A pulsar is a star that emits a regular pulse of energy, usually on the order of a few seconds up to hundreds of times a second. They were first discovered by Jocelyn Bell in 1967, and since then we’ve found more than 1,500 of them. While their source was once a mystery, we now know that they are caused by rotating neutron stars. All the pulsars we’ve found have been neutron stars, but does a pulsar have to be a neutron star? Nope, it turns out white dwarfs can be pulsars too. 

Animation of a pulsar. Credit: Jodrell Bank Centre for Astrophysics

Pulsars are produced when the strong magnetic fields of these stars interact with surrounding plasma to create beams of energy that stream out from the magnetic poles of a neutron star. When the magnetic poles of a neutron star are tilted a bit from its axis of rotation, the beams of energy can sweep around like a lighthouse. If a beam sweeps in the direction of Earth, we see it as a regular pulse.

Neutron stars make good pulsars because they are incredibly dense. A neutron star about twice the mass of our Sun would only be about 20 kilometers across. This means the magnetic field of a neutron star can be incredibly strong, since it is packed into such a small size, and it’s the magnetic field that makes the energy beams so powerful.

It’s long been speculated, however, that white dwarfs could also become pulsars. A white dwarf is similar to a neutron star, but instead of being collapsed down to the size of a small city, its stellar mass is compressed to about the size of Earth. White dwarfs aren’t as dense as neutron stars, and their magnetic fields aren’t quite as strong, but they are still strong enough to produce beams of energy from their magnetic poles. The problem is that a white dwarf pulsar would be harder to find, since their energy beams aren’t as strong and they would tend to rotate more slowly. But a team of astronomers thinks they’ve found one.

Artist animation of AR Scorpii. Credit: ESO

AR Scorpii is a binary system containing a red dwarf about half the mass of our Sun, and a white dwarf of roughly a solar mass. They are separated by a distance only 3 times that of the Earth to the Moon, and orbit each other every 3.6 hours. This kind of binary system is relatively common, but the team noticed the red dwarf was behaving unusually. The red dwarf pulses every two minutes. This is too fast for the variation to be due to the physics of the red dwarf itself. When the team analyzed the pulsations, they found it was highly polarized, which is the kind of thing that happens when material is illuminated by high energy beams. The kind of energy beams created by pulsars. It turns out that the white dwarf produces pulsar-like beams, and these sweep across the red dwarf, accelerating electrons in its upper atmosphere. The electrons then produce a pulse of ultraviolet, visible and radio light that we can observe here on Earth.

So this is the first example of a white dwarf acting like a pulsar, which is pretty cool.

Paper: Buckley, D. A. H. et al. Polarimetric evidence of a white dwarf pulsar in the binary system AR Scorpii. Nat. Astron. 1, 0029 (2017).

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You’re the captain of a Federation starship, ready to seek out new life and new adventures. As you travel through the galaxy, how do you know where you are? How do you find your way home? 

On Earth, your position is given by latitude and longitude. They are measured in angles about the center of the Earth, where latitude is the angle north or south of the equator, and longitude is the angle east or west of the Prime Meridian, which runs through Greenwich, England. It’s fairly easy to determine your latitude, particularly in the Northern Hemisphere. Since the north star Polaris is almost directly above the North Pole, you can simply measure the angle of Polaris above the horizon, and that’s your latitude. You can also use a sextant to measure the altitude of the Sun above the horizon at noon, and calculate your latitude from that.

Using a sextant to measure latitude. Credit: Joaquim Alves Gaspar (CC BY-SA 2.5)

Longitude is much more difficult. Since stars rise and set over the course of a night and the night sky shifts over the course of a year, there isn’t a fixed reference point against which you can measure longitude. Instead, early navigators either had to compare the shift of stars with measured distances between cities. It wasn’t particularly accurate, and you can see that in early maps of Europe. Things got easier when Galileo discovered the moons of Jupiter. Their clockwork motion could be used as a celestial clock, and by comparing their motion to the rotation of the Earth cartographers finally had an accurate tool for measuring longitude. Unfortunately, this method wasn’t useful at sea, so it took the development of accurate clocks to bring accurate longitude to sea-faring vessels. Nowadays we can simply use the global positioning system (GPS). The GPS consists of more than 30 satellites that continually transmit their location and time. By picking up the signal of at least four of these satellites, your phone can triangulate your position on Earth.

