Crab Nebula's Strange Pulsar Heart Slowly Going Off-Kilter

Dr Who Fan
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Topic 197251

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For the first time, astronomers have tracked the evolution of a pulsar's magnetic field over time, watching as it slowly tilts toward the dead star's equator. The new observations of the pulsar, located in the Crab Nebula, could offer clues to the long-standing problem of what slows pulsars' rotation.

"Most pulsars are millions or tens of millions of years old," said Andrew Lyne, emeritus professor of physics at the University of Manchester in the U.K., who led the study, which appears in the Nov. 1 issue of the journal Science. "So we don't expect to see significant changes. But we have been looking at this for a substantial portion of its lifetime, some 40 out of 1,000 years." The supernova that birthed the pulsar in the Crab Nebula occurred in A.D. 1054. Chinese and Arab astronomers both noted it.

Click here for full article from Space.com


astro-marwil
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Crab Nebula's Strange Pulsar Heart Slowly Going Off-Kilter

Hallo!
Thank you for the interesting figures in that article. That does mean amongst others, that the radiation period has increased within 22 years by astonishing 1% (305µs). Furthermore I learned, that the radio emission of the pulsar is polarized, as I mentioned for long time, but never found literature about this before.

Kind regards and happy crunching
Martin

Mike Hewson
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A caveat/spoiler : There are

A caveat/spoiler : There are many models for the emission behaviour. This group have chosen to represent the underlying signal mechanisms in a particular way, such that the simplest self consistent explanation is the increase in an angle between the primary and secondary pulses. This is what has increased and leads to the 'off-kilter' comment. You could use another model of course, but if you like Occam's razor has been applied. To be ( painfully ) exact : if one assumes the signal change is due to an inclination change then the figure would be about 6 degrees over the life of the pulsar since 1054. All well and good and they are probably right* as it is thought on other grounds - theory and observations elsewhere - that this should be true.

The basic idea is that the two pulses ( primary and secondary ) have different source locations. The angle b/w the axes of the two is assumed to be widening. This in turn impacts on the slowdown rate as, in effect, the angular momentum is being altered. What I found interesting was their consideration of the 'return' current to the pulsar surface. Meaning that if one accelerates charges from off the surface - hence producing the emissions we pick up - then such charge must eventually return to the star. This moves mass and momentum around the star slowly .... a complex sucker for sure. You see there is still a requirement for an 'electrical circuit' to exist !! What a scale this exists on !! If you were nearby it would quickly separate your spleen from your squeelly splooge** ..... :-)

[ I have access to the full text - alas it is subscription only, and I can't copy it here for you. My pet peeve is when qualifying comments are well discussed in an original paper ( here about 1/2 of the article ) and completely stripped from derived descriptions. ]

Cheers, Mike.

* What would I know anyway ?? :-)

** see SpongeBob SquarePants

I have made this letter longer than usual because I lack the time to make it shorter. Blaise Pascal

astro-marwil
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Hallo Mike! Thanks for your

Hallo Mike!
Thanks for your interesting comments.

Quote:
[ I have access to the full text - alas it is subscription only, and I can't copy it here for you. ... ]


You´r luccky to have access to that paper. I asked twice at the adress shown at the homepage for a copy of this paper, but didn´t get any answer.

I would be interested to know at what frequency they did measure this, and how much the degree of polarisation was.

Today I learned, that Cygnus A does emit also polarized radiation but in the infrared at 2µm, and this seems to be somewhat cluttered too.

Kind regards and happy crunching
martin

Mike Hewson
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Hi Martin !

Hi Martin ! :-)

Measurements made @ 610 MHz and 1400 MHz, but alas no specifics on the polarisations. Also of worthy note is the pulse components at those frequencies also correlate with measurements made at higher frequencies from optical through to gammas. I visualise/imagine this star as like a fountain of particles and radiation erupting from certain locales.

Cheers, Mike.

( edit ) I guess the idea is that polarisation - if intrinsic to the source, as opposed to something else b/w that and us - will relate to some physical structure/characteristic that defines the emission. But only some of the radiation is polarised, not all aspects. One can quite easily make polarised emissions artificially here on Earth by design of the physical arrangement of the transmitting apparatus. The trick in the astronomical realm would be to reverse engineer an 'effective emitter structure' that explains the observed behaviour. I think this lies within the area of study called 'magnetohydrodynamics' ( MHD ): a horribly complex beastie. MHD is where the position of the charged particles is fluid. In a metal lattice, for instance a copper wire, the modelling advantage is that the metal nuclei are essentially fixed in position ( phonons not withstanding ) so one studies electronic conduction with that background in place. Not so with plasmas.

I have made this letter longer than usual because I lack the time to make it shorter. Blaise Pascal

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