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A Multiscale View of Active Galactic Nuclei Jets: from the Formation and Acceleration to High Energy Outbursts
Full text link학위논문(박사)--서울대학교 대학원 :자연과학대학 천문학과,2019. 8. Sascha Trippe.Active Galactic Nuclei (AGNs) often produce highly collimated relativistic jets, one of the most energetic phenomena in the Universe. In this thesis, we probe the mechanism of launching, propagation, and energy dissipation of AGN jets by using various methodologies. We study the jet of a nearby radio galaxy M87 with very long baseline interferometry observations and find that the jet is collimated by the pressure of non-relativistic winds launched from hot accretion flows and accelerated to relativistic speeds by strong magnetic fields in the jet. We investigate the frequency dependence of Faraday rotation of many AGN jets and reveal that recollimation shocks in the jets may play an important role in dissipation of the jet kinetic energy. We examine the association of strong γ-ray flares occurred in 2015 in the jet of PKS 1510–089 and its peculiar kinematic behavior and find that the flares may originate from compression of the jet knots by a standing shock in the core. We study the long-term radio variability of many radio-loud AGNs by employing temporal Fourier transform of the light curves and reveal that the radio variability can be controlled by the accretion processes. We constrain the properties of the radio-emitting source known as Sagittarius A*, which is potentially powered by jets, by a very long baseline interferometry observation during the passage of the gas cloud G2 through the vicinity of the supermassive black hole.Abstract
List of Figures
List of Tables
1. Introduction. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1
1.1 Jets in Active Galactic Nuclei . . . . . . . . . . . . . . . . . . . . . . . . 1
1.1.1 Phenomenology . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1
1.1.2 How are AGN jets produced? . . . . . . . . . . . . . . . . . . . 10
1.1.3 Accretion flows and winds . . . . . . . . . . . . . . . . . . . . . . 19
1.1.4 Recollimation shocks and energy dissipation . . . . . . . . . . . . 27
1.1.5 M87: the best target for AGN jet astrophysics . . . . . . . . . . 37
1.2 The gas cloud G2 passing through the vicinity of Sagittarius A* . . . . 42
1.3 Very Long Baseline Interferometry . . . . . . . . . . . . . . . . . . . . . 44
1.4 Power spectrum of light curve . . . . . . . . . . . . . . . . . . . . . . . . 49
1.5 Thesis outline . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 52
2. Faraday Rotation in the Jet of M87 inside the Bondi Radius: Indication of Winds from Hot Accretion Flows Confining the Relativistic Jet . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 53
2.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 54
2.2 Archival data and data reduction . . . . . . . . . . . . . . . . . . . . . . 60
2.3 Analysis and Results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 66
2.3.1 RM maps . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 66
2.3.2 Radial RM profile . . . . . . . . . . . . . . . . . . . . . . . . . . 66
2.3.3 Contribution of RM sources outside the Bondi radius . . . . . . 69
2.3.4 Variability . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 69
2.3.5 The Faraday screen . . . . . . . . . . . . . . . . . . . . . . . . . 71
2.4 Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 81
2.4.1 Jet sheath vs hot accretion flows . . . . . . . . . . . . . . . . . . 81
2.4.2 Winds and the Faraday screen . . . . . . . . . . . . . . . . . . . 84
2.4.3 Jet collimation by winds . . . . . . . . . . . . . . . . . . . . . . . 85
2.4.4 Mis-alignment . . . . . . . . . . . . . . . . . . . . . . . . . . . . 86
2.4.5 Mass accretion rate . . . . . . . . . . . . . . . . . . . . . . . . . . 88
2.4.6 RM at HST-1 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 91
2.4.7 EHT observations . . . . . . . . . . . . . . . . . . . . . . . . . . 92
2.5 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 93
3 Intensive Monitoring of the M87 Jet with KaVA: Jet Kinematics based on Observations in 2016 at 22 and 43 GHz . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 97
3.1 Introduction. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 98
3.2 Observations and Data Reduction. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 101
3.3 Summary of Previous Studies of the M87 Jet Kinematics . . . . . . . . . 104
3.4 Jet Kinematics on Scales of . 20 mas Based on KaVA Observations . . 108
3.4.1 Modelfit with Circular Gaussian Components . . . . . . . . . . . 108
3.4.2 Modelfit with Point Sources and Grouping . . . . . . . . . . . . . 112
3.4.3 Wise . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 117
3.4.4 Jet Apparent Speeds and Comparison with Other Studies . . . . 119
3.5 Jet Kinematics on Scales of ≈ 340 − 410 mas Based on VLBA Archive Data . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 120
3.6 Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 126
3.6.1 Slow Jet Acceleration . . . . . . . . . . . . . . . . . . . . . . . . 128
3.6.2 Multiple Streamlines and Velocity Stratification . . . . . . . . . . 132
3.6.3 Current Limitations and Future Prospects . . . . . . . . . . . . . 133
