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High-Energy Electrons: Evidence for Dark Matter?

What is the unusual source of high-energy cosmic rays that has been discovered within 3000 light years of the Sun? Everyone loves a mystery, and this one has all the earmarks of a classic. The source was found by the Advanced Thin Ionization Calorimeter (ATIC) experiment, which was lofted to high altitude above Antarctica via helium-filled balloon. Behind the experiment was the goal of studying cosmic rays that are otherwise shielded from the surface by the Earth’s atmosphere, but among the results was an unexpected finding.

Cosmic ray electrons at 300 to 800 billion electron volts are simply too powerful to be regarded as standard fare, for these particles lose energy as they move through the galaxy. That means that a study like this should see fewer electrons at higher energies. Nearby sources, on the other hand, stand out, making it clear there is what principal investigator John Wefel (Louisiana State) calls “…a very interesting object near our solar system waiting to be studied by other instruments.” We’re talking about candidates ranging from a pulsar or a black hole to a supernova remnant or even a mini-quasar. See this news release for more.

Then again, could this be an indication of dark matter in the neighborhood? The annihilation of exotic particles caused by two of them colliding — some believe such particles are candidates for dark matter — could produce just what we are seeing, says Eun-Suk Seo (University of Maryland), who adds that the process would produce normal electrons and protons and their antimatter equivalents, positrons and antiprotons.

That latter possibility is highly theoretical, depending upon dark matter theories that invoke extra dimensions, but we still have to find out whether there is an unseen object that is accelerating electrons to these energy levels, surely the first step in characterizing this mysterious source. And here another mission may help us. For what ATIC has uncovered is roughly similar to data from the PAMELA (Payload for Antimatter Matter Exploration and Light-nuclei Astrophysics) satellite, which likewise found a source of energetic particles in the same range. From the paper on the PAMELA results:

Our results clearly show an increase in the positron abundance at high energy that cannot be understood by standard models describing the secondary production of cosmic-rays. Either a significant modification in the acceleration and propagation models for cosmic-rays is needed, or a primary component is present. There are several interesting candidates for a primary component, including the annihilation of dark matter particles in the vicinity of our galaxy. There may also be a contribution from near-by astrophysical sources, such as pulsars.

So what are these exotic particles? Here I want to quote from an article by Geoff Brumfiel on this work in Nature, running in the same issue as the ATIC results:

The exact nature of the dark-matter particles that produce electrons is uncertain, but one idea is that they may be ordinary particles that spend part of their lives in a compact extra dimension of space. Whereas the particles would appear relatively stationary to observers trapped in three spatial dimensions, they could be moving at ultra-high speeds in a fourth spatial dimension. At high speeds, they would create a gravitational force that could be felt by matter trapped in three dimensions of space-time.

I’ll opt for a nearby pulsar, as discussed in the above article, one whose magnetic fields would create the needed acceleration of electrons, but we may just have to wait for the Fermi Gamma-ray Telescope (originally called GLAST) to verify both ATIC and PAMELA. The paper on ATIC is Chang et al., “An excess of cosmic ray electrons at energies of 300–800 GeV,” Nature 456 (20 November 2008), pp. 362-365 (abstract). The PAMELA work is Adriani et al., “Observation of an anomalous positron abundance in the cosmic radiation,” submitted to Nature and available online.

Comments on this entry are closed.

  • James M. Essig November 20, 2008, 22:28

    Hi Paul;

    Interesting article.

    The existence of a natural supply of antimatter potentially bodes well for antimatter catalyzed fusion space craft such as improved or yet to be developed concepts of interstellar ramjets.

    The possible future ability to collect fusion fuel from the interstellar medium as well as material capable of hopefully, easily inducing nuclear fusion within a ramjet fusion engine, bodes well for high gamma factor manned space travel about the Milky Way Galaxy.

    Whether the ambient material or mattergy for powering high gamma factor manned intragalactic and intergalactic space craft takes the form of baryonic fusion fuel, baryonic fusion fuel ignited by ambient antimatter supplies, or perhaps some form of cold dark matter, etc., a knowledge of the real forms of mattergy in all its varieties is what it is, and so such knowledge can only benefit the cause of future manned interstellar space flight.



  • Adam November 21, 2008, 8:12

    If they find Dark Matter then I hope it really does make regular matter particles when it decays – might keep stars running for a few trillion more years. If Life can engineer that then maybe we can re-engineer the ultimate fate of the cosmos, whatever that might be.

  • Phil November 21, 2008, 8:24

    Isn’t a nearby Pulsar Dangerous? Damn, we’re all going to die again!!

