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.'"

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

[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

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.