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View Full Version : White Dwarf Pulses like a Pulsar



t@nn
2008-Jan-04, 05:52 AM
Scientists have found a white dwarf that emits pulses of x-rays, like a pulsar.

http://www.sciencedaily.com/releases/2008/01/080102155439.htm

loglo
2008-Jan-04, 04:08 PM
Cool. .. wonder if this will help explain some of the RRATs?

Kaptain K
2008-Jan-04, 06:16 PM
:confused: RRATs? :confused:

loglo
2008-Jan-04, 07:06 PM
Rotating Radio Transients (http://www.mikeoates.org/mas/history/lectures/20060921.htm)
They produce pulses like pulsars in radio except that they are extremely sporadic, emit less than a second per year. Less than a dozen known, some of which look like faint pulsars or AXP's in the X-ray range (http://adsabs.harvard.edu/abs/2007arXiv0708.1149M) but most haven't had anything found to explain them.

En Lite end
2008-Jan-04, 10:36 PM
from the link


White dwarfs and pulsars represent distinct classes of compact objects that are born in the wake of stellar death. A white dwarf forms when a star similar in mass to our sun runs out of nuclear fuel. As the outer layers puff off into space, the core gravitationally contracts into a sphere about the size of Earth, but with roughly the mass of our sun. The white dwarf starts off scorching hot from the starís residual heat. But with nothing to sustain nuclear reactions, it slowly cools over billions of years, eventually fading to near invisibility as a black dwarf.

snip


Instead, the astronomers were hoping to find out if white dwarfs could accelerate charged subatomic particles to near-light speed, meaning they could be responsible for many of the cosmic rays that zip through our galaxy and occasionally strike Earth.

snip


Some white dwarfs, including AE Aquarii, spin very rapidly and have magnetic fields millions of times stronger than Earthís. These characteristics give them the energy to generate cosmic rays

snip


The hard X-ray pulsations are very similar to those of the pulsar in the center of the Crab Nebula. In both objects, the pulses appear to be radiated like a lighthouse beam, and a rotating magnetic field is thought to be controlling the beam. Astronomers think that the extremely powerful magnetic fields are trapping charged particles and then flinging them outward at near-light speed. When the particles interact with the magnetic field, they radiate X-rays

Is this some sort of remnant gravitational effect, left over from the birth and then death and gravitational contraction of a star? Where is the energy to sustain such powerful magnetic fields if all of it's energy source is gone (thermonuclear)?

'cos the easiest way to accelerate charged particles (http://www.fieldp.com/cpa.html)according to what I've managed to get from the net is to place them in a large electric field, yet they only mention magnetic fields, why?


A particle accelerator is a device that uses electric fields to propel electrically-charged particles to high speeds and to contain them. An ordinary CRT television set is a simple form of accelerator. There are two basic types: linear (i.e. straight-line) accelerators and circular (i.e. circles) accelerators. WIKI (http://en.wikipedia.org/wiki/Particle_accelerator)

Is there something more going on here? 'cos there MUST be an extremely powerful electric field to accelerate particles to near light speed!

Fair assumption?

Neverfly
2008-Jan-05, 03:44 AM
Our digital friend (01011010) was right- You are indeed, injecting Electric Universe ATM Theories into regular threads- while playing off to be the innocent querent.

First, let's stop assuming things.

Learn about magnetic fields, learn the math involved before you speculate.

Second, Let's stop playing innocent shall we?:neutral:

I cordially invite you to open a proper thread in the ATM forum where you can debate your theories in the proper setting.
Bear in mind that there are stricter rules in the ATM section, however, you will not get away with sidestepping those rules by slipping ATM into other forums.

loglo
2008-Jan-05, 09:52 AM
'cos the easiest way to accelerate charged particles according to what I've managed to get from the net is to place them in a large electric field, yet they only mention magnetic fields, why?

