At the time of writing this thesis, there are more than 150 established VHE γ-ray sources 1. I
will give a brief description of all the types: Galactic sources
Pulsars:
Pulsars are rotating magnetized Neutron Stars (NSs) (we will give more details in Section §5.2). Particles get accelerated in specific regions of the NS magnetosphere. Photons are produced in a narrow emission beam. Since the rotation and the magnetic axes are usually not aligned, we observe EM emission only when the beam crosses the line of sight. The first pulsed VHE gamma rays were discovered by MAGIC in 2008 (Aliu et al. 2008). The spectrum was extended above 100 GeV by VERITAS (Aliu et al. 2011), up to 400 GeV by MAGIC (Aleksi´c et al. 2012b) and very recently up to TeV energies (Zanin 2014). Pulsed VHE gamma rays may be due to IC scattering. Recently, also pulsed emission from the
(a) Pion decay.
(b) Synchrotron radiation.
(c) Bremsstrahlung.
(d) Inverse Compton. Figure 1.4: VHE γ-ray production mechanisms.
Vela pulsar was reported by HESS (Brun, P. 2014), although the emission mechanism remains unclear.
SNRs
SNRs are the leftovers of Supernova (SN) explosions. CRs get accelerated through the Fermi mechanism in the shock that develops when the SN ejecta interact with the ISM. VHE gamma rays are thought to be of hadronic origin, although it remains to be confirmed (Gabici & Aharonian 2015). An example of a classical SNR where the origin of the emis- sion could not be determined can be found in Aharonian et al. (2004a). A more detailed description is given in Section §5.1.2.
PWNe
Pulsars lose their rotational energy mainly through an e±wind that interacts with the ISM
and the SNR where it is contained. Some leptons are eventually accelerated and emit EM radiation from radio up to gamma rays via synchrotron emission and VHE gamma rays via IC scattering of ambient photons. They are described in detailed in Chapter §5. There are currently 22 PWNe and PWN candidates detected at VHE gamma rays (all of them can be found in Table B.9 in Appendix B.5), the latest of which is 3C 58, described in Chapter§7.
γ-ray binaries
They consist of a compact object (a NS or a Black Hole (BH)) that is orbiting a massive
star. Five such objects are known to emit VHE gamma rays to date: LS I+61 303, HESS
J0632+057, HESS J1018-589, PSR B1259-63 and LS 5039. There are several explanations
for the observed VHE γ-ray emission. In the microquasar scenario, accretion into the compact object produces a jet where particles get accelerated. In the pulsar wind scenario, a pulsar orbits the massive star, the pulsar wind interacts with the companion wind, and a shock develops, where particles get accelerated.
Extragalactic sources AGN
They are galaxies hosting a supermassive BH in their center. Two plasma jets carrying part of the object’s angular momentum are emitted perpendicular to the accretion disk. The jet extends for several kpc distance. Particles get accelerated in shocks traveling along the jet. The behavior of the AGN seems to strongly depend on the viewing angle of the jet from the Earth.
Starburst galaxies
They are galaxies with a high star formation rate. As a consequence, the SN explosion rate and the CR density are larger than usual, providing shocks strong enough to accelerate particles that emit VHE gamma rays. Two starburst galaxies have been detected at VHE : NGC 253 and M82.
GRBs
They are the most energetic γ-ray outbursts known. No GRBs have been detected by IACTs so far, but Fermi-LAT has detected photons at 95 GeV from GRB 130427A (Ack- ermann et al. 2014b). Their origin is still under debate, although the main accepted mech- anisms are to be either hypernova explosions or the merger of two compact objects.
2
The imaging atmospheric Cherenkov
technique and the IACTs MAGIC and CTA
In this chapter we will describe the concept behind the detection of gamma rays through the imaging atmospheric Cherenkov technique. We will also describe the IACT arrays on which this thesis focuses: the Major Atmospheric Gamma-ray Imaging Cherenkov (MAGIC) telescopes and the Cherenkov Telescope Array (CTA). The hardware and techniques used to analyze the data of MAGIC will be described in detail. We will give an overview of the CTA project, together with a brief description of the telescope types involved in the project.
2.1
The imaging atmospheric Cherenkov technique
The EM spectrum spans more than 20 orders of magnitude in energy from radio to TeV gamma rays. The atmosphere is transparent to most of the radiation up to the UV, but higher energy photons do not penetrate into the atmosphere due to their interaction with the air molecules. A picture showing the bounds of the EM spectrum and the transparency of the atmosphere to all of them is shown in Figure 2.1. To detect those photons, one has to use satellites where they
have not been blocked yet. Unfortunately, due to the low fluxes, the collection area offered by
satellites is not large enough at energies exceeding 100 GeV.
Due to the interaction with the atmospheric nuclei, VHE particles produce cascades, also known as EASs. As the relativistic charged particles produced in the cascade move faster than the speed of light in the atmosphere, they produce Cherenkov light at wavelengths ranging from IR to UV. In the following, we will give a more detailed description of the shower development in the atmosphere and the way we can detect the Cherenkov light using optical telescopes. For a
Figure 2.1: The bounds of the EM spectrum (above) as seen from the altitude where photons are fully absorbed in the atmosphere (below). From Longair (1992); Moralejo (2000); Wagner (2006).
more detailed review, see Engel et al. (2011).