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Some satellites get all the glory. While Hubble, Chandra, and Spitzer frequently make headlines with their stunning images, many other space based observatories silently toil away. One of them, known as the Payload for Antimatter Matter Exploration and Light-nuclei Astrophysics (PAMELA) has been in orbit since 2006, but rarely receives media attention although a stunning discovery has led to the publication of over 300 papers within a single year. A new paper in that onslaught has proposed an interesting new object: pulsars powered by white dwarfs.
PAMELA isn’t a satellite in its own right. It piggybacks on another satellite. Its mission is to observe high energy cosmic rays. Cosmic rays are particles, whether they be protons, electrons, nuclei of entire atoms, or other pieces, that are accelerated to high velocities, often from exotic sources and cosmological distances.
Among the types of particles PAMELA detects is the elusive positron. This anti-particle of the electron is quite rare due to the scarcity of anti-matter in general in our universe. However, much to the surprise of astronomers, in the range of 10 – 100 GeV, PAMELA has reported an abundance of positrons. In even higher ranges (100 GeV – 1 TeV) astronomers have found that there is a rise in both electrons and positrons. The conclusion from this is that something is able to actually create these particles in these energy ranges.
A flurry of papers went to publication to explain this unexpected finding. Explanations ranged from showers of particles created by even higher energy cosmic rays striking the interstellar medium, to the decay of dark matter, to neutron stars, pulsars, supernovae, and gamma ray bursts. Indeed, many events that produce high energies are sufficient to spontaneously produce matter from energy through the process of pair production. However, the range of these ejected particles would be limited. Effects, such as synchrotron and inverse Compton emission would drain their energy over large distances and as such, by the time they reached PAMELA’s detectors would be too low energy to account for the excesses in the observed energy ranges. From this, astronomers are presuming the culprits are in the local universe.
Joining the long list of candidates, a new paper has proposed a mundane object could be responsible for the high energy necessary to create these energetic particles, albeit with an unusual twist. Neutron stars, one of the potential objects formed in a supernova, are known to release large amounts of energies when spinning quickly while creating a strong magnetic field in the form of pulsars, but the authors propose that white dwarfs, the products of the slow death from stars not massive enough to result in a supernova, may be able to do the same thing. The difficulty in creating such a white dwarf pulsar is that, since white dwarfs don’t collapse to such a small size, they don’t “spin up” as much as they conserve angular momentum and shouldn’t have the sufficient angular velocity necessary.
The authors, led by Kazumi Kashiyama at Kyoto University propose that a white dwarf may reach the necessary rotational speed if they undergo a merger or accrete a sufficient amount of mass. This idea is not unheard of since white dwarf mergers and accretion are already implicated in Type Ia Supernovae. The combination of this with the expectation that around 10% of white dwarfs are expected to have magnetic fields of 106 Gauss, the steps necessary to produce a pulsar from a white dwarf seem to be in place. They note that since white dwarfs tend to have weaker magnetic fields, they shed their angular momentum more slowly and would last longer. Although this duration is still far longer than humans can possibly watch, this may indicate that many of the pulsars observed in our own galaxy are white dwarfs.
Next, the authors hope to conclusively identify such a star. The creation of each of these types of pulsars may provide a clue: Since neutron stars form from supernovae, they are surrounded by a shell of gas that contains a shock front from the supernova itself, which is more dense than the interstellar medium in general. As particles pass through this shock front, some of them would be lost. The same would not be said for white dwarfs which formed from a more gentle release and aren’t impeded by the relatively high density area. This shift in energy distributions may be one distinguishing characteristic.
Some stars have even been tentatively proposed as candidates for white dwarf pulsars. AE Aquarii was seen to give off some pulsar-like signals. EUVE J0317-855 is another white dwarf that appears to meet the qualifications, although no signals have been detected from this star. This new class of stars would be able to explain the excess signal in the higher energy range detected by PAMELA and will likely be the target of further observational searches in the future.
