The most powerful way a star can die is through a supernova explosion. Through nuclear physics and the pull of gravity, a star can be ripped apart, shining brighter than an entire galaxy for a brief moment of cosmic time. There are several kinds of supernovae, but one particular kind known as Type Ia is particularly interesting to astronomers. And we still aren’t entirely sure how they occur. 

Type Ia supernovae are useful to astronomers because they always explode with about the same intensity. By measuring the apparent brightness of distant Type Ia supernovae, we can calculate the size of our universe, and how rapidly it’s expanding. But what makes a Type Ia supernova so consistent? Other supernovae can vary in brightness, so why not these?

The most popular theory is that they are caused by particular kind of binary star, where a white dwarf orbits a red giant. White dwarf stars typically have a mass about equal to our Sun. That’s because if they get much larger they reach what is known as the Chandrasekhar limit, and collapse under their own weight. If a white dwarf happens to orbit closely to a red giant, outer layer of the red giant can be captured by the gravitational pull of the white dwarf. This increases the mass of the white dwarf. And if its mass reaches the limit, then boom!

Evidence of an early helium flash before a supernova is triggered. Credit: Institute of Astronomy, the University of Tokyo

At least that’s the idea. The challenge is to work out the details. Just how does the gravitational collapse of a white dwarf trigger the type of nuclear reactions necessary to cause the star to explode? But new research shines more light on that question.

Using data from the Subaru telescope and the Gemini-North telescope, a team of astronomers captured the earliest moments of a Type Ia supernova. They found that a small initial flash occurred before the much larger supernova explosion. The spectrum of the initial explosion seems to indicate that it was caused by the rapid fusion of helium on the star’s surface. This effect has also been seen in computer simulations.

In this model, a build up of helium on the surface of the white dwarf causes it to start fusing rapidly. The explosive detonation of helium creates shock waves that propagate to the center of the star, where they trigger a chain reaction of fusion in its core. In this way, the explosion is not caused directly by reaching the Chandrasekhar limit, but by a chain reaction of rapid nuclear fusion. This would explain why there is some small variation in the brightness of Type Ia supernovae. It could also help us better refine how these supernovae are used to measure galactic distances.

We’ll need to observe more of these supernovae to learn whether this is the primary trigger mechanism for Type Ia supernovae, or if there are other ways these stars can be detonated. But for now it seems clear that with a bit of helium we can detonate a star.

Paper: Ji-an Jiang, et al. A hybrid type Ia supernova with an early flash triggered by helium-shell detonation. Nature 550, 80–83 (2017)

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In the spring 0f 1006 an object appeared in the night sky in the constellation Lupus. Its appearance was noted by astronomers across the world, and it was said to be extremely bright. By some accounts it could be seen during the day, and estimates put it at being 16 times brighter than Venus. It is thought to have been the brightest supernova to appear in recorded human history. 

Despite its clear record as a supernova, a remnant of the stellar explosion wasn’t easy to find. It wasn’t until 1965 that a radio source was observed showing a circular shell of radio emissions characteristic of a remnant. It has since been confirmed as originating with SN 1006. We now know that the supernova occurred about 7,000 light years away. Detailed searches of the region find no evidence of a remaining stellar companion, so it was probably caused by the merger of two white dwarfs, making it a type Ia supernova. It is this kind of supernova that we use as a standard candle to determine the distance of galaxies.

Despite the closeness and brightness of this supernova, there were no serious effects on Earth at the time, which goes to show that much of the “Betelgeuse will kill us” hype is largely nonsense. That isn’t to say there wasn’t any effect. The supernova did cause a flux of gamma rays that left a record in Antarctic ice. This is true for other close supernova events as well, where spikes in nitrate concentrations have been found to correlate with historical supernovae.

Most recently, a new historical observation of the supernova has come to light, made by the Persian astronomer Ibn Sina. From historical reports such as these we can try to get a better understanding of things such as its apparent color and light curve. Historical observations are all we have, since there hasn’t been a close supernova in the history of modern astronomy.

