eso0923 — Science Release

Milky Way's super-efficient particle accelerators caught in the act

25 June 2009

Thanks to a unique "ballistic study" that combines data from ESO's Very Large Telescope and NASA's Chandra X-ray Observatory, astronomers have now solved a long-standing mystery of the Milky WayòÀÙs particle accelerators. They show in a paper published today on Science Express that cosmic rays from our galaxy are very efficiently accelerated in the remnants of exploded stars.

During the Apollo flights astronauts reported seeing odd flashes of light, visible even with their eyes closed. We have since learnt that the cause was cosmic rays òÀÔ extremely energetic particles from outside the Solar System arriving at the Earth, and constantly bombarding its atmosphere. Once they reach Earth, they still have sufficient energy to cause glitches in electronic components.

Galactic cosmic rays come from sources inside our home galaxy, the Milky Way, and consist mostly of protons moving at close to the speed of light, the òÀÜultimate speed limitòÀÝ in the Universe. These protons have been accelerated to energies exceeding by far the energies that even CERNòÀÙs Large Hadron Collider will be able to achieve.

òÀÜIt has long been thought that the super-accelerators that produce these cosmic rays in the Milky Way are the expanding envelopes created by exploded stars, but our observations reveal the smoking gun that proves itòÀÝ, says Eveline Helder from the Astronomical Institute Utrecht of Utrecht University in the Netherlands, the first author of the new study.

òÀÜYou could even say that we have now confirmed the calibre of the gun used to accelerate cosmic rays to their tremendous energiesòÀÝ, adds collaborator Jacco Vink, also from the Astronomical Institute Utrecht.

For the first time Helder, Vink and colleagues have come up with a measurement that solves the long-standing astronomical quandary of whether or not stellar explosions produce enough accelerated particles to explain the number of cosmic rays that hit the EarthòÀÙs atmosphere. The teamòÀÙs study indicates that they indeed do and it directly tells us how much energy is removed from the shocked gas in the stellar explosion and used to accelerate particles.

òÀÜWhen a star explodes in what we call a supernova a large part of the explosion energy is used for accelerating some particles up to extremely high energiesòÀÝ, says Helder. òÀÜThe energy that is used for particle acceleration is at the expense of heating the gas, which is therefore much colder than theory predictsòÀÝ.

The researchers looked at the remnant of a star that exploded in AD 185, as recorded by Chinese astronomers. The remnant, called RCW 86, is located about 8200 light-years away towards the constellation of Circinus (the Drawing Compass). It is probably the oldest record of the explosion of a star.

Using ESOòÀÙs Very Large Telescope, the team measured the temperature of the gas right behind the shock wave created by the stellar explosion. They measured the speed of the shock wave as well, using images taken with NASAòÀÙs X-ray Observatory Chandra three years apart. They found it to be moving at between 10 and 30 million km/h, between 1 and 3 percent the speed of light.

The temperature of the gas turned out to be 30 million degrees Celsius. This is quite hot compared to everyday standards, but much lower than expected, given the measured shock waveòÀÙs velocity. This should have heated the gas up to at least half a billion degrees.

òÀÜThe missing energy is what drives the cosmic raysòÀÝ, concludes Vink.

More information

This research was presented in a paper to appear in Science: Measuring the cosmic ray acceleration efficiency of a supernova remnant, by E. A. Helder et al.

The team is composed of E.A. Helder, J. Vink and F. Verbunt (Astronomical Institute Utrecht, Utrecht University, The Netherlands), C.G. Bassa and J.A.M. Bleeker (SRON, Netherlands Institute for Space Research, The Netherlands), A. Bamba (ISAS/JAXA Department of High Energy Astrophysics, Kanagawa, Japan), S. Funk (Kavli Institute for Particle Astrophysics and Cosmology, Stanford, USA), P. Ghavamian (Space Telescope Science Institute, Baltimore, USA), K. J. van der Heyden (University of Cape Town, South Africa), and R. Yamazaki (Department of Physical Science, Hiroshima University, Japan). C.G. Bassa is also affiliated with the Radboud University Nijmegen, the Netherlands.

ESO, the European Southern Observatory, is the foremost intergovernmental astronomy organisation in Europe and the worldòÀÙs most productive astronomical observatory. It is supported by 14 countries: Austria, Belgium, the Czech Republic, Denmark, France, Finland, Germany, Italy, the Netherlands, Portugal, Spain, Sweden, Switzerland and the United Kingdom. ESO carries out an ambitious programme focused on the design, construction and operation of powerful ground-based observing facilities enabling astronomers to make important scientific discoveries. ESO also plays a leading role in promoting and organising cooperation in astronomical research. ESO operates three unique world-class observing sites in Chile: La Silla, Paranal and Chajnantor. At Paranal, ESO operates the Very Large Telescope, the worldòÀÙs most advanced visible-light astronomical observatory. ESO is the European partner of a revolutionary astronomical telescope ALMA, the largest astronomical project in existence. ESO is currently planning a 42-metre European Extremely Large optical/near-infrared Telescope, the E-ELT, which will become òÀÜthe worldòÀÙs biggest eye on the skyòÀÝ.

Links

• To request the Science paper: http://www.sciencemag.org/help/about/contact_info.dtl

Contacts

Eveline Helder
Astronomical Institute Utrecht
Utrecht, Netherlands
Tel: +31 30 253 5221
Email: E.A.Helder@uu.nl

Jacco Vink
Astronomical Institute Utrecht
Utrecht, Netherlands
Tel: +31 30 253 2513
Email: j.vink@uu.nl

Stefan Funk
Kavli Institute for Particle Astrophysics and Cosmology
Stanford, USA
Tel: +1 650 926 8979
Email: funk@slac.stanford.edu

Ryo Yamazaki
Hiroshima University
Hiroshima, Japan
Tel: +81-82-424-7362
Email: ryo@theo.phys.sci.hiroshima-u.ac.jp

Aya Bamba
Hiroshima University
Kanagawa, Japan
Tel: +81-42-759-8138
Email: bamba@astro.isas.jaxa.jp

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