PAMELA did find a large flux of postrons in the .1-1TeV range from the halo region near the galaxy center. So these particles do not appear to be the result of standard high energy events one might expect in the chaos at the center. The prospect then exists these are due to the decay of neutralino particles. These are strange particles, for they are due to the supersymmetric partner of the photon, Z and neutral Higgs. These are fermion particles with identical quantum numbers, and thus fill the same eigen-state. They are also Majorana fermions in that they are their own anti-particles and thus decay. They should have a decay signature around this energy.
It should not be that surprising that white dwarfs might emulate neutron stars, and if they are spun up by accreting matter from their parent star they could assume these properties. Studies of these should then be important in determining the accuracy of SNII’s as a standard cosmological candle. As these spin up they should approach the Chandrasekhar limit and produce a supernova. This might be a way of gauging this process.
Cheers LC
Could the vast positron belt near the center of the milky way be from black hole normal matter consumption, leaving the unconsumable anti-matter that is NOT annihilated in belts? If electrons are deflected by galactic black holes or interstellar medium magnetic fields, preventing annihilation with their opposite positrons, could this be the kind of process that forms neutralino particles, or how would it be described scientifically? will neutralions have a dark matter gravity effect?
LC:
Your post was very interesting. I wondered when you mentioned supersymmetric particles: Are these actually “real” particles, or are they particles theorized by String or M Theory?
The supersymmetric pairs of standard particles have been theoretically predicted since the mid-late 1970s. These should exist. Supersymmetry (not string theory yet) is a way of unifying external symmetries of spacetime (relativity) with the internal symmetries of particle physics, such as fermionic or gauge symmetries. There are reasons why this works, from the Coleman-Mandula theorem to theorems by Haig and Neiuenheusen.
String theory gets into the picture because the unification of external and and internal symmetries means gravity is in the supersymmetric picture. Yet the theories derived, so called supergravity, were not renormalizable and had huge problems. This is where string theory picked up the supersymmetry game. The supergravity theories were more workable in this string setting, hence the term supersymmetry.
LC
Good explaination — thank you, LC.
Guess I have less of a problem with supersymmetric particles than I do with String Theory.
String theory is almost demanded. The interaction of two particles in space and time looks a bit like a letter H. The vertical lines are two particles moving forward in time portrayed in the vertical direction and the horizontal line is the exchange of a gauge boson. The gauge boson comes about from the Heisenberg uncertainty principle, where a particle can come in and out of existence as a quantum fluctuation. This boson can pick up some momentum from one of the vertical particles and pass it on to the other. This is how forces occur in quantum mechanical language. Now the letter H can transform under Lorentz boosts of relativity, but there is a catch. The lines transform, but the two points connecting the vertical lines with the horizontal do not. Relativity is a transformation of vectors and tensors, but not scalars or points. This is a bit of a snag with quantum field theory. For certain reasons with gravity this is an even worse problem. The lines represent the motion of a point particle. If instead there is a loop, a closed string, then the letter H is a thickened thing, appearing to be made of welded pipes. Now the two vertex points are gone! Now the whole thing transforms continuously, and a whole host of infinities associated with that vertex are eliminated. This is the Veneziano amplitude, which replaces the annoying problems of vertex functions that diverge with an Euler Beta function.
I am not sure why this is the case, but it seems that everywhere people are becoming horribly conservative in a reactionary way. This is not just in politics, but in ideas of science, the rise of creationism, popular ideas that dismiss much of modern science, and curiously there is even a rise of geocentrism these days. This is particularly marked in the United States. To me this is pretty dismaying, for not only are Americans about to elect the greatest pack of know-nothings in history, but it appears that the eyesight of people are being cast downwards and away from things which are really wondrous.
LC
How is this suddenly news?I saw a news release from NASA dated 1/2/2008 that said the same thing?
Kammueller: There were 2008 stories that laid out much of the groundwork for this but was based almost solely on AE Aqu. This recent paper expands the number of potential objects since more candidates have been discovered since the initial proposal. This lends some credence to the hypothesis. Additionally, this new paper uses this proposal to potentially explain higher energy cosmic rays.
It’s not so much that the idea is new, but that it again becomes newsworthy because it has been fleshed out some more. But necessity dictate that much of the original information be reported again to be comprehensive.