Browsing by Author "Savolainen, Tuomas"
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- 180° rotations in the polarization angle for blazars
A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä(2020-04-21) Cohen, M.H.; Savolainen, TuomasRotations of the electric vector position angle (EVPA) in blazars are often close to an integral multiple of 180°. There are many examples of this in the literature, and we strengthen the evidence by showing that, in the RoboPol monitoring program, nπ rotations occur more frequently than otherwise expected by chance. We explain this using a model consisting of two polarized emission components: a “jet” that is constant in time and a “burst” that is variable. The EVPA of the combination is EVPAjet at both the beginning and the end of the burst, so the net rotation across the burst must be nπ. Several examples of this model are analyzed on the Stokes plane, where the winding number for the Stokes vector of the combination gives the value of n. The main conclusion is that the EVPA rotation can be much larger than the physical rotation of the emission region around the axis of the jet, but this requires the EVPAs of the jet and the burst to be nearly orthogonal. Shock-in-jet calculations can provide a physical model for our toy model and in addition they automatically give the required orthogonality. The model is illustrated with data from the literature on OJ 287. We suggest that the large rapid EVPA rotation seen in OJ 287 might be a phase effect and not representative of a physical rotation. - Active galactic nuclei imaging programs of the RadioAstron mission
A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä(2020-01-15) Bruni, Gabriele; Savolainen, Tuomas; Gómez, Jose Luis; Lobanov, Andrei P.; Kovalev, Yuri Y.Imaging relativistic jets in active galactic nuclei (AGN) at angular resolution significantly surpassing that of the ground-based VLBI at centimetre wavelengths is one of the key science objectives of the RadioAstron space-VLBI mission. There are three RadioAstron imaging key science programs that target both nearby radio galaxies and blazars, with one of the programs specifically focusing on polarimetry of the jets. The first images from these programs reach angular resolution of a few tens of microarcseconds and reveal unprecedented details about the jet collimation profile, magnetic field configuration, and Kelvin-Helmholtz instabilities along the flow in some of the most studied AGN (3C84, BL Lac, 3C273, S50836+710). Here we give an overview of the goals and strategy of these three ongoing programs, highlight their early results, and discuss the challenges of space-VLBI imaging. - Automatic segmentation and classification of seven-segment display digits on auroral images
A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä(2016-07-21) Savolainen, Tuomas; Keith Whiter, Daniel; Partamies, NooraIn this paper we describe a new and fully automatic method for segmenting and classifying digits in seven-segment displays. The method is applied to a dataset consisting of about 7 million auroral all-sky images taken during the time period of 1973-1997 at camera stations centred around Sodankylä observatory in northern Finland. In each image there is a clock display for the date and time together with the reflection of the whole night sky through a spherical mirror. The digitised film images of the night sky contain valuable scientific information but are impractical to use without an automatic method for extracting the date-time from the display. We describe the implementation and the results of such a method in detail in this paper. - Baseline Vector Repeatability at the Sub-Millimeter Level Enabled by Radio Interferometer Phase Delays of Intra-Site Baselines
