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Introduction

MAGIC is a system of two imaging atmospheric Cherenkov telescopes or IACTs. MAGIC-I started routine operation after commissioning in 2004. Construction of MAGIC-II has been completed in early 2009, and will be operational later this year. The project is funded primarily by the funding agencies BMFB (Germany), MPG (Germany), INFN (Italy), MICINN(Spain), and the ETH Zurich (Switzerland).

The picture to the left shows the MAGIC-I Telescope with its 236 m2 reflective surface, the largest of any operating IACT. The precursor experiment HEGRA used several telescopes of the same type, but of smaller size; one of them is shown in the photograph to the right.

The Roque de los Muchachos site is situated on the Canary island of La Palma, a volcanic island off the African coast at 28oN and 17oW. The site has excellent conditions for optical observations, and is run by the IAC. It is also under consideration for an ultra-large optical telescope.

The MAGIC site is at an altitude around 2200 above sea level.

Up to the rim at 2500m, multiple high-quality optical telescopes are installed, as seen in the photograph to left. It shows the MAGIC telescopes in 2009, with the MAGIC control building with a red roof and a dome for a LIDAR, and, above MAGIC, two optical telescopes, the Telescopio Nazionale Galielo (left), and the GTC, with its 11.3m segmented mirror the largest worldwide, and presently being commisssioned.

Imaging atmospheric Cherenkov telescopes

The cosmos and its evolution are studied using all radiation, in particular electromagnetic waves. The observable spectrum extends from radio waves (at wavelengths of several tens of meters, or energies of some .000 01 eV) to ultra-high enery gamma quanta (wavelengths of picometers or energies of 100 TeV). Observations at visible wavelengths (.5 to 1 micrometer) have a history of centuries, gamma astronomy by satellites (keV to few GeV) and ground-based telescopes (above 300 GeV) are end-of-20th century newcomers. IACTs are ground-based telescopes for the detection of very high energy (VHE) electromagnetic particles, in particular gamma rays. Having no electric charge, VHE gammas are not affected by magnetic fields, and can, therefore, act as messengers of distant cosmic events, allowing straight extrapolation to the source. Although high-energy gamma quanta get absorbed in the atmosphere, they can be observed indirectly. The absorption process proceeds by creation of a cascade or shower of high-energy secondary particles. The Cherenkov method uses the fact that the charged econdary particles emit radiation at a characteristic angle, the Cherenkov radiation. Cherenkov photons have energies in the visible and UV range, and pass through the atmosphere; thus they can be observed on the surface of the earth by sufficiently sensitive instruments. You will find more about IACTs on a dedicated page.

The physics case for IACTs

Most generally, the observation of gamma rays (electromagnetic radiation of high energy) is one aspect of astroparticle physics. Astroparticle physics is a new field developing as an intersection of Particle Physics, Nuclear Physics, Astrophysics, Gravitation and Cosmology. One of its cornerstones is Cosmic Ray Physics, which has its origins many decades in the past; then scientists observed in balloons and in mountain top laboratories the many charged particles impinging upon the earth. Today, the field has substantially widened, and includes all particles. Particularly in recent years, activities (and funding) have accelerated, with fundamental discoveries being made at an astonishing frequency. Using the understanding of particle interactions at very high energies, as derived from experiments in accelerator laboratories, the picture of how the universe developed since its earliest beginnings, some ten billion years ago, is changing fast. Theoretical models fuel multiple experiments, using different particles coming to earth from space.

Very high energy gamma astronomy using ground-based telescopes is a recent addition to the panoply of astroparticle physics instruments, and the number of established sources is small, so far. What can be expected to be the most interesting subjects of observation during the coming years, are

  • Active galactic nuclei:
recent results indicate that most if not all galaxies (including our own milky way) have an active center, in which a supermassive black hole is building up. Some of them (Mrk 421, Mrk 501) have been observed to be active in the VHE gamma region, with occasional outbreaks and even with quasi-periodic fluctuations. The preferred theory explains the VHE gammas as products of high acceleration fields (shock waves) in the jet that bundles charged particles along two directions at 180 degrees to each other. We currently believe that the VHE gamma rays are produced within the jets, close to the black hole.

The origin of the jets is not yet understood. Models relate the jet directions (seemingly constant over millions of years) to the spin axis (axis of rotation) of the black hole. Understanding more about these objects and the acceleration mechanisms both in the vicinity of the black holes and in intergalactic space is a task in which IACTs have an important role to play. MAGIC, in particular, with its emphasis on optimal light collection will be able to probe more deeply into the earlier part of the developing universe: the lower the energy threshold, the larger the observable redshifts (see related diagram).

