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SN 1006, a source of cosmic rays
How do supernova remnants produce X-rays?
The combination of imaging and spectroscopic capabilities makes ASCA
particularly suitable for the study of
supernova
remnants. In general, most X-rays that SNRs give off come from hot gas.
As the SNR shock wave goes through interstellar space, it heats up the gas it
plows into. This hot gas gives off X-rays via processes collectively known as
thermal emission.
In addition, a small amount of interstellar gas becomes trapped in the
SNR shock and bounces back and forth across the shock, a process known as
Fermi
acceleration. The more times the electrons, protons, and ions from the gas
bounce back and forth, the more energy they gain. Eventually, they become
cosmic rays,
which are subatomic particles (primarily electrons and protons) that travel in
space near the speed of light. The emission given off when the cosmic ray
electrons interact with the
magnetic
field is called synchrotron radiation.
Synchrotron radiation, from cosmic rays each with an energy of about a
billion
electron
volts (1 GeV), is widely seen from the shells of SNRs by
radio astronomers,
but it has not been observed elsewhere in the
electromagnetic
spectrum -- until recently with ASCA.
ASCA observations of one particular SNR, the supernova of 1006 A.D.
(SN 1006), may now be the first example of synchrotron radiation from cosmic
rays with energies about 100 trillion electron volts (100 TeV) within a SNR's
shell.
How do we know it is synchrotron radiation?
The spectrum from thermal X-rays generally shows a characteristic set of
lines, while synchrotron radiation forms a hard continuum which has no emission
lines. In addition, synchrotron X-rays from SNRs have always appeared in the
center of SNRs, and are believed to be powered by the
pulsar that was
created in the supernova explosion. However, SN 1006 does not fit neatly into
this picture, apparently showing extended emission of continuum X-rays. There
was much debate on whether this was synchrotron radiation, a peculiar kind
of thermal radiation, or something else.
Show me a viewgraph of Cosmic Ray Production in Supernova Remnants
ASCA observations (above) have solved this long-standing mystery through
the discovery of the expected thermal emission lines from hot gas and the
localization of the previously known featureless X-ray emission to the bright
rims of the remnant. This was inconsistent with the competing scenarios.
Therefore, electrons must have been accelerated to very high energies
in the SNR shock wave. This is the first observational link between particle
acceleration at a SN shock front and high-energy cosmic rays within our
Galaxy.
What are the broader implications?
In general, the rate at which cosmic rays are caught here at the Earth
tells us what rate they are made within the Milky Way. The only thing that is
both common and sufficiently energetic to produce the majority of cosmic rays
with energies up to 100 TeV are supernova explosions, but until now they have
only been associated with synchrotron radiation from electrons with GeV
energies. On the other hand, there are other objects that are less common or
have less energy than SNRs, but do clearly show X-ray synchrotron radiation
(e.g. pulsars &
active galactic
nuclei). Therefore astronomers were in a quandary, "Do SNRs
accelerate particles to higher energies, but we don't see it, or are the other
alternatives more energetic or more plentiful than we had thought?"
This result solves this quandary, because ASCA has clearly shown that SNR's
shock waves can accelerate particles all the way up to energies of at least
100 TeV.
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