Defining your position in the Milky Way can be done with galactic latitude and longitude. Simply define a galactic equator and a prime meridian, and determine your position relative to them. For galactic coordinates astronomers define the galactic equator (0° longitude) as the plane of the Milky Way running through its center. The prime meridian (0° latitude) is defined by a line running from the Sun to galactic center. In astronomy the sky can be treated as a celestial sphere, so the apparent position of a star can be given by its galactic coordinates. Your position in the galaxy could thus be given by three numbers: your galactic latitude, galactic longitude, and your distance from the Sun.

A map of the Milky Way using galactic coordinates. Credit: T. Jarrett (IPAC/Caltech)

The Star Trek universe is a bit fuzzy on the subject of galactic navigation, and there isn’t a definitively canon version. The most popular version is based upon the galactic coordinates astronomers use, with some slight differences. The prime meridian is still a line running from the Sun to galactic center, but rather than simply using galactic latitude and longitude, the Milky Way is divided into quadrants. Under this definition the Sun would lie on the line dividing the Alpha and Beta quadrant, and the Earth would cross between these two quadrants as it orbits the Sun. In the Next Generation episode “Relics” it is stated that Earth is in the Alpha quadrant, and is less than 90 light years from the Beta quadrant. That probably means the Alpha quadrant is extended around the Sun to put our local cluster of stars within the Alpha quadrant. We’ve done a similar thing with the international date line on Earth to ensure that countries aren’t divided by it.

Star Trek galactic quadrants based upon astronomical coordinates. Credit: NASA, with annotations.

Of course a coordinate system is only useful if you can determine what your coordinates are. Star Trek is again fuzzy on just how starships find their way. It’s possible that they calculate their position based upon some kind of stellar map, but identifying particular stars seems a bit impractical. It turns out there is an effective way to determine your position in the galaxy, and it’s one we’ve actually used.

It’s basically a galactic GPS. Neutron stars are dense old stars with strong magnetic fields. As a result beams of radio energy stream from their magnetic poles. As a neutron star rotates, those beams sweep across the sky like a lighthouse, and if their beams happen to point in our direction we see them as pulsing radio objects known as pulsars.

The plaque carried by Pioneer 10 gave the position of the Sun as reckoned by 14 pulsars.

The radio pulse pattern of each pulsar is unique, and we know there positions in the galaxy quite well. If your starship can detect enough known pulsars, you can use that information to calculate your position in the galaxy. So if the Enterprise drops out of warp unexpectedly, or if Q sends it to a strange part of the galaxy, the navigators just need to look for pulsars to find their way home. What’s interesting is that a neutron star’s rotation slows down over time, so older pulsars pulse more slowly than younger ones. This means you can use pulsars not only to determine your position in the galaxy, but also your stardate. It’s a useful trick when your plot of the week involves time travel.

This method has actually been used. Not to determine the position of a starship, but to give alien civilizations the location of our solar system. When Pioneer 10 was launched in 1972 it carried with it a plaque indicating 14 pulsar signals. The idea was that if an alien civilization were to come across Pioneer 10 as it traveled beyond the solar system, they would be able to determine just when and from where it was launched.

When it comes to the Star Trek universe, we still have to develop warp technology and subspace communication, discover wormholes, travel back in time to save the whales, and make contact with alien civilizations, but we already have a map to find our way.

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While general relativity is an extremely well tested theory, we’re always looking for more precise ways to test it further. Either relativity will pass the test yet again or we’ll find evidence of something beyond relativity. But tests of general relativity are hard to come by, particularly for the most subtle effect of the theory, gravitational waves.

The best tests we have of gravitational waves is through binary pulsars. The first indirect evidence came from the Hulse-Taylor system, which consists of a pulsar orbiting a neutron star. Most of the systems we can use to test GR contain a pulsar orbiting another object such as a neutron star or white dwarf. But a system known as J0737-3039 consists of two pulsars. One pulses at a rate of 23 milliseconds, while the other at about 2.8 seconds. Their orbital period is only about 2.4 hours, so they are quite close to each other.

The regular radio pulses of a pulsar allow us to determine their motion with great accuracy, and since we can measure the motion of both pulsars we can test relativity with greater precision. While Hulse-Taylor system confirms relativity to within 0.2% of the theory, J0737-3039 allows for confirmation to within 0.04%. So far, general relativity still wins.

Paper: Michael Kramer. Experimental tests of general relativity in binary systems. 28th Texas Symposium on Relativistic Astrophysics, (2015)

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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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How do you weigh a star? If two stars orbit each other, then we can determine their masses by their orbits. Since each star pulls gravitationally on the other, the size of their orbits and the speed at which they orbit each other allows us to calculate their masses using Kepler’s laws. But if a star is by itself, we have to use indirect methods such as its brightness and temperature to estimate their mass. While this can work reasonably well for main sequence stars like our Sun, it doesn’t work well for neutron stars due their small size and extreme density. But new work in Science Advances has found an interesting way to determine the mass of a type of neutron star known as a pulsar.

