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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About a year ago in Nature astronomers reported evidence of an anti-glitch in the magnetar 1E 2259+586. You might remember from yesterday’s post that a magnetar is a neutron star with an extremely strong magnetic field. This particular magnetar is also a pulsar, meaning that the intense x-ray beams that stream from the magnetar’s polar region happen to be aligned so that we see it flash regularly. You can think of a pulsar as a kind of cosmic lighthouse, if you will.

The rate at which a pulsar flashes is determined by the rate at which it rotates. So if you see a pulsar flash 10 times a second, you know it rotates once every tenth of a second. (It can be a bit more complicated than that, but you get the idea). The rotation of a pulsar is usually pretty regular. Pulsars can gradually slow down over thousands or millions of years due to its radiated energy, but this is a gradual process.

Occasionally, however, a pulsar will speed up just a bit in a very short time (on the order of minutes). This rapid speed-up is known as a glitch. After a glitch the pulsar will return to its previous speed within weeks or months, and then continue with its gradual, thousands-year slowdown.

It is generally thought that these glitches are due to changes in the shape of the neutron star. Because of their rotation, neutron stars should bulge a bit at their equator. The faster their rotation, the greater the bulge. As a neutron star gradually slows down, its equatorial bulge would tend to decrease. But it’s thought that the neutron matter in the crust of the star is fairly rigid. This means that as the pulsar gradually slows down, stresses would build up in the pulsar’s crust. This would eventually reach a breaking point. The crust would then collapse to form a more stable, less bulgy shape. Because of this new shape, the pulsar would speed up a bit, just like a spinning figure skater who spins faster when she pulls her arms inward.

We have observed lots of glitches in pulsars, and they all behave in a similar way. But the Nature article presents an observation of an anti-glitch. In other words, researchers observed a rapid slowdown in this particular magnetar. You can see this slowdown in the graph below. Such an anti-glitch is very strange, because it means either it was caused by an external interaction, or our understanding of neutron stars will need to be revised.

The slowdown of this magnetar actually happened in two stages. There was an initial anti-glitch, and then a second shift that could be modeled as a glitch or a second anti-glitch, depending on how you look at it. This means the magnetar had a quick slowdown, then a second adjustment a little while later. This could have been caused by a twisting and reconnection of the star’s magnetic field. On the Sun, such phenomena causes solar flares and coronal mass ejections. If a similar process occurred on the magnetar, then it could slow it down by transferring some of the angular momentum from the star to the material ejected and the magnetic field. But such a process would likely cause an x-ray burst during both the initial anti-glitch and the secondary readjustment. Such activity was observed during the first anti-glitch, but not the second, so this doesn’t seem very likely.

The alternative is that the magnetar is rotating differentially. That is, the superfluid core of the magnetar is rotating at a different speed than its crust. If that is the case, then as the crust shifted it could some rotation from the superfluid to the crust. The resulting transfer would cause the initial slowdown, and then the secondary shift would be caused by later readjustment into a more stable state. This model matches the observations, but it would mean that neutron stars can rotate differentially, which we didn’t think happened. If that’s the case, then we will need to reexamine the glitch model as well, since that may be caused by differential rotation as well.

So it looks like we’ll need to look at pulsar glitches a bit more closely.

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One of the differences between astronomy and astrophysics is that astronomy is based upon observation, while astrophysics is about the underlying mechanism behind those observations. For this reason, many types of phenomena in the universe have multiple names depending on how we observe them. The reason for this is that typically astronomers start observing different phenomena, give them names, and then only later do astrophysicists figure out that they are different examples of the same thing. By then the names have already stuck.

I’ve talked about this before, where radio galaxies, quasars and blazars are all caused by supermassive black holes in the center of galaxies. A similar thing occurs with neutron stars. A neutron star is the remnant of a supernova. When a large star explodes, the core of the star collapses into an object so dense that it is comprised almost entirely of neutrons. Depending on its formation, orientation and surrounding environment, it can also be known as a pulsar, magnetar or x-ray binary.

