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Cosmic Ray Interactions: Bridging High and Low Energy Astrophysics
Cosmic rays are an energetically important ingredient of the interstellar medium (ISM), but until recently it was difficult to map the cosmic ray content of the Galaxy. With the coming of age of GeV/TeV gamma-ray astronomy this situation has dramatically changed, because the radiative effects of cosmic rays can be imaged directly with arcminute resolution. At the same time there is a wealth of information from radio, submm, and infrared wavebands on the magnetic field configuration, and the distribution and abundance of dense matter in the Galaxy with which cosmic rays interact. This provides vital information for the researchers working on GeV/TeV observations, in understanding the targets for accelerated particles. In parallel, researchers working in low energy astrophysics need information about the low-energy end of the cosmic ray distribution because it provides an important, perhaps sometimes even the only, source of ionization in dense, cold, molecular clouds. Thus even though the low and high energy cosmic rays are separated by decades in energy, by using both gamma-ray and molecular cloud chemistry as tracers, a new lever arm for understanding source physics and cosmic ray acceleration can be exploited. At the same time, understanding the complications of UV vs. X-ray vs cosmic ray ionization is a burgeoning new field.
During this workshop all the above mentioned aspects were discussed. Both the organizers and the participants were very satisfied with the workshop, as there are otherwise few occasions where such different scientific communities interact.
The format of the workshop consisted of many long talks intended to bring both communities up to date, and some contributed talks on recent, interesting results. At the last day four groups, two from each community, presented their ideas of what they think researchers from the other community could investigate. During the week time was reserved for the groups to work on these final talks. There was some confusion concerning this idea, as participants were not quite sure how detailed their proposals for research should be, but in the end participants were pleased by what this format had accomplished, namely lively discussions during the group sessions, on what they had just learned from fields that were new to them.
During the workshop the high energy astrophysicists became much more aware of the molecular tracers of cosmic rays interactions, with H3+ being one of the most important molecules. They also became aware of the importance of low energy cosmic rays (up to 10 MeV). These cosmic rays are usually ignored by cosmic ray scientists because they are difficult to detect, due to interference caused, e.g. by solar modulations, although future MeV gamma-ray satellite missions could fill that gap.
The low energy astrophysicists were brought up to date on the recent developments in GeV and TeV astronomy, and learned how these fields have revealed the presence of cool regions in the ISM that escaped the attention of radio, submm and infrared astronomy. Participants also became more aware of the potential diagnostic power of Fe-K fluorescent lines around 6.4 keV, which could be induced by interactions between cold gas and MeV protons. These lines were also discussed during the meeting in the context of cosmic ray interactions in the Galactic Center region. The 6.4 keV feature is detected from many clouds there, but there is a debate on whether it is induced by a past bright X-ray flare from the central black hole, or whether it is caused by low energy electron or ion cosmic rays. In fact, different cloud complexes within the Galactic Center prefer different ionization mechanisms, respectively.
The impression of the organizers, which is supported by comments and e-mails from the participants, is that this was an extremely useful and timely workshop. Both communities learned a lot from each other. The talks were in general very good, and speakers tried to bring across their arguments keeping in mind that they were addressing a diverse community. It was also interesting to see how the different communities have sometimes overlapping jargon with different meanings. An example was the confusion about the term “secondary electrons”, which for low energy astrophysicists means the electrons liberated after ionizations, whereas for a cosmic ray physicists means the electron created after the decay of pions, which themselves are created after high energy proton-proton collisions.
Some ideas that came out of this workshop are: the use of H3+ molecules to trace the distribution and origin of low energy cosmic rays; the importance of future MeV satellite telescopes to do the same; the potential use of 6.4 keV line emission to trace low energy cosmic ray interactions.
The general feeling at the end of the workshop was that we should stay in touch as an interdisciplinary community, and exchange more information about ideas and potential observing campaigns. To that end we are in the process of setting up a Wiki. All Lorentz Center participants will have access to the Wiki, and already several non participants have asked to be involved. We will post there the presentations and papers from our Lorentz Center workshop website, as well as start new pages for questions and observation organization. There was also a call for trying to do a similar meeting in two years time, so our sense is that we have put something in motion with this workshop that will have lasting effects, perhaps even leading to larger conferences, since the interest level was so high.