3.7 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 135
4 Revealing the Nature of Blazar Radio Cores through Multi-Frequency Polarization Observations with the Korean VLBI Network . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 137
4.1 Introduction. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 138
4.2 Observations and Data Reduction. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 142
4.3 results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 147
4.3.1 RM at radio wavelengths . . . . . . . . . . . . . . . . . . . . . . 147
4.3.2 Optical EVPAs from the Steward observatory . . . . . . . . . . . 163
4.3.3 fractional polarization . . . . . . . . . . . . . . . . . . . . . . . . 168
4.4 discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 170
4.4.1 RM distributions at different frequencies . . . . . . . . . . . . . . 170
4.4.2 Change of core opacities from optically thick to thin . . . . . . . 173
4.4.3 The Faraday screen. . . . . . . . . . . . . . . . . . . . . . . . . 174
4.4.4 RM sign change. . . . . . . . . . . . . . . . . . . . . . . . . . . 175
4.4.5 Optical subclasses . . . . . . . . . . . . . . . . . . . . . . . . . . 177
4.4.6 Intrinsic polarization orientation . . . . . . . . . . . . . . . . . . 178
4.4.7 Multiple recollimation shocks in the cores . . . . . . . . . . . . . 179
4.5Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 181
5 Ejection of Double knots from the radio core of PKS 1510–089 during the strong γ-ray flares in 2015. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 185
5.1 Introduction. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 186
5.2 Multi-wavelength Light Curves . . . . . . . . . . . . . . . . . . . . . . . 190
5.2.1 iMOGABA . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 190
5.2.2 SMA . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 192
5.2.3 Radio Spectral Index . . . . . . . . . . . . . . . . . . . . . . . . . 192
5.2.4 Optical Photometric Data . . . . . . . . . . . . . . . . . . . . . . 193
5.2.5 Fermi-LAT . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 193
5.3 Jet kinematics and linear polarization analysis . . . . . . . . . . . . . . 194
5.4 Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 200
5.4.1 Comparison of the γ-ray flares in 2015 with previous flares . . . 200
5.4.2 Double-knot Jet Structure . . . . . . . . . . . . . . . . . . . . . . 201
5.4.3 Acceleration motions and Spine-sheath Scenario . . . . . . . . . 204
5.4.4 Origin of the 2015 γ-ray flare . . . . . . . . . . . . . . . . . . . . 207
5.5 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 209
6 Radio Variability and Random Walk Noise Properties of Four Blazars. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 211
6.1 Introduction. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 212
6.2 Target Selection and Flux Data . . . . . . . . . . . . . . . . . . . . . . . 214
6.3 Analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 215
6.3.1 Lightcurves . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 215
6.3.2 Spectral indices. . . . . . . . . . . . . . . . . . . . . . . . . . . 215
6.3.3 Time offsets among spectral bands . . . . . . . . . . . . . . . . . 217
6.3.4 Periodograms. . . . . . . . . . . . . . . . . . . . . . . . . . . . 220
6.3.5 Simulated lightcurves and significance levels. . . . . . . . . . . 222
6.4 Results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 224
6.4.1 3C 279 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 224
6.4.2 3C 345 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 225
6.4.3 3C 446 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 225
6.4.4 BL Lac . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 226
6.5 Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 226
6.5.1 Spectral indices . . . . . . . . . . . . . . . . . . . . . . . . . . . . 226
6.5.2 Spectral time delays . . . . . . . . . . . . . . . . . . . . . . . . . 227
6.5.3 Power spectra . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 228
6.6 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 229
7 The long-term centimeter variability of active galactic nuclei: A new relation between variability timescale and accretion rate. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 231
7.1 Introduction. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 232
7.2 Sample and Data . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 236
7.3 Analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 240
7.3.1 Lightcurves and Power Spectra . . . . . . . . . . . . . . . . . . . 240
7.3.2 Fractal Dimension . . . . . . . . . . . . . . . . . . . . . . . . . . 243
7.3.3 Fitting Lightcurves Piecewise with Gaussian Peaks . . . . . . . . 248
7.3.4 Derivatives of Lightcurves . . . . . . . . . . . . . . . . . . . . . . 250
7.3.5 Black Hole Masses and Accretion Rates . . . . . . . . . . . . . . 250
7.4 Results and Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . 254
7.4.1 General Features of Power Spectra . . . . . . . . . . . . . . . . . 254
7.4.2 Distributions of Fractal Dimension . . . . . . . . . . . . . . . . . 256
7.4.3 β as an Indicator of Variability Timescale . . . . . . . . . . . . . 257
7.4.4 Relation between β and the Accretion Rate . . . . . . . . . . . . 262
