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Observations of the Active Galactic Nucleus 1ES 1218+304 with the MAGIC-Telescope

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Observations of the Active Galactic Nucleus 1ES 1218+304 with the MAGIC-Telescope Observations of the Active Galactic Nucleus 1ES 1218+304 with the MAGIC-telescope Dissertation zur Erlangung des naturwissenschaftlichen Doktorgrades der Bayerischen Julius-Maximilians-Universitat¨ Wurzb¨ urg THOMAS BRETZ aus Frankfurt am Main Wurzb¨ urg 2006 Abstract The astronomical exploration at energies between 30 GeV and 350 GeV was the main motiva- tion for building the MAGIC-telescope. With its 17 m mirror>it is the worldwide largest imag- ing air-Cherenkov telescope. It is located at the Roquede los Muchachos at the Canary island of San Miguel de La Palma at 28.8◦ N, 17.8◦ W, 2200 m a.s.l. The telescope detects Cherenkov light produced by relativistic electrons and positrons in air showers initiated by cosmic gamma-rays. The imaging technique is used to powerfully reject the background due to hadronically induced air showers from cosmic rays. Their inverse power-law energy-distribution leads to an increase of the event rate with decreasing energy threshold. For MAGIC this implies a trigger rate in the order of 250 Hz, and a correspondingly large data stream to be recorded and analyzed. A robust analysis software package, including the general framework MARS, was developed and commissioned to allow automation, necessary for data taken under variable observing conditions. Since many of the astronomical sources of high-energy radiation, in particular the enigmatic gamma-ray bursts, are of a transient nature, the telescope was designed to allow repositioning in several tens of seconds, keeping a tracking accuracy of 0.01◦. Employing a starguider, a tracking accuracy of 1.3 minutes of arc was obtained. > The main class of sources> at very high gamma-ray energies, known from previous imaging air- Cherenkov telescopes, are Active Galactic Nuclei with relativistic jets, the so-called high-peaked Blazars. Their spectrum is entirely dominated by non-thermal emission, spanning more than 15 orders of magnitude in energy, from radio to gamma-ray energies. Predictions based on radiation models invoking a synchrotron self-Compton or hadronic origin of the gamma-rays suggest, that a fairly large number of them should be detectable by MAGIC. Promising candidates have been chosen from existing compilations, requiring high (syn- chrotron) X-ray flux, assumed to be related to a high (possibly inverse-Compton) flux at GeV energies, and a low distance, in oder to avoid strong attenuation due to pair-production in inter- actions with low-energy photons from the extragalactic background radiation along the line of sight. Based on this selection the first AGN, emitting gamma-rays at 100 GeV, 1ES 1218+304 at a redshift of z = 0:182, was discovered, one of the two farthest known AGN emitting in the TeV energy region. In this context, the automated analysis chain was successfully demonstrated. The source was observed in January 2005 during six moonless nights for 8.2 h. At the same time the collaborating KVA-telescope, located near the MAGIC site, observed in the optical band. The lightcurve calculated showed no day-to-day variability and is compatible with a constant 7 2 1 flux of F ( > 100 GeV) = (8:7 1:4) 10− m− s− within the statistical errors. A differen- · tial spectrum between 87 GeV and 630 GeV was calculated and is compatible with a power law 7 3:0 0:4 1 2 1 of F (E) = (8:1 2:1) 10− (E=250 GeV)− TeV− m− s− within the statistical er- E · rors. The spectrum emitted by the source was obtained by taking into account the attenuation due to pair-production with photons of the extragalactic background at low photon energies. A homogeneous, one-zone synchrotron self-Compton model has been fitted to the collected multi- wavelength data. Using the simultaneous optical data, a best fit model could be obtained from which some physical properties of the emitting plasma could be inferred. The result was com- pared with the so-called Blazar sequence. Observations of the Active Galactic Nucleus 1ES 1218+304 with the MAGIC-telescope I Zusammenfassung Die wesentliche Motivation zum Bau des MAGIC-Teleskops war die Untersuchung astronomi- scher Objekte zwischen 30 GeV und 350 GeV. Mit einem 17 m Spiegel ist es das weltweit großte¨ abbildende Luft-Cherenkov Telesk> op. Es steht auf der Kanarischen Insel San Miguel de La Palma auf dem Roque de los Muchachos. Das Teleskop detektiert Cherenkov Licht von Elek- tronen und Positron aus Luftschauern, erzeugt von kosmischer Gammastrahlung. Die abbildende Technik ist das Hauptmerkmal einer guten Unterdruckung¨ des Untergrundes, der hauptsachlich¨ aus Luftschauern der kosmischen Strahlung besteht. Ihre Energieverteilung, beschrieben durch ein steil abfallendes Potenzgesetz, fuhrt¨ zu einem starken Anstieg der Ereig- nisrate bei sinkender Energieschwelle. Fur¨ MAGIC bedeutet das Raten von etwa 250 Hz, was zu einem großen Datenstrom fuhrt,¨ der verarbeitet werden muss. Dazu wurde eine robuste Analyse Software, einschließlich des Grundgerustes¨ MARS, entwickelt und ihre Anwendung automati- siert. Dies ist notig¨ um wechselnde Beobachtungsbedingungen berucksichtigen¨ zu konnen.