Is a Nearby Object in Space Beaming Cosmic Rays at Earth?
Written by Nancy Atkinson
Fermi Telescope. Credit: NASA
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
Filed under: Missions, Physics
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May 4th, 2009 at 11:36 am
Another great story, Nancy, on this recently detected positron-electron flux being detected. A version of this paper can be found here: http://arxiv.org/PS_cache/arxiv/pdf/0905/0905.0025v1.pdf . 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.
May 4th, 2009 at 5:44 pm
That's really interesting. I'm looking forward to when they figure out the location.
May 4th, 2009 at 6:55 pm
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.
May 4th, 2009 at 9:28 pm
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.
May 4th, 2009 at 9:30 pm
They probably originate in an AGN which redshift alone indicates to be at cosmic distances, but is relatively nearby by cosmic standards.
May 5th, 2009 at 1:56 am
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…
May 5th, 2009 at 7:04 am
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: http://arxiv.org/PS_cache/arxiv/pdf/0905/0905.0105v1.pdf . More references to follow.
May 5th, 2009 at 7:14 am
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: http://arxiv.org/PS_cache/arxiv/pdf/0811/0811.3894v2.pdf . 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.
May 5th, 2009 at 10:38 am
@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: http://sci.esa.int/science-e/www/object/index.cfm?fobjectid=35853).
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?
May 5th, 2009 at 11:48 am
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:
http://www.google.com/url?sa=t&source=web&ct=res&cd=1&url=http%3A%2F%2Ftrs-new.jpl.nasa.gov%2Fdspace%2Fbitstream%2F2014%2F39209%2F1%2F04-0335.pdf&ei=mIQASrreCaP4tAOdu534BQ&usg=AFQjCNFm398QtPtT4W0wV6soTCJgAMnR1A
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.
May 5th, 2009 at 12:11 pm
@solrey: what is the toroidal magnetic field, or (and?) ring current, that surrounds the Sun?
And have you heard of anomalous cosmic rays?
May 5th, 2009 at 4:53 pm
Feenix: No the detector does not detect neutralinos. In fact the Tevatron CDF and D0 detectors don't detect top quark hadrons directly either. These detect the decay "secondaries," which are low mass muons, pions etc. PAMELA detects gamma rays, which are final decay products predicted for neutralinos. All that can be done is to benchmark the measured flux and energy spectra of gamma rays and compare that to phenomenological predictions of SUSY neutralino decays. The results are suggestive, but in no way final.
Heliosheaths don't accelerate particles that appreciably. Seriously, cosmic rays are in the GeV (billion electron volt) to 10^9 GeV energy range. The solar produced charged particles are in the KeV (thousand eV) range of energy, and most of this energy is produced by ECM at the sun. For the solar magnetic field to accelerate charged particles it must vary. Please everyone, review Maxwell's equations! So if the solar magnetic field were oscillating like mad with high frequencies it might shove KeV charged particles to the MeV range and so forth. This is not happening. The solar magetodyndrodynamic system is not capable of producing cosmic rays with energies orders of magnitude larger than the LHC energy.
Lawrence B. Crowell
May 5th, 2009 at 7:35 pm
@lbc
That would be magnetohydrodynamics. It's not an oscillating solar magnetic field that boosts the particles to such high energies. The electrons are likely guided by the toroidal magnetic field surrounding the solar system and accelerated by the double layer of the heliosheath, much like ions are accelerated by the double layer surrounding the sun, (which continue to accelerate beyond the orbits of the inner planets, btw)
Regardless of the voracity of my hypothesis, the heliosheath is not well understood, as indicated in the paper I linked in my previous post. Considering that pretty much all of the data collected lately is of an electro-magnetic origin, extrapolating known plasma phenomena into a system of larger scale, such as the heliopause, is just as valid as any hypothesis involving mysterious 'dark matter'.
If you read that linked paper at all, mainstream btw, you'd see what they describe as an equatorial 'jet sheet' ACCELERATING from the heliosheath, outward into the ISM. It's logical to conlcude that a corresponding and equal energetic acceleration of much lighter particles, electrons, would occur inward from the heliosheath.
@nereid
NASA actually identifies the toroidal magnetic field around the sun as an 'equatorial belt'. Around Earth, we have the 'Van-Allen' belts.
May 5th, 2009 at 8:09 pm
Changing magnetic fields are what change the energy of a particle. A constant magnetic field will not change the energy of a particle. It is simple to see. A constant field will have
F = qvxB = charge*velcoity x mag-field
where x is cross product. The energy this force results in is the work
E = int F*dr
where * = dot product and dr a displacement. Calculus tells us
dr = vdt and so the energy the magnetic field imparts is
E = int qvxB*vdt
but the dot product times a cross product with the same vector is zero. No work done! This is a math theorem vxB*v = 0.
Please, I wish these plasam physics people would stop displaying their confusion and ignorance here.
