Astronomers have found compelling evidence that a supernovashock wave has produced a large amount of cosmic rays, particles ofmysterious origin that constantly bombard the Earth. This discovery,made with NASA's Chandra X-ray Observatory, supports theoreticalarguments that shock waves from stellar explosions may be a primarysource of cosmic rays.
This finding is important forunderstanding the origin of cosmic rays, which are atomic nuclei thatstrike the Earth's atmosphere with very high energies. Scientistsbelieve that some are produced by flares on the Sun, and others bysimilar events on other stars, or pulsars or black hole accretiondisks. But, one of the prime suspects has been supernova shock waves.Now, a team of astronomers has used Chandra observations of Tycho'ssupernova remnant to strengthen the case for this explanation.
"Withonly a single object involved we can't state with confidence thatsupernova shock waves are the primary source of cosmic rays," said JohnP. Hughes of Rutgers University in Piscataway, New Jersey, and coauthorof a report to be published in an upcoming issue of The AstrophysicalJournal. "What we have done is present solid evidence that the shockwave in at least one supernova remnant has accelerated nuclei to cosmicray energies."
In the year 1572, the Danish astronomer TychoBrahe observed and studied the sudden appearance of a bright "new star"in the constellation Cassiopeia. Now known as Tycho's supernovaremnant, the event created a sensation in Tycho's time because itexploded the myth that stars never change.
Four centuries later,the Chandra results on Tycho's remnant show that some modern ideas ofthe aftermath of supernova explosions may have to be revised. Thereport by Hughes and colleagues demonstrates that the shock waveproduced by the explosive disruption of the star behaves in a way thatcannot be explained by the standard theory.
The supernova debrisis observed to expand at a speed of about six million miles per hour.This rapid expansion has created two X-ray emitting shock waves - onemoving outward into the interstellar gas, and another moving inwardinto the stellar debris. These shock waves, analogous to the sonic boomproduced by supersonic motion of an airplanes, produce sudden, largechanges in pressure, and temperature behind the wave.
Accordingto the standard theory, the outward-moving shock should be about twolight-years ahead of the stellar debris (that's half the distance fromour sun to the nearest star). What Chandra found instead is that thestellar debris has kept pace with the outer shock and is only abouthalf a light-year behind.
"The most likely explanation for thisbehavior is that a large fraction of the energy of the outward-movingshock wave is going into the acceleration of atomic nuclei to speedsapproaching the speed of light," said Jessica Warren, also of RutgersUniversity, and the lead author of the report in the AstrophysicalJournal.
Previous observations with radio and X-ray telescopeshad established that the shock wave in Tycho's remnant was acceleratingelectrons to high energies. However, since high-speed atomic nucleiproduce very weak radio and X-ray emission also, it was not knownwhether the shock wave was accelerating nuclei as well. The Chandraobservations provide the strongest evidence yet that nuclei are indeedaccelerated, and that the energy contained in high-speed nuclei isabout 100 times that in the electrons.
Hughes also pointed outthat the Chandra result for Tycho's remnant significantly changesastronomers' view of the evolution of supernova remnants. A largecomponent of cosmic ray nuclei alters the dynamics of the shock wave,and may require changing the way that astronomers estimate theexplosive energy of a supernova from the properties of its remnant.
NASA'sMarshall Space Flight Center, Huntsville, Ala., manages the Chandraprogram for the agency's Science Mission Directorate. The SmithsonianAstrophysical Observatory controls science and flight operations fromthe Chandra X-ray Center in Cambridge, Mass.
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