SNR and Cosmic Ray Acceleration
Cosmic Rays: What Are They?
Cosmic rays are the atomic nuclei (mostly protons) and electrons that are
observed to strike the Earth's atmosphere with exceedingly high energies.
Cosmic rays can be over 1021eV, which is a
billion times more energetic than high energy particles created on earth in the most
powerful particle accelerators. They are moving at nearly the speed of
They were first discovered by Victor Hess, during a balloon
flight. Although Hess did not know what the particles were or where they
came from, he observed a source of radiation (which he thought, at the
time, were gamma-rays). He noticed that there was more radiation
the higher up he rose and concluded that the Earth could not therefore be the
source of the emission. This was the first time that an external source of energetic
particles had been discovered. Cosmic Rays have been widely observed
since then and are found just about everywhere in our Galaxy.
The big question is: how and where are normal protons and electrons accelerated to these
Where are Cosmic Rays Accelerated? Current Theory...
The standard belief is that some cosmic rays are accelerated in the Galaxy and some are
accelerated outside the Galaxy.
One characteristic which provides a clue as to how cosmic rays are
accelerated is the spectrum of particles we catch here on Earth. The cosmic ray spectrum is
fairly well described by a broken power law, that is a power law with two
"kinks". A "power law" has the form F ~ Ep. The
"spectral index", p, is the slope of the graph of the data (in
logarithmic units). The slope is flatter at high energies than the
thermal spectrum, for example, so that there are more cosmic rays at
high energies. Such a spectrum is said to be "hard" (as opposed to
"soft", which means the numbers fall off rapidly at high energies). The cosmic
ray spectrum steepens around 3 X 1015 eV (the "knee") and flattens around
3 X 1018 eV (the "ankle"). Any theory of cosmic ray origins must account for
this shape. A successful model must produce the right numbers of
particles as a function of energy; in other words, the spectrum constrains models of
cosmic ray production. |
The "knee" of the cosmic ray spectrum
The origin of the really high energy cosmic rays (above
the knee) is a mystery. Nobody knows exactly where they are produced and a
source has yet to be precisely detected. They are hard to pin down, since at such high energies there are so few of them
that it is hard to tell exactly where they come from. For example,
cosmic ray particles with energies greater than 1019eV hit
the Earth at a rate of one per square kilometer per century (see the
page by the HiRes project to study high energy cosmic rays (http://www.cosmic-ray.org)). It is difficult
enough to observe these particles, never mind determine directly where their
origin is. For a number of reasons, it is suspected that these cosmic rays
above the ankle are extragalactic in origin, perhaps generated in the cores of
Active Galactic Nuclei, in powerful radio galaxies, or by cosmic
strings. These sources are known to have the tremendous amounts of energy needed
particles to such high energies, though a direct correlation has yet to be
This is an area of active research, and as more and more
sensitive detectors come on line and more evidence is gathered, scientists
will have a better
picture of where these extraordinarily high energy particles are generated.
The particles below the ankle have lower energies and are thought to be produced in the Galaxy. Furthermore,
there is reason to believe that at least up to about
1014eV, if not all the way to the knee or to the ankle, most of the
particles are accelerated in the shocks of
supernova remnants (http://heasarc.gsfc.nasa.gov/docs/objects/snrs/snrs.html). In this
model, particles are scattered across the shock fronts of a SNR, gaining
energy at each crossing. Until the last two years, evidence
to support this claim was circumstantial, based on theory and logic
rather than on observations. For example, it seemed reasonable
that SNR shocks could accelerate particles to the desired energies.
The kinetic energy released by supernova explosions is more than enough
to account for the Galactic cosmic rays up to 1015 eV. And
supernovae are fairly
common and occur throughout the Galaxy, so it is reasonable that they
could be responsible for cosmic rays, which are also plentiful and found
throughout the Galaxy. However, astronomers strove to find more
direct evidence for shock acceleration of particles in SNR.
Puzzle Pieces Fall Into Place...
What would be the best direct evidence to support this model? Is there a
way to "see" high energy, accelerated particles and associate them with SNRs? In fact,
in a sense, there is. The key is synchrotron radiation. Synchrotron
radiation is emitted when fast moving ions are
accelerated in magnetic
fields. A magnetic field will force an energetic particle to
travel in a helical path and emit radiation. We know that magnetic fields exist near and
around SNR, for example in the ambient ISM or
around the neutron star that can remain after a SNe. So if there are fast
moving charged particles, they should produce synchrotron emission which
we should be able to observe. Where in the electromagnetic spectrum do
we expect this synchrotron emission from Galactic cosmic rays to occur?
