The Odd Physics of Pulsars

Updated on November 5, 2018
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Leonard Kelley holds a bachelor's in physics with a minor in mathematics. He loves the academic world and strives to constantly explore it.

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Neutron stars are crazy to begin with. 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, and they are the subject of another article.

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 mean companion possibly stealing some of the pulsar's spotlight!
The mean companion possibly stealing some of the pulsar's spotlight! | Source

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").

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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").

The intensity of a pulsar visualized.
The intensity of a pulsar visualized. | Source

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").

Antimatter Factory

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).

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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").

PSR B1259-63/LS 2883 taking care of business.
PSR B1259-63/LS 2883 taking care of business. | Source

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).

Magnetic Attraction

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 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").

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….

Works Cited

Chandra X-ray Observatory Team. "Pulsar Punches Hole in Stellar Disk." Astronomy.com. Kalmbach Publishing Co., 23 Jul. 2015. Web. 16 Feb. 2017.

Ferron, Karri. "Pulsar Challenges Theories." Astronomy Feb. 2015: 12. Print.

Francis, Matthew. "Neutron superfluid may put the brakes on pulsars’ spins." ars technica. Conte Nast., 03 Oct. 2012. Web. 30 Oct. 2015.

Hall, Shannon. "Warp In Space-Time Swallows Pulsar." space.com. Space.com, 04 Mar. 2015. Web. 16 Feb. 2017.

Klesman, Alison. "Amateur Astronomer Sheds Light on Pulsar Companion's Odd Behavior." Astronomy Apr. 2017. Print. 18.

---. "Pulsars May Be Churning Out Small Reserves of Antimatter." Astronomy.com. Kalmbach Publishing Co., 07 Mar. 2017. Web. 30 Oct. 2017.

---. "White Dwarfs Can Be Pulsars, Too." Astronomy Jun. 2017. Print. 16.

Kruesi, Liz. "An Evolutionary Link for Pulsars." Astronomy Jan. 2014: 16. Print.

---. "Millisecond Pulsar Put on the Brakes." Astronomy Jun. 2012: 22. Print.

O'Neill, Ian. "Pulsar Punches Hole Through Star's Disk." Seekers.com. Discovery Communications, 22 Jul. 2015. Web. 16 Feb. 2017.

Max Planck Institute for Radio Astronomy. “The Art of Recycling Pulsars.” Astronomy.com. Kalmbach Publishing Co., 06 Feb. 2012. Web. 09 Jan. 2015.

Naeye, Robert. "New Pulsar Result Supports Particle Dark Matter." Astronomy.com. Kalmbach Publishing Co., 16 Nov. 2017. Web. 14 Dec. 2017.

NASA. “Swift Reveals New Phenomenon in a Neutron Star.” Astronomy.com. Kalmbach Publishing Co., 30 May 2013. Web. 10 Jan. 2015.

NRAO. "Neutron Stars Strike Back at Black Holes in Jet Contest." Astronomy.com. Kalmbach Publishing Co., 04 Aug. 2015. Web. 16 Sept. 2016.

---. “Pulsars: The Universe’s Gift to Physics.” Astronomy.com. Kalmbach Publishing Co., 20 Feb. 2012. Web. 09 Jan. 2015.

Rzetelny, Xaq. "Strange X-ray Source is the Brightest Pulsar Ever Observed." arstechnica.com. Conte Nast, 22 Oct. 2014. Web. 16 Feb. 2017.

Scoles, Sarah. "Pulsar System Validates Einstein." Astronomy Aug. 2013: 22. Print.

---. "Pulsars Flip-Flop Their Radio Waves and X-Rays." Astronomy May 2013: 18. Print.

© 2015 Leonard Kelley

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