7.4.5 Broken Power-law Periodograms . . . . . . . . . . . . . . . . . . 274
7.4.6 Comparison with Other Studies . . . . . . . . . . . . . . . . . . . 282
7.5 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 286
8 No asymmetric outflows from Sagittarius A* during the pericenter passage of the gas cloud G2. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 293
8.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 294
8.2 Observations and data analysis . . . . . . . . . . . . . . . . . . . . . . . 296
8.3 Discussion. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 299
Bibliography. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 305
Appendix. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 338
A Appendices for Chapter 2. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 339
A.1 Errors in linear polarization quantities . . . . . . . . . . . . . . . . . . . 339
A.2 Significance level of RM . . . . . . . . . . . . . . . . . . . . . . . . . . . 343
A.3 RM maps for all observations . . . . . . . . . . . . . . . . . . . . . . . . 346
A.4 Radial RM profiles for the northern and southern jet edges . . . . . . . 348
B Appendices for Chapter 3. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 349
B.1 WISE Analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 349
C Appendices for Chapter 4. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 353
C.1 D-Term calibration and evolution. . . . . . . . . . . . . . . . . . . . . 353
C.2 Reliability check of KVN polarimetry. . . . . . . . . . . . . . . . . . . 358
Conclusion. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 363
요 약. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 369Docto
한국 전파간섭계와 한일 공동 전파간섭계를 이용한 근거리 전파 AGN의 플라스마 물리적 특성 연구
Full text link학위논문 (석사)-- 서울대학교 대학원 : 물리·천문학부, 2015. 2. Sascha Trippe.Nearby bright Active Galactic Nuclei (AGN) are important sources to study relativistic jets of complex structure in detail. Improving our physical understanding about launching, acceleration, and propagation of the structured outflow requires information on (i) internal conditions of the plasma such as particle densities and degree of turbulence and (ii) the strength and geometry of magnetic fields pervading the jets. Multi-frequency VLBI observations – especially with polarimetry – have been crucial to spatially resolve the jet structures on different scales and to extract
detailed physical information from emission features. In this work, we show results and analysis of the first-epoch data obtained from the Plasma physics of Active Galactic Nuclei project, which is an ongoing program aimed at studying spectral and polarimetric properties of parsec-scale radio jets in AGN. Seven radio-bright nearby AGN were observed at frequencies of 22, 43, 86, and 129 GHz in dual polarization
with Korean VLBI Network (KVN). Also, for higher angular resolution, the targets are observed with a joint VLBI array made by KVN and VERA (KaVA) at 43 GHz. Total intensity source maps are obtained successfully for most of the
observations. At 22 and 43 GHz KVN detected polarizations from 3C 111, 3C 120, BL Lac, and DA 55. The observations have three main results. Firstly, apparent brightness temperatures of the jet components and radio cores measured by KaVA are found to be comparable to or lower than the plasma equipartition temperature (∼ 10^11K). This indicates that the observed unpolarized synchrotron radiation can be explained without exotic emission mechanisms such as inverse Compton radiation. Secondly, as for polarization properties, observed degree of polarization mpol for each source was relatively low, i.e. less than 10%. By simple calculation, it is shown that strong turbulences in the jet is able to suppress the intrinsic mpol. In the light of this model, we analyzed an exceptionally high degree of polarization detected from an extended blob of BL Lac at 43 GHz (mpol ∼ 40%). In order to observe the high level of polarization the magnetic field configuration in the jet component should be highly ordered by additional physical processes. Assuming that there is a transverse shock front traveling to the downstream of a jet, we found that combination of properties of the shock front such as its observed orientation and the shock strength nshocked/nunshocked can explain the high degree of polarization. Finally, by using the fluxes of the cores and jets obtained over a wide range of frequencies by KVN, spectral indices a of the components are measured. Interestingly, spectral index of the core at high frequencies (i.e. a86,129GHz) became quite steep. If other side effects such as (a) coherence loss of the radio emissions at high frequencies or (b) different UV coverages of KVN at the high frequencies do not affect the data significantly, the observed steepening pattern implies that the AGN radio
cores at mm-wavelengths could have different characteristics compared to the cmwavelength cores. Clearly, the multi-frequency polarimetric data obtained by KVN and KaVA revealed complicated spatial and spectral evolution pattern of the jets. Second- and third-epoch data from the Plasma physics of Active Galactic Nuclei project with the VLBI arrays in East Asia will probe spectral and polarimetric properties in more detail.I. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1
