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Non-Thermal Emission from Galaxy Clusters

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Non-Thermal Emission from Galaxy Clusters University of Amsterdam MSc Physics Gravitation AstroParticle Physics Amsterdam Master Thesis Non-Thermal Emission from Galaxy Clusters Predictions for X-Ray Satellites by Richard Tony Bartels 10508333 June 2014 60 ECTS August 2013 - June 2014 Supervisor: Daily Supervisor: Examiner: Dr Shin'ichiro Ando Dr Fabio Zandanel Dr Jacco Vink GRAPPA and IoP Abstract Clusters of galaxies are the largest gravitationally bound structures in the Universe and the latest ones to form. Large-scale diffuse synchrotron emission is observed in many clusters proving the presence of relativistic electrons and magnetic fields in the intra-cluster medium. The same population of electrons can inverse-Compton scatter off the photons of the cosmic microwave background. This can generate non-thermal hard X-ray emission, on top of the thermal X-ray bremsstrahlung observed in all clusters. However, so far, non-thermal hard X-ray detections have been claimed in just a few clusters and are not confirmed. A definitive detection of the inverse-Compton emission from galaxy clusters would allow us to disentangle the magnetic fields and relativistic electron distributions. Upper limits on this emission can be used to place lower limits on the magnetic field. In this Master's thesis, we estimate upper limits for the volume averaged magnetic field that still allow for a detectable non-thermal hard X-ray signal by next-generation X-ray telescopes, in particular ASTRO-H and the already launched NuSTAR, for all known radio halos and relics. Acknowledgements There are various people that I would like to thank for different reasons. First of all, I would like to thank Tera for another year of support and especially for cheering me up when frustration gets the better of me. Also the kittens you offered a temporary home at our place, without me being aware of that, where a nice form of distraction in the final weeks of this project. Finally, there are certain people I have to thank at the university. Thanks to Jacco Vink for being willing to be my second reader and for useful suggestions. Thanks to everyone in Shin'ichiro's 'thursday' group for listening to my presentation. Especially to Irene, who has been very helpful. Also thanks to the other Master's students, with whom I had a lot of fun this year. Thanks to the faculty at GRAPPA, Christoph and Shin'ichiro in particular, for having faith in my future research career. Finally, thanks to my supervisors for all their help. To Shin'ichiro for being willing to guide my project, even though he already had 4 students under his wings. And Fabio, to whom I am most thankful, for being the best supervisor I could have wished for. ii Contents Abstract i Acknowledgements ii Contents iii List of Figuresv List of Tables vi Abbreviations vii Physical Constants ix Symbols x 1 Introduction1 2 Theoretical Background: Radiative Processes6 2.1 Synchrotron Radiation.............................6 2.1.1 Motion of a Particle in a Magnetic Field...............6 2.1.2 Spectrum of a Single Electron.....................7 2.1.3 Spectrum for a Distribution of Electrons...............8 2.1.4 Magnetic Field Orientation......................9 2.1.5 Photon Spectrum............................ 10 Photon spectral index..................... 10 2.2 Inverse Compton Radiation.......................... 11 2.2.1 IC scattering in the Thomson limit.................. 11 2.2.1.1 Klein-Nishina limit..................... 13 2.2.2 Inverse Compton Spectrum...................... 13 Photon spectral index..................... 14 2.3 The Electron Spectrum............................ 14 2.3.1 Loss Functions............................. 17 2.3.1.1 γmin .............................. 18 2.3.1.2 γmax .............................. 20 2.4 Analytical Magnetic Field Estimates..................... 20 Correction for Isotropically Distributed Magnetic Fields.. 21 iii Contents iv 3 Methods 24 3.1 Cluster Selection and Data.......................... 24 Radio Data........................... 25 3.1.1 Non-Thermal X-Ray Data....................... 25 3.2 Background Modelling............................. 26 3.2.1 APEC model.............................. 26 3.2.1.1 Thermal Gas Density.................... 27 Clusters without a gas density model............. 28 Thermal emission in the halo region.............. 28 Thermal emission in the relic region.............. 29 3.2.1.2 PyXspec........................... 31 3.3 ASTRO-H Sensitivity............................. 33 3.4 Analysis of the Spectrum........................... 36 4 Results and Discussion 38 4.1 Promising Targets............................... 43 4.1.1 Comments on Good Targets...................... 45 4.2 Spectra with a Spectral Break......................... 