  • Supernova remnants:
in the wake of a certain class of supernova explosions, the so-called SN of types II and Ia, gas clouds expand and a very dense core develops; the core may be a spinning neutron star or a black hole. In the example of the Crab nebula (the picture to the right is taken in the X-ray spectrum), the neutron star is observed as a pulsar, because it rotates at 30 cycles and 'pulses' in the X-ray domain; it is also observed at optical and UV wavelengths. A rather constant radiation at higher energy, in the TeV range, has also been observed by IACTs. Supernova remnants may be VHE gamma sources of different types. According to the standard model of cosmic ray origin, the shell type supernova remnants (radiating from the expanding cloud) are sites of acceleration of nuclei to very high energies: if so, they not only are the main accelerators of charged cosmic rays, but should also copiously produce gamma rays. SN remnants of the plerion type, instead, are expected to radiate from the core. A systematic high-sensitivity scan of candidates, most of them lying in our own galaxy, has still to be done.
  • Sources found at lower energies but not yet identified:
All-sky surveys of wide-angle searching experiments in satellites have discovered a large number of lower energy gamma-ray emitters. The angular resolution of these detectors is limited, so that for more than half of these sources, it was not yet possible to relate them to known sources observed at different wavelengths. The (third) 1999 catalogue of sources established by the (no longer operating) EGRET detector is a well-known book of astrophysics riddles: it lists (green spots in the picture to the left, click on it to see the details) 170 unidentified sources, along with 101 whose origin is thought to be understood. Many of the unidentified populate the galactic equator, hence can be expected to be in our own galaxy. Clearly, there is much uncharted territory in the sky! IACTs will collect many more gamma rays from these sources, and thus pinpoint their positions and contribute to their identification.
  • Gamma Ray Bursts:
every day, a few powerful stellar explosions illuminate the sky from all directions. Usually, they do not get recorded at visible wavelengths, or only a very weak optical signal is seen. Gamma ray bursts last for seconds to minutes only (click on the picture to the right to see the time scale of one observation). Sometimes, an afterglow in the optical or X-ray domain is observed after much longer delays. The energy observed makes them the beacons of most likely the most energetic events known in the universe. Discovered 30 years ago, these Gamma Ray Bursts have been objects for research and speculation ever since. One theory, the fireball or hypernova model, posits that they are indicative of extremely violent explosions releasing in excess of 1051 ergs (or 1044 J), and creating violent shockwaves as the materials flowing out from the explosion at different velocities collide.

Today, a few thousand gamma ray bursts have been carefully charted, mostly by the BATSE satellite experiment, now removed from orbit. These objects cover the entire sky, seem spatially uncorrelated, many of them have large redshifts, i.e. we observe them at billions of years in the past, in the period of active star formation. Observations in the very high-energy gamma domain are not available, so far, but are expected to help clarifying these mysterious phenomena.

  • Other contributions to cosmology and fundamental physics:

Observations of VHE gammas, if done systematically, will also allow to formulate constraints on stellar formation in the early universe, by measuring the extragalactic infrared radiation field. They will further allow searches for the stable lightest supersymmetric particle, expected (if it exists) to annihilate with its own self-conjugate antiparticle into photons in areas of high gravitational field, e.g. in the vicinity of the black holes of galactic centers (including our own). Quantum gravity effects might become apparent if subtle time differences can be detected in the arrival of gammas from a given source, at different wavelengths. If they occur in nature, the MAGIC telescopes have the capability to record such phenomena.

Multi-wavelength observations

Already in the one time diagram shown just above for a gamma ray burst, the combination of results obtained at different wavelengths is a crucial ingredient. In the example, one deals with gammas in different energy brackets and with visible light, The same relevance of simultaneous observations can be safely predicted for most future astrophysics observations. The correlation in time and in amplitude for expected signal fluctuations will give important clues to the mechanisms of production, acceleration, and transport through space. As an example, take the observation by HEGRA, RXTE/ASM, and by optical and radio telescopes (right, click on the thumbnail image). Time correlations of the fluctuations in the source (Mkn501) intensity can be seen between VHE gammas and X-rays, whereas the optical and radio intensities stayed constant over the period (6 months in 1997). Such effects still need explaining. Multiple measurements are equally important in obtaining curves of total flux over a large band of energies (the example to the left goes over some 20 orders of magnitude!): there, again, conclusions about the production and the transport of radiation is entirely dependent on the collaboration between multiple experiments at different wavelengths.

MAGIC will participate in multi-wavelength campaigns (simultaneous observations) wherever promising, and is also preparing for a close collaboration with several X-ray, optical, and radio experiments.

Technical solutions chosen for MAGIC

Much of the revived interest in astrophysics experiments is driven by technologies which were not available a generation ago: detectors are being developed with the know-how of new techniques often taken from accelerator experiments, and now deployed in space, on earth, and deep underground; the availability of fast electronics and automatic control allows to economically build devices of astounding performance and complexity; computers and networks are available with sufficient capacity to record and reconstruct large volumes of data and find their interrelations. The MAGIC project falls very logically into this line of development of detectors for Cosmic Rays.

Read more about the physics motivations of pushing to lower values the threshold of observable energy in MAGIC and in future telescopes. You will find more about the technical aspects and performances of the MAGIC telescopes in dedicated pages.


This page was created by Robert Wagner. Last modification 20.05.2009 by Robert Wagner.
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