Categories: MissionsPhysics

Is a Nearby Object in Space Beaming Cosmic Rays at Earth?

Data from several different space and ground based observatories imply the presence of a nearby object that is beaming cosmic rays our way. Scientists with the Fermi Space Telescope say an unknown pulsar may be close by, sending electrons and positrons towards Earth. Or another more exotic explanation is that the particles could come from the annihilation of dark matter. But whatever it is, the source is relatively close, surely in our galaxy. “If these particles were emitted far away, they’d have lost a lot of their energy by the time they reached us,” said Luca Baldini, a Fermi collaborator.

Comparing data from the Fermi space telescope with results from the PAMELA spacecraft and the High Energy Stereoscopic System (H.E.S.S.) ground-based telescope, the three observatories have found surprisingly more particles with energies greater than 100 billion electron volts (100 GeV) than expected based on previous experiments and traditional models.

Fermi is primarily a gamma ray detector, but its Large Area Telescope (LAT) is also tool for investigating the high-energy electrons in cosmic rays.

Video of the LAT detecting high energy particles.

Cosmic rays are hyperfast electrons, positrons, and atomic nuclei moving at nearly the speed of light. Unlike gamma rays, which travel from their sources in straight lines, cosmic rays wend their way around the galaxy. They can ricochet off of galactic gas atoms or become whipped up and redirected by magnetic fields. These events randomize the particle paths and make it difficult to tell where they originated. But determining cosmic-ray sources is one of Fermi’s key goals.

Using the LAT, which is sensitive to electrons and their antimatter counterparts, positrons, the telescope looked at the energies of 4.5 million cosmic rays that struck the detector between Aug. 4, 2008, and Jan. 31, 2009 and found more of the high-energy variety than expected, those with more than 1 billion electron volts (eV).

A spokesman from the Goddard Space Flight Center said the exact number of how many more is not currently available, due to peculiarities of the data.

But results from Fermi also refute other recent findings from a balloon-borne experiment. The Advanced Thin Ionization Calorimeter (ATIC) captured evidence for a dramatic spike in the number of cosmic rays at energies around 500 GeV from its high atmospheric location over Antarctica. But Fermi did not detect these energies.

“Fermi would have seen this sharp feature if it was really there, but it didn’t.” said Luca Latronico, a team member at the National Institute of Nuclear Physics (INFN) in Pisa, Italy. “With the LAT’s superior resolution and more than 100 times the number of electrons collected by balloon-borne experiments, we are seeing these cosmic rays with unprecedented accuracy.”

“Fermi’s next step is to look for changes in the cosmic-ray electron flux in different parts of the sky,” Latronico said. “If there is a nearby source, that search will help us unravel where to begin looking for it.”

Source: NASA

Nancy Atkinson

Nancy has been with Universe Today since 2004. She is the author of a new book on the Apollo program, "Eight Years to the Moon," which shares the stories of 60 engineers and scientists who worked behind the scenes to make landing on the Moon possible. Her first book, "Incredible Stories from Space: A Behind-the-Scenes Look at the Missions Changing Our View of the Cosmos" tells the stories of those who work on NASA's robotic missions to explore the Solar System and beyond.

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  • Another great story, Nancy, on this recently detected positron-electron flux being detected. A version of this paper can be found here: . While these preliminary observations from Fermi look promising for possible detection of DM annihilation, I agree more data is needed to discriminate this probable positron excess as being from nearby pulsars or from DM annihilation. But I do hope the answer may come in as little as a year, fingers crossed. Detection of DM annihilation, especially its mode of decay (and type of DM !), would be a great achievement.

  • That's really interesting. I'm looking forward to when they figure out the location.

  • I should think that a pulsar source would be both more collomated with time variations, while DM annihilations from the galactic halo or nucleus would be spread out and constant in time.

    The energy reported here is in line with the ~ 1TeV neutralino. This certainly gives the LHC something to look for.

  • The heliosheath is likely a plasma sheath (double layer) that separates the solar plasma environment of the heliosphere from the ISM. Just as the Sun has a surrounding toroidal magnetic field, or ring current, and the Van Allen Radiation belts are similar, the entire solar system should also have a ring current surrounding the heliosphere. If a point source for those cosmic rays is not found, the distribution of cosmic rays might be in the form of a mostly evenly distributed belt due to the particle acceleration in the double layer of the heliosheath, guided by the EM field of the surrounding toroidal magnetic field. The distribution might even have "hot spots" analogous to sunspots and would likely follow the same cycle. ;)

  • They probably originate in an AGN which redshift alone indicates to be at cosmic distances, but is relatively nearby by cosmic standards.

  • Great article, lots of stimulus for "creative speculation"!
    Could the 500GeV rays detected by the balloon experiments (i.e. the ones which Fermi didn't see) be slowed down versions of the >1TeV particles which Fermi saw? Did the balloon experiments see any >1 TeV rays at all?

    Can Fermi detect neutralinos? That's a fascinating notion...

    a funny little typo:
    "Fermi is primarily a gammy ray detector"
    "gammy", at least in English as spoken in Ireland, means "gone off" or "moldy"... giving rise to the notion that Fermi's task is to filter out rays which have gone beyond their "best before" date... ;)

  • Recently research papers exploring the ultra-high energy spectrum of this electron excess by means of observations with imaging atmospheric Cherenkov telescope (IACT) arrays, most recently by H.E.S.S. A recent paper by the H.E.S.S. collaboration on the observed electron spectrum down to 340 GeV can be found here: . More references to follow.

  • An earlier paper by the H.E.S.S. collaboration that explored this electron spectrum from 600 GeV to 5 TeV can be found here: . It seems that ground based IACT arrays like H.E.S.S., MAGIC & VERITAS may provide a crucial window on this ultra-high energy electron spectrum and may provide crucial clues to the source of the electron-positron excess. Hopefully, we will know the answer to this question soon.

  • @solrey: I don't think that idea will fly.

    Have you heard of anomalous cosmic rays (ACRs)? While the acceleration mechanism(s) for them is rather an open question, I somehow doubt your idea could be modelled, quantitatively, to account for the large amount of ACR data; for example, ACRs are isotropic (modulo well-understood local effects), but would not be under your idea.

    Then there's the data on the flow of neutral He from the ISM (see this ESA article, for example:

    But why not develop your idea into a quantitative model, write a paper, and get it published?

    Oh, and didn't Fraser say that promoting your personal theories in UT story comments was to be banned?

  • Nereid,
    I'm not 'promoting' anything, just discussing my ideas about a subject that doesn't have definitive answers.

    Magnetic effects change our view of the heliosheath:

    Not that this paper proves what I'm saying, just that using MHD modeling as opposed to simple mechanical fluid modeling produces a more accurate picture of the heliosphere and the interaction with the ISM. It's just a small step in the direction of my hypothesis.

    I did not interpret the new comment rules as declaring that we have to simply blindly agree with everything in every article, or that we have to produce our own peer-reviewed papers in order to legitimize our thoughts.

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