How a Pulsar Gets Its Spin

brian0918 writes "Until now, the assumption has been that the rapid spin of a pulsar comes from the spin of the original star. The problem was that this only explained the fastest observed pulsars. Now, researchers at Oak Ridge have shown that the spin of a pulsar is determined by the shock wave created when the star's massive iron core collapses. From the article: 'That shock wave is inherently unstable, and eventually becomes cigar-shaped instead of spherical. The instability creates two rotating flows — one in one direction directly below the shock wave and another, inner flow, that travels in the opposite direction and spins up the core. The asymmetrical flows establish a 'sloshing' motion that accounts for the pulsars' observed spin velocities from once every 15 to 300 milliseconds.'"

Comments

Pulsar gets it's spin

[DittoBox]

For each and shock that waves
Another Pulsar gets it's spin
So show you care and let your iron core collapse
And help a pulsar spin tonight...

a-z

[Anonymous Coward]

Mumble mumble... angular momentum...
Therefore, God exists.

Bad reporting

[exp(pi*sqrt(163))]

From 'the first plausible explanation' to 'researchers at Oak Ridge have shown that the spin of a pulsar is determined by the shock wave created when the star's massive iron core collapses'. Shows how poor journalists give scientists a bad name by making their claims seem much stronger than they are. The press release is very careful in how it makes its statements. The /. story isn't.

Re:Bad reporting

[Surt]

Bah, unstable shockwave giving a pulsar its spin! Ridiculous! Pulsars obviously spin through Intelligent Exploding!

The sad thing about this argument is that it's clear in the original Aramaic that they meant Imploding.

Re:Bad reporting

[Hal_Porter]

The sad thing about this argument is that it's clear in the original Aramaic that they meant Imploding.

God spoke the Queen's English, not some obscure semitic language.

Pulsars as GPS

[mrcaseyj]

The most interesting application of pulsars I've heard of is using them like GPS transmitters. Since pulsars are about the most precise timing devices known, if you time the arrival of the pulse from at least four of them you can use the time differences to triangulate your position precisely anywhere in the solar system.

Re:Pulsars as GPS

[swebster]

This idea was used on the Pioneer plaque (http://en.wikipedia.org/wiki/Pioneer_plaque).

Re:Pulsars as GPS

[mrcaseyj]

I think the idea of Pulsar GPS is primarily intended for interplanetary spacecraft. For terrestrial stuff GPS probably works better. One problem with Pulsars is that you would have to know which pulse to measure the time difference relative to the other Pulsars. if a pulsar pulses every 300ms then the pulses will be about a third of a light second apart or about 100km. If you go 100km relative to the pulsar you might get mixed up as to which pulse to lock onto and think you hadn't moved. The problem gets worse as the pulse rate gets higher. GPS doesn't have this problem because the absolute time is encoded within the signal from each satellite so you know the date and time each pulse left the GPS satellite.

One way to deal with the problem is to carefully keep continuous track of your position so you can keep straight which pulse to measure. If you can figure out where you are accurately enough by some other means you can figure out which pulse to measure. You can decrease the uncertainty of which pulse to use by using more than four pulsars. For example if one pulsar pulses every 7ms and another pulses every 11ms you can increase your window to 77ms by watching as they go into and out of sync.

I'm guessing that another problem is that the pulsars probably have a rather faint signal. You may have to have four or more large high gain dish antennas pointed in different directions to pick up the signals. This would be impractical on earth for most applications and would be a lot of extra weight for a spacecraft also. You might be able to have just one dish and point it at each pulsar in turn.

I don't see anything about this on Wikipedia. I think I'll add it one of these days.

Re:Pulsars as GPS

[iluvcapra]

a third of a light second apart or about 100km

I think you mean 100,000 km.

If you go 100km relative to the pulsar you might get mixed up as to which pulse to lock onto and think you hadn't moved.

