The Odd Physics of Neutron Stars, Pulsars, and Magnetars
Stars come in all different sizes and shapes, but none are as unique as the family of neutron stars. In this group, we find an example of an object that is so dense that a tablespoon of material would weigh millions of tons! How could nature have cooked up something so bizarre? Like black holes, neutron stars find their birth begins with a death.
How Neutron Stars Are Made
Massive stars have lots of fuel, initially in the form of hydrogen. Through nuclear fusion, hydrogen is transformed into helium and light. This process happens to helium as well and up and up we go on the periodic table until we get to iron, which cannot be fused together in the interior of the sun. Normally, electron degeneracy pressure, or its tendency to avoid being near other elections, is enough to counter gravity but once we get to iron the pressure is not as great as the electrons are pulled closer to the nucleus of the atom. The pressure decreases and gravity condenses the star’s core to the point where an explosion releases incredible amounts of energy. Depending on the size of the star, anything between 8-20 sun masses will become a neutron star while anything larger becomes a black hole.
So why the name neutron star? The reason is surprisingly simple. As the core collapses, gravity condenses everything so much that the protons and electrons combine to become neutrons, which are charge neutral and thus are happy to be bunched up with one another without care. Thus the neutron star can be quite small (about 10 km in diameter) and yet have as much mass as nearly 2 or 3 Suns! (Seeds 226)
Even more amazing is that pulsars and magnetars are special types of neutron stars. A pulsar is a spinning neutron star that seemingly emits pulses at a regular interval. These flashes are because of the magnetic field of the star sending gas to the poles, exciting the gas and emitting light in the form of radio and X-rays. Moreover, if the magnetic field is strong enough it can cause cracks in the surface of the star, sending gamma rays out. We call these stars magnetars.
The Spin Doesn't Lie
Now that we are somewhat familiar with these stars, let’s talk about the spin of a pulsar. It arises from the supernova that created the neutron star, for the conservation of angular momentum applies. The matter that was falling to the core had a certain amount of momentum that was transferred to the core and thus pumped up the rate the star was spinning. It is similar to how an ice skater increases their spin as they pull themselves in.
But pulsars don’t just spin at any rate. Many are what we call millisecond pulsars, for they complete a single revolution in 1-10 milliseconds. Put another way, they spin hundreds to thousands of times a second! They achieve this by taking material away from a companion star in a binary system with the pulsar. As it takes material from it, it increases the spin rate because of conservation of angular momentum, but does this increase have a cap? Only when the material falling in dies down. Once this happens, the pulsar decreases its rotational energy by as much as half. Huh? (Max Planck)
The reason lies with what is called the Roche-lobe decoupling phase. I know, it sounds like a mouthful but hang in there. While the pulsar is pulling material into its field, the inbound matter is accelerated by the magnetic field and is emitted as X-rays. But once the material falling in dies down, the radius of the magnetic field, in a spherical shape, starts to increase. This pushes charged material away from the pulsar and thus robs it of momentum. It also decreases the rotational energy and thus lowers the X-rays into radio waves. That expansion of the radius and its consequences is the decoupling phase in action and helps resolve the mystery of why some pulsars appear too old for their system. They have been robbed of their youth! (Max Planck, Francis "Neutron").
But surprisingly, more millisecond pulsars should have been found with a faster spin rate than theory initially predicted? What gives? Is it something even odder than we have seen before? According to Thomas Jauris (from the University of Bonn in Germany) in a February 3rd issue of Science, maybe not so weird as initially suspected. You see, most pulsars are in a binary system and steal material away from their companion, increasing their rate of rotation through conservation of angular momentum. But computer simulations show that the magnetosphere of the companion object (a region where charged particles of a star are governed by magnetism) actually prevents material from going to the pulsar, thus further robbing it of spin. In fact, almost 50% of the potential spin that a pulsar could have is taken away. Man, these guys can't catch a break! (Kruesi "Millisecond").
Gravity Rules Over All
Okay, so I promised some odd physics. Isn’t the above enough? Of course not, so here is some more. How about gravity? Are there better theories out there? The key to that answer is the orientation of the pulses. If alternate theories of gravity, which work just as well as relativity, are correct then details of the interior of the pulsar should affect the pulses scientists witness because it would fluctuate the motion of the pulses seen, like a swiveling pivot. If relativity is correct then we should expect those pulses to be regular, which is what has been observed. And what can we learn about gravity waves? These movements in space-time caused by moving objects are elusive and hard to detect, but fortunately nature has provided us with pulsars to help us find them. Scientists count on the regularity of the pulses and if any changes in the timing of them are observed then it could be because of the passage of gravity waves. By noting anything massive in the area, scientists could hopefully find a smoking gun for some gravity wave production (NRAO "Pulsars").
