CTA performance

In order to achieve the ambitious goals described in Sec. 2.4.2.1, the CTA design target is to achieve certain goals, regarding different aspects of the observatory performance:

• Sensitivity: CTA is required to reach milli-Crab (mCrab) sensitivity in 50 hours of observation on a point-like source between ≈ 100 GeV to 10 T eV . This would improve the sensitivity of current generation of IACTs by a factor 10 inside the CTA core energy range. This unprecedented sensitivity will allow the study of fainter objects, already observed by Fermi-LAT, discover new source populations of even fainter ones and reduce the selection bias of the different γ-ray source types.

• Angular resolution: In order to resolve extended sources such as SNRs, CTA should be able to reach angular resolutions below 2 arc minutes for en- ergies above 1 TeV, the best resolution ever achieved in this range of energies, improving by a factor 3 the usual values attained in current instruments.

• Energy range: The observable energy range should extend well below 100 GeV and up to more than 200 TeV, with improved energy resolution with respect to current IACTs. CTA will be able to observe more than 3 orders of magnitude in energy, crucial for discerning between different emission mechanisms scenarios in AGNs or distinguish different hypothesis of gamma ray production in SNRs.

• Temporal resolution: CTA large effective area and improved angular res- olution will open a new window to a wide variety of transient phenomena. Current IACTs have already measured short time-scale variations of few minutes in the most rapid flux variations ever observed in AGNs, constrain- ing the emitting region size [51]. CTA will be able to resolve sub-minute time scales, improving constraints on AGN emitting region and significantly increase the chances of detecting extreme transients such as the very high energy component of GRBs, never observed from ground.

These performance goals represent an enormous technological challenge. We can divide the CTA energy range in three regions, each one corresponding to a different limiting factor:

The low energy range (E < 100 GeV): γ-rays below 100 GeV produce showers with low Cherenkov photon density, typically concentrated in a ring of ra- dius R ≈ 120m. Individual IACTs need large reflecting areas in order to integrate enough photons to discern between Cherenkov photons and Night-Sky Background Light (NSBL). In addition, the overwhelming amount of low energy hadronic show- ers produce images similar to the ones from γ-rays, turning background rejection into a difficult task. In this energy range sensitivity is limited by background rejection and signal to noise ratio.

CTA will push the low energy threshold down to some tens of GeV using a system of LST, each one equipped with a large mirror area (23 m of diameter) to collect enough Cherenkov photons from low energy showers. Since at low energies event rates are high, large collection areas are not needed, and a few number (≈ 4) of LSTs placed at short distances (≈ 100 m) will be able to provide enough effective area (∼ 104 _m2_{) and decent background rejection.}

The medium energy range (100 GeV < E < 10 TeV): Corresponds to the core energy range of CTA. This region is well understood from the experi- ence gained by the current generation of instruments. Cascades generate higher Cherenkov photon density, consequently reflectors with very large areas are not needed anymore. Images, composed of more pixels, reflect more features of the shower development through the atmosphere, helping to discern between hadron and γ-ray events. Sensitivity is mainly limited by the effective area and the quality of stereo reconstruction.

To maximize the core energy of CTA, a system of medium-sized telescopes (MST) of about 12m diameter will be spread over an area of ≈ 5 · 105 m2. Each cascade should be stereoscopically imaged by as many telescopes as possible in order to improve the quality of shower reconstruction, so telescope separation should range between 100 − 150m. For the first time, the area covered by the IACT system will be significantly larger than the Cherenkov light pool size, and therefore most of the events in this energy range will have their impact point reconstructed inside the array, improving the quality of the reconstruction.

The high energy range ( E > 10 TeV): At these energies, cascades generate a huge amount of Cherenkov photons, becoming observable at larger distances. The images show clear distinctive features in the EAS development between γ-ray and cosmic rays, turning background rejection into a simple task. As a consequence, sensitivity is only limited by the total effective area telescopes are able to cover, only constrained by the requirement of stereo shower reconstruction.

To maximize effective area, CTA will use small-sized telescopes (SST) of small reflecting area (∼ 10 m2) separated by a wider distance than other telescope types. This distance is yet to be optimized, but it will range between 200 and 400 m, depending on the final model chosen.

This thesis work is devoted to the evaluation of CTA performance through de- tailed MC simulations and the analysis of its future scientific impact in γ-ray as- trophysics. Chapter 3 describes the analysis performed and shows the impact on the sensitivity of different elements involved in the design phase of CTA, studying the effect of parameters associated with the construction site (altitude of construc-

tion, geomagnetic field intensity or Night-Sky Background (NSB) level) or array layout: different telescope types, spacing and distribution. Chapter 5 is devoted to explore MLA within γ-ray astronomy, evaluating their current applications and probe their implementation of new purposes. In chapter 4 CTA capabilities are evaluated in different physics cases, estimating the final scientific impact of the observatory.

Chapter 3

Sensitivity studies for the CTA

This chapter is the central part of this work, and gathers all the sensitivity stud- ies performed through MC simulations and analysis for the CTA. First, section 3.1.1 introduces the IACT technique, describing the main characteristics of these telescopes and how CTA is planned to improve the performance of the current generation of γ-ray detectors. Then, section 3.2 describes the different software packages involved in the CTA MC simulations, from the development of the EAS to the response of each telescope measuring the Cherenkov radiation. Section 3.3 describes the different tools used for the analysis of the CTA MC productions, both the software used in this work and the existing alternatives. An overview of the different large-scale MC productions performed by the CTA collaboration together with their configuration is performed in section 3.4. The two last sec- tions present the main results of this work: Section 3.5 analyzes CTA performance in detail, showing the different Instrument Respond Functions (IRFs) describing the observatory capabilities and how these are affected by the MST types used in the telescope layout. Section 3.6 concludes this chapter by evaluating the dif- ferent construction sites proposed for the CTA-N observatory, studying the effect on performance of the location related parameters such as the altitude or the Geo-magnetic field.

3.1

Imaging Atmospheric Cherenkov Telescopes

IACTs currently provide the best sensitivity among the different detection tech- niques in the VHE range. As previously introduced in section 2.2.2, Cherenkov photons from EASs are collected with wide mirror surfaces and focused on a cam- era, were these photons are measured creating images of the shower development. They operate in similar fashion as optical telescopes, although the short timescale of the EAS Cherenkov flashes and the overwhelming cosmic ray background im- pose certain key differences, mainly regarding the camera nature and data analysis needed. The technique is considered rather new, as they have been operating for less than half a century, but the low cost of the experiments compared to space detectors, and the benefits they provide in terms of effective area and sensitivity pushed this technique forward, specially in the last decades.

A general overview of the IACT technique and data analysis will be done in the following sections. For a deeper description, the reader is encouraged to glance over more specific experiment overviews, such as [123] of the MAGIC telescope.