A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä(2023-03) Xu, Ming H.; Savolainen, Tuomas; Bolotin, Sergei; Bernhart, Simone; Plötz, Christian; Haas, Rüdiger; Varenius, Eskil; Wang, Guangli; McCallum, Jamie; Heinkelmann, Robert; Lunz, Susanne; Schuh, Harald; Zubko, Nataliya; Kareinen, NikoWe report the results of position ties for short baselines at eight geodetic sites based on phase delays that are extracted from global geodetic very-long-baseline interferometry (VLBI) observations rather than dedicated short-baseline experiments. An analysis of phase delay observables at X band from two antennas at the Geodetic Observatory Wettzell, Germany, extracted from 107 global 24-hr VLBI sessions since 2019 yields weighted root-mean-square scatters about the mean baseline vector of 0.3, 0.3, and 0.8 mm in the east, north, and up directions, respectively. Position ties are also obtained for other short baselines between legacy antennas and nearby, newly built antennas. They are critical for maintaining a consistent continuation of the realization of the terrestrial reference frame, especially when including the new VGOS network. The phase delays of the baseline WETTZ13N–WETTZELL enable an investigation of sources of error at the sub-millimeter level. We found that a systematic variation of larger than 1 mm can be introduced to the Up estimates of this baseline vector when atmospheric delays were estimated. Although the sub-millimeter repeatability has been achieved for the baseline vector WETTZ13N–WETTZELL, we conclude that long term monitoring should be conducted for more short baselines to assess the instrumental effects, in particular the systematic differences between phase delays and group delays, and to find common solutions for reducing them. This will be an important step toward the goal of global geodesy at the 1 mm level. - Characterizing and Mitigating Intraday Variability: Reconstructing Source Structure in Accreting Black Holes with mm-VLBI
A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä(2022-05-01) Broderick, Avery E.; Gold, Roman; Georgiev, Boris; Pesce, Dominic W.; Tiede, Paul; Ni, Chunchong; Moriyama, Kotaro; Akiyama, Kazunori; Alberdi, Antxon; Alef, Walter; Algaba, Juan Carlos; Anantua, Richard; Asada, Keiichi; Azulay, Rebecca; Bach, Uwe; Baczko, Anne Kathrin; Ball, David; Baloković, Mislav; Barrett, John; Bauböck, Michi; Benson, Bradford A.; Bintley, Dan; Blackburn, Lindy; Blundell, Raymond; Bouman, Katherine L.; Bower, Geoffrey C.; Boyce, Hope; Bremer, Michael; Brinkerink, Christiaan D.; Brissenden, Roger; Britzen, Silke; Broguiere, Dominique; Bronzwaer, Thomas; Bustamante, Sandra; Byun, Do Young; Carlstrom, John E.; Ceccobello, Chiara; Chael, Andrew; Chan, Chi Kwan; Chatterjee, Koushik; Chatterjee, Shami; Chen, Ming Tang; Chen, Yongjun; Cheng, Xiaopeng; Cho, Ilje; Christian, Pierre; Kim, Dong Jin; Ramakrishnan, Venkatessh; Savolainen, Tuomas; Wagner, Jan; , Event Horizon Telescope CollaboratThe extraordinary physical resolution afforded by the Event Horizon Telescope has opened a window onto the astrophysical phenomena unfolding on horizon scales in two known black holes, M87* and Sgr A*. However, with this leap in resolution has come a new set of practical complications. Sgr A* exhibits intraday variability that violates the assumptions underlying Earth aperture synthesis, limiting traditional image reconstruction methods to short timescales and data sets with very sparse (u, v) coverage. We present a new set of tools to detect and mitigate this variability. We develop a data-driven, model-agnostic procedure to detect and characterize the spatial structure of intraday variability. This method is calibrated against a large set of mock data sets, producing an empirical estimator of the spatial power spectrum of the brightness fluctuations. We present a novel Bayesian noise modeling algorithm that simultaneously reconstructs an average image and statistical measure of the fluctuations about it using a parameterized form for the excess variance in the complex visibilities not otherwise explained by the statistical errors. These methods are validated using a variety of simulated data, including general relativistic magnetohydrodynamic simulations appropriate for Sgr A* and M87*. We find that the reconstructed source structure and variability are robust to changes in the underlying image model. We apply these methods to the 2017 EHT observations of M87*, finding evidence for variability across the EHT observing campaign. The variability mitigation strategies presented are widely applicable to very long baseline interferometry observations of variable sources generally, for which they provide a data-informed averaging procedure and natural characterization of inter-epoch image consistency. - Collimation of the Relativistic Jet in the Quasar 3C 273
