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24 3. Intergalactic magnetic fields 18:00.0 -23:20:00.0 Declination 22:00.0 24:00.0 26:00.0 28:00.0 500 kpc 50.0 40.0 20:03:30.0 20.0 10.0 Right ascension Figure 3.2: Example of a radio halo. Chandra image of the hot ICM in the cluster RXCJ 2003.5– 2323 overlaid on contours of 235 MHz radio emission from the Giant Metrewave Radio Telescope (GMRT). Taken from Giacintucci et al. (2009). relativistic protons have loss timescales of the order of the Hubble time, thus they are able to travel a large distance through the cluster. The observed link between radio halos and relics and the unrelaxed state of clusters supports the scenario of re-acceleration in situ of particles by turbulence produced during cluster mergers e.g. Schlickeise, Sievers & Thiemann 1987; Brunetti et al. 2001; Petrosian 2001; Fujita, Takizawa & Sarazin 2003). Cluster mergers are indeed able to drive turbulence, shocks and compression in the intracluster medium. The energy dissipated in cluster mergers can be channeled into the amplification of magnetic fields and particle acceleration (e.g. Carilli & Taylor 2002; Dolag et al. 2002; Brüggen et al. 2005; Brunetti & Lazarian 2007; Pfrommer, Enßlin & Springel 2008). However, not all dynamically disturbed clusters possess radio halos. One of the reasons is that they are difficult to observe, due to their low surface brightness and steep spectrum. Cassano & Brunetti (2005) have shown that only the most massive clusters, where the energy density of the turbulence is high enough, should possess radio halos produced by primary models. 3.1.2 Magnetic fields from diffuse radio sources Secondary models predict gamma-ray emission from neutral pion decay from the same hadronic collisions that create relativistic electrons. So far, such diffuse emission, which would corroborate the secondary models for the origin of radio halos, has not been detected from clusters, and only upper limits can be inferred (e.g. Ackermann et al. 2010). The expected gamma-ray flux is related 24
3.1. Introduction 25 to the relativistic electron energy spectrum, which together with the magnetic field strength also determines the observed radio emission. Consequently, an upper limit to the gamma-ray flux corresponds to a lower limit on the intra-cluster field strength. To reproduce the observed radio halo emissivity, current gamma-ray upper limits give average field strength of the order of several µG (e.g. Jeltema & Profumo 2011). When halos or relics are observed, minimum energy assumptions offer an alternative approach to estimate the magnetic field strength B eq averaged over the whole halo/relic volume. This is the only estimate available for the preferred primary models. Multiple uncertainties affect this approach (Sec. 1.3.4). In particular, equipartition calculations can grossly underestimate the field strength as they are based on the assumption of a homogeneous magnetic field throughout the halo/relic volume, in contrast with evidence for its radial decline (Sec. 3.5). With standard assumptions (ζ= 1,Φ=1, emitting frequency range=10 MHz÷10 GHz) B eq ranges from 0.1 to 2µG in cluster halos and from 0.5 up to 6µG in relics (Ferrari et al. 2008 and reference therein, van Weeren et al. 2010). 3.1.3 Diffuse inverse Compton emission As in radio galaxies, the observation of diffuse IC flux from radio halos and relics would allow a straightforward determination of both magnetic field strength and relativistic particle density (Sec. 1.2.3). IC emission from radio halos and relics, should be observable at hard X-ray energies (the “HXR excess”) where the exponential decline of the thermal bremsstrahlung, dominating at soft keV energies, is steeper than the expected non-thermal spectrum (Rephaeli 1977). So far, highly significant IC emission has been detected only in the Ophiuchus cluster (e.g. Eckert et al. 2008), which possesses a mini-halo and the implied averaged magnetic field is≈0.3µG (Murgia et al. 2010). Earlier, less significant, detections gave predicted magnetic fields strength in the range 0.1-0.7µG, reaching the higher values in relics (e.g. Slee et al. 2001; Rephaeli, Gruber & Arieli 2006; Eckert et al. 2008). Similar or slightly higher fields are derived as lower limits from non-detections of IC emission (e.g. Lutovinov et al. 2008; Wik et al. 2011). Estimates of magnetic fields from IC emission in clusters are problematic for a number of reasons. • It is difficult to distinguish the HXR excess from thermal emission. In some clusters, the observed spectrum can instead be reproduced by thermal model with a single or multiple gas temperatures (e.g. Lutovinov et al. 2008; Wik et al. 2011). • An HXR excess might also be interpreted as synchrotron emission from highly relativistic electrons (≈PeV, Timokhin et al. 2004) or non-thermal bremsstrahlung from supra-thermal 25