1.1 Radio jets of Active Galactic Nuclei and Role of VLBI Observations 1
1.2 VLBI arrays in East Asia . . . . . . . . . . . . . . . . . . . . . . . 4
1.3 Structure of the thesis . . . . . . . . . . . . . . . . . . . . . . . . . 6
II. Observations and Data Analysis . . . . . . . . . . . . . . . . . . . . 9
2.1 Sample Selection . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
2.2 Outline of Observations . . . . . . . . . . . . . . . . . . . . . . . . 14
2.3 Data Processing for Aperture Synthesis – 1 . . . . . . . . . . . . . 16
2.3.1 Illustration of Degree of Coherence . . . . . . . . . . . . . 16
2.3.2 Corrections of the relative phase differences f12 . . . . . . . 18
2.3.3 UV data – Definition . . . . . . . . . . . . . . . . . . . . . 21
2.3.4 UV data – KVN and KaVA . . . . . . . . . . . . . . . . . . 22
2.4 Data Processing for Aperture Synthesis – 2 . . . . . . . . . . . . . 23
2.4.1 Calibrations of UV data for Stokes I . . . . . . . . . . . . . 23
2.4.2 Construction of Source Maps . . . . . . . . . . . . . . . . . 35
2.4.3 Further calibrations for full Stokes maps . . . . . . . . . . . 41
2.5 Final Source Mapping with modelfit . . . . . . . . . . . . . . . . . 49
2.6 Development of VIMAP for Spectral Analysis . . . . . . . . . . . . 52
III. Results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 55
3.1 Global properties of whole samples . . . . . . . . . . . . . . . . . 55
3.2 3C 111 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 64
3.2.1 Intensity Distributions . . . . . . . . . . . . . . . . . . . . 66
3.2.2 Spectral Index Distribution . . . . . . . . . . . . . . . . . . 69
3.2.3 Polarization Properties . . . . . . . . . . . . . . . . . . . . 75
3.3 3C 120 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 78
3.3.1 Intensity Distributions . . . . . . . . . . . . . . . . . . . . 78
3.3.2 Spectral Index Distribution . . . . . . . . . . . . . . . . . . 82
3.3.3 Polarization Properties . . . . . . . . . . . . . . . . . . . . 88
3.4 3C 84 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 91
3.4.1 Intensity Distributions . . . . . . . . . . . . . . . . . . . . 91
3.4.2 Spectral Index Distribution . . . . . . . . . . . . . . . . . . 96
3.5 4C +01.28 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 100
3.5.1 Intensity Distributions . . . . . . . . . . . . . . . . . . . . 100
3.5.2 Spectral Index Distribution . . . . . . . . . . . . . . . . . . 101
3.6 4C +69.21 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 104
3.6.1 Intensity Distributions . . . . . . . . . . . . . . . . . . . . 104
3.7 BL Lac . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 108
3.7.1 Intensity Distributions . . . . . . . . . . . . . . . . . . . . 108
3.7.2 Spectral Index Distributions . . . . . . . . . . . . . . . . . 115
3.7.3 Polarization Properties . . . . . . . . . . . . . . . . . . . . 116
3.8 DA 55 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 122
3.8.1 Intensity Distributions . . . . . . . . . . . . . . . . . . . . 122
3.8.2 Polarization Properties . . . . . . . . . . . . . . . . . . . . 124
IV. Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 127
4.1 Correlations between physical properties . . . . . . . . . . . . . . . 127
4.2 Plasma opacity and degree of polarization . . . . . . . . . . . . . . 134
4.3 Brightness temperature Tb,app of all samples . . . . . . . . . . . . . 141
4.4 Intensity structures and polarization asymmetries . . . . . . . . . . 144
V. Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 153
References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 157Maste
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Variations on the Author
Full text link“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Appropriate Similarity Measures for Author Cocitation Analysis
Full text linkWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
블레이저들의 전파/감마선 상관관계
Full text link학위논문(박사) -- 서울대학교대학원 : 자연과학대학 물리·천문학부(천문학전공), 2021.8. Sascha Trippe.Relativistic jets in Active Galactic Nuclei (AGN) are one of the most powerful, persistent
sources of energy in the Universe. Investigation of AGN jets is valuable and promising as they
play an important role in not only the fields of high-energy astrophysics, but also the evolution
of galaxies and clusters. Radio-loud AGNs with their relativistic jets directed toward us (e.g.,
with a viewing angle of ∼5 degrees), are classified as blazars. One of the well-known characteristics of
blazars is strong γ-ray emission originating from their relativistic jets. Because the spatial res-
olution of high-energy telescopes is inadequate, however, our understanding of the high-energy
emission is limited and thus the production site of blazar γ-ray flares is a matter of active de-
bate. To explore the high-energy emission processes and its origin, I studied several individual
blazars that recently showed strong γ-ray emission: BL Lacertae, OT 081 (1749+096), 3C 273,
and 0716+714. In these studies, I analyzed their multi-wavelength (radio-to-γ-ray) light curves
and Very Long Baseline Interferometry (VLBI) datasets on both the time-domain and image-
plane to investigate the variations in emission, structure, and kinematics of the jets during a
number of γ-ray flaring periods.
