2.2 The MAGIC telescopes
2.2.3 Performance of the MAGIC telescopes
The performance of the MAGIC telescopes, expressed in terms of sensitivity, energy threshold, energy resolution and angular resolution, is summarised in Fig. 2.12 (pre- upgrade: [39], post-upgrade: [42]).
The angular resolution is the standard deviation of a two-dimensional Gaussian fit to the distribution of reconstructed event direction of the signal and corresponds to a radius containing 39% of γ rays of a point source. The angular resolution at 250 GeV is ∼ 0.07◦ and it improves at higher energies reaching ∼ 0.04◦. With the upgrades a factor ∼ 5 − 10% improvement has been achieved (Fig. 2.12a).
The energy resolution is defined as the standard deviation of a Gaussian fit to (Eest− Etrue)/Eest and its mean value is the energy bias introduced by this method
and whose effects are corrected with the unfolding procedure (see Section 2.2.2). The energy resolution of the MAGIC telescopes, after the upgrades of 2011-2012, in the energy range of a few hundred GeV is 15% and the energy bias is below a few percent. The energy bias is larger both at low energies, due to energy overestimation (lower photon number, higher relative noise and worse arrival direction estimation) and at high energies, due to energy underestimation (truncated shower images). As shown in Fig. 2.12b, the upgrades of 2011 and 2012 lead to an improvement in the energy resolution at low and medium energies.
The sensitivity is the minimum flux that can be detected with a statistical sig- nificance of 5 σ, for a defined observation time1. The differential sensitivity of the MAGIC telescopes, before and after the upgrade is compared in Fig. 2.12c: the post- upgrade system has a better sensitivity, in particular at low energies (E < 100 GeV) where the same sensitivity of the pre-upgrade system can be achieved reducing the observation time by a factor of 2.5.
1Here the sensitivities are given for 50 hours of effective time of observations (S
50 h). For other
Estimated Energy [GeV]
2
10 3
10 4
10
Angular resolution [degree]
0 0.02 0.04 0.06 0.08 0.1 0.12
2D Gaus fit, Crab, 2010, < 30deg MC
2D gaus fit, Crab, 2013, < 30deg MC
2D gaus fit, Crab, 2013, 30-45deg MC
(a) Angular resolution;
/ GeV
true
E 60 100 200 300 1000 2000 10000 20000
Energy bias and resolution
-0.05 0 0.05 0.1 0.15 0.2 0.25 0.3 0.35 ) ° Bias (2010, < 30 Resolution ) ° Bias (2013, < 30 Resolution ) ° -45 ° Bias (2013, 30 Resolution
(b) energy resolution and bias;
Energy [GeV] 2 10 3 10 104 D if fe re n ti a l s e n s it iv it y [ % C .U .] 1 2 10 20 30 40 50 Stereo, (2010), Zd. < 30° ° Stereo Upgd., (2013), Zd. < 30 ° Stereo Upgd., (2013), Zd. 30-45 (c) differential sensitivity.
Figure 2.12: The angular resolution (a), the energy resolution and bias (b), and the differential sensitivity for 50 hours of effective time of observations (c) of the MAGIC telescopes after the upgrade for low zenith angles (< 30◦ red) and medium zenith angles (30◦− 45◦ blue). For comparison, the performace of the pre-upgrade
3
Active Galactic Nuclei
Active galactic nuclei (AGN) are galaxies which exhibit an unusual activity in their nuclear region. They are not powered by starlight, like “normal” galaxies, but by the release of gravitational energy of stellar material falling into a hole. They are the most luminous persistent sources in our universe, with luminosities that can exceed the ones of “normal” galaxies by factors of 104 concentrated in a tiny volume, which
can be less than ∼ 1 pc3.
In this chapter some basic properties and observational characteristics of AGN and blazars in particular are presented. The general structure of an AGN together with the historical discoveries and the present time classification will be introduced first. In the following section the experimental features of blazars will be outlined, together with a review of the emission models for blazars. In the final section exper- iments whose data are combined with MAGIC observations into multiwavelength light curves and spectral energy distributions are briefly listed.
