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Prime candidate to explain cosmic ray
sea runs short of energy
Scientists have been advocating for decades that cosmic rays are accelerated in the remnants of supernova explosions.
However the MAGIC telescopes have now observed that one of the best candidates, the supernova remnant Cassiopeia A, falls very short of the required energy.
MAGIC telescopesand a multiband image of the supernova remnant Cassiopeia A (Cas A)obtained with the three big NASA observatories[(Spitzer (Infrared), HST (optical) and Chandra (X-ray)]. (M.Lopez / IAC / NASA)
Cassiopeia A is a famous supernova remnant, the product of a gigantic explosion of a massive star about 350 years ago. Although discovered in radio observations 50 years ago, now we know that its emitted radiation spans from radio through high-energy gamma rays. It is also one of the few remnants for which the birth date and the type of supernova are known. It was a type IIb, the result of a core collapse supernova explosion -. The precise knowledge of its nature makes Cassiopeia A one of the most interesting and investigated objects in the sky, and in particular the study of its connection with the cosmic rays, sub-atomics particles that fill our Galaxy with energies higher than anything achievable in laboratories on Earth.
The very
high-energy part of the spectrum of Cassiopeia A results from the cosmic rays
(either electrons or protons) within the remnant. Until now, this range of
energy could not be measured with sufficient precision to pinpoint its origin.
Sensitive observations above 1 Tera-electronvolts (TeV) were required but achieving them was daunting. An
international team led by scientists from the Institute for Space Sciences (ICE -
IEEC-CSIC, Spanish National Research Council-CSIC), the Institut
de Fisica dÕAltes Energies (IFAE) and the Institute of Cosmos
Sciences of the
University of Barcelona (ICCUB), in
Spain, has finally succeeded in doing those observations with the MAGIC telescopes (short for Major Atmospheric
Gamma-ray Imaging Cherenkov Telescope). More
than 160 hours of data were recorded between December 2014 and October 2016,
revealing that Cassiopeia A is an accelerator of massive particles, mostly
hydrogen nuclei (protons). However,
even when those particles are 100 times more energetics than the ones we can reach
in artificial accelerators such the one in CERN, their energy is not hugh
enough to explain the cosmic ray sea that fills our Galaxy.
ÒCassiopeia A is the
perfect object to be a PeVatron, that is, an
accelerator of particles up to PeV energies (1 PeV = 1.000 TeV): it is young,
bright, with a shock expanding a great velocity and with very large magnetic
fields that can accelerate cosmic rays up to at least,
conservatively, 100 or 200 teraelectronvoltsÓ
explains Emma de O–a Wilhelmi,
scientist of CSIC in the Institute for Space Sciences, ÒBut contrary to what we
expected, in Cassiopeia A the particle energies do not reach more than a few
tens of tera-electronvolts.
At these energies, the radiation suddenly drops and the emission stops
abruptly: Either the remnant cannot accelerate the particles to higher
energies, which challenge our knowledge of shocks acceleration, or maybe, the
fastest ones escaped quickly the shock, leaving only the slowest ones for us to
observeÓ, adds Daniel Guberman, at the Institut de Fisica dÕAltes Energies.
ÒThose supernovae are natural accelerators of particles, therefore the perfect laboratory to study charge particles and plasma in conditions that are not possible in our labs in Earth", remarks Daniel Galindo, working at Institute of Cosmos Sciences of the University of Barcelona (ICCUB). ÒTo understand the origin of the cosmic rays implies to unveil the origin of our own GalaxyÓ, concludes Razmik Mirzoyan, MAGIC Spokeperson from the Max Planck Institute for Physics (MPP) in Munich (Germany).
MAGIC
telescopes
MAGIC telescopes are located at the Roque de los Muchachos Observatory, in La Palma (Canary Islands).
MAGIC, a system of two
17m diameter Cherenkov telescopes, is currently one of the three major imaging
atmospheric Cherenkov instruments in the world. It is designed to detect
photons tens of billions to tens of trillions times more energetic than visible
light. MAGIC also uses a novel technique to reduce the effect of the Moonlight
in the camera, allowing for observations during moderated Moonlight nights.
MAGIC has been built
with the joint efforts of an international collaboration that includes about
160 researchers from Germany, Spain, Italy, Switzerland, Poland, Finland,
Bulgaria, Croatia, India, Japan, Armenia and Brazil.
For more information on MAGIC, visit: https://wwwmagic.mpp.mpg.de/
Published in the Monthly Notices
of the Royal Astronomical Society (MNRAS, 2017): MAGIC Collaboration (M.
L. Ahnen et al.) "A cut-off
in the TeV gamma-ray spectrum of the SNR Cassiopeia A". DOI:
10.1093/mnras/stx2079
CONTACTS:
Dr. Razmik Mirzoyan
Max-Planck-Institute for Physics, Foehringer Ring 6
80805 Munich, Germany
Tel.: (+49)(89)-323-54-328
e-mail: Razmik.Mirzoyan@mpp.mpg.de
e-mail: Mirzoyan.Razmik@gmail.com