An Old Friend: Supernova Remnant RX J1713.7–3946
The constellation Scorpius hosts one of the most important sources of very-high-energy gamma rays: the supernova remnant RX J1713.7-3946. Possibly associated with a star explosion seen by Chinese astronomers in the year AD393 (see Fig. 1), this object is one of the brightest sources of gamma rays in the H.E.S.S. energy range in the sky and its detailed studies (see SOM January 2005 and references -) earmark the breakthrough of ground-based gamma-ray astronomy with Cherenkov telescopes some twelve years ago. This was the first resolved gamma-ray source in the sky, it allowed us to study a Galactic particle accelerator in detail, and by comparisons with measurements at other wavelengths (X-ray and radio data) it allowed us to probe the supernova remnant (SNR) paradigm as an explanation for the origin of Galactic cosmic rays. We are now about to release an updated H.E.S.S. measurement of this supernova remnant, 9 years after our previous publication , using a larger dataset and much improved analysis tools more than doubling the previous sensitivity. As seen in Fig. 2, the morphology measurement and angular resolution have improved dramatically over the years, gamma-ray astronomy is turning into precision science!
Speaking about the SNR paradigm - how can we prove that this beast accelerates protons and not only electrons? If we knew precisely what the ambient magnetic field strength as well as the ambient matter and photon field densities were, we could simply calculate the spectral energy distributions and compare these to data. Unfortunately, since none of the above parameters is really precisely known, we can only make assumptions and derive parameters from model curves that match the data. This is illustrated in the plot of the H.E.S.S. and the Fermi-LAT data, compared to electron and proton models, shown in Fig. 3.
So what's the point? As you can see from the figure, both models can in principle fit the data, and these fits tell us the corresponding physical parameters of the source. This is where the work of an experimental collaboration like H.E.S.S. stops and the work of theorists will start. Questions like: is 10 micro-Gauss a reasonable magnetic field strength, or a density of 1 particle per cm3, can anyone understand why there is a break in the particle spectrum at 2-3 TeV, are the energetics reasonable, are we looking at gamma rays from only electrons or only protons, or a mix of both? Time and lots of theory papers will hopefully tell.
And here is a final interesting bit of the H.E.S.S. result that is really new: Fig. 4 illustrates that when comparing the radial size of the supernova shell of RX J1713.7-3946 it clearly extends further when seen in gamma rays than in X-rays!
Such effects from accelerated particles leaving the main shock region have long been predicted in Diffusive Shock Acceleration theory , but have so far never been seen. Well, that has changed now! We believe that the gamma rays extending further out than the X-rays are exactly this: the X-rays mark the end of the shock region, the gamma rays are either from completely detached (escaped) particles or else from particles in the forward shock (shock precursor) region. Even if it is too early to tell with certainty which one of the two scenarios we are seeing, this is an important achievement: we are ticking off an item on the long list of cosmic-particle-acceleration theorists by showing them for the first time gamma-ray images of particles in the process of leaving the main accelerating shock!
 Shen, Y. 500, Sung Shu (History of the Sung Dynasty), 25.
 Wang, Z. R., Qu, Q.-Y., Chen, Y., A&A, 318, L59 (1997).
 H.E.S.S. Collaboration, Aharonian et al., Nature, 432, 75 (2004).
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 H.E.S.S. Collaboration, Aharonian et al., A&A, 464, 235 (2007).
 CTA web page at www.cta-observatory.org
 See e.g. the review by Hillas, A. M., Journal of Physics G: Nuclear and Particle Physics, 31, R95 (2005).