Paper: Yuko Motizuki, et al. An Antarctic ice core recording both supernovae and solar cycles. arXiv:0902.3446 [astro-ph.HE]

Paper: Ralph Neuhaeuser, et al. An Arabic report about supernova SN 1006 by Ibn Sina (Avicenna).  arXiv:1604.03798 [astro-ph.SR]

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Virtually all the elements beyond hydrogen and helium come not from the big bang, but from the astrophysical processes of stars. The Earth and Moon are largely made of stardust, as the saying goes. We generally think of this process as having occurred long before our solar system formed, but it is an ongoing process we can see occurring more recently as well.

Evidence of cosmologically young elements on Earth can be seen in ocean floor sediment. In layers about 2 million years old there are concentrations of iron-60. This particular isotope of iron is interesting because it is seen in supernova remnants, and is is mainly supernova explosions that produces it. It also has a half life of about 2.6 million years. Any iron-60 that was around during the formation of our solar system billions of years ago would have long since decayed into cobalt-60. So the spike of iron-60 in ocean sediment would seem to be due to a supernova that occurred a couple million years ago. To be present on Earth, it would also need to have occurred fairly close to our solar system.

But if that’s the case, then we should see similar evidence on other solar system bodies. A recent paper in Physical Review Letters finds just such evidence on the surface of the Moon. It comes from lunar samples gathered by the Apollo missions. Measuring the isotope levels of lunar material, a team found that it had concentrations of iron-60 about 10 times higher than you would expect normally, similar to the spike in layers of ocean sediment on Earth. The team also found elevated levels of manganese-56 which is produced when cosmic rays strike iron atoms. This is important because iron-60 can also be produced by cosmic rays. By comparing the manganese and iron isotopes, it became clear that cosmic rays couldn’t account for such a high level of iron-60. Thus it must be due to a local supernova.

From the lunar levels, this supernova likely occurred roughly 2 million years ago about 300 light years away from Earth, which is surprisingly close on a cosmic scale.

Paper: L. Fimiani et al. 2016. Interstellar Fe⁶⁰ on the Surface of the Moon. Phys. Rev. Lett. 116, 151104; doi: 10.1103/PhysRevLett.116.151104

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When we observe a supernova, we can identify its type by looking at its spectrum. Supernovae that have strong hydrogen lines are known as type II, and are caused by the core collapse of a star at the end of its life. Stars without a strong hydrogen line are known as type I, and there’s a variation of type I supernovae with a strong silicon line known as type Ia. This last variety is particularly useful for astronomers, because they always have about the same absolute magnitude. This means they can be used as “standard candles” to determine the distance of a galaxy. While we know type Ia supernovae are not caused by the core collapse of a large star, we aren’t entirely sure what triggers them.  

There are two main models for type Ia supernovae. The first is that they are caused when a white dwarf star closely orbits a companion star. The outer layer of the companion star can be gravitationally captured by the white dwarf, which causes a runaway fusion process that results in a supernova. The other is that they are triggered by the collision of two white dwarfs. Understanding the origin of these supernovae is important, particularly in light of the fact that there is some evidence they might not be as standard as we thought.

One of the challenges with studying these supernovae is that they typically occur in other galaxies. There hasn’t been a clear supernova in our own galaxy for several hundred years, even though there’s evidence that one occurs in our galaxy about every 50 years. This is likely due to the fact that much of our galaxy is blocked by gas and dust, making them difficult to observe.  But recently observations from the Chandra x-ray observatory have found the remnant of a supernova in our galaxy that occurred about 110 years ago. Since it’s in a region of the galaxy rich in dust, the visible light from the supernova was obscured, which is why astronomers at the time didn’t see it. Looking through archive data from Chandra, a team found the remnant has been increasing in brightness at x-ray and radio wavelengths as the remnant interacts with interstellar gas and dust. Such an increase is what you’d expect from the collision model of type Ia supernovae, but not the binary model.

While this supports the idea that type Ia supernovae can occur from white dwarf mergers, it doesn’t necessarily mean that they can’t occur through a dance of binary stars as well. It’s possible that both mechanisms can produce supernovae of this type. It will take more observations to determine whether mergers are the only cause.