But it should be noted that another confirmation of relativity was secured from evidence gathered by the Green Bank Telescope as well as optical and radio telescopes in Chile, Canary Islands, and Germany. Published in an April 26 issue of Science, Paulo Freire was able to show that the expected orbital decay that relativity predicts in fact occurred in a pulsar/white dwarf binary system. Unfortunately, no insights into quantum gravity were to be gleamed, for the scale of the system is too large. Shucks (Scoles "Pulsar System").
Let The Weirdness Begin
Okay, so gravity. Big deal right? What about a potential new form of matter? It is possible, for the conditions in a neutron star are unlike anywhere else in the Universe. Matter has been condensed to as maximum an extreme as possible. Anymore, and it would have become a black hole upon the supernova. But the form matter takes inside a neutron star has been compared to pasta. Yum?
This was proposed after scientists noticed that no pulsars seem to exist that can have a spin period longer than 12 seconds. Theoretically it could be slower than that but none have been found. Some models showed that the matter inside the pulsar could be responsible for this. When in a pasta formation, the electric resistivity increases which thus causes the electrons to have a difficult time moving around. Electron movement is what causes magnetic fields to form and if the electrons have a hard time moving in the first place then the ability of the pulsar to radiate EM waves is limited. Thus, the ability for the angular momentum to decrease is also limited, for one way to decrease spin is to radiate energy or matter (Moskowitz).
In fact, scientists witnessed a magnetar going through such a loss of angular momentum. Neutron star 1E 2259+586 (catchy, right?), which is in the direction of the constellation Cassiopeia about 10,000 light-years away, was found to have a rotation rate of 6.978948 seconds based off X-ray pulses. That is, until April of 2012 when it decreased by 2.2 millionths of a second, then sent out a huge burst of X-rays on April 21. Big deal, right? In this magtnetar, however, the magnetic field is several magnitudes greater than a normal neutron star and the crust, which is mostly electrons, encounters great electric resistivity. It thus gains an inability to move as fast as the material underneath it and this causes strain on the crust, which cracks and releases X-rays. As the crust reconstitutes itself, the spin increases. 1E went through such a spin down and a spin up, adding some evidence to this model of neutron stars, according to the May 30, 2013 issue of Nature by Neil Gehrels (from the Goddard Space Flight Center) (NASA, Kruesi "Surprise").
And guess what? If a magnetar slows down enough, the star will lose its structural integrity and it will collapse...into a black hole! We have mentioned above such a mechanism to lose rotational energy, but the powerful magnetic field can also rob energy by speeding along EM waves on their way out of the star. But the neutron star has to be big - as massive as 10 suns minimum - if gravity is to condense the star into a black hole (Redd).
But what if the material inside a neutron star isn't that pasta-property material? Several models have been proposed for what the core of a neutron star really is. One is a quark core, where remaining protons are condensed with the neutrons to break apart and are just a sea of up and down quarks. Another option is a hyperon core, where those nucleons are not broken but instead have a high amount of strange quarks because of the high energy present. Another option is quite catchy - the kaon condensate core, where quark pairs of strange/up or strange/down exist. Figuring out which (if any) are viable is tough because of the conditions needed to generate it. Particle accelerators can make some of them but at temperatures that are billions, even trillions, of degrees warmer than a neutron star. Another standstill (Sokol).
But a possible test to determine what models work best was devised using glitches of a pulsar. Every once and a while, a pulsar should experience a sudden change in speed, a glitch, and change its output. These glitched likely arise from interactions between the crust and a super fluid interior (which moves about with low friction) exchanging momentum, just like 1E 2259+586, or from magnetic field lines breaking. But when scientists watched the Vela pulsar for three years, they had a chance to see the before and after glitch moment, something missing before. Only one glitch was seen through that time. Before the glitch occurred, a "weak and very broad pulse" in polarization was sent, then 90 milliseconds later...no pulse, when one was expected. Then the normal behavior returned. Models are being built with this data to see which theory works best (Timmer "Three").
Pulsar or Black Hole?