Neutron stars have a mass greater than our Sun, but are only about 20 kilometers (12 miles) wide. They are so dense that their magnetic fields are incredibly strong. So strong that they channel lots of radio energy away from their magnetic poles. As a neutron star rotates, these beams of radio energy sweep around like a lighthouse. If the beam is oriented toward Earth, then we observe these beams as short pulses of radio energy. Each pulse marks a single rotation of the star (known as its rotational period). We can measure the timing of these pulses very precisely, and one thing we notice is that period of a pulsar gradually lengthens as its rotation slows over time.

Basic structure of a neutron star. Image by Wikipedia user Brews ohare. CC BY-SA 3.0.

But every now and then the rotational period of pulsar will jump a bit, indicating that its rotation has increased. These jumps are known as glitches, and they are due to interactions between the core of the neutron star and its outer crust. As a neutron star loses energy, it is the crust that slows down over time. The interior of the star is a superfluid, and so continues to rotate at a steady rate. Over time the difference in rotation becomes severe enough that the interior transfers some of its rotational speed to the crust, slowing down the core and speeding up the crust so that the two are more in sync.

How a pulsar glitches.

Just how much rotation is transferred, and how often such a glitch occurs, depends upon the exact nature of a neutron star’s interior. That’s where this new work comes in. The team took glitch data from the Vela pulsar spanning 45 years, and compared it to several models of neutron star interiors. They found that only one model matched the observed glitches. When they compared this model to another pulsar (PSR J0537−6910) spanning about 14 years, it also agreed with the same model. From the glitch data the team was able to pin down the interior structure of these neutron stars.

What’s interesting about this result is that the superfluid model that fits the glitch data can be used to determine the mass of a pulsar. Since the interior of a neutron star must be below a critical temperature to be superfluid, the glitch data tells us about the internal temperature of the star. Since neutron stars don’t produce heat through fusion like main sequence stars, they gradually cool over time. Larger (more massive) neutron stars cool more slowly than smaller ones. If we know how old the neutron star is (and thus how long it has been cooling) then we can use its age and the critical temperature to determine the mass of the neutron star. Often we can determine the age of a neutron star by studying the remnant of the supernova that formed it, or by using x-ray observations to study its surface temperature.

Since the ages of the Vela pulsar and PSR J0537−6910 are known, the team calculated their masses. They found the Vela pulsar has a mass of 1.5 Suns, and PSR J0537−6910 has a mass of 1.8 Suns. More pulsars will need to be studied to see if their glitch patterns follow the same model, but if the method holds up we’ll be able to determine the mass of a pulsar even when it’s all alone.

This article originally appeared on Forbes.

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Cue the dramatic music. A pulsar will be making a close approach to a star in 2018.

The pulsar was discovered in 2012, but over the past few years it’s become apparent that it is in a 25-year orbit with a large companion star. This not only makes it the pulsar with the largest known binary orbit, it gives us a specific time range for when a close approach with its companion will occur. As a result we have a heads up on when a burst of gamma rays will occur.

This is important because transient bursts can be difficult to observe when you don’t know they’re going to occur. Since this one is known well in advance, we have plenty of time to prepare. So while the teaser is over dramatic and tongue-in-cheek, it is an exciting event to look forward to.

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The pulsar J1906+0746 has gone silent, and that’s good news for general relativity.

A pulsar is a rapidly spinning neutron star. Neutron stars have incredibly strong magnetic fields. As a charged particles are trapped near the magnetic poles, they give off intense beams of radio waves. The rotation of the neutron star (and thus the poles) sweeps the beam around, much like the light from a lighthouse. When the beam is facing us, we see a pulse.

This particular pulsar is a close binary, and orbits another star of similar mass. The two stars are separated by about the width of our Sun, and orbit each other ever four hours. They are so close to each other that their orbits are affected by general relativity, including the effect of gravitational waves.

Over the past five years, a team has monitored the pulsar continuously, capturing about a billion pulses from J1906+0746. The goal was to compare the orbital predictions of general relativity with the observed orbital behavior of the pulsar. Not only did the observations match, the team also observed the pulsar fade over time, which is another prediction from general relativity.

The pulsar hasn’t actually stopped emitting energy, but its beam no long sweeps in our direction. That’s because the axis of rotation has shifted, through a process known as precession. We see this effect with Earth, where due to the gravitational pull of the Sun and Moon, our north pole drifts relative to the stars. This is why our current north star hasn’t always been the north star. In general relativity precession also occurs, but it happens at a slightly different rate. In the case of this pulsar, that is about two degrees of drift every year.