A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä(2022-11-01) Okino, Hiroki; Akiyama, Kazunori; Asada, Keiichi; Gómez, José L.; Hada, Kazuhiro; Honma, Mareki; Krichbaum, Thomas P.; Kino, Motoki; Nagai, Hiroshi; Bach, Uwe; Blackburn, Lindy; Bouman, Katherine L.; Chael, Andrew; Crew, Geoffrey B.; Doeleman, Sheperd S.; Fish, Vincent L.; Goddi, Ciriaco; Issaoun, Sara; Johnson, Michael D.; Jorstad, Svetlana; Koyama, Shoko; Lonsdale, Colin J.; Lu, Ru Sen; Martí-Vidal, Ivan; Matthews, Lynn D.; Mizuno, Yosuke; Moriyama, Kotaro; Nakamura, Masanori; Pu, Hung Yi; Ros, Eduardo; Savolainen, Tuomas; Tazaki, Fumie; Wagner, Jan; Wielgus, Maciek; Zensus, AntonThe collimation of relativistic jets launched from the vicinity of supermassive black holes (SMBHs) at the centers of active galactic nuclei (AGNs) is one of the key questions to understand the nature of AGN jets. However, little is known about the detailed jet structure for AGN like quasars since very high angular resolutions are required to resolve these objects. We present very long baseline interferometry (VLBI) observations of the archetypical quasar 3C 273 at 86 GHz, performed with the Global Millimeter VLBI Array, for the first time including the Atacama Large Millimeter/submillimeter Array. Our observations achieve a high angular resolution down to ∼60 μas, resolving the innermost part of the jet ever on scales of ∼105 Schwarzschild radii. Our observations, including close-in-time High Sensitivity Array observations of 3C 273 at 15, 22, and 43 GHz, suggest that the inner jet collimates parabolically, while the outer jet expands conically, similar to jets from other nearby low-luminosity AGNs. We discovered the jet collimation break around 107 Schwarzschild radii, providing the first compelling evidence for structural transition in a quasar jet. The location of the collimation break for 3C 273 is farther downstream from the sphere of gravitational influence (SGI) from the central SMBH. With the results for other AGN jets, our results show that the end of the collimation zone in AGN jets is governed not only by the SGI of the SMBH but also by the more diverse properties of the central nuclei. - Comparison of Polarized Radiative Transfer Codes Used by the EHT Collaboration
A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä(2023-06-01) Ramakrishnan, Venkatessh; Savolainen, Tuomas; , Event Horizon Telescope CollaboratInterpretation of resolved polarized images of black holes by the Event Horizon Telescope (EHT) requires predictions of the polarized emission observable by an Earth-based instrument for a particular model of the black hole accretion system. Such predictions are generated by general relativistic radiative transfer (GRRT) codes, which integrate the equations of polarized radiative transfer in curved spacetime. A selection of ray-tracing GRRT codes used within the EHT Collaboration is evaluated for accuracy and consistency in producing a selection of test images, demonstrating that the various methods and implementations of radiative transfer calculations are highly consistent. When imaging an analytic accretion model, we find that all codes produce images similar within a pixel-wise normalized mean squared error (NMSE) of 0.012 in the worst case. When imaging a snapshot from a cell-based magnetohydrodynamic simulation, we find all test images to be similar within NMSEs of 0.02, 0.04, 0.04, and 0.12 in Stokes I, Q, U, and V, respectively. We additionally find the values of several image metrics relevant to published EHT results to be in agreement to much better precision than measurement uncertainties. - The Connection between the Radio Jet and the Gamma-ray Emission in the Radio Galaxy 3C 120