Page 1: Alma Mater Studiorum Università de
Page 5: To Hypatia from Alexandria in Egypt
Page 9 and 10: Contents Abstract i 1 Physics of ra
Page 11 and 12: 4.6.3 The outer scale of the magnet
Page 13 and 14: Abstract The existence of diffuse m
Page 15 and 16: 2. Two-dimensional Monte Carlo simu
Page 17 and 18: Chapter 1 Physics of radio galaxies
Page 19 and 20: 1.2. Non-thermal emission mechanism
Page 21 and 22: 1.2. Non-thermal emission mechanism
Page 23 and 24: 1.3. Radio galaxies 7 FR II sources
Page 25 and 26: 1.3. Radio galaxies 9 (a) (b) Figur
Page 27 and 28: 1.3. Radio galaxies 11 pressure, th
Page 29 and 30: Chapter 2 The X-ray environment of
Page 31 and 32: 2.2. The thermal component 15 Figur
Page 33 and 34: 2.3. Radio source - environment int
Page 35 and 36: 2.3. Radio source - environment int
Page 37 and 38: Chapter 3 Magnetic fields in the ho
Page 39: 3.1. Introduction 23 Figure 3.1: Ex
Page 43 and 44: 3.2. Faraday Rotation 27 The RM can
Page 45 and 46: 3.2. Faraday Rotation 29 Figure 3.4
Page 47 and 48: 3.3. Depolarization 31 3.3 Depolari
Page 49 and 50: 3.5. The magnetic field profile 33
Page 51 and 52: 3.6. Tangled magnetic field 35 rad
Page 53 and 54: 3.7. The magnetic field power spect
Page 59 and 60: Chapter 4 Structure of the magneto-
Page 61 and 62: 4.1. The radio source 3C 449: gener
Page 63 and 64: 4.3. The Faraday rotation in 3C 449
Page 65 and 66: 4.3. The Faraday rotation in 3C 449
Page 67 and 68: 4.4. Depolarization 51 Figure 4.4:
Page 69 and 70: 4.4. Depolarization 53 1.25 arcsec
Page 71 and 72: 4.5. Two dimensional analysis 55 sm
Page 73 and 74: 4.5. Two dimensional analysis 57 Fi
Page 75 and 76: 4.6. Three-dimensional analysis 59
Page 83 and 84: 4.7. Summary and comparison with ot
Page 89 and 90: Chapter 5 Ordered magnetic fields a
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5.1. The Sample 75 35 49 00 ROSAT P
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5.1. The Sample 77 5.1.1 0206+35 02
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5.2. Analysis of RM and depolarizat
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5.3. Rotation measure images 81 IMA
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5.3. Rotation measure images 83 cor
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5.4. Depolarization 85 Therefore, a
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5.5. Rotation measure structure fun
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5.6. Rotation-measure bands from co
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5.7. Rotation-measure bands from a
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5.7. Rotation-measure bands from a
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5.8. Discussion 99 5.8 Discussion 5
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5.8. Discussion 101 line−of−sig
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5.8. Discussion 103 flat slopes. I
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5.9. Conclusions and outstanding qu
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5.9. Conclusions and outstanding qu
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Chapter 6 Faraday rotation in two e
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6.2. Two-dimensional analysis: rota
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6.3. Two-dimensional analysis: stru
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6.4. Preliminary conclusions 121 Ta
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6.4. Preliminary conclusions 123
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Summary and conclusions The goal of
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6.5. Summary 127 responsible for th
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6.6. General conclusions 129 radio
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6.7. Future prospects 131 and is th
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Appendix 133
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136 Data reduction and imaging of l
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144 BIBLIOGRAPHY Bîrzan, L., Raffe
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146 BIBLIOGRAPHY Ensslin, T.A. & Go
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148 BIBLIOGRAPHY Jeltema, T.E. & Pr
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150 BIBLIOGRAPHY Owen, F.N., 1989,
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152 BIBLIOGRAPHY 152
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