Animation of the Crab Nebula pulsar.
Credit: S. Klepser, MAGIC Collaboration

A pulsar is neutron star that appears to pulse rapidly, usually at radio frequencies, but sometimes in the visible and even x-ray spectrum. You can see an example in the animated crab pulsar image here. A neutron star typically has a very strong magnetic field. This magnetic field can interact with surrounding ionized plasma to create intense electromagnetic beams that are directed from its magnetic poles. The magnetic poles are typically offset a bit from the neutron stars rotational poles, so the poles (and the beams) rotate around, similar to the way a lighthouse sweeps around a beam of light. If the beam is oriented so that it sweeps in our direction, we see a pulse of energy. The rate of rotation for the neutron star determines the rate of pulses we see.

A magnetar is a neutron star with an extremely strong magnetic field. They also tend to rotate more slowly than other neutron stars. Because of the strength of their magnetic fields, their polar beams are typically x-rays and gamma rays. They also geologically active, and can have starquakes (similar to earthquakes on Earth). These realignments of the magnetar’s crust can create large gamma ray flares, such as seen in soft gamma repeaters.

If a neutron star has a companion star, then it can become an x-ray binary. In this case the neutron star captures some of the material from the outer layers of the companion star. As the material falls toward the neutron star it is accelerated and heated, causing it to emit x-rays. The result is an x-ray source that is part of a binary system. X-ray binaries can also be caused by white dwarfs and black holes.

Sometimes what appears to be radically different phenomena can have a similar cause.

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A magnetar is a neutron star with an extremely strong magnetic field, a billion times stronger than the strongest fields we can create on Earth.  As a neutron star, magnetars also have very strong gravitational fields, with a surface gravity a hundred billion times that of Earth.  Such a high gravity would seem to ensure that a magnetar is spherical, but a magnetar’s strong gravitational field could distort the star, making it more of an oblate spheroid.  We’ve suspected that such a magnetic distortion could occur with magnetars, but now a research team seems to have found an example of this phenomenon.

The result was published in Physical Review Letters, and analyzes data from a magnetar known as 4u 0142+61, which is also a pulsar.  As you might remember, pulsars are neutron stars that emit strong energy pulses in our direction as they rotate. Because these pulses are of very short duration, we can make very accurate measurements of their timing.  This allows us to measure the rotation of a pulsar with great precision.  The measurement is so precise that we can observe their how rotational periods gradually lengthen as the neutron star loses rotational energy.  We can even observe small glitches in their rotation due to starquakes.

The precession of the magnetar due to its oblate shape. Credit: K. Makishima, et al.

Normally the rotational period of a pulsar will gradually lengthen, with the occasional glitch that causes a sudden shortening of its period, only to continue its gradual lengthening.  But in the case of 4u 0142+6, the authors discovered something rather odd. They measured the pulses of x-rays from the magnetar, and found that sometimes the pulses arrived a bit earlier than expected, and at other times a bit later.  It appeared that the rotational period had a slight oscillation to it. This shouldn’t be possible for a spherical neutron star, but it is possible for an oblate neutron star. That’s because an oblate spheroid precesses as it rotates.

Precession is the effect you see with a spinning top, where it wobbles as it spins on its axis.  We see the effect with the Earth because it is an oblate spheroid, which is why the north star of today is not the same as the north star thousands of years ago. The magnetar 4u 0142+6 seems to wobble in a similar way, which is why its pulses are sometimes sooner or later than expected.  For the Earth, the precessional period is about 26,000 years.  For  4u 0142+6, it is about 15 hours.  This means the magnetar differs from a sphere by about 1 part in 5000. When the authors calculated the magnetic field strength necessary for such a deviation, they found that it was about a trillion tesla.  That is higher than most magnetars, but within a reasonable range.

So it seems that this particular magnetar is indeed warped by its own magnetic field, which gives it is weeble wobble motion.

Paper: K. Makishima, et al. Possible Evidence for Free Precession of a Strongly Magnetized Neutron Star in the Magnetar 4U 0142+61. Phys. Rev. Lett. 112, 171102 (2014).

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