A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä(2015) Casadio, Carolina; Gómez, José L.; Grandi, Paola; Jorstad, Svetlana G.; Marscher, Alan P.; Lister, Matthew L.; Kovalev, Yuri Y.; Savolainen, Tuomas; Pushkarev, Alexander B. - The Connection between the Radio Jet and the γ-ray Emission in the Radio Galaxy 3C 120 and the Blazar CTA 102
A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä(2016-09-27) Casadio, Carolina; Gómez, José; Jorstad, Svetlana G.; Marscher, Alan P.; Grandi, Paola; Larionov, Valeri M.; Lister, Matthew; Smith, Paul S.; Gurwell, Mark A.; Lähteenmäki, Anne; Agudo, Iván; Molina, Sol N.; Bala, Vishal; Joshi, Manasvita; Taylor, Brian; Williamson, Karen E.; Kovalev, Yuri; Savolainen, Tuomas; Pushkarev, Alexander B.; Arkharov, Arkady A.; Blinov, Dmitry A.; Borman, George A.; Di Paola, Andrea; Grishina, Tatiana S.; Hagen-Thorn, Vladimir A.; Itoh, Ryosuke; Kopatskaya, Evgenia N.; Larionova, Elena G.; Larionova, Liudmila V.; Morozova, Daria; Rastorgueva-Foi, Elizaveta; Sergeev, Sergey G.; Tornikoski, Merja; Troitsky, Ivan; Thum, Clemens; Wiesemeyer, HelmutWe present multi-wavelength studies of the radio galaxy 3C 120 and the blazar CTA 102 during unprecedented γ-ray flares for both sources. In both studies the analysis of γ-ray data has been compared with a series of 43 GHz VLBA images from the VLBA-BU-BLAZAR program, providing the necessary spatial resolution to probe the parsec scale jet evolution during the high energy events. To extend the radio dataset for 3C 120 we also used 15 GHz VLBA data from the MOJAVE sample. These two objects which represent very different classes of AGN, have similar properties during the γ-ray events. The γ-ray flares are associated with the passage of a new superluminal component through the mm VLBI core, but not all ejections of new components lead to γ-ray events. In both sources γ-ray events occurred only when the new components are moving in a direction closer to our line of sight. We locate the γ-ray dissipation zone a short distance from the radio core but outside of the broad line region, suggesting synchrotron self-Compton scattering as the probable mechanism for the γ-ray production. - Constraints on black-hole charges with the 2017 EHT observations of M87∗
A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä(2021-05-20) Kocherlakota, Prashant; Rezzolla, Luciano; Falcke, Heino; Fromm, Christian M.; Kramer, Michael; Mizuno, Yosuke; Nathanail, Antonios; Olivares, Héctor; Younsi, Ziri; Akiyama, Kazunori; Alberdi, Antxon; Alef, Walter; Algaba, Juan Carlos; Anantua, Richard; Asada, Keiichi; Azulay, Rebecca; Baczko, Anne Kathrin; Ball, David; Baloković, Mislav; Barrett, John; Benson, Bradford A.; Bintley, Dan; Blackburn, Lindy; Blundell, Raymond; Boland, Wilfred; Bouman, Katherine L.; Bower, Geoffrey C.; Boyce, Hope; Bremer, Michael; Brinkerink, Christiaan D.; Brissenden, Roger; Britzen, Silke; Broderick, Avery E.; Broguiere, Dominique; Bronzwaer, Thomas; Byun, Do Young; Carlstrom, John E.; Chael, Andrew; Chan, Chi Kwan; Chatterjee, Shami; Chatterjee, Koushik; Chen, Ming Tang; Chen, Yongjun; Chesler, Paul M.; Cho, Ilje; Christian, Pierre; Kim, Dong Jin; Ramakrishnan, Venkatessh; Savolainen, Tuomas; Wagner, Jan; , Event Horizon Telescope CollaboratOur understanding of strong gravity near supermassive compact objects has recently improved thanks to the measurements made by the Event Horizon Telescope (EHT). We use here the M87∗ shadow size to infer constraints on the physical charges of a large variety of nonrotating or rotating black holes. For example, we show that the quality of the measurements is already sufficient to rule out that M87∗ is a highly charged dilaton black hole. Similarly, when considering black holes with two physical and independent charges, we are able to exclude considerable regions of the space of parameters for the doubly-charged dilaton and the Sen black holes. - Constraints on Particles and Fields from Full Stokes Observations of AGN
