The MAGIC telescope has been built with the clear goal to lower the energy threshold for gamma rays. For many unanswered physics questions, a low energy threshold holds the key. As the MAGIC telescope reaches its design performance, it is likely that, beyond the physics that has been predicted, new avenues will open.

The areas of interest which MAGIC can address are given graphically to the left. Below, we give more detailed arguments why they are particularly interesting for a device with low energy threshold.

The general argument is the absolute necessity to explore the electromagnetic spectrum at all wavelengths, and the absence, at the present time, of any instrument exploring the energy region between some tens of GeV and several hundred GeV with adequate sensitivity. At lower energies, satellite experiments, in particular EGRET, have contributed substantial knowledge. Their energy range and sensitivity will be much improved by the EGRET successor, GLAST, (launch foreseen in 2007), but even that will have the limit of detector size, and will have to be supplemented by complementary terrestrial observations. Pre-MAGIC gamma-ray telescopes, on the other hand, have typically an energy threshold of several hundred GeV.

The physics domains of MAGIC

Fundamental physics

• Gamma Ray Horizon

  The Gamma Ray Horizon is commonly defined as that energy for each redshift, for which the photon flux is reduced by a factor of e. Gamma rays from distant sources interact in the intergalactic space with infrared photons and other low-energy photon fields, and are substantially attenuated. Observations at lower energy threshold give access to gamma-ray sources with higher redshifts, hence a larger sensitivity for tests of light propagation effects.

• Cold dark matter

  Cosmology provides strong arguments for some 25% of the energy in the Universe to be in the form of Dark Matter; most of it should be 'cold', non-baryonic, and weakly interacting, Weakly Interacting Massive Particles or WIMPs. The Standard Model of Particle Physics has no candidate for WIMPs, and the nature of Dark Matter can only be explained by going beyond the Standard Model. Supersymmetry assumes symmetry between the fermionic and bosonic degrees of freedom, and thus provides a quite natural candidate for the WIMP, the neutralino. The neutralino could be the Lightest Supersymmetric Particle and should, therefore, be stable. Neutralinos might be indirectly detected by their annihilation through different channels, producing high energy gammas; in particular, gamma/gamma or p Z/gamma pairs provide ideal observational results (monochromatic annihilation lines). Another possible reaction is through hadronic jets producing several gammas; in this reaction, neutralinos could be observed as an energy distribution in gammas different from the power laws characteristic for the cosmic acceleration mechanisms. In any case, lowering the gamma energy threshold to identify the continuum signature extends the sensitivity to many competing theoretical neutralino production models.

• Quantum gravity

  Models of Quantum Gravity naturally contain quantum fluctuations of the gravitational vacuum, and lead to predictions of an energy-dependent velocity for electromagnetic waves. In other words, gammas of different energy produced simultaneously in an extragalactic object, arrive on Earth at different times due to their propagation through the gravitational vacuum. Gamma telescopes could be suitable instruments. Their figure of merit, in order to be sensitive to such an effect, is to provide sensitivity to large expected time delays between gammas in a wide range of energies. Also, any time delay effect must be studied over a wide range of redshifts, in order to disentangle any Quantum Gravity effect from source-dependent time delay emission effects. For this reason, a large number of sources at different redshifts and different characteristics must be detected and measured with precision. A low-energy threshold and high-sensitivity detector has clear advantages.

Astrophysics

• Gamma Ray Bursts

  Nearly 3000 Gamma Ray Bursts (GRBs) have been observed by BATSE, but the phenomena at their origin are not fully understood. GRBs seen in the gamma ray domain last between a fractions of a second and minutes; the X-ray emission typically runs on a scale of days, and the optical one even on a scale of weeks. Multiwavelength observations of GRBs have been made made possible by fast distribution of the information coming mainly from satellites, in real time. Some BATSE observations (less than a percent) were complemented by EGRET, at ~1 GeV of energy; however, both the field of view (FOV) and the sensitivity of EGRET sets severe limits. Encouraging is the fact that two of the EGRET-detected GRBs (GRB930131 and GRB940217) lasted longer inside the EGRET energy window than the observation in BATSE, so that an afterglow at higher energies can not be excluded, at least for some GRBs. The existence of exceptionally long GRBs, with the gamma ray emission lasting clearly longer than emisssion in the hard X-ray range, is particularly challenging for theoretical models, that have to deal with a continuous acceleration process at a substantially higher energy than that of the prompt emission. The successful simultaneous observation of some GRBs at different wavelengths has favored, at least for longer lasting bursts, among the existing theoretical models that of a collapsing supermassive star. The emission mechanism, however, is still under discussion. While the "fireball shock model" is commonly accepted for the prompt emission, the origin of the longer-lived afterglow is explained by various models, differing substantially in their predictions for high-energy gamma emission. Experimental data for higher-energy gamma-rays are not available; MAGIC has been constructed (light weight = fast slewing) to provide this information, for the moment a totally new territory.

