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 GroupJournal Club Jan. 31, 2005

as interpreted by J.W. MartinUniversity of Winnipeg

Cosmic Rays

• Discovered by Victor Hess, Nobel Prize 1936.– not from Earth– not from Sun– source? composition?

• How do we answer these questions?

Cosmic Ray Energy

Spectrum• near 1/E3

dependence

• relatively featureless

• goes to incredibly high energy

from CHICOS website

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

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.

HESS is not a conventional telescope

• 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%

• 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.

• 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.

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:

1. independent telescope operation with GPS synchronization offline.

2. 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

Count map for this object

• smeared with 3 arcmin resolution of device

Comparison to Image in X-rays

• good overlap of interesting regions

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.

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.

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)

Conclusions

• Significant step towards finding the source of cosmic rays.

• First resolved image from an air-Cherenkov telescope.