Paper: Sayan Chakraborti, et al. Young Remnants of Type Ia Supernovae and Their Progenitors: A Study of SNR G1.9+0.3. The Astrophysical Journal, Volume 819, Number 1 (2016)  arXiv:1510.08851 [astro-ph.HE]

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What do you do when you see the flash? In the case of a dying star, you know a supernova is coming. 

A core-collapse supernova occurs when a large star runs out of hydrogen and other elements to fuse. The fusion of elements in a star’s core creates the heat and pressure necessary to prevent a star from collapsing under its own weight. When a star stops fusing elements, its core cools rapidly and collapses. The heavy elements in the core slam into each other and recoil, creating a shockwave that rips the star apart. This causes a dramatic brightening of the star over a period of days, which is what we observe as a supernova. But before the star explodes the shockwave ripples through the layers of the star. It first causes instabilities in the star’s surface, such as plasma jets. When the full shockwave reaches the surface, it liberates a tremendous amount of photons from the star’s interior, causing an initial flash of light before the star begins to brighten.

At least that was the theory. The problem is the initial flash only lasts a few minutes, and it occurs before the star swells into a supernova. Usually we don’t notice a supernova until the star has already brightened a bit. To see the initial shockwave flash, you need to be watching a star before it goes supernova. Since we have no way of predicting when a particular star will begin to explode, we haven’t been able to catch the initial flash. That is, until now, when the Kepler space observatory happen to observe a supernova in its earliest moments.

The Kepler space observatory was designed to find exoplanets. It does this by observing stars for long periods of time, measuring their brightnesses to look for small dips in brightness. Such dips can indicate that a planet is passing in front of the star. It just so happened that a star in its field of view began to go supernova, and so Kepler caught the initial flash of the shockwave. It was really just blind luck, but it confirms the shockwave of a core-collapse supernova.

As we continue to make large scale sky surveys, the chances of observing the early stages of a supernova such as this become more likely. That’s important because it’s only by studying the early stages of a supernova that we will gain a better understanding of their triggering mechanisms.

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A superluminous supernova is an immense supernova more than ten times that of the type Ia supernovae used to measure cosmic distances. They are so intense that they challenge our understanding of just how they occur. Two possible mechanisms include the idea that they may be caused by magnetic heating as the core collapses into a magnetar, or that it’s intensity is strengthened by pair-instability reactions in its core. The evidence leaned toward the magnetar model, but observations of a new supernova challenge that idea. 

The new supernova is known as ASASSN-15lh, and it was more luminous than any supernova ever seen. About 20 times more luminous than the entire Milky Way. It’s light has traveled for about 2.7 billion years, so it’s apparent brightness in our sky wasn’t particularly bright, but in terms of absolute magnitude it was about three times as bright as other known superluminous supernovae. It is also unusual in that it occurred in a bright galaxy where there is not much new star formation. Other superluminous supernovae occur in active dwarf galaxies.

The team observing the spectra of this supernova found that it was low in hydrogen. This is indicative of a star that has cast off its hydrogen-rich outer later, and would seem to support the magnetar model. But the extreme energy of ASASSN-15lh puts it at the upper limit of the model. If this was indeed a magnetar supernova, then it was at the upper limit of the hypothetical energy range. That seems a bit unusual, and it raises the question of whether the magnetar model might be flawed.

The key to solving this mystery will be the discovery of similar superluminous supernovae. This particular supernova was discovered by the All Sky Automated Survey for SuperNovae (ASASSN) which is a collection of small (14 centimeter) telescopes in Chile and Hawaii. It’s a relatively low cost project that lays the groundwork for larger projects such as LSST. So over time we’re bound to find similar supernovae.

Paper: Subo Dong, et al. ASASSN-15lh: A highly super-luminous supernova. Science Vol. 351, Issue 6270, pp. 257-260 (2016)

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The Crab Nebula is a pulsar that’s only about 1,600 light years away. It is the remnant of a supernova that occurred in 1054, and was recorded by Chinese astronomers. Pulsars are rotating neutron stars that produce bursts of energy we observe as pulses. Most pulsars are observed at radio wavelengths, but the Crab Nebula pulsar can also be observed in the visible. Because of its relative proximity, it’s also one of the few neutron stars we can observe directly. You can see it as the bright central dot in the x-ray image above.