The blazar BL Lacertae which is the prototypical BL Lac object (a subclass of blazar), was
explored using VLBI datasets obtained from the Korean VLBI Network (KVN). Properties
of the radio jet are presented with light curves of the radio core (i.e., the VLBI core seen
by the KVN) at 22, 43, 86, and 129 GHz. Our observations covered the decaying part of a
strong radio flare. The timescales (τ) of the exponential decays show the following relationship:
τ ∝ ν−0.2 , with ν being observing frequency. This is much shallower than the one expected
from opacity effects (i.e., the core shift). Simultanous multi-frequency observations of the KVN
allow us to perform spectral analysis of the radio emission. The spectral indices versus time
and radio frequency, support the models of recollimation shocks (Marscher et al. 2008) and the
generalized shock evolution (Valtaoja et al. 1992).
OT 081 is a blazar with a compact radio jet. In many VLBI images, the source shows
a simple point-like feature without any notable extended structures. It had been consistently
bright at radio wavelengths (e.g., a few Jy), but without noteworthy strong γ-ray outbursts. However, there was a historically strong γ-ray outburst in 2016 in this source. To investigate
this phenomenon, multi-waveband data were used: KVN and OVRO (radio), ASAS-SN (opti-
cal), Swift-XRT (X-ray), and Fermi-LAT (γ-ray). It was revealed that the 2016 γ-ray outburst
is highly correlated with emission at lower frequencies from radio to X-ray. By using VLBA
observations, we found that this γ-ray event was accompanied by the emergence of a moving
polarized knot from the radio core which propagates further downstream of the flow. Combin-
ing all the evidence, we conclude that the radio core is the origin of the γ-ray outburst.
Blazars can be divided into two subclasses: flat-spectrum radio quasars (FSRQ) and BL Lac
objects, based on the presence/absence of broad optical emission lines. Recent studies sup-
ported that γ-rays of FSRQs originate from a region beyond the broad-line region (BLR), sug-
gesting distances of a few parsecs from the central engine where the radio core is thought to
be located. Motivated by this, I investigated two recent γ-ray outbursts of the FSRQ 3C 273
which is one of the most powerful and famous blazars. Analysis were done with data obtained
from the ALMA, VLBA, and Fermi-LAT. In order to check the correlation between radio and
γ-ray emission, the discrete correlation function (DCF) was employed. Our results indicate
that the compact features (i.e., multiple standing shocks) are responsible for the observed γ-ray
outbursts in the jet of 3C 273.
0716+714 is known to have extreme variability over the entire electromagnetic spectrum.
Our preliminary findings of an unusual anti-correlation between radio and γ-ray emission in
this source, lead us to start a detailed study of the radio/γ-ray connection in the jet of 0716+714.
Archival multi-frequency data (i.e., SMA, Metsähovi, OVRO, Fermi-LAT, and VLBA) were
employed and the correlation analysis between the datasets was performed using the techniques
of modeling and simulating the light curves. As a result, we found three significant radio-to-
γ-ray correlations: two anti-correlations and one positive correlation. We also analyzed VLBA
datasets to investigate the parsec-scale jet activity during the γ-ray flares. With all the evidence,
we constrain the origin of the γ-ray flares in the jet and suggest internal-shock interactions
induced by the passage of a moving disturbance through the radio core as the mechanism
behind the observed correlated behaviors.