The answer depends on a couple of factors. The energy of synchrotron emission
on the energies of the fast charged particles doing the emitting and the
strength of the magnetic field doing the bending. For the energies of the cosmic rays (as
expected based on cosmic rays observed on Earth) and the strength of the
magnetic fields (deduced from radio measurements), cosmic rays' synchrotron
emission should fall into the X-ray range. In fact, it appears that the best
way at present to look for direct evidence for particle acceleration
in SNRs (to at least ~ 1014 eV) is to look in the vicinity of
SNR shocks for X-ray
synchrotron radiation from electrons that have ~ 1014 eV of energy.
Radio synchrotron radiation in SNR had long been observed and optical
synchrotron radiation had been observed in the 1950s, but these
observations could not account for the range of energetic cosmic rays seen
with energies up to 1015eV.
The characteristic spectrum of synchrotron radiation is featureless, following
a more or less straight line. This is in contrast to a spectrum from a
hot radiating gas, which has many bumps and peaks corresponding to
emission from particular atoms at particular energies.
Despite this characteristic shape, synchrotron radiation in the
X-ray region of the spectrum is not easy to identify.
There are many reasons for this: until recently, other theories
have successfully accounted for the observed shape of the SNR spectrum
at X-ray energies (such as the Non Equilibrium Ionization, or NEI,
models of Hughes and Helfand (http://adsabs.harvard.edu/cgi-bin/nph-bib_query?bibcode=1985ApJ...291..544H&db_key=AST&high=3ff0e4969c26605) or of Hamiltonet al. (http://adsabs.harvard.edu/cgi-bin/nph-bib_query?bibcode=1983ApJS...51..115H&db_key=AST&high=3fdff706f826238)); unlike in the radio, where polarization information
is available, there is no independent way of confirming that the
radiation is definitely synchrotron; the X-ray synchrotron spectrum was
not just an extension of the radio synchrotron spectrum.
The technique of looking for X-ray synchrotron radiation is actually somewhat
amusing since people have bashed away,
unsuccessfully, at the problem for years by looking for gamma-ray emission
Synchrotron radiation in SNRs was thought by many to be important only in the radio
part of the electromagnetic spectrum. Other processes such as thermal
emission from hot gas, nonthermal bremsstrahlung, or inverse Compton
scattering were what astronomers usually thought of when they thought of what
processes create high energy X-ray photons. The problem was, the spectra for
the X-rays produced
by these processes looked nothing like the observed emission from SNR
unless some very ad hoc assumptions were made about the ionization and
abundances of the SNR gas!
The shape of the distribution was wrong and the energies were way off.
Obviously these high energy photons were coming from some other
process occurring in the remnant. That process is synchrotron emission. That X-rays could be produced
by synchrotron emission by cosmic rays came as a surprise to
many people although Steve Reynolds and Roger Chevalier (http://adsabs.harvard.edu/cgi-bin/nph-bib_query?bibcode=1981ApJ...245..912R&db_key=AST&high=3fdff706f826105) suggested the idea in 1981 to explain
the spectrum of SN 1006.
When ASCA observed SN1006 in 1995, something unexpected was found. Key
were ASCA's observations of the center of the remnant, separate from
the X-ray bright rims. When the
emission from the lobes was omitted, what remained was a thermal
spectrum, dominated by X-ray lines. This result conflicted with the
NEI model of Hughes and Helfand, but was in agreement with Reynold and
Chevalier's synchrotron model. In previous observations, the flat synchrotron
emission in the rims overwhelmed the thermal emission in the center, and
so the different models seemed to be equally valid.
As a result of these observations, Koyama et al. published a paper about SN 1006 in which they showed very
strong evidence of synchrotron emission
because they could produce separate spectra for the two lobes of the shell
and for the interior of the remnant.
If the strength of the magnetic field is known, the energy of the ions
that are responsible for the synchrotron emission can be calculated from
the X-ray synchrotron energies.
Given some uncertainty about the strength of the magnetic field
in SN 1006, it appears to accelerate electrons (and presumably protons) to
energies ~ 1014 eV. This is the right amount of energy to explain the
cosmic-ray power spectrum up to the knee. Lower mass particles such as
have up to 1014eV of energy while heavier particles such as
iron, at the same velocities, would be up
to 3 X 1015 eV (the knee). The shape of a synchrotron
spectrum produced by a population of Fermi accelerated particles matches
observations, and furthermore, an estimate of the total amount
of energy in the accelerated particles (given some more uncertainty in some
of the assumptions) is close to what you expect if all supernova remnants
do the same thing and if they produce all of the Galactic cosmic rays.