60 4.3 Discussion.................................... 64 4.3.1 Low Energy Cutoff: Potential in EUV/SXR and Low Frequency Radio Emission............................. 64 4.3.2 NuSTAR and Background Modelling................. 65 4.3.3 Primary Targets in a Broader Science Perspective......... 67 5 Conclusion 69 A Some (Astro-)Physics 71 A.1 Cosmology................................... 71 A.1.1 Distance scales............................. 72 A.2 Equipartition Magnetic Field......................... 73 A.3 Parameter Dependence on Cosmology.................... 75 B Radio Data 77 C Comments on Less Good Targets 86 D Cluster Spectra 93 Bibliography 103 List of Figures 1.1 Abell 1689....................................2 1.2 Radio emission in clusters...........................2 1.3 Non-thermal emisson..............................4 2.1 Cooling processes................................ 15 2.2 Loss timescales................................. 19 3.1 APEC metallicity dependence......................... 27 3.2 Normalisation for radio halos......................... 29 3.3 APEC normalisation for relics......................... 30 3.4 ASTRO-H sensitivity curves.......................... 34 3.5 ASTRO-H sensitivity curve scaling...................... 35 3.6 Spectrum Analysis............................... 36 4.1 1E0657-56.................................... 46 4.2 A0085...................................... 47 4.3 AS753...................................... 48 4.4 A1367...................................... 49 4.5 Coma...................................... 51 4.6 A1914...................................... 52 4.7 A2255...................................... 53 4.8 A2319...................................... 54 4.9 A2744...................................... 56 4.10 A3667...................................... 57 4.11 A4038...................................... 58 4.12 MACSJ0717.5+3745.............................. 59 4.13 ZwCl0008.8-5215................................ 61 4.14 Broken Power Law Spectra.......................... 63 4.15 NuSTAR vs. ASTRO-H............................ 66 A.1 Equipartition condition............................ 73 v List of Tables 2.1 γ for maximum loss timescale......................... 18 3.1 Thermal data.................................. 33 3.2 ASTRO-H Properties............................. 33 4.1 Cluster Sample................................. 39 4.2 Results for halos................................ 41 4.3 Results for relics................................ 42 4.4 Equipartition magnetic field estimates.................... 44 4.5 Results Broken Power Law.......................... 62 4.6 NuSTAR Properties.............................. 65 B.1 Radio Data: Halos............................... 77 B.2 Radio Data: Relics............................... 80 vi Abbreviations AGN Active Galactic Nucleus ATCA Australia Telescope Compact Aarray cgs centimetre gram second CMB Cosmic Microwave Background CR Cosmic Ray CRe Cosmic Ray electron CRp Cosmic Ray proton DM Dark Matter d.o.f. Degree of Freedom EoM Equation of Motion EUV Extreme UltraViolet FoV Field of View FWHM Full Width at Half Maximum HPD Half-Power Diameter HXI Hard X-ray Imager HXR Hard X-Rays IC Inverse Compton ICM Intra-Cluster Medium KN Klein-Nishina LOFAR Low-Frequency Array for Radio Astronomy NFW Navarro-Frenk-White NuSTAR Nuclear Spectroscopic Telescope Aarray NVSS NRAO VLA Sky Survey RM Rotation Measure REXCESS Representative XMM-Newton Cluster Structure Survey vii Abbreviations viii SKA Square Kilometre Array SXI SoftX-ray Imager SXR Soft X-Rays SXS SoftX-ray Spectrometer VLA Very Large Array VSSRS Very Steep Spectrum Radio Source WENSS Westerbork Northern Sky Survey WSRT Westerbork Synthesis Radio Telescope Physical Constants speed of light c = 2:997 924 58 1010 cm s−1 × elementary charge e = 4:803 205 10−10 esu × gravitational constant G = 6:673 10−8 cm3 g−1 s−1 × Planck constant h = 6:626 068 85 10−27 erg s × − Planck constant, reduced ~ = 1:054 571 73 10 27 erg s × −16 Boltzmann constant kb = 1:380 649 10 erg K × −28 electron mass me = 9:109 382 15 10 g × classical electron radius r = 2:817 940 29 10−12 cm 0 × Thomson cross-section σT = 0:665 245 856 barn present day CMB temperature T0 = 2:726 K −13 4 −3 Average CMB energy density UCMB = 4:19 10 (1 + z) erg cm × ix Symbols a acceleration cm s−2 B Magnetic field esu cm−2 γ Lorentz factor νc Critical frequency Hz νg Gyration frequency Hz P Power erg s−1 q Charge esu r200 Virial radius kpc rc Core radius kpc −3 UB Magnetic energy density erg cm x Chapter 1 Introduction Clusters of galaxies are the largest virialized structures in the universe and as such the latest ones to form according to current paradigm of ΛCDM and hierarchical structure 15 formation. Their mass is typically of the order of 10 M , most of which consists of ∼ dark matter, about 70 80%. The remaining
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