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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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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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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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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One of the advantages of radio astronomy is that you can connect observations from radio telescopes thousands of miles apart.  Done in the right way, this creates a radio interferometer that effectively makes a virtual telescope as big as the separation (baseline) of the individual telescopes.  The bigger your telescope (or virtual telescope), the finer the detail of your image.  When we talk about the detail level of an astronomical image, we usually talk about the angle of separation between two distinctly resolvable points.  So a resolution of a tenth of a degree would mean you could resolve two points of light (such as stars) separated by at least that angle.  

In modern astronomy our resolution is much better than degrees, and we usually measure things in terms of arcseconds. An arcsecond is 1/3600 of a degree. It comes from dividing a degree into 60 minutes of arc, and each minute into 60 seconds of arc. Yes, the terms stem from their historical relation to time measurements.  Beyond arcseconds, we simply divide them into parts by base 10.  So a thousandth of an arcsecond is a milliarcsecond and so on.

Given the baseline of radio telescope interferometers, the upper resolution is typically on the order of milliarcseconds. But now a new paper in the Monthly Notices of the Royal Astronomical Society has resolved a radio pulsar to picoarcseconds. That is, a trillionth of an arcsecond, or about a separation of 5 kilometers 2000 light years away.

The method they used is quite clever. As the pulsar rotates, it radiates radio energy in beams from the magnetic poles.  These radio beams sweep through the surrounding interstellar media.  As the radio beams travel through the interstellar media in the general region of the pulsar, they are distorted.  By measuring the motion of the pulsar itself, the team was able to use the distorted radio signals from the radio beams to create an astronomical interferometer. But instead of having a baseline of a few thousand kilometers, the effective baseline is about 5 AU, or the distance from the Sun to Jupiter. With such a precise resolution the team found that the emissions of the pulsar’s radio waves are closer to the surface of the pulsar than we had thought.  This could help us understand how pulsars generate such strong radio signals.

This trick will only work with radio bright objects like pulsars, but it does demonstrate how clever observations can produce results that are far better than we ever thought possible.

Paper: Ue-Li Pen, et al. 50 picoarcsec astrometry of pulsar emission. MNRAS (May 01, 2014) 440 (1): L36-L40. doi: 10.1093/mnrasl/slu010

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In 1967 a PhD student named Jocelyn Bell detected a radio signal with an odd regularity. Patterns can be heard in all sorts of radio signals, but this particular signal was unusual in that it was a pulse with a period of 1.33 seconds. You can see this pattern in the figure above, and you can hear what the signal sounds like here.

Together with her advisor Antony Hewish, they found that the signal came from the same location in the sky, and followed sidereal time, which meant it was not caused by some terrestrial source. They half jokingly referred to the signal as LGM-1, where LGM stood for “Little Green Men”. The signal was so incredibly regular that the idea that the signal was produced by an alien intelligence crossed their minds.

Soon other similar signals were discovered, and it was clear that they were due to a natural process. By 1968 it was determined that these signals were produced by rotating neutron stars.

Neutron stars have intense 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, and the rate of that pulse is the rate at which the neutron star spins.

These objects are now known as pulsars, and their discovery led to Hewish (but not Bell) being awarded the Nobel prize in physics in 1974.

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Recently popular-science websites have been buzzing with news of a new pulsar putting Einstein’s theory of gravity to its greatest test yet. In particular, some tout it as a test of alternatives to general relativity. While the attention this work has gotten in the press implies this is a new breakthrough, that’s not quite the case. So what’s the real deal on these latest findings?

The results have been recently published in Science. In the paper the team presents observations of a binary system known as PSR J0348+0432. You can see this system in the lower right of the figure below, compared with PSR J0737-3039A/B, which consists of two pulsars, and PSR B1913+16, also known as Hulse-Taylor or H-T.

Orbits of various known binary pulsar systems.
Credit: Norbert Wex/MPIfR

The Hulse-Taylor system is perhaps the most famous pulsar with a binary companion, since it provided the first observational evidence of gravitational waves. In Newtonian gravity, objects orbiting each other should continue to do so basically forever. This is because in Newton’s gravity stable binary orbits have no way to gain or lose energy. But Einstein’s theory of general relativity treats gravity as a curvature of space. This means that orbiting masses should create ripples in space known as gravitational waves (similar to the way stirring your coffee creates ripples). These gravitational waves carry energy, so over time orbits should lose energy by radiating gravitational waves. As a result, members of a binary system will move closer and closer to each other until they collide.

For most objects, such as binary star systems, this effect is far too small to matter. But if the members of a binary system are massive and close, then it can have a measurable effect. As you can see in the figure these binary systems are not that much bigger than the Sun, but the masses of these objects are as large or larger than the Sun. So they provide a good test of general relativity.