When the supernova occurred in 1054, it had a maximum magnitude of about -6, which is much brighter than the brightest stars in the sky, and even brighter than Venus at its maximum. It should have been easily visible across the globe, and yet there is limited confirmed recording of the event in historical records. There are some hints of recordings, but the Chinese observation is the only one with sufficient accuracy to confirm. It’s an interesting example of how transient events in the sky didn’t always gain attention.

One of the mysteries about the Crab Nebula is that the calculated mass is about 3 solar masses. Combined with the mass of the neutron star itself (about 2 solar masses), the estimated mass of the original star would be about 5 solar masses. However to create a supernova of this size and chemical composition, the original star should have been about 9 – 11 solar masses. We’re still not sure where the missing mass went, though a good possibility is that the outer layers of the star were pushed away by the star before the supernova occurred. We see this happen with Wolf-Rayet stars.

The Crab Nebula itself can be seen with the naked eye, and some of its structure can be observed with binoculars or a small telescope. So if you get the chance, it’s worth checking out.

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In August of last year a star briefly brightened by a factor of 5 in a single day. Known as Gaia 14aae is a cataclysmic variable. It is a binary where the two stars orbit closely enough for the outer layers of one star to be captured by the other. Not only do these stars orbit each other every 50 minutes, their orbits are aligned so that the larger star totally eclipses the smaller one  with each orbit. 

The light curve of the binary system. Credit: Campbell et al.

The smaller star actually has the greater mass. It is a white dwarf with a mass about 80% that of our Sun. The larger star only has a mass 15% of our Sun, but has expanded to 125 times the diameter of our Sun as it approaches the end of its life. One of the striking features about this binary is that it doesn’t contain much hydrogen. This is exactly what you’d expect if the white dwarf has already stripped off much of the outer layer from its companion star. As the white dwarf continues to capture material from the other star, we can expect more flare ups similar to the brightening of last year.

But what makes this system particularly interesting is that it could provide clues as to just how type Ia supernovae might form. These “standard candle” supernovae are used to determine the distances of the earliest galaxies, but we still aren’t entirely sure what causes them. One possibility is the stellar dance of close binaries like Gaia 14aae.

Paper: H. C. Campbell et al. Total eclipse of the heart: the AM CVn Gaia14aae/ASSASN-14cn. MNRAS 452 (1): 1060-1067 (2015)

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In general relativity, gravity is represented not as a force, but as a curvature of spacetime. This was first verified in 1919 when Arthur Eddington observed the deflection of starlight during a solar eclipse. Since then we’ve observed lots of examples of such gravitational lensing.  Often these occur when a distant quasar is obscured by a closer galaxy, though the effect is also seen when measuring the positions of stars. Usually, the gravitational lensing we see by galaxies are largely static, since the background quasar or galaxy is a fairly uniform light source, but a new paper in Science presents the observation of a distant supernova gravitationally lensed by a closer galaxy. This has produced multiple images of the explosion not only at different points in space, but also at different points in time.

Multiple appearances of the supernova.
Credit: NASA/Hubble

The team happened to observe a supernova that occurred about 9 billion years ago. Since it happened to be located almost exactly behind a closer galaxy, it was seen in multiple locations due to the gravitational effect of the galaxy. The gravitational lensing also made the supernova appear brighter than it would on its own, since gravitational lensing can magnify an image similar to the way a telescope can. We’ve seen this kind of thing before, but the difference is that a supernova brightens quickly and then fades, unlike a distant galaxy or quasar. This meant the light reaching us was only visible for a short period of time. But since light traveling around one side of the foreground galaxy travelled a different distance than light taking a different path around the galaxy, the flash of the supernova appeared not only at different locations, but at different times. The team saw four different images of the supernova over the course of a few weeks.