• Supernova remnants (SNRs) and plerions

  Shell-type supernova remnants have long been suggested to be sites for cosmic ray acceleration below 100~TeV, mainly on the basis of general energetics arguments. We know from their synchrotron, radio and X-ray emission that electrons are accelerated to TeV energies. There is, however, no solid evidence for proton acceleration. A possible signature of proton acceleration would be the spectrum of neutral pi decay from collisions of cosmic ray protons and nearby matter like high density molecular clouds. A number of shells have been observed by EGRET at 0.1-10~GeV energies and by gamma ray telescopes at TeV energies; the energy region in between still has to be pinned down experimentally. Also, the origin of this radiation remains uncertain, due to the contamination of gamma-rays produced by leptonic processes (Bremsstrahlung or Inverse-Compton). A low threshold and high sensitivity will allow spectral studies in the range above 10 GeV, where the different mechanisms are expected to show different spectral shapes; it also will allow to pin down the exact positions of possible spectral cutoffs. Plerions or Pulsar-Wind Nebulae are SNRs in which a pulsar wind injects energy into its surroundings. A bubble is inflated out to a radius where it is confined by the expanding shell, as already suggested for the Crab Nebula. Particle acceleration is expected in the wind termination shock. A precise measurement of the spectrum in the energy range below 100 GeV is crucial to constrain the model parameters and to ascertain if another source of photons is necessary, possibly Bremsstrahlung from dense regions of gas.

• Pulsars

  The observation of gamma-ray pulsars in the GeV domain is of special interest: in this range, the EGRET pulsar spectra cut off due to limited sensitivity; observations of the differential spectra of gamma-ray pulsars in this energy domain will give us clues about the different proposed models ( polar cap and outer gap), which explain the gamma-ray emission in neutron stars. The two models predict different cut-off energies: below 40 GeV for the polar cap, up to 100 GeV for the outer gap model. Moreover, many of the unidentified sources recorded in the 3rd EGRET catalogue are believed to be radio-quiet Geminga-like pulsars. Besides the EGRET sources all radio pulsars are expected to lose part of their rotational energy through gamma-ray emission. A large collection area and a low energy threshold will allow the detection of such possibly faint emission. MAGIC will also be sensitive to the weak gamma-ray flux expected from millisecond pulsars, which are predicted to have spectra extending up to a few hundred GeV; these telescopes will contribute to resolve this issue.

• Active Galactic Nuclei

  Galaxy formation and evolution in the early universe are among the main open questions of extragalactic astronomy. The conventional understanding is based on a co-evolution of galaxies and super-massive black holes, which power the AGN phenomenon. Recent observations, however, seem to challenge this understanding, by showing early star and galaxy formation, compared to a later evolution of AGNs. Gamma ray observations of AGNs at different redshifts (i.e.with a low energy threshold) will help in understanding the evolution of AGNs and their central engines, super-massive black holes. A low threshold will enable us to study the gamma ray emission of a large population of AGNs, up to redshifts above z~2, as the lower-energy gammas can reach us largely unabsorbed by the meta-galactic radiation field.

• Diffuse photon background

  The diffuse photon background may be classified according to its origin: The Extragalactic Background Radiation (EBR) and the Diffuse Galactic Emission (DGE). EBR is essentially isotropic, and is well established up to energies of some 50 GeV. In the energy range from 30 GeV to 100 TeV, a large part of the EBR may be due to the direct emission from Active Galactic Nuclei, which have not yet been resolved. Direct measurements of the DGE exist up to energies of ~70 GeV. The data are explained by the interaction of cosmic-ray electrons and hadrons with the interstellar radiation fields and with the interstellar matter. The production mechanisms are synchrotron radiation of electrons, high-energy electron bremsstrahlung, inverse Compton scattering with low-energy photons, and pi-0 production by nucleon-nucleon interactions. More recently, measurements by INTEGRAL seem to show that rather than diffuse, galactic emission is associated to multiple point sources. New measurements of the gamma radiation (either diffuse or from point-like or extended sources) in the energy region below 300 GeV will help to better understand the different sources of the diffuse gamma ray background :

  • By the detection or identification of new AGNs one will test the unresolved-blazar model for the origin of the apparent EBR. The basic assumption is an average linear relationship between gamma ray and radio fluxes. Such a relation is suggested if the same high-energy electrons are invoked as the source of both the radio and gamma ray emission.
  • A deep observation of new blazars, with measurement of redshift, energy spectrum, and cutoff energy, will allow a more reliable determination of the collective luminosity of all gamma ray blazars. A good knowledge of this contribution is a precondition for future tests of the predictions of the cascading models for the EBR.
  • The detection or identification of new SNRs and pulsars will allow better estimates of their contribution to the DGE.

  More subjects

• Unidentified EGRET sources

  this is an enormously rich field of activity for detailed studies, possible with modest observation times on nearly half of the observable EGRET sources.

• Nearby galaxies

  their expected steep energy spectrum makes observations at low energy a particularly good argument, as they allow enough flux to be detected in the gamma ray domain.