The pulsar hasn’t disappeared forever, though. We should start seeing it pulse again around 2170.

Paper: J. van Leeuwen, et al. The Binary Companion of Young, Relativistic Pulsar J1906+0746. 2015 ApJ 798 118 (2015)

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An ultraluminous x-ray source (ULX) is an  intense, localized sources of x-rays. They are generally powered by solar-mass black holes, similar to the way quasars and blazars are powered by supermassive black holes. We’ve generally thought only black holes could provide enough power to generate such powerful x-rays, but now it seems that isn’t always the case. New results have been published in Nature that show some of them might be powered by accreting neutron stars.

The team looked at x-ray data from NuSTAR taken of M82, which is a galaxy with several ultraluminous x-ray sources. What they noticed that instead of being a constant level of brightness, there was some flickering of brightness in the data. Some variation in brightness is expected, but in this case the flickering was both regular and rather fast. They tracked the source of the flickering down to a ULX known as M82X-2, and found that it flickered with a period of about 1.4 seconds.

A black hole wouldn’t cause such a flicker, but a neutron star would.  When they do, we call them pulsars. So despite having the x-ray intensity of a black hole, this particular x-ray source is a neutron star. Just how a neutron star could emit such intense energy is not clear. One idea is that it is accreting massive amounts of material, the heating of which drives the x-ray production. But if that’s the case, this particular neutron star may be eating its way toward becoming a black hole.

The interesting thing about this new work is that it was only possible because of a recent supernova in M82, causing several telescopes to focus their attention there. Because of the wealth of high-resolution data, the team was able to distinguish this flicker. So it seems other ultraluminous x-ray sources may be worth a closer look.

Paper: M. Bachetti, et al. An ultraluminous X-ray source powered by an accreting neutron star. Nature 514 (7521): 202 (2014)

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Yesterday I talked about millisecond pulsars, and the way in which they might gain such rapid rotation. Another property of millisecond pulsars is that they demonstrate very clearly that pulsars are neutron stars. It all has to do with their rapid rotation and the physics of centripetal (or centrifugal) force.

When you stand on the Earth, there are two basic forces acting on you. The first is gravity, where the attraction of Earth’s mass tries to pull you to the center of our planet. The second is the ground, which prevents you from moving through it by pushing up on you. This is often called the normal force. Standing on the ground, these two forces are in balance. The weight of gravity is countered by the normal force of the ground, which is why you can stay put.

If the Earth weren’t rotating, then these two forces would be equal in magnitude. But since the Earth rotates, the normal force is very slightly smaller than the attraction of gravity. Just how much smaller depends on where you are on the Earth, but it is smallest if you are standing on the equator. You can see why that is if you’ve ever been in a car going around a corner. As the car turns, it feels like you are being pulled slightly outward, away from the turning car. This is sometimes called a centrifugal force, and it has to do with the fact that your body (like any object with mass) would like to keep moving in a straight line at a constant speed. To change your direction (and keep you in your seat) the car has to push you in the direction of the turn. Whenever you are pushed in some direction, it feels like you are being pulled in the opposite direction, so it feels like you are being pulled outward. A similar thing occurs when your car accelerates, and it feels like you are being pulled back into your seat.

As you stand upon our rotating planet, a similar thing occurs. You are moving around in a circle, once per day, so your direction is always changing. Your body would like to keep moving in a straight line, but Earth’s gravity keeps changing your direction. Earth’s gravity is more than strong enough to overcome your centrifugal force, so the normal force of the ground also acts to keep you on the ground. But the normal force doesn’t have to counteract all of gravity, just the extra gravity beyond the centrifugal force.

Diagram of a pulsar Source: NRAO

Now suppose we could spin our spherical world faster and faster. As we are standing on its surface, we would have a tendency to fly off the world due to its rotation. The gravity is stronger than the centrifugal force, so we stay put. But if we could spin the world faster and faster, we would reach a point where the centrifugal force is equal to the force of gravity. Spin the world any faster, and we would fly off. Earth’s gravity wouldn’t be strong enough to hold us. The surface gravity of a planet or star depends upon its density, so far a given density for a planet/star, there is a maximum rate of rotation. Any faster and it would fly apart. Put another way, if we measure the rotation of a planet or star, we know its minimum density.

When we do the math, we find we can calculate that minimum density in grams per cubic centimeter by taking 140 million and dividing it by the square of an object’s period in seconds. So for the Earth, the period is 24 hours or 86,400 seconds. Plug that into our equation and we get a minimum density of about 0.02 g/cc. The Earth’s real density is about 5 g/cc, well above that minimum.