    Seriously though, I am glad there are people researching this kind of thing. I hope the followup experiment to figure out the source is already underway!! Go Fermi!!

  • ljk November 21, 2008, 14:53

    Other than dark matter, what else could be generating these
    particular cosmic rays?

  • Administrator November 21, 2008, 15:49

    Larry, other than a pulsar, a supernova remnant seems a possibility, and they do mention an intermediate-sized black hole as a candidate. Also, and I think you already know about this one:


    Hall and Hooper, “Distinguishing Between Dark Matter and Pulsar Origins of the ATIC Electron Spectrum With Atmospheric Cherenkov Telescopes,” goes into all this. From the abstract:

    “Emission from a local pulsar and dark matter annihilations have each been put forth as possible origins of this signal. In this letter, we consider the sensitivity of ground based atmospheric Cherenkov telescopes to electrons and show that observatories such as HESS and VERITAS should be able to resolve this feature with sufficient precision to discriminate between the dark matter and pulsar hypotheses with considerably greater than 5 sigma significance, even for conservative assumptions regarding their performance. In addition, this feature provides an opportunity to perform an absolute calibration of the energy scale of ground based, gamma ray telescopes.”

  • James M. Essig November 22, 2008, 11:12

    Hi Adam;

    I would love to believe that cold dark matter decays into baryonic matter. This would potentially provide a source of protons, or hydrogen atoms, for stellar formation. It would be nice if some Cold Dark Matter species had a half life on the order of trillions if not 10s of trillions of years so that the cosmic supply of potentially star forming matter could last the additional few trillion years you mentioned above.

    An interesting possibility, as you mentioned, would entail a future human and/or ETI ability to engineer the universe for sustained human and ETI presence essentially forever. Just as in some theories of the Big Bang, the net mattergy of the universe may be relatively very small if not zero wherein the energy associated with the expansive properties of space is balanced by the gravitational force and the real mattergy content of the universe, perhaps this balancing mechanism can permit the bootstrapping of the total negative energy and the total positive energy within the universe in to higher levels perhaps leading to the production of additional baryonic matter for an eternal process of star formation.

    Perhaps in some casual manner, the presense of CDM and perhaps Hot Dark Matter is related to the Dark Energy that seems to be increasing the rate of cosmic expansion.



  • ljk November 25, 2008, 0:02

    November 24, 2008

    Sources of Earth-Bombarding Cosmic Rays May Have Been Located

    Written by Nancy Atkinson

    The cosmic ray hot spots were identified in the two red-colored regions near the constellation Orion. Courtesy John Pretz, LANL

    Last week’s announcement of a puzzling and unknown source of high energy cosmic rays bombarding the Earth is now joined by another discovery of two sources of unexpected cosmic rays from nearby regions of space.

    A Los Alamos National Laboratory cosmic-ray observatory has seen for the first time two distinct hot spots that appear to be bombarding Earth with an excess of cosmic rays.

    “These two results may be due to the same, or different, astrophysical phenomenon, said Jordan Goodman, principal investigator for the Milagro observatory, commenting on last week’s announcement by the ATIC experiment and the new discovery by his team.

    “However, they both suggest the presence of high-energy particle acceleration in the vicinity of the earth. Our new findings point to general locations for the localized excesses of cosmic-ray protons.” T

    he cosmic rays appear to originate from an area in the sky near the constellation Orion.

    Researchers used Los Alamos’ Milagro cosmic-ray observatory to peer into the sky above the northern hemisphere for nearly seven years starting in July 2000.

    The observatory is unique in that it monitors the entire sky above the northern hemisphere. Because of its design and field of view, Milagro was able to record over 200 billion cosmic-ray collisions with Earth’s atmosphere.

    Full article here:


  • ljk November 25, 2008, 13:57

    The Case for a 700+ GeV WIMP: Cosmic Ray Spectra from ATIC and PAMELA

    Authors: Ilias Cholis, Gregory Dobler, Douglas P. Finkbeiner, Lisa Goodenough, Neal Weiner

    (Submitted on 24 Nov 2008)

    Abstract: Multiple lines of evidence indicate an anomalous injection of high-energy e+e- in the Galactic halo. The Advanced Thin Ionization Calorimeter (ATIC) has detected an excess bump in the electron cosmic ray spectrum from 300-800 GeV, falling back to the expected E^{-3.2} power law at 1 TeV and above.

    The recent positron fraction spectrum from the Payload for Antimatter Matter Exploration and Light-nuclei Astrophysics (PAMELA), shows a sharp rise up to 80 GeV. Excess microwaves towards the Galactic center in the WMAP data are consistent with hard synchrotron radiation from a population of 10-100 GeV e+e- (the WMAP “haze”).

