After the second half of the XX century, many experiments attempted to apply the IACT technique in astronomy. The first detection was carried out by the Whipple telescope, detecting the Crab Nebula [110] after 20 years of operation. A few years later stereoscopic imaging was developed by the HEGRA [111] collaboration, greatly improving IACTs sensitivity. All competitive experiments from the current generation of IACTs, shown with their specifications in Table 2.2, incorporate this technique with different approaches, such as increasing the individual telescope size or the total number of telescopes used. Nowadays IACTs provide the best sensitivity among the techniques used in the VHE range. The IACT technique and analysis will be explained in more detail in Sec. 3.1.
Table 2.2: Specifications of the different generations of IACTs
Instrument Lat. Long. Alt. Telescopes Construction FoV Eth ∆E ∆Ω Sensitivity
Units Area in 50h
[◦] [◦] [m] [m2] [year] [◦] [GeV] [%] [◦] [% Crab]
Whipple 32 -111 2300 1 75 1968 2.3 300 30 0.1 15 HEGRA 29 18 2200 5 8.5 1987 4.3 500 15 0.1 5 CAT 42 2 1650 1 17.8 1996 4.8 250 20 0.14 15 H.E.S.S. -23 16 1800 4 107 2003 5 100 15 0.1 0.7 H.E.S.S. II -23 16 1800 1 616 2012 3.7 20 10-40 0.3 0.7 MAGIC 29 18 2225 2 234 2004 3.5 50 15 0.07 0.8 VERITAS 32 -111 1275 4 106 2007 3.5 100 15 0.1 0.7 CTA (North) 30 – ∼ 2000 ∼ 30 400/100 2017-2020 5-8 30 7 0.03 0.05 CTA (South) -25 – ∼ 2000 ∼ 125 400/100/15 2017-2020 5-8 30 6 0.03 0.05
Specifications of past, current and future generation of IACTs. Showing site location (latitude, longitude and altitude), total number of telescopes and individual mirror sur- face, date of first light, FoV, low energy threshold, energy and angular resolution and sensitivity (in the most sensitive energy, in Crab Units). Adapted from [112].
All IACT experiments share certain key scientific objectives. Some examples are the observation of SNRs, considered to be the main accelerators of cosmic rays, understanding the emission processes in γ-ray pulsars and discerning between pro- posed models, observing AGNs to understand the physical processes taking place in the vicinity of their super massive black holes and measuring the absorption affecting γ-rays from distant extragalactic sources due to the EBL, or searching for Dark Matter signals from regions where it is expected to accumulate such as
the Galactic Center or Dwarf Spheroidal Galaxies and detecting the high energy end of GRBs.
IACTs observable energy range overlaps with γ-ray space telescopes. First, with EGRET results, and later on in coalition with Fermi-LAT, IACTs provide improved angular resolution to classify Fermi unidentified sources (generally caused by the superposition within Fermi’s wide PSF of several possible sources) and extend their detected spectra up to the TeV energy range. At the same time Fermi-LAT provides essential information of interesting targets for ground based detectors, triggering observation proposals.
The current generation of IACTs is represented by MAGIC, HESS, and VERITAS.
• MAGIC: The Major Atmospheric Gamma-ray Imaging Cherenkov Tele- scopes (MAGIC) is located at the Roque de los Muchachos Observatory (La Palma, Spain) at 2200 m above sea level. It is a system composed by two IACTs of 17 m of diameter separated by 85 m. With a total mirror surface of 472 m2 it represents the most sensitive IACT experiment between 30 and
300 GeV in the northern hemisphere, and also the biggest IACTs on Earth until HESS II was built.
Due to the huge size of the MAGIC telescopes, low energy events are ob- served and accurately reconstructed using stereo imaging, widening the low energy range of IACTs. This low energy threshold makes the experiment ideal for the detection of high redshift AGNs and γ-ray pulsar observations. MAGIC telescopes also have the fastest repositioning time among the IACTs, becoming the best candidate to observe the highest energy component of a GRB from Earth.
• H.E.S.S.: High Energy Stereoscopic System (H.E.S.S.) (named after Vic- tor Hess) is formed by a system of 5 IACTs located in Khomas Highland (Namibia) at 1800 m above sea level, being the only major IACT experiment present in the southern hemisphere. The telescope layout is distributed as follows: four 12m of diameter telescopes form a perfect square of side equal to 120 m forming the classical HESS I system, and a 28m of diameter tele-
scope named HESS II sits in the center of the square. This system allows the observation of γ-rays with energies between 30 GeV to 100 TeV.
HESS II saw it’s first light on the 26th of July 2012, becoming the largest IACT on Earth, greatly improving H.E.S.S. sensitivity below 300 GeV. It’s location in the southern hemisphere allows to observe key sky regions such as the galactic plane and galactic centre, the most interesting and popu- lated areas of the sky in the TeV range. In 2009, H.E.S.S. was considered among the top 10 observatories worldwide ranked by their scientific impact by Nature [113].
• VERITAS: Very Energetic Radiation Imaging Telescope Array System (VER- ITAS) is built on Mount Hopkins (Arizona, USA) at 1268 m above sea level with an array of four 10 m of diameter IACTs placed in a diamond-like distribution with distances ranging between 80 to 120 m. This system of IACTs is able to observe from 50 GeV to 50 TeV, and is the most sensitive experiment in the northern hemisphere above 300 GeV.
2.3.2.1 Scientific results
Along with Fermi, γ-ray astronomy is mainly driven by IACTs. In the last decade, MAGIC, H.E.S.S. and VERITAS showed great performance with outstanding sci- entific results. As shown in Fig. 2.6, astronomy in the VHE regime is following a similar trend as other wavelengths underwent in the past in terms of number of detected sources.
Some scientific achievements of IACTs are briefly listed here:
• Galactic: The H.E.S.S. collaboration performed the Galactic Plane Survey [115], the first survey performed in the VHE range, detecting up to 14 pre- viously unknown sources [116] and extending Fermi spectra located in that region to the TeV range. Another mayor discovery was the detection of the Crab Pulsar by MAGIC [25] and later on by VERITAS [117], constraining emission models and unveiling the origin of pulsed radiation.
• Extragalactic: The MAGIC collaboration observed fast variability from IC310 [51] in the VHE range constraining the size of the radiating region to be less
Figure 2.6: Kifune plot showing the number of detected sources for different astronomy energy bands as a function of the year. Updated version (22nd April 2015) based on [114].
than 20% of the gravitational radius of its central supermassive black hole, suggesting pulsar-like γ-ray emission processes. In addition, the detection of VHE gamma-ray spectra of high redshift AGNs performed by H.E.S.S. [52], MAGIC [53] [54] and VERITAS [55], [56] placed strong upper limits on the EBL density, indirectly measuring star formation rates along the history of the universe.
• Fundamental physics: IACTs also faced fundamental physics problems such as the search of Dark Matter, providing competitive constraints for certain super symmetric models of self annihilation WIMP particles through obser- vations of the galactic center by H.E.S.S. [118] or dSph galaxies by H.E.S.S. [81], MAGIC [119] and VERITAS [120]. H.E.S.S. collaboration also per- formed measurements of the cosmic ray electron component [121] extending
the known electron-positron spectrum up to 5 TeV.