that it has a wide field of view with a fairly precise resolution. This means over time it can scan the entire sky and know just where gamma rays originate. With it we’ve learned quite a bit about gamma ray bursts. We’ve also learned some other interesting things, but I’ll talk about that next time.

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In the 1960s a series of satellites were built as part of Project Vela.  Project Vela was intended to detect violations of the 1963 ban on above ground testing of nuclear weapons.  The Vela satellites were designed to detect bursts of gamma rays, which are high energy electromagnetic waves (light) produced by radioactive decay.  If any nuclear weapon was detonated in space, the resulting radioactive decay would release a large amount of gamma rays which would be detected by the Vela satellites.

In 1967, two of the Vela probes detected a large spike of gamma rays.  But the signature of this spike was very different from those of a nuclear explosion.  Soon more gamma ray spikes were detected, and these likewise differed from the expected signature of a nuclear test.  Since the bursts were observed by multiple satellites, the Vela team was able to compare the arrival of the bursts between different satellites, and it soon became clear that the bursts had an extraterrestrial source.  Of course the Vela project was classified, so it wasn’t until 1973 that the results were declassified and published in Astrophysical Journal.  It was only then that astronomers were made aware of these gamma ray bursts (GRBs).

We now know that GRBs are very common.  On average, about one gamma ray burst occurs every day.   They appear randomly in all directions of the sky, and this means they aren’t produced in our galaxy.  If they were, then GRBs would mostly be found along the plane of the Milky Way.

Some gamma ray bursts (known as long bursts) can last more than two seconds.  These bursts have afterglow caused by gamma rays colliding with interstellar material near the event, causing the emission of light at other wavelengths.  This afterglow allows us to measure the redshift of these events, and what we find is that they are quite distant.  The closest observed gamma ray burst occurred at a distance of 100 million light years, and many occurred billions of light years away.

We aren’t entirely sure what causes a gamma ray burst.  Because of their distance, and apparent brightness, they must be extraordinarily energetic, with about 100 times more energy than a supernova.  They may be caused by huge supernova explosions known as hypernova, or they may be caused by supernova explosions occur with a rotational axis pointing in our direction, causing a jet-like burst of energy.  Short burst GRBs, lasting less than 2 seconds, may be due to collisions between neutron stars.  Observing gamma rays with precision is a challenge, and we are only just beginning to gather good observations.

Given the huge energy of GRBs, one might wonder if one could occur in our galaxy.  Given the average rate of GRBs and the huge distances at which they typically occur, the rate at which one happens in our galaxy is probably about once every 5 million years.

So there is no need to fear the big one.

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Type Ia supernovae are brilliant stellar explosions that can outshine an entire galaxy. They also have the useful property of always exploding with a similar brightness. This makes them useful in determining the distances of galaxies. By comparing the observed brightness of a type Ia supernova to its standard brightness, we can calculate the distance of the supernova, and the galaxy in which it resides. But while we know type Ia supernovae have a consistent brightness, we aren’t entirely sure why.

There are two main models regarding how Type Ia supernovae occur. The first is that a white dwarf is in close orbit with a red giant star. Matter from the outer layers of the red giant is captured by the white dwarf, which raises its mass to the point where it collapses, which triggers a supernova explosion. The second is that two white dwarfs are in orbit with each other. As their orbits decay over time, they eventually collide and merge, triggering a supernova.

Both of these models could account for the standard brightness of type Ia supernovae, and both would account for the type of elements seen in supernova remnants. Generally the latter model has been a bit more favored, since binary white dwarfs seem more common than two stars orbiting just close enough to exchange material. White dwarfs capturing material from a companion could also explode in ways that aren’t quite type Ia supernovae.

Now a new paper in Nature finds evidence that points toward the captured material model. The team looked at gamma ray emission from a supernova known as SN2014J. This particular supernova was only 11 million light years away, which is pretty close as supernovae go, so the team was able to measure spectral lines from the element cobalt-56. This particular isotope has a half-life of only 77 days, so it normally isn’t seen directly. From these they determined that about 0.6 solar masses of cobalt-56 was produced by the supernova. This agrees with model of a single white dwarf exploding after capturing matter from a companion.

The authors stress that certain kinds of binary white dwarf collisions could produce a similar result, so this doesn’t definitively answer the mystery. But given their data, the capture model now seems more probable.

Paper: E. Churazov, et al. Cobalt-56 γ-ray emission lines from the type Ia supernova 2014J. Nature 512, 406–408 (2014)

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In the ongoing search for dark matter particles, the most popular are efforts to detect them directly here on Earth.  These experiments involve underground detectors that try to observe the impacts between dark matter and regular matter.  While there have been some interesting hints, the efforts so far have turned up nothing, and have even eliminated many of the low-mass options for dark matter WIMPs (Weakly Interacting Massive Particles).  Another way to look for dark matter particles is to look for the by-product of their collisions with each other.  A recent paper posted on the arxiv has done just that, and they think they’ve found a signal.

The paper hasn’t yet been peer reviewed, so the results should be considered tentative, but they are interesting.  What the team did was analyze data from the Fermi gamma ray telescope, specifically gamma rays emitted in the region of the supermassive black hole in the center of our galaxy.  Assuming dark matter particles exist, they must interact with things gravitationally, and that means near the black hole in the center of our galaxy they would be orbiting rather quickly.  Because of this they would also tend to collide at high speed, causing them to either emit gamma rays directly, or decay into particles that would then emit gamma rays.

The challenge is that there are several sources of gamma rays in the region of the black hole, such as rapidly rotating neutron stars known as millisecond pulsars.  The team eliminated the expected sources of gamma rays, and found that there was a signal that still remained.  You can see this in the image below (taken from their paper) where the left image is the raw gamma ray signal, and the right image is the “extra” signal remaining when the expected sources are removed.

There are a few things that are interesting about this result.  The first is that the strength of the signal follows the expected profile for the distribution of dark matter near galactic center.  The second is that the strength and wavelength of the signal matches matches the predictions for dark matter WIMPs with a mass of 35 GeV, or about 35 times more massive than a proton.  So the results look a bit promising.

But we can’t declare victory quite yet.  As I mentioned, the paper hasn’t gone through peer review.  But even assuming it does, the statistical strength of the signal is only at 3 – 4 sigma, meaning that it has a 1 in 100,000 chance of being a fluke.  That might seem pretty certain, but we have seen direct detections of dark matter particles with at a similar statistical strength fade away upon further investigation.  It is still possible that this result will also fade away under further investigation.  To really be sure of the result we’d like to see a 5-sigma outcome, which would be less than a one in a million chance of being a fluke.

What is clear is that we are starting to get various hints of dark matter from different sources.  So hopefully soon we will either have a conclusive dark matter discovery, or clear evidence that we need to look elsewhere.

Paper: Tansu Daylan, et al. The Characterization of the Gamma-Ray Signal from the Central Milky Way: A Compelling Case for Annihilating Dark Matter. arXiv:1402.6703 [astro-ph.HE] (2014)

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