    We argue that dark matter annihilations can provide a consistent explanation of all of these data, focusing on dominantly leptonic modes, either directly or through a new light boson. Normalizing the signal to the highest energy evidence (ATIC), we find that similar cross sections provide good fits to PAMELA and the Haze, and that both the required cross section and annihilation modes are achievable in models with Sommerfeld-enhanced annihilation.

    These models naturally predict significant production of gamma rays in the Galactic center via a variety of mechanisms. Most notably, there is robust inverse-Compton scattered (ICS) gamma-ray signal arising from the energetic electrons and positrons, detectable at Fermi/GLAST energies, which should provide smoking gun evidence for this production.

    Comments: 28 pages

    Subjects: Astrophysics (astro-ph); High Energy Physics – Phenomenology (hep-ph)

    Cite as: arXiv:0811.3641v1 [astro-ph]

    Submission history

    From: Douglas P. Finkbeiner [view email]

    [v1] Mon, 24 Nov 2008 20:56:28 GMT (86kb)


  • ljk December 10, 2008, 1:10

    arXiv:0812.1286 (*cross-listing*)

    Date: Sat, 6 Dec 2008 13:24:53 GMT (45kb)

    Title: The Dark Universe Riddle

    Authors: A. J. S. Capistrano, P.I. Odon

    Categories: gr-qc astro-ph

    Comments: 38 pages, no figures

    In this work we review some of the theoretical efforts and experimental vidences related to Dark matter and Dark energy problems in the universe.

    These dilemmas show us how incomplete our knowledge of gravity is, and how our concepts about the universe must at least be revised. Mainly, on the Wilkinson Microwave Anisotropy Probe (WMAP) fifth year, the data indicates that more than 90% of the total energy density of the universe is dark.

    Here we discuss the impact of these phenomena imprint on gravitational and quantum field theory’sstandard history. Moreover, we point out some recent and upcoming projects on Cosmology in a quest to understand theses issues thoroughly.

    http://arxiv.org/abs/0812.1286 , 45kb

  • ljk January 7, 2009, 1:37

    Dark Matter

    Authors: Jaan Einasto

    (Submitted on 6 Jan 2009)

    Abstract: A review of the development of the concept of dark matter is given. I begin the review with the description of the discovery of the mass paradox in our Galaxy and in clusters of galaxies.

    In mid 1970s the amount of observational data was sufficient to suggest the presence of a massive and invisible population around galaxies and in clusters of galaxies. The nature of the dark population was not clear at that time, but the hypotheses of stellar as well as of gaseous nature of the new population had serious difficulties. These difficulties disappeared when non-baryonic nature of dark matter was suggested in early 1980s.

    In addition to the presence of Dark Matter, recent observations suggest the presence of Dark Energy, which together with Dark Matter and ordinary baryonic matter makes the total matter/energy density of the Universe equal to the critical cosmological density.

    There are various hypothesis as for the nature of the dark matter particles, and generally some form of weakly interactive massive particles (WIMPs) are strongly favored. Both Dark Matter and Dark Energy are the greatest challenges for modern physics since their nature is unknown.

    Comments: UNESCO EOLSS ENCYCLOPEDIA (accepted) 25 pages, 17 figures

    Subjects: Astrophysics (astro-ph)

    Report number: Tartu Observatory 2009.1

    Cite as: arXiv:0901.0632v1 [astro-ph]

    Submission history

    From: Jaan Einasto [view email]

    [v1] Tue, 6 Jan 2009 11:54:26 GMT (2521kb)


  • ljk January 20, 2009, 0:09

    A Decade of Dark Energy: 1998 – 2008

    Authors: Ruth A. Daly

    (Submitted on 18 Jan 2009)

    Abstract: The years 1998 to 2008 were very exciting years for cosmology. It was a pleasure to accept this invitation to describe my contributions to the development of our knowledge and understanding of the universe over the course of the past decade.

    Here, I begin by describing some of my work on radio galaxies as a modified standard yardstick and go on to describe model-independent studies of the accelerating universe and the properties of the dark energy.

    During the course of these studies, I came upon interesting ways to study the spin and other properties of supermassive black holes, some of which are briefly mentioned.