ULX M82 X-2 is the catchy name of a pulsar located in M82, otherwise known as the Cigar Galaxy, by NuSTAR and Chandra. What has X-2 done to be on our list of notable stars? Well, based on the x-rays that were coming off of it scientists had thought for years that it was a black hole eating at a companion star, formally classifying the source as an ultra-luminous x-ray source (ULX). But a study led by Fiona Harrison of the California Institute of Technology found that this ULX was pulsing at a rate of 1.37 seconds per pulse. Its energy output is 10 million suns worth which is 100 times as much as current theory allows for a black hole. At since it comes in at 1.4 solar masses, it is just barely a star based on that mass (for it is close to its Chandrasekhar limit, the point of no return for a supernova), which may account for the extreme conditions witnessed. The signs point to a pulsar, for while these conditions mentioned challenge it being that, the magnetic field around one would allow for these observed properties. With that in account, the Eddington limit for in falling matter would allow for the observed output (Ferron, Rzetelny).
A different pulsar, PSR J1023+0038, is for sure a neutron star but it exhibits jets that rival the output of a black hole. Normally, the pulses are much weaker simply because of the lack of strength that gravitational tidal forces and magnetic fields are found at around a black hole, plus all the material around a neutron star further inhibits jet flow. So why did it begin to jet at levels comparable to a black hole so suddenly? Adam Deller (from the Netherlands Institute for Radio Astronomy), the man behind the study, is not sure but feels additional observations with the VLA will reveal a scenario to match observations (NRAO "Neutron").
Pulsars have other jet properties too (of course). Because of the high magnetic field around them, pulsars can accelerate material to such a speed that electron-position pairs are created, according to data from the High-Altitude Cherenkov Observatroy. Gamma rays were seen from a pulsar that corresponded to electrons and positrons striking the material around the pulsar. This has huge implications for the matter/antimatter debate that scientists still have no answer to. Evidence from two pulsars, Geminga and PSR B0656+14, seem to point to the factory not being able to explain away the excess positrons seen in the sky. Data taken by the water tanks at HAWC from November 2014 to June 2016 looked for Cherenkov radiation that is generated from gamma-ray hits. By back-tracking to the pulsars (which are 800 to 900 light-years away), they calculated the gamma-ray flux and found that the number of positrons needed to make that flux wouldn't be enough to account for all the stray positrons seen in the cosmos. Some other mechanism, like dark matter particle annihilation, may be responsible (Klesman "Pulsars", Naeye).
Neutrinos and Neutron Stars
Still not sold on this whole odd physics yet? Alright, I think I may have something that may satisfy. It involves that crust we were just mentioning, and it also involves energy release. But you will never belief what is the agent of the energy takeaway. It is one of nature’s most elusive particles that hardly interact with anything at all and yet here plays a big role. That’s right; the tiny neutrino is the culprit.
And a potential problem exists because of that. How? Well, sometimes matter falls into a neutron star. Usually, its gas that gets caught in the magnetic field and sent to the poles but occasionally something can encounter the surface. It will interact with the crust and fall under enormous pressure, enough for it to go thermonuclear and release an X-ray burst. However, for such a burst to occur also requires that the material be hot. So why is that a problem? Most models show the crust to be cold. Very cold. Like nearly absolute zero. This is because a region where double beta-decay (where electrons and neutrinos are released as a particle breaks down) occurs frequently has been potentially found below the crust. Through a process known as Urca, those neutrinos take energy away from the system, effectively cooling it down. Scientists propose a new mechanism to help reconcile this view with the thermonuclear explosion potential that neutron stars have (Francis "Neutrino").
Flipping Between X-Rays and Radio Waves
PSR B0943+10 is one of the first pulsars discovered that somehow switches from emitting high x-rays and low radio waves to the opposite - without any recognizable pattern. The January 25, 2013 issue of Science by project leader W. Hermsen (from the Space research Organization) detailed the finding, with the change of state lasting for a few hours before switching back. Nothing known at the time could cause that transformation. Some scientists even propose it could be a low-mass quark star, which would be even weirder than a pulsar. Which I know is hard to believe (Scoles "Pulsars Flip").
But no need to fear, for insights were not too far in the future. A variable x-ray pulsar in M28 found by ESA's INTEGRAL and further observed by SWIFT was detailed in the September 26 issue of Nature. Initially found on March 28, the pulsar was soon found to be a millisecond variant as well when XXM-Newton found a 3.93 second x-ray source there as well on April 4. Named PSR J1824-2452L, it was further examined by Alessandro Papitto and found to switch between states over a timeframe of weeks, way too fast to conform with theory. But scientists soon determined that 2452L was in a binary system with a star 1/5 the mass of the Sun. The x-rays scientists had been seeing were in fact coming from the material of the companion star as it was heated by tidal forces of the pulsar. And as the material fell onto the pulsar, its spin increased, resulting in its millisecond nature. With the right concentration of buildup, a thermonuclear explosion could occur that would blow material away and slow down the pulsar again (Kruesi "An").
Stars Within Stars?
