Steady-state emission of blazars at very high energies [Elektronische Ressource] / vorgelegt von Daniel Höhne-Mönch

Sujets

• Astronomie

Informations

• Publié par: julius-maximilians-universitat_wurzburg
• Publié le: 01 janvier 2010
• Nombre de lectures: 33
• Langue: Deutsch
• Poids de l'ouvrage: 8 Mo

Extrait

Steady-state emission of blazars
at very high energies
Dissertation zur Erlangung des
naturwissenschaftlichen Doktorgrades
der Bayerischen Julius-Maximilians-Universit¨at Wurzburg¨
vorgelegt von
Daniel H¨ohne-M¨onch
aus Werneck
Wurzburg¨ 2010Eingereicht am
bei der Fakult¨at fur¨ Physik und Astronomie
1. Gutachter: Prof. Dr. K. Mannheim
2. Gutachter: Prof. Dr. F. Reinert
3. Gutachter:
der Dissertation.
1. Prufer:¨ Prof. Dr. K. Mannheim
2. Prufer:¨ Prof. Dr. F. Reinert
3. Prufer:¨
im Promotionskolloquium
Tag des Promotionskolloquiums:
Doktorurkunde ausgeh¨andigt am:
2Contents
Abstract 7
Zusammenfassung 9
Introduction 11
1 Evolution of blazars 13
1.1 Active galactic nuclei . . . . . . . . . . . . . . . . . . . . . . . . 13
1.1.1 Empirical classification . . . . . . . . . . . . . . . . . . . 13
1.1.2 Unified scheme for AGN . . . . . . . . . . . . . . . . . . 15
1.2 Blazars . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16
1.2.1 Emission models . . . . . . . . . . . . . . . . . . . . . . 16
1.2.2 Blazar sequence . . . . . . . . . . . . . . . . . . . . . . . 18
1.2.3 BL Lac objects . . . . . . . . . . . . . . . . . . . . . . . 20
1.3 Blazar unification by evolution . . . . . . . . . . . . . . . . . . . 21
2 The very high energy γ-ray and cosmic ray connection 23
2.1 Cosmic rays . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23
2.2 Very high energy γ-rays . . . . . . . . . . . . . . . . . . . . . . 25
2.2.1 Production mechanisms . . . . . . . . . . . . . . . . . . 26
2.2.2 Intergalactic absorption of γ-rays . . . . . . . . . . . . . 27
2.3 Sources. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28
2.3.1 γ-ray bursts . . . . . . . . . . . . . . . . . . . . . . . . . 28
2.3.2 Starburst galaxies . . . . . . . . . . . . . . . . . . . . . . 29
2.3.3 Compact objects . . . . . . . . . . . . . . . . . . . . . . 30
2.3.4 Active galactic nuclei . . . . . . . . . . . . . . . . . . . . 30
2.3.5 Diffuse emission . . . . . . . . . . . . . . . . . . . . . . . 33
3 Imaging atmospheric Cherenkov technique 35
3.1 Extensive air showers . . . . . . . . . . . . . . . . . . . . . . . . 35
3.2 Cherenkov effect . . . . . . . . . . . . . . . . . . . . . . . . . . 36
3.3 Imaging technique . . . . . . . . . . . . . . . . . . . . . . . . . 38
3.4 MAGIC telescope . . . . . . . . . . . . . . . . . . . . . . . . . . 39
3Contents
3.4.1 Structure and Reflector . . . . . . . . . . . . . . . . . . . 39
3.4.2 Camera . . . . . . . . . . . . . . . . . . . . . . . . . . . 41
3.4.3 Data acquisition and trigger system . . . . . . . . . . . . 41
3.4.4 Observation modes and file types . . . . . . . . . . . . . 42
3.4.5 Monte Carlo simulations . . . . . . . . . . . . . . . . . . 43
4 Analysis chain 45
4.1 Signal extraction and calibration . . . . . . . . . . . . . . . . . 45
4.1.1 Signal extraction . . . . . . . . . . . . . . . . . . . . . . 45
4.1.2 Calibration . . . . . . . . . . . . . . . . . . . . . . . . . 46
4.1.3 Bad pixel treatment . . . . . . . . . . . . . . . . . . . . 47
4.2 Event image reconstruction . . . . . . . . . . . . . . . . . . . . 48
4.2.1 Software trigger . . . . . . . . . . . . . . . . . . . . . . . 48
4.2.2 Image cleaning . . . . . . . . . . . . . . . . . . . . . . . 48
4.2.3 Image parametrisation . . . . . . . . . . . . . . . . . . . 49
4.3 Background rejection . . . . . . . . . . . . . . . . . . . . . . . . 51
4.3.1 Quality cuts . . . . . . . . . . . . . . . . . . . . . . . . . 52
4.3.2 γ – hadron separation cuts . . . . . . . . . . . . . . . . . 52
4.4 Spectrum . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 52
4.4.1 Energy estimation . . . . . . . . . . . . . . . . . . . . . 53
4.4.2 Effective collection area . . . . . . . . . . . . . . . . . . 53
4.4.3 Ee observation time . . . . . . . . . . . . . . . . . 54
4.4.4 Energy spectrum . . . . . . . . . . . . . . . . . . . . . . 54
4.5 Lightcurves . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 54
4.6 Observations of the Crab Nebula . . . . . . . . . . . . . . . . . 55
4.6.1 Data selection and automatic analysis. . . . . . . . . . . 55
4.6.2 Background rejection . . . . . . . . . . . . . . . . . . . . 56
4.6.3 Energy spectrum . . . . . . . . . . . . . . . . . . . . . . 56
5 Observations and analysis results 59
5.1 Search for TeV candidate BL Lac objects . . . . . . . . . . . . . 60
5.1.1 TeV flux estimation . . . . . . . . . . . . . . . . . . . . . 60