A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä(2018-01-29) Homan, Daniel C.; Hovatta, Talvikki; Kovalev, Yuri Y.; Lister, Matthew L.; Pushkarev, Alexander B.; Savolainen, TuomasCombined polarization imaging of radio jets from Active Galactic Nuclei (AGN) in circular and linear polarization, also known as full Stokes imaging, has the potential to constrain both the magnetic field structure and particle properties of jets. Although only a small fraction of the emission when detected, typically less than a few tenths of a percent but up to as much as a couple of percent in the strongest resolved sources, circular polarization directly probes the magnetic field and particles within the jet itself and is not expected to be modified by external screens. A key to using full Stokes observations to constrain jet properties is obtaining a better understanding of the emission of circular polarization, including its variability and spectrum. We discuss what we have learned so far from parsec scale monitoring observations in the MOJAVE program and from multi-frequency observations of selected AGN. - A Decade of Multiwavelength Observations of the TeV Blazar 1ES 1215+303: Extreme Shift of the Synchrotron Peak Frequency and Long-term Optical–Gamma-Ray Flux Increase
A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä(2020-03-10) Ramazani, Vandad Fallah; Hovatta, Talvikki; Savolainen, Tuomas; Lähteenmäki, Anne; Tornikoski, Merja; , Fermi Large Area Telescope Collaboration; , VERITAS CollaborationBlazars are known for their variability on a wide range of timescales at all wavelengths. Most studies of TeV gamma-ray blazars focus on short timescales, especially during flares. With a decade of observations from the Fermi-LAT and VERITAS, we present an extensive study of the long-term multiwavelength radio-to-gamma-ray flux-density variability, with the addition of a couple of short-time radio-structure and optical polarization observations of the blazar 1ES 1215+303 (z = 0.130), with a focus on its gamma-ray emission from 100 MeV to 30 TeV. Multiple strong GeV gamma-ray flares, a long-term increase in the gamma-ray and optical flux baseline, and a linear correlation between these two bands are observed over the ten-year period. Typical HBL behaviors are identified in the radio morphology and broadband spectrum of the source. Three stationary features in the innermost jet are resolved by Very Long Baseline Array at 43.1, 22.2, and 15.3 GHz. We employ a two-component synchrotron self-Compton model to describe different flux states of the source, including the epoch during which an extreme shift in energy of the synchrotron peak frequency from infrared to soft X-rays is observed. - Eniac, Univac, IBM, CDC ja CRAY virtauslaskijan apuvälineinä
Insinööritieteiden korkeakoulu | Bachelor's thesis(2015-04-10) Savolainen, Tuomas - The Event Horizon General Relativistic Magnetohydrodynamic Code Comparison Project
A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä(2019-08) Savolainen, Tuomas; , Event Horizon Telescope CollaboratRecent developments in compact object astrophysics, especially the discovery of merging neutron stars by LIGO, the imaging of the black hole in M87 by the Event Horizon Telescope, and high- precision astrometry of the Galactic Center at close to the event horizon scale by the GRAVITY experiment motivate the development of numerical source models that solve the equations of general relativistic magnetohydrodynamics (GRMHD). Here we compare GRMHD solutions for the evolution of a magnetized accretion flow where turbulence is promoted by the magnetorotational instability from a set of nine GRMHD codes: Athena++, BHAC, Cosmos++, ECHO, H-AMR, iharm3D, HARM-Noble, IllinoisGRMHD, and KORAL. Agreement among the codes improves as resolution increases, as measured by a consistently applied, specially developed set of code performance metrics. We conclude that the community of GRMHD codes is mature, capable, and consistent on these test problems. - Event Horizon Telescope imaging of the archetypal blazar 3C 279 at an extreme 20 microarcsecond resolution