¨ Da viele Quellen von Hochenergie Gammastrahlen, insbesondere die ratselhaften¨ Gamma-Ray Bursts, stark veranderliche¨ Phanomene¨ sind, wurde das Teleskop so ausgelegt, dass es Neuaus- richtungen innerhalb weniger Sekunden ermoglicht,¨ und trotzdem eine Nachfuhr¨ genauigkeit von 0.01◦ erreicht, die mit einem Starguider auf 1.30 verbessert wird. >Die wichtigsten Quellen sehr hochenergetischer> Gammastrahlung sind Aktive Galaktische Ker- ne mit relativistischen Jets, die sog. high-peaked Blazars. Ihr Spektrum erstreckt sich uber¨ mehr als 15 Großenordnungen,¨ vom Radio- bis zum Gammabereich, und ist vollstandig¨ durch nicht- thermische Strahlung dominiert. Modelle, die synchrotron selbst-Compton Strahlung oder Strah- lung aus hadroninduzierten Kaskaden zu Grunde legen, sagen voraus, dass eine große Anzahl dieser Quellen von MAGIC zu detektieren sein mussten.¨ Vielversprechende Kandidaten wurden aus existierenden Katalogen ausgesucht mit der An- forderung eines großen Rontgenflusses¨ (Synchrotronstrahlung), von dem erwartet wird, dass er einen großen Fluss bei GeV-Energien zur Folge hat. Außerdem wurden nur nahe Quellen aus- gewahlt¨ um nicht von der Abschwachung¨ des Flusses durch Paarbildung an der extragalaktischen Hintergrundstrahlung entlang der Sichtlinie betroffen zu sein. Basierend auf dieser Auswahl wur- de zum ersten Mal Gammastrahlung von 100 GeV bei einem AGN (1ES 1218+304,z = 0:182) nachgewiesen. Hierbei wurde die automatische Analysekette erfolgreich demonstriert. Die Quel- le wurde im Januar 2005 wahrend¨ sechs mondloser Nachte¨ 8.2 Stunden lang beobachtet. Zeit- gleich wurden Daten im optischen Bereich vom mitarbeitenden KVA-Teleskop aufgenommen. Die berechnete Lichtkurve ist innerhalb der statistischen Fehler mit einem konstanten Fluss 7 2 1 von F ( > 100 GeV) = (8:7 1:4) 10− m− s− vertraglich.¨ Das differentielle Spektrum wur- · de zwischen 87 GeV und 630 GeV bestimmt und ist innerhalb der statistischen Fehler mit ei- 7 3:0 0:4 1 2 1 nem Potenzgesetz F (E) = (8:1 2:1) 10− (E=250 GeV)− TeV− m− s− vereinbar. E · Daraus wurde das von der Quelle emittierte Spektrum errechnet, indem die Paarbildung am ex- tragalaktischen Hintergrund entlang der Sichtlinie berucksichtigt¨ wurde. An das resultierende Spektrum und weitere gesammelte Multiwellenlangendaten¨ wurde ein homogenes einzonen syn- chrotron selbst-Compton Modell angepasst. Unter Berucksichtigung¨ der zeitgleichen optischen Daten konnten physikalische Großen¨ des emittierenden Plasma bestimmt werden. Das Resultat wurde mit der sog. Blazar-Sequenz verglichen. II Observations of the Active Galactic Nucleus 1ES 1218+304 with the MAGIC-telescope Contents Abstract . I Zusammenfassung . II 1. Introduction 1 2. Hardware Framework 11 2.1. Tracking system . 12 2.1.1. Requirements . 12 2.1.2. Implementation . 13 2.1.3. Astrometric corrections . 14 2.1.4. Calibration – Pointing correction . 14 2.1.5. Tracking accuracy . 15 2.2. Starguiding system . 17 2.3. Data acquisition . 19 3. Software Framework 21 3.1. Requirements . 21 3.2. Implementation . 22 3.2.1. Layer 0: Kernel . 23 3.2.2. Layer 1: Analysis Methods, Tools . 28 3.2.3. Layer 2: IACT standard algorithms . 29 3.2.4. Layer 3: Detector Geometry . 30 3.2.5. Layer 4: MAGIC specific part . 30 4. Analysis 33 4.1. Calibration . 33 4.1.1. Signal Extraction . 34 4.1.2. Pedestal Treatment . 35 4.1.3. Camera Inhomogeneity . 36 4.2. Bad pixel treatment . 39 4.3. Artificial triggers . 39 4.3.1. Trigger logic . 39 4.3.2. Software trigger . 40 4.4. Image Cleaning . 40 III Contents Contents 4.5. Image Parameters . 42 4.5.1. Classical statistic-parameters . 42 4.5.2. Additional parameters . 43 4.5.3. Semi classical parameters . 44 4.5.4. Muon Parameters . 45 4.6. Determination of the source position . 49 4.6.1. On-/off-observation mode . 50 4.6.2. Wobble observation mode . 51 4.6.3. Nominal source position calculation . 52 4.6.4. Source position correction . 53 4.6.5. Quality cuts . 53 4.7. Background suppression . 55 4.8. Effective observation time . 55 4.9. Monte Carlo . 55 4.9.1. Effective collection area . 60 4.9.2. Energy reconstruction . 60 4.9.3. Spill-over correction coefficients . 62 4.10. Spectrum calculation . 65 5. Automatic analysis 67 5.1. Data selection . 68 5.2. Analysis chain . 68 5.3. Database . 69 6. Results 71 6.1. Dataset . 71 6.2. Significance of the detection . 71 6.3. Sky map . 71 6.4. Lightcurve . 72 6.5. Measured spectrum . 73 7. Intrinsic spectrum 77 7.1. Basic principle . 77 7.2. Metagalactic radiation field . 78 7.2.1. Parameters . 79 7.2.2. Optical depth, Unfolding . 79 7.3. Intrinsic spectrum of 1ES 1218+304 . 80 8. Active Galactic Nuclei 83 8.1. Overview . 84 8.1.1. Classification . 84 8.1.2. AGN jets . 86 8.1.3. Jet origin . 87 IV Observations of the Active Galactic Nucleus 1ES 1218+304 with the MAGIC-telescope Contents Contents 8.2.
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