Physics of the relativistic jets in blazars is tricky and complicated due to the extreme physical conditions and various scenarios/possibilities. More detailed observations of the jets with
high-resolution VLBI arrays, are currently the best way to resolve the issues of the jet physics.
This thesis presents new observational data and results on the nature of blazar γ-ray flares and
contributes to the scientific community by supplying the wealth of information for the cases
of four remarkable blazars. The individual studies presented in this thesis, conclude as follow:
(1) blazar γ-ray flares have multiple emission regions in the jets (i.e., subpc/pc-scales distances
from the central black hole) and (2) the propagation of shocks/disturbances along the jet in the
subpc/pc-scale regions, causes γ-ray flares (particularly when they pass through the standing
shock features; e.g., the radio core).활동성은하핵 (AGN)들의 상대론적 제트는 우주에서 가장 강력하며 영속적인 에너지 소스들 중에 하나이다. 고에너지 천체물리분야 뿐만 아니라 은하와 성단의 진화에 중요한 역할을 함에 따라, AGN 제트들에 대한 연구는 가치있으며 전도유망하다. 전파에서 밝은 AGN (radio-loud AGN)들 중, 그들의 제트가 뻗어나가는 축과 우리의 시선방향 사이의 각도가 매우 작은 (약 5도 이내) AGN들은 블레이저 (Blazar)로써 분류된다. Blazar들의 대표적인 특징 중 하나는 그들의 제트로부터 나오는 강력한 감마선 방출이다. 하지만 고에너지 망원경들의 공간분해능이 충분하지 못하기 때문에, 그러한 고에너지 빛 방출에 대한 우리의 이해는 제한되어 있고, 이에 따라 Blazar 감마선 폭발의 기원은 현재 활발한 논쟁 중에 있다. 그러한 고에너지 빛의 방출기작과 기원을 탐구하기 위해서, 본 저자는 최근 강한 감마선 방출을 보인 몇 개의 개별 Blazar들을 연구하였다: BL Lacertae, OT 081 (1749+096), 3C 273, 그리고 0716+714. 이 연구들에서 본 저자는 감마선 폭발 기간 동안에 제트들의 빛, 구조, 그리고 운동학이 어떻게 변화하는지를 조사하였으며, 이를 위해 그들의 다파장 (전파-감마선) 변광곡선 및 최장기선 간섭계 (VLBI) 데이터들을 시간도메인과 이미지면 위에서 분석하였다.
Blazar의 한 유형인 BL Lac object에 속하며, 또한 해당 타입의 원형이기도 한 BL Lacertae가 한국 VLBI 관측기인 KVN를 이용해 연구되어졌다. 22, 43, 86, 그리고 129 GHz에서 얻어진 전파코어의 변광곡선을 이용하여 얻어진 제트의 특성들이 나타내어졌다. 우리의 관측데이터는 하나의 강력한 전파 폭발의 감쇄 부분을 포함한다. 지수함수적 감쇄의 시간규모 (tau) 들은 다음의 관계식을 따른다: tau 비례 nu^-0.2, nu는 관측주파수를 나타낸다. 이는 불투명도 효과 (Core shift)로부터 예상되는 결과에 비해 매우 얕다. KVN의 다주파수 동시관측은 전파 방출 빛에 대한 분광학적 분석을 가능하게 해준다. 시간과 전파 주파수에 대한 분광지수 (Spectral index)의 변화는 Recollimation shock 모델과 Generalized shock 모델들을 지지한다.
OT 081은 조밀하고 소형인 제트를 지닌 Blazar이다. 많은 VLBI 이미지들에서 해당 제트는 바깥쪽으로 연장된 눈에 띄는 구조들 없이 단순한 포인트와 같은 형태의 구조를 보여준다. 이 소스는 주목할 만한 감마선 폭발없이 전파에서 지속적으로 밝아왔었다. 그러나 2016년도에 이 타겟소스에 대해서 역사적으로 강력한 감마선 폭발이 발생하였었다. 이 현상을 조사하기 위해, 다파장 데이터들이 사용되었다: KVN과 OVRO (전파), ASAS-SN (광학), Swift-XRT (엑스선), 그리고 Fermi-LAT (감마선). 2016 감마선 폭발이 낮은 주파수대 (전파부터 엑스선까지)의 방출 빛들과 상당한 상관관계를 가지고 있음이 드러났다. VLBA 관측데이터를 이용함으로써, 우리는 또한 이 감마선 폭발이 전파코어로부터 나와 제트의 하류로 전파/이동해나가는 편광 컴포넌트 (Knot)의 출현을 수반했었음을 찾았다. 이러한 모든 증거들을 조합해봄으로써, 우리는 전파코어가 감마선 폭발의 기원이라 결론지었다.