3.1
Structure, history and classification of active
galactic nuclei
According to our present understanding [e.g. 65, 155], AGN (see Fig. 3.1) host in their central region a super massive black hole (M ∼ 108 − 109M). The sur-
rounding stellar material is accreted into a disc, the accretion disc. In the case of a 47
central black hole with mass ∼ 108M and radius r ∼ 10−5pc1, the accretion disc
is located within the first ∼ 10−4 − 10−3 pc. It typically rotates and develops two
diametrically opposite and narrowly collimated jets which can extend over several pc (from 10−1pc to ∼ 4 Mpc of the giant radio galaxy 3C 236 [166]), and are clearly visible in the images at radio wavelengths. A hot, optically-thin corona surrounds the optically-thick accretion disc [70]. In AGN whose optical spectra show broad emission lines, fast moving gas clouds (∼ 10−2 − 10−1pc from the central region)
are found, forming the so-called broad line region (BLR). Moving out, (∼ 0.1 pc), a thick and cold torus of dust and molecules, which obscures the accretion disc, is present. Even farther from the central region (∼ 1 − 102pc), in some cases, there is the narrow line region: a region of less dense clouds than the ones in the broad line region which cause absorption features in the optical spectra. The radio jets originate in the central region of the AGN and they may be regarded as a continuous with the core. They stream out energetic particles and magnetic field energy from the nucleus with relativistic velocities, so that the radiation is enhanced through Doppler boosting. The composition of the jet is still unknown: it could be an electron-positron pair plasma, proton-electron plasma or a combination of the two (electron-positron jets [83, 123]; protons/ions energetically dominated jets but with uncertain electron-proton number [66, 169, 170]). The energetic jets interact with the ionized gas which fills and surrounds the host galaxy; they are confined by a medium through which they drive a tunnel. Eventually, the ram pressure of the diffused gas stops the jets and hot spots of radiation form and originate radio lobes. The first optical spectrum of an AGN was obtained by E. A. Farth in 1908 and the first class of AGN was recognized in 1943 by Carl Seyfert [167]. He was studying the optical spectra of a group of galaxies, thought to be bright nebulae and now called Seyfert galaxies, with star-like cores and the appearance of spiral galaxies. Their emission lines were broader and wider than the lines observed in other galaxies, with the hydrogen lines broader than the others. Later in 1955, two of these objects, NGC 1068 and NGC 1275, were detected as radio emitters. At the end of the 1950s, with the first radio surveys, the identification of the strongest radio sources with optical objects started. Quasars (quasi stellar radio sources), radio sources with stellar appearance, great luminosity and showing variability were discovered during such surveys. 3C 273 was among the first ones to be detected. It is the brightest known quasar, more that 100 times more luminous than our galaxy but with the redshift z = 0.158. In the following years other types of AGN were discovered: the radio-quiet quasars and the BL Lacertae objects.
The flow of history and the spread of discoveries over several decades resulted in a sort of zoology of AGN, where the sources were classified according to the
1The size of the various components given in the following description refer to a black hole
Figure 3.1: The structure of an AGN: the central black hole is surrounded by an accretion disc and a torus of dust. Two diametrically opposite jets propagate over distances of several parsecs. Depending on the observer’s viewing angle, AGN are classified into different types ( http://www.nasa.gov/centers/goddard/news/ topstory/2007/active_galaxy.html).
observed properties. Only recently it has been possible to collect all these objects under the phenomenology of AGN in the “unified model” [186] and explain their different emission features in terms of orientation with respect to the observer. In addition, there is also great variety among objects belonging to the same class. Indeed, the features of such complicated objects depend on the relative importance of the various components.
Two big classes of AGN depending on the nature of the emitted radiation exist [7, 186]. If it is thermal radiation originating from in-falling material and it is mainly concentrated in the UV, optical and X-ray band, we call these sources thermal/disc- dominated. These represent the vast majority (∼ 90%) of AGN. If the emission has non-thermal origin, being generated by magnetic fields and high energetic, rela- tivistic particles, the AGN are called non-thermal/jet-dominated. In the latter case, radiation is produced all over the electromagnetic spectrum, from radio waves to very energetic γ-ray photons. This class corresponds to the one of radio-loud AGN. Non-thermal AGN are further classified according to the orientation of their jets with respect to the line of sight into blazars (one of the jets is pointing towards the observer) and non-aligned non-thermal dominated AGN (radio loud AGN with jets pointed at intermediate and large angles ∼ 15◦ − 40◦ with the line of sight).
quasars (FSRQs). Even though their continuum spectra are similar, they display diversity: sources belonging to the former class are characterised by a continuous spectrum with weak or no emission lines in the optical regime, while FSRQs show broad emission lines. The border line is the width of the strongest optical emission line: < 5˚A in BL Lacs [186].