I'm not wishing to get involved in any EU discussions but section 5.3 of the paper (http://arxiv.org/abs/0711.2716) discusses possible acceleration mechanisms including how the electric field potential could be maintained in the comparatively high density plasma near white dwarfs. Binary WD's accrete a lot but regular pulsars don't have the same issue. They talk about a "propellor effect" caused by the high rotation speed clearing a cavity out of the accreting material. I think that is caused by the rotating magnetic field so that is likely why it is mentioned over the electric potential. Just sticking a plasma in an electric field alone won't do it, a magnetic field is also required to keep it going. I guess that is why they conclude:-
"that the hard X-ray pulse is probably caused by relativistic electrons accelerated by the fast rotation of the magnetized WD. The most promising emission mechanism is the synchrotron emission in the strong magnetic fields of the WD."

Seems reasonable to my (non-expert) eyes. Any corrections happily received.

Regarding my RRAT suggestion, the relatively long rotation period of 33s seems to rule them out though IIRC there were a couple with really uncertain periods which may still fit. End-of life, spun-down pulsars are one of the candidates for them so perhaps its not so far-fetched that fast rotating WDs like this one could look like them.

Neverfly
2008-Jan-05, 10:48 AM
Seems reasonable to my (non-expert) eyes. Any corrections happily received.


As my non-expert self I would say you did pretty well;)
Also, consider how the rays are emitted- Not in a uniform pattern like light is from the sun - but directional.

Jerry
2008-Jan-07, 04:31 AM
The more we learn about white dwarfs, the less confidence we can place in the assumption that they supernova into well-defined supernovae with predictable magnitudes.

trinitree88
2008-Jan-07, 06:43 PM
The more we learn about white dwarfs, the less confidence we can place in the assumption that they supernova into well-defined supernovae with predictable magnitudes.

Jerry. Agreed. It'll be interesting to see if there's a pulsation period vs. / luminosity relationship similar conceptually to the one for Cepheid variables, and whether that too, is subject to viewing angle. I'd like eventually (though I'll never live to see it...barring being abducted by aliens)....to see a match of type 1a white dwarf progenitors' pulsation periods and x-ray emissivity to the subsequent type 1a's projected absolute magnitudes. neat. pete.

Jerry
2008-Jan-08, 12:42 AM
Actually you might. The Russians are concentrating on supernovae science now, and they are not nearly as streamlined in their treatment of supernova events as the Americans and Europeans. They are also very good at X-ray astronomy.

tusenfem
2008-Jan-08, 11:28 AM
I'm not wishing to get involved in any EU discussions but section 5.3 of the paper (http://arxiv.org/abs/0711.2716) discusses possible acceleration mechanisms including how the electric field potential could be maintained in the comparatively high density plasma near white dwarfs. Binary WD's accrete a lot but regular pulsars don't have the same issue. They talk about a "propellor effect" caused by the high rotation speed clearing a cavity out of the accreting material. I think that is caused by the rotating magnetic field so that is likely why it is mentioned over the electric potential. Just sticking a plasma in an electric field alone won't do it, a magnetic field is also required to keep it going. I guess that is why they conclude:-
"that the hard X-ray pulse is probably caused by relativistic electrons accelerated by the fast rotation of the magnetized WD. The most promising emission mechanism is the synchrotron emission in the strong magnetic fields of the WD."

Seems reasonable to my (non-expert) eyes. Any corrections happily received.


Not having read the paper yet, I can easily imagine that a fast rotating white dwarf with a strong magnetic field will act similar to a fast rotating neutron star. I will see if I find some time to read the paper. For neutron stars (pulsars) there are several mechanisms by which the emission can be created, one of which has to do with accelerated particles in the electric field at the polar cap of the pulsar. Indeed, I myself, have written a paper on this topic, assuming there is an electrostatic double layer in the polar region (do not try to search for it you can only find it in my PhD thesis, as it never got published because of moving away from my Alma Mater and then improving on the paper etc. etc.)



The more we learn about white dwarfs, the less confidence we can place in the assumption that they supernova into well-defined supernovae with predictable magnitudes.


Jerry you might want to read over this sentence again, and then edit it, because as it is now, it does not make any sense.



It'll be interesting to see if there's a pulsation period vs. / luminosity relationship similar conceptually to the one for Cepheid variables, and whether that too, is subject to viewing angle.


If the pulsation is like a neutron star (although the 0.2 cycle separation is rather difficult in that case, unless the emission does not come from the poles) there will most definitely be a viewing angle phenomenon, such that you will not see it if the angle is not correct.
About the light curve, you will have to find more fast rotating pulsating magnetized white dwarfs first.