    Comments: Proceedings of the 2008 UCLA Conference “Dark Matter and Dark Energy in the Universe,” submitted to AIP Conference Proceedings, 6 pages

    Subjects: Astrophysics (astro-ph)

    Cite as: arXiv:0901.2724v1 [astro-ph]

    Submission history

    From: Ruth A. Daly [view email]

    [v1] Sun, 18 Jan 2009 18:48:19 GMT (86kb)


  • ljk May 6, 2009, 12:48

    From Cosmic Variance:

    Fermi Waffles on Dark Matter

    For the last few months there’s been some excitement among particle-astrophysicists about intriguing results from the PAMELA satellite experiment and the ATIC balloon experiment. (We also blogged about it here and here.) PAMELA claimed to see an excess in the number of high-energy cosmic positrons (anti-electrons) over what you would expect from conventional astrophysical sources, while ATIC (which can’t distinguish between positrons and electrons) saw an overall rise in the number of positrons and electrons combined, more or less consistent with what PAMELA saw.

    One dramatic but plausible explanation for this result is that the positrons are produced when dark matter particles and antiparticles annihilated with each other, which would certainly be exciting. But it wasn’t quite a home run, because there was no evidence for the corresponding excess of anti-protons you would probably also expect. (Although that is not a deal-breaker; with a little ingenuity, particle physicists are able to come up with models that produce positrons but not anti-protons.)

    There was also some controversy when theorists wrote papers trying to fit the data before the data were even published, by snapping pictures of plots shown at conferences with their cell-phone cameras. More than enough drama for a TV movie, I would say.

    A tinfoil-hat conspiracy theorist might imagine that all the excitement was intentionally manufactured, just so people would pay more attention to the first measurement from the new Fermi (formerly GLAST) gamma-ray telescope. And now those results are in! (Other Fermi results have already appeared, but not about this particular question.)

    Sadly, the results are “in” in the sense of being published in Physical Review Letters, which helpfully charges $25 if you’re not a subscriber. (Presumably it will be on arxiv soon, probably tonight.) The best summary of the results, although somewhat technical, is by Bruce Winstein and Kathryn Zurek at Physics, the American Physical Society’s in-house journal that highlights interesting results.

    And here are those results.


  • ljk June 15, 2009, 22:27

    Capture of dark matter by the Solar System

    Authors: I.B.Khriplovich, D.L.Shepelyansky (BINP, Novosibirsk & CNRS, Toulouse)

    (Submitted on 13 Jun 2009)

    Abstract: We study the capture of galactic dark matter by the Solar System. The effect is due to the gravitational three-body interaction between the Sun, one of the planets, and a dark matter particle. The total mass of the captured dark matter particles is found. The estimates for their density are less reliable. The most optimistic of them give an enhancement of dark matter density by about three orders of magnitudes compared to its value in our Galaxy. However, even this optimistic value remains below the best present observational upper limits by about two orders of magnitude.

    Comments: 4 pages, 3 tables

    Subjects: Solar and Stellar Astrophysics (astro-ph.SR); Earth and Planetary Astrophysics (astro-ph.EP)

    Cite as: arXiv:0906.2480v1 [astro-ph.SR]

    Submission history

    From: Dima Shepelyansky L [view email]

    [v1] Sat, 13 Jun 2009 14:34:30 GMT (27kb)


  • ljk July 9, 2009, 13:04


    Thursday, July 09, 2009

    Cosmic Ray Moon Shadow Could Reveal Dark Matter

    If a strange excess of positrons hitting Earth are created by dark matter, then the way the Moon blocks these impacts could help confirm the idea

    The Earth is constantly bombarded by high energy positrons and electrons. These bombardments generate showers of secondary particles that light up our skies at night, if you have the right equipment to see ’em: so-called Imaging Atmospheric Cherenkov Telescopes. The ratio of electrons to positrons is predicted fairly precisely by our models of the way cosmic rays interact with objects in the Milky Way.

    But here’s a conundrum. Various space-based experiments such as PAMELA have recently found an excess of positrons out there, particularly at energies above 10 GeV. That’s totally unexpected and difficult to square with the conventional model.

    The PAMELA measurement generated excitement because the dark matter brigade pounced on the result as evidence that dark matter particles must annihilating each other, producing the excess positrons in the centre of our galaxy. These guys were forced to put the champagne back on ice when other astrophysicists pointed out that the positrons could equally be created by particle cascades in the magnetospheres of nearby pulsars.

    What’s needed, of course, is more measurements of positron/electron ratios, particularly at energies up to a few TeV that cannot yet be made by space-based experiments.

    Can the growing number of Imaging Atmospheric Cherenkov Telescopes help? On the face of it, that looks unlikely because there is no way to tell apart the showers created by positrons and electrons when they hit the atmosphere. At least, until now.