You wouldn't use the phase of the pulsar signal to get a distance reading, but you could use it's frequency as a way of identifying it uniquely, and then you would triangulate your position by getting a bearing on three or four pulsars; this is essentially how the starburst pattern on the Mariner Plaque is supposed to work for aliens trying to find Earth. It gives the frequency of pulsars near the earth and their bearing to Sol relative to the galactic center. The aliens would identify the pulsars by their frequency, and then use their knowledge of the pulsars' absolute locations to work backward and triangulate the position of Sol (assuming these aliens, a million years from now, are able to backtrack the locations of all the pulsars, as they will all have moved relative to Sol and will have slowed down in their spinning, causing an error in our reported frequencies).

Re:Pulsars as GPS

[lowen]

Being that we do pulsar research here at PARI, I'll comment on this timing thing. Some pulsars are quite regular; most however do have what are known as 'glitches' and in the case of the Crab pulsar 'giant pulses'; both of these phenomena are unpredictable and skew any timing you might receive from the pulsar. Also, pulsar timing requires some fairly extensive integration of the incoming pulses, as most pulsars miss 'beats' frequently, and pulses vary somewhat in terms of amplitude. Some pulsars exhibit odd phasing effects as well.

Also, pulses have to be dedispersed, which is somewhat complicated to do in real-time, especially in the ideal frequency range for observations (we observe in the 300-350MHz range, with our best sensistivity occurring in the 327 window); the larger the dispersion measure (DM) the more complicated dedispersion becomes. This dispersion makes the initial impulse signal become a chirp signal, and it needs to be 'dechirped'. This makes it difficult to find the instant of the pulse arrival, as it is 'smeared' not a hard edge.

Pulsars are almost uniformly weak sources; with our 26 meter (85 feet) dishes they are still a challenge to receive at 327MHz; and they get weaker as the frequency goes up. Although the difficulty of dedispersing the pulse becomes easier as frequency rises.

We use several pieces of equipment to receive pulsars: RFspace's SDR14 with gating modification is one, and the GNUradio project's USRP is another. We have a specialized receiver that uses analog filter banks to do the dedispersion, but it is not currently in operation.

The most accurate timing soures commercially available that I know of are hydrogen masers; the timing from them easily exceeds the accuracy of pulsar timing. Cesium and Rubidium, along with ovenized quartz, are increasingly inaccurate, but if a pulsar can be accurately timed and the timing fluctuations of the pulsar measured by a source as inaccurate as a Rubidium standard, then the Rubidium standard is more accurate than the pulsar as to timing. That is, in order of accuracy: hydrogen>cesium>rubidium>oven-quartz; the pulsar accuracy is typically between the quartz and the rubidium.

If you are looking for position information (as Global Positioning System implies) triangulation upon several sources (you mentioned four) in the sky is doable; however, at radio frequencies these are not point sources; at 327MHz, for instance, an 85 foot parabolic dish has a 3dB beamwidth of about 2.4 degrees of arc; this is too wide to use for accurate positioning. The common and bright 1420 spectral sources are likewise gas clouds; much continuum radiation isn't from point sources. You will not be able to recieve pulsars with anything but a highly directional antenna; they are just too weak. Of course, in a spacecraft it is easy getting the cold temperatures to cool a low noise amplifier down to get the noise figure where you need it to be; I would question if the Johnson noise in available LNA devices would be low enough even then to make receiving a pulsar (even the brightest) with an omnidirectional antenna possible.

Now, terrestrial observations of pulsars get doppler effects due to the earth's revolution around the sun (also due to rotation, but the diurnal doppler swamps out the daily doppler) that depend upon your location on the earth; to determine position from the observed pulsar's timing would require you to do the local standard of rest calculation backwards, yielding time and location from the variance in pulsar timing and sky location. While this might be possible if you have four pulsars being timed, it would be rather difficult. Timing a single pulsar will not help you, as that is insufficient information to solve for time and location by calculating the local standard of rest in reverse. (that is, you take the measured pulsar period of several (but not too many) pulses, integrate, compare to theoretical yielding the measured doppler, then reverse the doppler calculation ordinarily yielded by the L

wait a minute.