Cosmic Rays and SNR - Making the Connection
- The X-ray spectrum of SNR is not fit by emission from processes
such as thermal emission, nonthermal Bremsstrahlung or inverse Compton
- Thermal emission in the center of the remnant (discovered by ASCA)
rules out NEI emission by hot gas as a possible source
- With other theories ruled out, the shape of the spectrum tells us that
the strong X-ray emission in the
rims must be from synchrotron emission by accelerated, charged particles
- From the energy of the X-rays and an estimate of the strength of the
magnetic field we can calculate the energy of the accelerated particles
- Energies correspond to observed cosmic ray energies (at
Earth) up to the "knee"
Conclusion Cosmic rays up to 1015 are accelerated in the
shocks of SNR and become visible in the X-rays when they emit
Why Do We Want to Know?
In general anything which is both extreme and plentiful is fascinating.
Cosmic rays are found in such large numbers everywhere in our galaxy and
they can be incredibly energetic. We are not able to generate anything
anywhere nearly as energetic, so it becomes an interesting question to
explore what physical processes CAN generate them.
Do cosmic rays affect our everyday lives, or are they too remote to
worry about? Although you may never yourself be bombarded by a primary cosmic
ray (we are shielded from them by the Earth's magnetosphere), we are bombarded all the time by the
secondary cascades of cosmic rays that are created when cosmic rays
interact with Earth's atmosphere. These secondary cosmic rays are not as
energetic as the primary CRs which exist in space, but are responsible
nonetheless for a constant background radiation to which we are all
constantly exposed. Cosmic rays produce Carbon 14, a small source of
radiation but one which is critical for dating (for example establishing
the age of fossils).
Spacecraft and high
altitude planes certainly feel their effects. With their high energy
concentrated in such a small bundle, cosmic rays can disrupt computer
hardware or sensitive electronics and these instruments have to be
shielded in vehicles traveling above the atmosphere.
If a cosmic ray passes though a sensitive part of a semiconductor chip,
for example, the logical state of the bit ("on" or "off") can be
flipped. This is called a single-event upset (SEU).
A single-event upset can also result from a cosmic ray hitting
the nucleus of an atom in a
sensitive component location. The nuclear interaction can cause the
nucleus to split, or spallate. The broken pieces of the nucleus then
carry away most of the cosmic ray's energy. These bits of debris can
then flip the bit state.
This problem is most commonly seen in the South Atlantic Anomaly (http://heasarc.gsfc.nasa.gov/docs/rosat/gallery/display/saa.html). The distribution of errors (http://www.estec.esa.nl/wmwww/WMA/) from the UoSAT-3
spacecraft in a polar orbit can be seen at the European Space Agency's
web page. The errors at high latitudes are primarily caused by cosmic
rays striking the spacecraft.
Similarly, cosmic rays also corrupt observations of space made with CCDs (Charged Coupled
Devices, a kind of digital telescope). The cosmic rays have to be subtracted from the data.
To read more about Galactic cosmic rays and cosmic rays that are
generated from gamma-rays in the Earth's atmosphere, check out "Imagine the Universe!".
Observational Clues: Current and Future Work
One technique to to search for X-ray synchrotron radiation is
"spatially-resolved spectroscopy" with ASCA (http://heasarc.gsfc.nasa.gov/docs/asca/ascagof.html) and with
the Chandra X-ray Observatory (CXO) (http://chandra.harvard.edu/about/axaf_mission.html). CXO was launched in July 1999 and will provide even better resolution and more coverage. Right now it is not clear how many remnants this will work for. One other remnant, RX
JJ1713.7-3946, appears to be similar to SN 1006, but most other
remnants appear to be dominated by thermal emission from their shells. In
order to pull out the synchrotron emission, it is necessary to have
a broad-band X-ray spectrum that extends to
many tens of keV because the synchrotron radiation is expected to dominate
above about 10 keV. This latter technique has been used to show the
presence of synchrotron emission for Cas A, using
RXTE (and SAX).
IC 443, the only other remnant with any evidence of synchrotron radiation
that is known now, is sort of a mixture of the two techniques in the sense
that a small part of the remnant appears to emit synchrotron radiation, but
the evidence lies in the broad-band spectrum. That is, the synchrotron
emission does not dominate the spectrum.
We've only just begun looking for X-ray synchrotron radiation. It will
be interesting to discover if all SNRs produce X-ray synchrotron
radiation and what fraction of Galactic cosmic rays are produced in
Thank you to Glenn Allen for contributing to this article.