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Jerry
2009-Dec-22, 07:37 PM
Discovery of a New Torque Reversal of the Accreting X-ray Pulsar 4U 1626-67 by Fermi/GBM

http://arxiv.org/abs/0912.3809v1


Recent X-ray observations by Fermi/GBM discovered a new torque reversal of 4U 1626-67 after 18 years of steady spinning down. Using Swift/BAT observations we were able to center this new torque reversal on Feb 4 2008, lasting approximately 150 days. From 2004 up to the end of 2007, the spin-down rate averaged at a mean rate of ~dnu/dt=-4.8e-13 Hz s-1 until the torque reversal reported here. Since then it has been following a steady spin-up at a mean rate of ~dnu/dt= 4e-13 Hz s-1. The properties of this torque reversal, as well as the lack of correlation between the X-ray flux and the torque applied to the neutron star before this transition, challenges our understanding of the physical mechanisms operating in this system.

What the hell? This is right out of Men in Black. Is somebody playing with a dreidel?

hhEb09'1
2009-Dec-22, 08:24 PM
If I understand that abstract right, it was spinning down, at a rate where it would stop in about 80,000 years? Now, it's stopped slowing, and is actually spinning up a little faster than that? Maybe it was expanding before, and now it's contracting? I know, it says there is a lack of correlation with the x-ray flux...maybe it's getting ready to blow.

ETA: Just a minute, what did you mean by the title of this thread, Dipole about face?

mike alexander
2009-Dec-23, 05:03 AM
If you open up the abstract page you will find a .pdf pointer to the whole paper. Insufficient background for me to really understand, but it sounds like the torque reversal is referring to events and interactions between the accretion disk and the neutron star, not the neutron star in isolation. As I say, the actual discussion is well beyond my expertise.

01101001
2009-Dec-23, 05:52 AM
Note that it's a new torque reversal. There was an old torque reversal. Perhaps many more before that.

I never trusted torque.

hhEb09'1
2009-Dec-23, 06:09 AM
The pdf says, additionally, that before the 18 years of spin down mentioned in the OP, for the nine years after its discovery, it was spinning up. Now, it's back to spinning up.

The spin hasn't changed direction though.

tusenfem
2009-Dec-23, 08:51 AM
The pdf says, additionally, that before the 18 years of spin down mentioned in the OP, for the nine years after its discovery, it was spinning up. Now, it's back to spinning up.

The spin hasn't changed direction though.

I will have to read it, but with an accreting neutron star many things can happen, albeit it with no signature in the Xray flux one will have to be careful.

Thank goodness the direction did not change, or too bad it did not, that would have been something!

The title though, is highly over the top.

captain swoop
2009-Dec-23, 09:10 AM
Almost 'atm'

George
2009-Dec-23, 03:22 PM
It looks like quite a dynamic accretion process with some luminosity quirks to boot.

Jerry
2009-Dec-23, 08:42 PM
ETA: Just a minute, what did you mean by the title of this thread, Dipole about face?
A torque reversal implies, actually requires, a ~180 deg change in one of the force vectors. I'm wondering out-loud if there could a change in the internal orientation of the magnetic dipole, reversing the magnetic vector. Similar explanations have been advanced for the twisting of jets. Very curious event!

antoniseb
2009-Dec-23, 08:49 PM
A torque reversal implies, actually requires, a ~180 deg change in one of the force vectors....

Not quite. In this case there is most likely the usual torque that slows down all pulsars, and a variable torque from variable amounts of material falling in... Sometimes it is larger in magnitude that the slowing down torque, and sometimes less. There's no evidence that anything physical switched directions

George
2009-Dec-23, 11:40 PM
Wouldn't this star be a great place to study magnetic coupling to disks? The Sun's lack of expected angular momentum is no small issue.