[User 956]

Now, researchers at Oak Ridge have shown that the spin of a pulsar is determined by the shock wave created when the star's massive iron core collapses. From the article: 'That shock wave is inherently unstable, and eventually becomes cigar-shaped instead of spherical. The instability creates two rotating flows -- one in one direction directly below the shock wave and another, inner flow, that travels in the opposite direction and spins up the core. The asymmetrical flows establish a 'sloshing' motion that accounts for the pulsars' observed spin velocities from once every 15 to 300 milliseconds.'

Hey now, it's not an asymmetrical flow that establishes an observed spin velocity. It's 'Intelligent Exploding'.

Strange summary...

[imsabbel]

The current theories also explain the slow pulsars.
By the simple way of energy loss.
Take the crab nebular pulsar as example. Currently spinning about 30 times per second, it will be down to 20 in a few thousand years. Those thing have huge magnetic fields, and they can couple out energy amounts into the planetary nebular even OB stars usually can only dream of.

So no, its not like everybody was totally stupid before this theory.

Re:Strange summary...

[Gospodin]

...even OB stars usually can only dream of.

Do pulsars dream of magnetic sheep?

Re:Strange summary...

[iamlucky13]

Thanks for explaining that.

However, the submission and the article are still a little strange in their word choices (mostly the submission...If you re-read the article a few times, it makes more sense). They seem to suggest that the assymetric shockwave gives the pulsars their sping, when according to the theory, it actually takes away spin. I clicked on the article wondering how in the world they were postulating massive, exploding objects with very low or possibly non-existant angular momentums.

If I'm reading it right, the ejected debris from the rebound should spiral more in this case to account for the net loss of angular momentum, no?

Anyway, the worst part was this line:

According to three-dimensional simulations they performed at the Leadership Computing Facility, located at ORNL, the spin of a pulsar is determined not by the spin of the original star, but by the shock wave created when the star's massive iron core collapses.

What they really meant is the original spin plus the shockwave, not just the latter instead of the former. Other than that though, the press release is a fresh breath of clear writing.

Alt. theory : Pulsar leans to the extreme right

[Anonymous Coward]

...and FOX "NEWS" does the spinning for it.

Pulsars get sloshed every 15 to 300 milliseconds?

[Picass0]

Sounds like that's where the party is at.

From Kipling's "Just-so Stories"

[sehlat]

"How the Pulsar Got Its Spin"

In the galaxy, once upon a time, O my Best Beloved, there was a
star, and it gave off light and a stellar wind. It fused hydrogen
to helium, and developed turblence and spots, and slowly grew old,
turning to burning helium, then heavier and really truly heavier
elements, until it grew a Great Iron Core. ...

Re:Not trying to discredit....

[servognome]

But it seems that most of these science articles, particularly the ones related to astronomy, have forgotten the word "theory".

All science is theory, so it would be redundant.

Or, in legal circles...

[mangu]

"theory" is for science journalists as "allegedly" is for the crime pages. We all know it's true, but no one wants to risk the liability of a libel and slander suit.

Similar Concept

[Rhesusmonkey]

Isn't that pretty much the same process by which Stella Got her Groove Back?

Spindowns

[Shitok]

I think the story ought to acknowledge the larger effect which comes from the stars initial angular momentum.

Our Sun (for example) rotates at the rate of around once per 25 days and has a radius of around 1 million km. If
it was to collapse into a neutron star without losing any mass the moment of inertia would go down by a factor
of (1,000,000 km/10 km)^2 = 10 billion. So the rotation rate would go up to 4500 times per second. The principle
is the same one that makes figure skaters spin-up when they bring their arms and legs closer to their bodies.

Clearly, it would not retain all of its matter when collapsing and you need to be several times heavier than the
sun in order to collapse into a neutron star. The fastest pulsars still only rotate at ~600x per second. But its
still a significant factor in the spin rate calculation.

Of course, then they spin down becomes of electromagnetic radiation. Some of them probably even spin down because
they are asymmetric and lose energy in the form of gravitational radiation.