Blazar들은 두 가지 유형으로 나뉘어 질 수 있다: 넓은 광학 방출선의 존재/부재에 따라, Flat-Spectrum Radio Quasar (FSRQ) 그리고 BL Lac object. 근래의 연구결과들은 FSRQ들로부터 나오는 감마선들이 Broad-Line Region (BLR) 너머의 지역에서 기원함을 시사했었다. 이는 전파코어가 위치해 있을 것으로 여겨지는, 중앙의 블랙홀로부터 몇 파섹 (parsec) 떨어진 거리를 암시한다. 이러한 아이디어에 착안하여, 본 저자는 가장 강하며 유명한 Blazar들 중 하나인 3C 273 (FSRQ)로부터 발생한 최근의 두 감마선 폭발들을 조사하였다. 분석은 ALMA, VLBA, 그리고 Fermi-LAT 데이터들을 이용하여 수행되어졌다. 전파와 감마선 변광곡선 사이의 상관관계를 알아보기 위해, Discrete Correlation Function (DCF)가 사용되었다. 우리의 결과들은 매우 조밀한 Multiple standing shock들이 3C 273의 제트 안에서 관측된 감마선 폭발들의 기원임을 나타낸다.
0716+714는 모든 전자기파 영역에서 극적인 변광성을 보이는 것으로 알려져 있다. 우리의 사전조사결과로 밝혀진 해당 소스에서의 전파와 감마선 방출 빛 사이의 역상관관계 (anti-correlation)는 우리가 0716+714 제트 안에서 전파/감마선 연관성에 대한 자세한 연구를 개시하게 되는 동기가 되었다. 기록 보관된 (archival) 다파장 데이터들 (SMA, Metsahovi, OVRO, Fermi-LAT, 그리고 VLBA)이 사용되었으며, 데이터들 간의 상관관계 분석은 변광곡선들의 모델링과 시뮬레이션을 이용해 수행되어졌다. 그 결과, 우리는 세 개의 중대한 전파/감마선 상관관계를 찾았다: 두 개의 역상관관계들과 하나의 양적 상관관계. 우리는 또한 감마선 폭발들이 발생하는 동안 파섹 규모에서의 제트가 어떠한 활동성을 보이는지를 알아보기 위해 VLBA 데이터를 분석하였다. 결과들로부터 얻어진 모든 증거를 토대로 우리는 제트 내에서 감마선 폭발들의 기원을 한정하고, 관측된 상관관계들의 배경기작으로써 이동하는 섭동이 전파코어를 지나면서 유발되는 Internal-shock interaction을 제안한다.
극적인 물리적 상태와 다양한 시나리오 및 가능성들 때문에, Blazar의 상대론적 제트들에 대한 물리는 복잡하며 까다롭다. 고분해능 VLBI 어레이들을 통한 제트의 자세한 관측은 현재 제트물리에서의 쟁점들을 해결할 가장 좋은 방법이다. 본 학위논문은 Blazar 감마선 폭발들의 특성에 대한 새로운 관측적 데이터와 결과들을 보여주며, 4개의 주목할 만한 Blazar들의 경우들에 대하여 풍부한 정보를 제공함으로써 학계에 기여한다. 본 학위논문에 수록된 개별 연구들은 다음과 같은 결론을 내린다: (1) Blazar의 감마선 폭발들은 제트 내에서 다중 방출 영역을 가진다 (중앙 블랙홀로부터 subpc/pc 규모의 거리) 그리고 (2) subpc/pc 규모의 거리에서 제트의 하류를 따라 전파해나가는 Shock/섭동들의 이동이 감마선 폭발을 유발한다 (특별히 그들이 Standing shock 구조들을 지나갈 때).I. Introduction 1
1.1 Radio jets in Active Galactic Nuclei (AGN) 1
1.1.1 Active Galactic Nuclei 1
1.1.2 Formation of AGN jets 3
1.1.3 Jet structures and evolution 6
1.1.4 Beaming effects 13
1.1.5 Importance in astrophysics 16
1.2 Multi-waveband observations of AGN 18
1.2.1 Very Long Baseline Interferometry 18
1.2.2 Fermi-LAT 25
1.3 High energy gamma-ray emission in blazars 28
1.3.1 Nonthermal emission 28
1.3.2 The radio/γ-ray connection 32
1.4 Thesis outline 37
II. The Millimeter-Radio Emission of BL Lacertae During Two γ-Ray Outbursts 39
2.1 Introduction 40
2.2 Observation and Data reduction 41
2.3 Results 45
2.3.1 Radio morphology of BL Lac seen by the KVN 46
2.3.2 Radio Light Curves 48
2.3.3 Spectral Indices and Spectrum of the Core 51
2.4 Discussion 53
2.4.1 Variability and Cooling Time Scales 55
2.4.2 Shock Evolution in The Core Region 59
2.4.3 The Radio-γ-Ray Connection 60
2.5 Summary 63
III. Exploring The Nature of The 2016 γ-Ray Emission in The Blazar 1749+096 65
3.1 Introduction 66
3.2 Observations and Data 67
3.2.1 KVN 22/43/86/129 GHz & VLBA 43 GHz 67
3.2.2 OVRO 15 GHz 68
3.2.3 ASAS-SN 69
3.2.4 Swift-XRT 69
3.2.5 Fermi-LAT 69
3.3 Results and Analysis 71
3.3.1 Multi-waveband light curves 71
3.3.2 Multi-wavelength flux correlations 72