Lawrence B. Crowell
May 5th, 2009 at 8:27 pm
Reference please solrey (thanks for taking the time to reply).
One more:
What 'double layer' would that be solrey? Where is it to be found? What are the parameters to describe it (e.g. thickness)?
Again, references please.
May 5th, 2009 at 8:33 pm
@ Lawrence B. Crowell:
I don't think it's accurate to use the word 'physics' to apply to them; it's obvious to me that Anaconda and solrey have an extremely poor grasp of the subject (and Anaconda's familiarity with the relevant history seems just as bad), and I've only been here a few days!
Here's a question though: where does their evident zeal come from? What makes them so enthusiastic about writing comments to UT stories, based on what they must surely realise is not even a first year university science student's grasp of physics (much less astronomy)?
May 5th, 2009 at 9:21 pm
@lbc
"Changing magnetic fields are what change the energy of a particle. A constant magnetic field will not change the energy of a particle."
"Please, I wish these plasam physics people would stop displaying their confusion and ignorance here."
If you're gonna go there, then I'll say that it is you who is confused and mis-informed. Magnetic fields are generated by electric current. It's voltage potential that drives/accelerates charged particles (current) and their paths are guided by magnetic attraction/repulsion. Particles are accelerated by the voltage potential between the two, opposing polarity, charge sheaths that constitute a double layer.
Also, a conductor moving through a magnetic field will induce particle flow in the conductive medium. The heliosphere is a conductive medium (plasma), with a voltage potential in relation to the surrounding ISM, that is moving through the galactic magnetic field, which provides an inductive force, in addition to the voltage potential, to charged particle velocity. Certain high energy particles will get an additional, strong boost via acceleration through the heliosheath. Within a diffuse plasma in near 0 pressure, which limits collission probability, further reduced by orderly particle flow, resulting in minimal resistance to particle velocity/eV retention, the voltage pressures required are significantly lower than your attempted calculations.
May 6th, 2009 at 4:48 pm
Solrey, if you seriously think that a static magnetic field can increase the energy of a particle then you are seriously confused over sophmore level physics. A static magnetic field accelerates particles in a direction perpendicular to both the velocity of the particle and the field itself. This is then a centripetal acceleration which does not involve a change in the kinetic energy.
If you are not straight on this it is then apparent you are hopelessly confused.
Lawrence B. Crowell
May 6th, 2009 at 7:19 pm
@lbc
I said nothing of static magnetic fields. Everything I talked about involved voltage potentials and electromagnetic fields, which are dynamic, not static. The double layer of the heliosheath accelerates particles because of the voltage potential between the two sheaths that constitute the DL, and the charge density of the particles, the ISM and the sun.. Depending on velocity, charge density and angle of incidence, both positive and negative charged particles will either be repelled back into the ISM, or accelerated into the heliosphere, due to the opposing polarities of the sheaths that constitute a double layer. Hans Alfven thought that DL's should be classified as a specific cosmological phenomenon.
May 7th, 2009 at 6:19 am
@ solrey: More information on this 'toroidal magnetic field' that supposedly encircles the solar system would be appreciated. Wouldn't this field polarize light passing through it on its way to Earth? This would seem to be easy to detect, yet I've not seen any published work mentioning a band of aligned polarized light seen across the sky (presumably along the ecliptic) as seen from Earth. Any references or citations for that claim of a 'toroidal magnetic field' surrounding the heliosheath ?
May 7th, 2009 at 6:44 am
@Jon Hanford (actually solrey): a couple of days back I politely asked solrey for some (NASA) references on the Sun having a toroidal magnetic field around it, which he says NASA calls an ‘equatorial belt’.
I haven't got an answer yet (but I'm patient).
I also asked him about "the double layer surrounding the sun", which is what he says in an earlier comment; I asked where it is to be found, what the parameters to describe it (e.g. thickness) are, and for some references so that I (and any other reader interested) can learn more.
I haven't got an answer yet (but I'm patient).
I do hope he (she?) is not like Total Science, who, when asked to provide references for what he claimed were quotes sometimes did not do so (and, for the record, when he (she?) did we could all see that he'd mis-quoted, or worse).
May 7th, 2009 at 4:14 pm
Solrey, Ok so even if you admit that static magnetic fields don't do work on a charge, yoiu still have a huge problem. The electric potentials you refer to must come from somewhere. An oscillating magnetic field would induce an oscillating electric field that would do work on a charge. Yet the solar magnetic field far removed from the sun is very small, < 1 Gauss, though it extends far in space. The EM field or magnetoydrodynamic system would have to contain enough energy to accelerate particles up to the 10^{9} GeV range in energy. As a plasma this would mean the constituents would have energies far larger than what is obtained in Tokamaks, though more diffuse obviously.
Nobody who knows what is going on is entertaining this sort of idea. You are beating a dead horse.
Lawrence B. Crowell