The Hulse-Taylor system consists of two neutron stars, one of which is a pulsar. Since pulsars give off radio pulses at a very precise rate, we can measure the signal to know just how these two objects are orbiting. Hulse and Taylor observed this system over many years, and showed that they were slowly moving closer together just as predicted by general relativity and gravity waves. This won them the Nobel prize in physics in 1993.

This new system (PSR J0348+0432) is hitting the press because it’s a little bit different. Most close binary systems like Hulse-Taylor consist of two neutron stars. Neutron stars typically have about the same mass (about 1 – 2 solar masses), so that means they tend to be rather symmetric. This new system consists of a pulsar (neutron star) of about 2 solar masses, and a white dwarf companion about a fifth the mass of our Sun. So this system is very non-symmetric. That means it might be able to distinguish between general relativity and alternative theories of gravity.

General relativity has passed every experimental test so far, but there are alternative theories such as scalar-tensor-vector gravity, or tensor-scalar-vector gravity (yes, they are two different models) that also make similar predictions for gravity waves. These alternative models are more complex than general relativity, and they are typically proposed as a modified gravity to explain things like the galaxy rotation curves that are commonly attributed to dark matter. They aren’t popular models among astronomers, but without a direct observation of dark matter, we can’t entirely rule them out.

In addition to explaining away dark matter, these alternative gravitational models make different predictions about gravitational waves. This means the orbital decay predicted by general relativity and these alternative models differ in a measurable way. But to really see these differences we would want to observe a close binary system where the two members differ significantly in mass. Ideally they would also be in very elliptical orbits. This latest system has the non-symmetric aspect we want, but their orbits are very circular.

This initial paper mainly just verifies the masses and orbits of this pulsar and its companion. It confirms that the two masses are very different, and it demonstrates that (so far) general relativity is still confirmed. That’s not really a big deal, so the attention it has gotten in the press is a bit overhyped. But the work also places some loose constraints on alternative models of gravity. It’s not nearly enough to verify or disprove them yet, but over the next decade or two it could provide a real test of these alternative models. That’s another reason for some of the hype. It is a way for the team to claim this particular pulsar system as theirs.

So it is an interesting system, and it may provide an interesting test of gravity in the future. But it isn’t yet the “greatest test of general relativity” as many popular articles claim.

The post Passing the Test appeared first on One Universe at a Time.

The Guitar Nebula is a guitar-shaped nebula in the constellation Cepheus.  It is a faint nebula, about 6,500 light years away, but it is unusual because it is a bow shock nebula.  This particular bow shock nebula is produced by a pulsar moving about 800 kilometers per second through the surrounding interstellar media.  As the pulsar plows through the interstellar gas it disrupts it, much like a boat on a still pond produces waves.  The nebula was first discovered in 1993, and it is among the strongest evidence a phenomenon known as a pulsar kick.

Pulsars are neutron stars, formed when a large star explodes as a supernova.  Because of this, one would expect a pulsar to lie within the surrounding supernova remnant, and to move at the same relative speed.  But this is not the case with the Guitar Nebula.  It seems that something must have caused the pulsar to move at great speed relative to the remnant. Given it a kick, as it were, hence the term pulsar kick (or neutron star kick).  Given the mass of a neutron star (greater than that of our Sun) the only thing that could have provided such a kick would be the supernova itself.

Computational simulation of an asymmetrical supernova. Credit: J. Nordhaus, et al

Just how the supernova can kick the neutron star isn’t clear, but there are a couple of ideas.  One is that during the collapse of the progenitor star, the core becomes slightly off-center.  This causes the surrounding star to compress asymmetrically, which means the resulting supernova exerts a large force on the core, causing it to race off at great speed.  Another is that it is an electromagnetic effect.  If the magnetic field of a pulsar is off-center, the jets of energy produced by the pulsar would also be off-center, and thus would push the neutron star like a rocket.

There is a third, much more hypothetical possibility which involves the sterile neutrinos I mentioned yesterday.  The basic idea is that there is an asymmetry between regular neutrinos and sterile neutrinos.  Since intense nuclear interactions occur within a supernova, it is possible that both regular and sterile neutrinos could be produced.  But because of the asymmetry between the two, this would result in a kick being given to the resulting pulsar.  We still aren’t sure of the nature of sterile neutrinos (or even if they exist), but some theoretical papers show that it could solve the pulsar kick mystery.

At the moment, the asymmetrical supernova model is the most popular. It will take more study to determine if that is indeed the solution.

Paper: James M. Cordes, Roger W. Romani and Scott C. Lundgren. The Guitar nebula: a bow shock from a slow-spin, high-velocity neutron star. Nature 362, 133 – 135 (11 March 1993); doi:10.1038/362133a0

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