3.3.3 LAT γ-ray photon indices 75
3.3.4 Linear polarization at 43 GHz 77
3.3.5 Flux evolution near the core 79
3.4 Discussion 82
3.4.1 γ-ray activity 82
3.4.2 Multi-wavelength correlations 83
3.4.3 Origin of the γ-ray outburst 83
3.4.4 The enhanced γ-ray emission in 2016 October 86
3.5 Summary 86
IV. Investigating The Connection between γ-Ray Activity and The Relativistic Jet in 3C 273 during 2015–2019 89
4.1 Introduction 90
4.2 Observations 91
4.2.1 Fermi-LAT 91
4.2.2 ALMA band3 92
4.2.3 VLBA 43 GHz 92
4.3 Results 94
4.3.1 Light curves 94
4.3.2 Photon indices from weekly and monthly γ-ray light curves 97
4.3.3 Correlation between the radio and γ-ray light curves 99
4.3.4 Parsec-scale jet near the 43 GHz core 102
4.3.5 Polarization 106
4.4 Discussion 108
4.4.1 Positional variations of the stationary components 108
4.4.2 2016 γ-ray outburst 109
4.4.3 2017 γ-ray outburst 112
4.4.4 γ-ray spectra 114
4.5 Summary 115
V. Radio and γ-Ray Activity in The Jet of The Blazar S5 0716+714 117
5.1 Introduction 118
5.2 Observations 120
5.2.1 cm-wavelength data 120
5.2.2 SMA 230 GHz (1.3 mm) 120
5.2.3 γ-ray flux 120
5.3 Results 121
5.3.1 Radio and γ-ray light curves 121
5.3.2 Correlation analysis 121
5.3.2.1 Long-term correlation with the 37 GHz data 121
5.3.2.2 Optimization of the probable time ranges 125
5.3.2.3 DCF curves over the T1, T2, and T3 periods 136
5.3.3 Jet kinematics 140
5.4 Discussion 149
5.4.1 Internal shock interactions 149
5.4.2 Frequency dependence in the time lags 151
5.4.3 Timing of the knot ejections in T1 and T2 153
5.4.4 Location of the γ-ray production site 154
5.4.5 Evolution of the parsec scale jet 155
5.5 Summary 157
VI. Conclusion 161
Bibliography 167
Appendix 183
A Appendices for Chapter4 183
A.1 Gaussian model-fit parameters 183
요약 189
감사의 글 193박
Dispelling the Myths Behind First-author Citation Counts
Full text linkWe conducted a full-scale evaluative citation analysis study of scholars in the XML research field to explore just how different from each other author rankings resulting from different citation counting methods actually are, and to demonstrate the capability of emerging data and tools on the Web in supporting more realistic citation counting methods. Our results contest some common arguments for the continued
use of first-author citation counts in the evaluation of scholars, such as high correlations between author rankings by first-author citation counts and other citation
counting methods, and high costs of using more realistic citation counting methods that are not well-supported by the ISI databases. It is argued that increasingly available digital full text research papers make it possible for citation analysis studies to go beyond what the ISI databases have directly supported and to employ more
sophisticated methods
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