5.1.2 Source catalogues and compilations . . . . . . . . . . . . 61
5.1.3 Selection criteria . . . . . . . . . . . . . . . . . . . . . . 62
5.2 Observation campaign . . . . . . . . . . . . . . . . . . . . . . . 66
5.2.1 Known sources from the selected sample . . . . . . . . . 67
5.2.2 Tentative redshift measurements . . . . . . . . . . . . . . 69
5.3 Analysis results . . . . . . . . . . . . . . . . . . . . . . . . . . . 70
5.3.1 Results of the analysis chain . . . . . . . . . . . . . . . . 70
5.3.2 Upper limit calculation . . . . . . . . . . . . . . . . . . . 76
5.3.3 Significance distribution . . . . . . . . . . . . . . . . . . 76
5.3.4 Source stacking . . . . . . . . . . . . . . . . . . . . . . . 77
4Contents
5.3.5 Crosscheck analysis . . . . . . . . . . . . . . . . . . . . . 81
6 Steady state emission of blazars 83
6.1 Spectral characteristics . . . . . . . . . . . . . . . . . . . . . . . 83
6.1.1 multi-wavelength data . . . . . . . . . . . . . . . . . . . 83
6.1.2 EBL correction . . . . . . . . . . . . . . . . . . . . . . . 86
6.1.3 Result . . . . . . . . . . . . . . . . . . . . . . . . . . . . 87
6.2 Comparison with known steady state sources . . . . . . . . . . . 91
6.2.1 HBLs measured in a low emission state . . . . . . . . . . 91
6.2.2 Broad-band spectral indices . . . . . . . . . . . . . . . . 93
6.2.3 Spectral energy distribution . . . . . . . . . . . . . . . . 93
7 Conclusions and outlook 97
A Data compendium 99
2B ϑ -distributions 121
C Lightcurves 133
List of figures 145
List of tables 147
Bibliography 149
List of publications 161
Curriculum vitae 169
Acknowledgements 171
5Summary
One key scientific program of the MAGIC telescope project is the discovery
and detection of blazars. They constitute the most prominent extragalactic
source class in the very high energy (VHE) γ-ray regime with 29 out of 34
1known objects . Therefore a major part of the available observation time was
spentinthelastyearsonhigh-frequencypeakedblazars. Theselectioncriteria
werechosentoincreasethedetectionprobability. AstheX-rayfluxisbelieved
to be correlated to the VHE γ-ray flux, only X-ray selected sources with a
flux F > 2μJy at 1keV were considered. To avoid strong attenuation of theX
γ-rays in the extragalactic infrared background, the redshift was restricted to
values between z < 0.15 and z < 0.4, depending on the declination of the
objects. The latter determines the zenith distance during culmination which
◦ ◦should not exceed 30 (for z <0.4) and 45 (for z <0.15), respectively.
Between August 2005 and April 2009, a sample of 24 X-ray selected high-
frequencypeakedblazarshasbeenobservedwiththeMAGICtelescope. Three
of them were detected including 1ES 1218+304 being the first high-frequency
peaked BL Lacertae object (HBL) to be discovered with MAGIC in VHE γ-
rays. One previously detected object was not confirmed as VHE emitter in
this campaign by MAGIC. A set of 20 blazars previously not detected will be
treated more closely in this work. In this campaign, during almost four years
∼450hrs or∼22% of the available observation time for extragalactic objects
were dedicated to investigate the baseline emission of blazars and their broad-
band spectral properties in this emission state. For the sample of 20 objects
in a redshift range of 0.018<z <0.361 integral flux upper limits in the VHE
range on the 99.7% confidence level (corresponding to 3 standard deviations)
were calculated resulting in values between 2.9% and 14.7% of the integral
flux of the Crab Nebula.
As the distribution of significances of the individual objects shows a clear
shift to positive values, a stacking method was applied to the sample. For the
whole set of 20 objects, an excess of γ-rays was found with a significance of
4.5 standard deviations in 349.5 hours of effective exposure time. For the first
1As of April 2010
7Summary
time a signal stacking in the VHE regime turned out to be successful. The
measured integral flux from the cumulative signal corresponds to 1.4% of the
CrabNebulafluxabove150GeVwithaspectralindexα =−3.15±0.57. None
of the objects showed any significant variability during the observation time
and therefore the detected signal can be interpreted as the baseline emission
of these objects.
Fortheindividualobjectslowerlimitsonthebroad-bandspectralindicesαX−γ
between the X-ray range at 1keV and the VHE γ-ray regime at 200GeV were
calculated. The majority of objects show a spectral behaviour as expected
from the source class of HBLs: The energy output in the VHE regime is in
general lower than in X-rays. For the stacked blazar sample the broad-band
spectral index was calculated to α = 1.09, confirming the result foundX−γ
for the individual objects. Another evidence for the revelation