Having observed this, the team figured that other images of the supernova should have appeared at different times before. Looking through past observations of the galaxy, they found other appearances of the supernova in 1964 and 1995. They also estimated that another appearance should occur in a few years. Because of the great cosmic distances, different paths of the supernova’s light can vary in their travel times by decades, so we can observe this single stellar explosion again and again as the light reaches us along different paths.  We’ve known this kind of thing should happen in theory, but it’s really neat to actually observe the effect.

As we see more appearances of the supernova, it will give us detailed information about the distribution of matter around the galaxy. Combining information about the amount of lensing with the light travel time, we can make a map of the shape of spacetime near the galaxy, and that will tell us about the distribution of both regular and dark matter in the region.

Paper: Kelly et al. Multiple images of a highly magnified supernova formed by an early-type cluster galaxy lens. 347 (6226): 1123-1126 (2015)

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A couple weeks ago I wrote about the discovery of a supernova ten times more powerful than so-called type Ia supernova. This superluminous supernova known as SN2007bi was not only extremely bright, but it remained bright for about five months. Such a long brightness period is indicative of a type of supernova known as a pair instability supernova, so called because the intense gamma rays produced in the core during the explosion form pairs of electrons and positrons (electron pairs).

It’s generally thought that SN2007bi is a clear example of a pair instability supernova due to its intensity and long brightness period, but now new supernova observations suggest that SN2007bi wasn’t a pair-instability supernova after all. But if it wasn’t, then how could it remain so bright for so long?

In a recent paper in Nature (behind a paywall, but the arxiv version is here), a research team looked at data from two new superluminous supernovae, known as PTF 12dam and PS1-11ap. Both of these were extremely bright and had long brightness periods, just like SN2007bi. But while SN2007bi was only observed once it was close to maximum brightness, the new supernovae were observed as they grew brighter.

This is important because pair instability supernovae should not only stay bright for a long time, they should also take a long time to become bright. This is because the debris from a pair-instability supernova is so massive and dense that it takes time (on the order of a year) for the heat and radiation from the explosion to filter through the debris.

If you’ve ever cooked with a heavy cast-iron pan as opposed to an aluminum pan, you know that the cast iron pan takes much longer to heat up, but once hot it stays hot for a long time. A pair-instability supernova is similar. It’s so massive that it takes a long time to rise to maximum brightness, and once bright it takes a long time to fade.

These two new supernovae did not take a long time to reach maximum brightness. They reached their maximum relatively quickly (about two months). This means they can’t be pair instability supernovae. But they still took a long time to fade, just like SN2007bi. This means our best pair-instability candidate might not have been one after all. But if it wasn’t, then how could it remain so bright for so long?

The answer seems to be a magnetic heating effect. When a supernova explodes, the core typically collapses into a neutron star. The magnetic fields of a neutron star can be quite strong, and in their most extreme case the neutron star is often referred to as a magnetar. The intense magnetic field of a magnetar can interact with surrounding material, causing it to superheat. This is similar to a magneto, where a rotating magnet can produce pulses of electric current.

Credit: M. Nicholl et al.

The authors propose that a large supernova (hypernova) can produce a magnetar when it explodes. Since the hypernova isn’t as massive as a pair-instability supernova, it can brighten fairly quickly. Normally, it would fade more quickly as well, but the intense magnetic field of the magnetar heats the surrounding debris, causing it to remain hot (and therefore bright) for much longer. When the team compared the theoretical brightness curve of a magnetar hypernova with the observed brightness of PTF 12dam, they found it matched really well. You can see this in the figure below (taken from the paper). The black curve shows the theoretical brightness of a magnetar supernova, and the black circles show the observed brightness. The other lines are theoretical pair-instability brightness curves of different masses.

Not only does the data match the magnetar theory quite well, it is also clear that a magnetar supernova fades slowly in a way that looks a lot like a pair instability supernova. This means we can’t prove a supernova is a pair-instability one simply by observing it after maximum brightness. We also need to observe its brightening period. Since we didn’t observe the brightening of SN2007bi, we can’t be sure about it. It may have been a pair instability supernova, or it may have been a magnetar supernova.

So we’re now not sure whether we’ve observed a pair instability explosion or not, but we now have a better understanding of superluminous supernovae.

Sometimes in science you have to take a step back before you can take a step forward.

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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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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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