    Today, Pierre Colin and pals at the Max-Planck-Institut fur Physik in Munich have come up with an ingenious idea that should be able to tell them apart. Most of the electrons and positrons come from the galactic centre. Colin and co point out that that when the Moon comes between us and the electron/positron source, it creates a shadow that is already used to calibrate Imaging Atmospheric Cherenkov Telescopes.

    But here’s the interesting idea: Colin and co say the shadow of charged particles should be deflected by the Earth’s magnetic field. The electron shadow should be shifted eastward and the positron shadow westward. These Imaging Atmospheric Cherenkov Telescopes should therefore be able to spot the separate shadows, allowing the measurement of positron/electron ratios at energies up to several TeV, well beyond what space-based experiments can achieve.

    What’s more, Imaging Atmospheric Cherenkov Telescopes ought to be able to spot these shadows now as long as they can make measurements in the glare of the Moon. One such instrument called MAGIC, built by the Max-Planck-Institut fur Physik at Roque de los Muchachos in the Canary islands, exactly fits the bill.

    The measurements will still be tricky however, particularly of the positron shadow which may well be superimposed on the shadow created by positively charged atoms in the cosmic ray spectrum. However, Colin and co think they ought to be able to pick out the electron shadow with just 50 hours of observing (although that may take several years given that the shadows occur only at certan times of the year).

    That’s an ingenious idea that may well give astronomers a way of determining what role dark matter plays, if any, in the creation of these excess positrons.

    Ref: http://arxiv.org/abs/0907.1026: Observation of Shadowing of the Cosmic Electrons and Positrons by the Moon with IACT

  • ljk July 9, 2009, 13:20

    Absolute electron and positron fluxes from PAMELA/Fermi and Dark Matter

    Authors: C. Balázs, N. Sahu, A. Mazumdar

    (Submitted on 27 May 2009 (v1), last revised 7 Jul 2009 (this version, v3))

    Abstract: We extract the positron and electron fluxes in the energy range 10 – 100 GeV by combining the recent data from PAMELA and Fermi LAT. The {\it absolute positron and electron} fluxes thus obtained are found to obey the power laws: $E^{-2.65}$ and $E^{-3.06}$ respectively, which can be confirmed by the upcoming data from PAMELA.

    The positron flux appears to indicate an excess at energies $E\gsim 50$ GeV even if the uncertainty in the secondary positron flux is added to the Galactic positron background.

    This leaves enough motivation for considering new physics, such as annihilation or decay of dark matter, as the origin of positron excess in the cosmic rays.

    Comments: Accepted by JCAP

    Subjects: High Energy Physics – Phenomenology (hep-ph); High Energy Astrophysical Phenomena (astro-ph.HE); General Relativity and Quantum Cosmology (gr-qc)

    Cite as: arXiv:0905.4302v3 [hep-ph]

    Submission history

    From: Narendra Sahu [view email]

    [v1] Wed, 27 May 2009 15:56:34 GMT (67kb)

    [v2] Thu, 28 May 2009 19:05:18 GMT (68kb)

    [v3] Tue, 7 Jul 2009 19:04:00 GMT (68kb)


  • ljk September 9, 2009, 12:46

    Results from PAMELA, ATIC and FERMI : Pulsars or Dark Matter ?

    Authors: Debtosh Chowdhury, Chanda J. Jog, Sudhir K Vempati

    (Submitted on 7 Sep 2009)

    Abstract: It is well known that the dark matter dominates the dynamics of galaxies and clusters of galaxies. Its constituents remain a mystery despite an assiduous search for them over the past three decades.

    Recent results from the satellite-based PAMELA experiment detect an excess in the positron fraction at energies between 10-100 GeV in the secondary cosmic ray spectrum. Other experiments namely ATIC, HESS and FERMI show an excess in the total electron (\ps + \el) spectrum for energies greater 100 GeV.

    These excesses in the positron fraction as well as the electron spectrum could arise in local astrophysical processes like pulsars, or can be attributed to the annihilation of the dark matter particles.

    The second possibility gives clues to the possible candidates for the dark matter in galaxies and other astrophysical systems. In this article, we give a report of these exciting developments.

    Comments: 11 pages,3 figures, Latex, pdftex. Pedagogical review submitted to Current Science

    Subjects: High Energy Astrophysical Phenomena (astro-ph.HE); Cosmology and Extragalactic Astrophysics (astro-ph.CO)

    Cite as: arXiv:0909.1182v1 [astro-ph.HE]

    Submission history

    From: Sudhir Vempati [view email]

    [v1] Mon, 7 Sep 2009 09:19:12 GMT (220kb,D)