Pulsars are kind of like cosmic light houses. As they rotate they sweep out a beam of radio waves from their magnetic poles. If those poles are pointing in our direction, we can hear each rotation as a pulse. The fastest millisecond pulsar has a rotation of 716 times a second, or a rotational period of 1.4 milliseconds. Plug that into our equation, and you get a minimum density of about 7 x 10¹³ g/cc. Thats that’s about 7 billion kilograms per sugar cube volume, which is an unimaginably high density. It also happens to be a bit less than the density of atomic nuclei.

So pulsars are as dense as atomic nuclei, only several kilometers in diameter. We call them neutron stars.

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A millisecond pulsar is a neutron star that is rotating about 600 to 700 times a second. Because of their strong magnetic fields, they produce strong beams of radio energy from the regions of their magnetic poles, and as they rotate these beams can point in our direction. As a result, we observe these neutron stars as radio bursts that pulse every 1 – 10 milliseconds. Hence their name.

Millisecond pulsars are rotating about as fast as neutron star can rotate, which makes them a bit of a mystery. Left by themselves, a pulsar gradually slows down over time. That means millisecond pulsars are either very young neutron stars that formed at near maximal rotation, or there must be some mechanism that causes them to spin more rapidly.

It’s generally thought that the latter process is the more common. A neutron star that is part of a binary system with a red giant companion can capture material from the companion star. As the material is captured, the angular momentum (rotation) of the material is transferred to the neutron star, thus increasing its rotation. This would explain why millisecond pulsars are often old pulsars with a companion. While this mechanism was initially proposed decades ago, over the years we’ve gathered a lot of evidence to support it.

When neutron stars are actively capturing material from their companion, the energy released as it falls to the neutron star produces intense x-rays. Such x-ray producing systems are known as x-ray binaries. These x-ray binaries can be quite active, but the radiation emitted by them tends to push the accreting material away. Thus over time an active x-ray binary will become less active, eventually entering a quiet period after which it may become active again. In the late 1980s it was observed that some x-ray binaries in the late stage of their active period contain radio millisecond pulsars. In 1998 a millisecond pulsar was observed within an active x-ray binary. Then in 2009 an accretion disk was discovered around a millisecond pulsar, indicating that the pulsar had been accreting material in the past.

Then last year in Nature new evidence was presented that further verifies the mechanism. The paper presents observations of an x-ray transient known as IGR J18245–2452. An x-ray transient is an object that emits x-rays for a time, then goes quiet for a time. There are several types of x-ray transients, but this particular one is a neutron star with a companion. In the past it had been observed as a radio pulsar. It then entered an active period and begin emitting x-rays with millisecond pulsations. After an active period of about a month, the x-rays went quiet, and the neutron star began to emit radio pulses again.

This not only demonstrates a clear connection between x-ray binaries and millisecond pulsars, but that these objects can shift between the two states on a fairly rapid pace. It seems that some neutron stars really do eat and run.

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A while back I wrote about how general relativity predicts gravitational waves. While we haven’t yet observed gravity waves directly, we know they exist. That’s because gravitational waves carry energy away from their source, just as light waves carry light energy.

When two stars orbit each other, they produce gravitational waves. The gravity waves in turn take away some of the energy from the binary system. This means that the two stars gradually move closer together, an effect known as inspiralling. As the two stars inspiral, their orbital period gets shorter (because their orbits are getting smaller). Eventually they become so close that they merge, usually creating a supernova or black hole.

For regular binary stars this effect is so small that we can’t observe it. However in 1974 two astronomers (Hulse and Taylor) discovered an interesting pulsar. You may remember that pulsars are rapidly rotating neutron stars that happen to radiate radio pulses in our direction. The pulse rate of pulsars are typically very, very regular. Hulse and Taylor noticed that this particular pulsar’s rate would speed up slightly then slow down slightly at a regular rate. They showed that this variation was due to the motion of the pulsar as it orbited a star. They were able to determine the orbital motion of the pulsar very precisely, calculating its orbital period to within a fraction of a second. As they observed their pulsar over the years, they noticed its orbital period was gradually getting shorter. The pulsar was inspiralling, and would eventually merge with its companion star in about 300 million years.

In the figure above, I’ve plotted the measured orbital periods of pulsar with the theoretical period shortening due to gravity waves. As you can see, the data lines up almost exactly. The system is losing energy just as general relativity predicts. Hulse and Taylor had demonstrated that gravity waves exist. Which is why they were awarded the Nobel prize in physics in 1993.

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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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