Possibly one of the strangest concepts a neutron star is involved in is a TZO. This hypothetical object is simply put a neutron star inside a super red giant star and arises from a special binary system where the two merge. But how could we spot one? Turns out, these objects have a shelf life, and after a certain number of years the super red giant layer is cast off, resulting in a neutron star that spins too slow for its age, courtesy of a transfer of angular momentum. Such an object may be like 1F161348-5055, a supernova remnant that is 200 years old but is now an x-ray object and spins at 6.67 hours. This is way too slow, unless it was a part of a TZO in its former life (Cendes).
Blasting Away Space
Pulsars are rather good a cleaning up their local area of space. Take for example PSR B1259-63/LS 2883 and its binary companion, located about 7,500 light-years away. According to observations by Chandra, the pulsar's proximity and orientation of the jets relative to the disc of material around the companion star push clumps of material out of it, where it then follows the magnetic field of the pulsar and is then accelerated away from the system. The pulsar completes an orbit every 41 months, making the pass through the disc a periodic event. Clumps moving as fast as 15 percent the speed of light have been seen! Talk about a speedy delivery (O'Neill "Pulsar," Chandra).
In a feat of amateur astronomy, Andre van Staden examined pulsar J1723-21837 for 5 months in 2014 using a 30cm reflector telescope and record the light profile from the star. Andre noticed that the light profile went through the dips we expect it to but found that it "lagged" behind comparable pulsars. He sent the data to John Antoniadis to see what was going on, and in December 2016 it was announced that a companion star was to blame. Turns out, the companion was sunspot heavy and therefore had a high magnetic field, tugging at the pulses we saw from Earth (Klesman "Amateur").
A Pulsar or a Magnetar?
Can a neutron star switch between these two identities? Yes, yes it can, as PSR J1119-6127 has been seen to do. Observations made by Walid Majid (JPL) show that the star switches between a pulsar and a magnetar, one driven by spin and the other by high magnetic field. Big jumps between emissions and magnetic field readings have been seen to support this view, making this star a unique object. So far (Wenz).
A White Dwarf Pulsar?
So we cave a duel role neutron star. How about a white dwarf pulsar? Professor Tom Marsh and Boris Gansicke (University of Warwick) and David Buckley (South African Astronomical Observatory) released their findings in a February 7 Nature Astronomy detailing AR Scorpi, a binary system. It is 380 light-years away and consists of a white dwarf and a red dwarf that orbit each other every 3.6 hours at an average distance of 870,000 miles. But the white dwarf has a magnetic field over 10,000 that of Earth, and it spins fast. This causes the red dwarf to be bombarded with radiation and that generates an electric current we see on Earth. So it this really a pulsar? No, but it does have pulsar behavior and is interesting to see it emulated in a much less dense star (Klesman "White").
Symbiotic X-ray Binary
Another type of red star is involved in another weird system. Located in the direction of the Milky Way's center, a red giant star was spotted in the vicinity of an X-ray burst. Upon closer examination, a neutron star was spotted near the giant, and scientists were surprised when they did some number crunching. Turns out, the outer layers of the red giant that are naturally shed off at this stage in its life are being powered by the neutron star and sent out as a burst. Based off the magnetic field readings, the neutron star is young...but the red giant is old. It is possible that the neutron star was initially a white dwarf that gathered enough material to surpass its weight limit and collapse into a neutron star rather than forming from a supernova (Jorgenson).
Evidence for a Quantum Effect
One of the biggest predictions of quantum mechanics is the idea of virtual particles, which rise from differing potentials in vacuum energy and have huge implications for black holes. But as many will tell you, testing out this idea is tough, but fortunately neutron stars offer an easy (?) method of detection of the effects of virtual particles. By looking for vacuum birefringence, an effect arising from virtual particles being affected by an intense magnetic field which causes light to scatter like in a prism, scientists have an indirect method of detecting the mysterious particles. Star RX J1856.5-3754, located 400 light-years away, seems to have this predicted pattern (O'Neill "Quantum").
Evidence for a Relativity Effect
Another hallmark of science would have to be Einstein's theory of relativity. It has been tested over and over again, but why not do it again? One of those predictions is the precession of perihelion of an object close to a huge gravitational field, like a star. This is because of the curvature of spacetime causing the objects orbit to move as well. And for pulsar J1906, located 25,000 light-years away, its orbit has precessed to the point where its pulses are no longer oriented to us, effectively blinding us to it activity. It has for all intents and purposes....disappeared...(Hall).
So, how was that for some odd physics? No? Can’t convince everyone I guess….
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© 2015 Leonard Kelley