High-energy particle acceleration in the shell of a supernova remnant

1. High-energy particle acceleration in the shell of a supernova remnant F.A. Aharonian et al (the HESS Collaboration) Nature 432, 75 (2004) Nuclear Physics Group Journal Club Jan. 31, 2005 as interpreted by J.W. Martin University of Winnipeg

2. Cosmic Rays • Discovered by Victor Hess, Nobel Prize 1936. • not from Earth • not from Sun • source? composition? • How do we answer these questions?

3. from CHICOS website Cosmic Ray Energy Spectrum • near 1/E3 dependence • relatively featureless • goes to incredibly high energy

4. Some current experiments on cosmic rays • charged particle observatories • low energy (ACE, AMS), UHECR air showers (Fly’s Eye, CHICOS, Agasa, Auger) • neutrino observatories • solar (SNO), atmospheric (SuperK), cosmic (IceCube, ANTARES) • photons • conventional telescopes (many), X- (Chandra), Air-Č (Whipple, HESS), air shower (Milagro) from Milagro website

5. Interesting things about HESS • HESS = High Energy Stereoscopic System, an air-Čerenkov telescope, sensitive to air showers initiated by TeV gamma rays. • Built mainly by MPI-Heidelberg, telescope is located in Namibia. Completed in Dec. 2003. • First air-Č telescope to resolve an image of anything. • The image shows that a source of high-energy cosmic rays is in the shell of a supernova remnant.

6. HESS is not a conventional telescope • Consists of four 13 m telescopes on corners of 120 m square (stereoscopic) • detects Čerenkov light from air shower • light is reflected from mirror onto bank of phototubes. • 960 phototubes per camera • field of view of 5 degrees • angular resol of few arcmins • energy range of 100 GeV – 10 TeV • energy resol. of 15-20%

7. map of Cherenkov light seen by the bank of phototubes • stereoscopic info from multiple telescopes allows determination of incident angle of primary • energy calibration very important, air quality monitoring etc.

8. Details for this measurement • 26 h data from May to August 2003 (they don’t say it, but they need clear, moonless sky) • after data quality and deadtime: 18.1 h • only two telescopes used • two datasets: • independent telescope operation with GPS synchronization offline. • array-level trigger. (multiplicity in two telescopes) • trigger level 250 GeV – 150 GeV. • “hard” cuts (only well-reconstructed events) effectively raised threshold to 800 GeV but improved angular resolution. • Construct “count map” based on reconstructed RA and Dec for each

9. Count map for this object • smeared with 3 arcmin resolution of device

10. Comparison to Image in X-rays • good overlap of interesting regions

11. Energy Spectrum of Gammas from this Source • well-described by power law 1/E with =2.2. • previous =2.8 for only NW region. • more data to come to get  vs. RA and Dec.

12. Relationship to X-ray observers • Has already been seen in X-rays, • most plausible source of those X-rays is synchrotron rad from 100-TeV electrons. • Alternate explanations are not ruled out (thermal model of X-ray emission). • So, TeV-gamma ray observations were necessary for conclusive proof.

13. Relationship to previous TeV measurements and the origin of cosmic rays • Another TeV -component arises from accelerated protons striking nearby dense molecular clouds. • CANGAROO data + models suggest this is happening in the NW region. In agreement with XMM Newton data. • Total flux in TeV band consistent with picture where SNR’s generate all cosmic rays (some assumptions). • Multitude of competing processes stresses need for TeV spectroscopy with spatial resolution. (i.e. please keep funding us)

14. Conclusions • Significant step towards finding the source of cosmic rays. • First resolved image from an air-Cherenkov telescope.