A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä(2020-08-01) Kim, Jae Young; Krichbaum, Thomas P.; Broderick, Avery E.; Wielgus, Maciek; Blackburn, Lindy; Gómez, José L.; Johnson, Michael D.; Bouman, Katherine L.; Chael, Andrew; Akiyama, Kazunori; Jorstad, Svetlana; Marscher, Alan P.; Issaoun, Sara; Janssen, Michael; Chan, Chi Kwan; Savolainen, Tuomas; Pesce, Dominic W.; Özel, Feryal; Alberdi, Antxon; Alef, Walter; Asada, Keiichi; Azulay, Rebecca; Baczko, Anne Kathrin; Ball, David; Baloković, Mislav; Barrett, John; Bintley, Dan; Boland, Wilfred; Bower, Geoffrey C.; Bremer, Michael; Brinkerink, Christiaan D.; Brissenden, Roger; Britzen, Silke; Broguiere, Dominique; Bronzwaer, Thomas; Byun, Do Young; Carlstrom, John E.; Chatterjee, Shami; Chatterjee, Koushik; Chen, Ming Tang; Chen, Yongjun; Cho, Ilje; Christian, Pierre; Conway, John E.; Cordes, James M.; Crew, Geoffrey B.; Cui, Yuzhu; Davelaar, Jordy; De Laurentis, Mariafelicia; Deane, Roger; Dempsey, Jessica; Desvignes, Gregory; Dexter, Jason; Doeleman, Sheperd S.; Eatough, Ralph P.; Falcke, Heino; Fish, Vincent L.; Fomalont, Ed; Fraga-Encinas, Raquel; Friberg, Per; Fromm, Christian M.; Galison, Peter; Gammie, Charles F.; García, Roberto; Gentaz, Olivier; Georgiev, Boris; Goddi, Ciriaco; Gold, Roman; Gómez-Ruiz, Arturo I.; Gu, Minfeng; Gurwell, Mark; Hada, Kazuhiro; Hecht, Michael H.; Hesper, Ronald; Ho, Luis C.; Ho, Paul; Honma, Mareki; Huang, Chih Wei L.; Lei, Huang; Hughes, David H.; Ikeda, Shiro; Inoue, Makoto; James, David J.; Jannuzi, Buell T.; Jeter, Britton; Wu, Jiang; Jimenez-Rosales, Alejandra; Jung, Taehyun; Karami, Mansour; Karuppusamy, Ramesh; Kawashima, Tomohisa; Keating, Garrett K.; Kettenis, Mark; Kim, Junhan; Kim, Jongsoo; Kino, Motoki; Koay, Jun Yi; Koch, Patrick M.; Koyama, Shoko; Kramer, Michael; Kramer, Carsten; Kuo, Cheng Yu; Lauer, Tod R.; Lee, Sang Sung; Li, Yan Rong; Li, Zhiyuan; Lindqvist, Michael; Lico, Rocco; Kuo, Liu; Liuzzo, Elisabetta; Lo, Wen Ping; Lobanov, Andrei P.; Loinard, Laurent; Lonsdale, Colin; Lu, Ru Sen; Macdonald, Nicholas R.; Mao, Jirong; Markoff, Sera; Marrone, Daniel P.; Martí-Vidal, Iván; Matsushita, Satoki; Matthews, Lynn D.; Medeiros, Lia; Menten, Karl M.; Mizuno, Yosuke; Mizuno, Izumi; Moran, James M.; Moriyama, Kotaro; Moscibrodzka, Monika; Musoke, Gibwa; Müller, Cornelia; Nagai, Hiroshi; Nagar, Neil M.; Nakamura, Masanori; Narayan, Ramesh; Narayanan, Gopal; Natarajan, Iniyan; Neri, Roberto; Ni, Chunchong; Noutsos, Aristeidis; Okino, Hiroki; Olivares, Héctor; Ortiz-León, Gisela N.; Oyama, Tomoaki; Palumbo, Daniel C.M.; Park, Jongho; Patel, Nimesh; Pen, Ue Li; Piétu, Vincent; Plambeck, Richard; Popstefanija, Aleksandar; Porth, Oliver; Ben, Prather; Preciado-López, Jorge A.; Psaltis, Dimitrios; Pu, Hung Yi; Ramakrishnan, Venkatessh; Rao, Ramprasad; Rawlings, Mark G.; Raymond, Alexander W.; Rezzolla, Luciano; Ripperda, Bart; Roelofs, Freek; Rogers, Alan; Ros, Eduardo; Rose, Mel; Roshanineshat, Arash; Rottmann, Helge; Roy, Alan L.; Ruszczyk, Chet; Ryan, Benjamin R.; Rygl, Kazi L.J.; Sánchez, Salvador; Sánchez-Arguelles, David; Sasada, Mahito; Schloerb, F. Peter; Schuster, Karl Friedrich; Shao, Lijing; Shen, Zhiqiang; Small, Des; Sohn, Bong Won; Soohoo, Jason; Tazaki, Fumie; Tiede, Paul; Tilanus, Remo P.J.; Titus, Michael; Toma, Kenji; Torne, Pablo; Trent, Tyler; Traianou, Efthalia; Trippe, Sascha; Tsuda, Shuichiro; Van Bemmel, Ilse; Van Langevelde, Huib Jan; Van Rossum, Daniel R.; Jan, Wagner; Wardle, John; Ward-Thompson, Derek; Weintroub, Jonathan; Wex, Norbert; Wharton, Robert; Wong, George N.; Wu, Qingwen; Yoon, Doosoo; Young, André; Young, Ken; Younsi, Ziri; Feng, Yuan; Yuan, Ye Fei; Zensus, J. Anton; Zhao, Guangyao; Zhao, Shan Shan; Zhu, Ziyan; Algaba, Juan Carlos; Allardi, Alexander; Amestica, Rodrigo; Anczarski, Jadyn; Bach, Uwe; Baganoff, Frederick K.; Beaudoin, Christopher; Benson, Bradford A.; Berthold, Ryan; Blanchard, Jay M.; Blundell, Ray; Bustamente, Sandra; Cappallo, Roger; Castillo-Domínguez, Edgar; Chang, Chih Cheng; Chang, Shu Hao; Chang, Song Chu; Chen, Chung Chen; Chilson, Ryan; Chuter, Tim C.; Rosado, Rodrigo Córdova; Coulson, Iain M.; Crowley, Joseph; Derome, Mark; Dexter, Matthew; Dornbusch, Sven; Dudevoir, Kevin A.; Dzib, Sergio A.; Eckart, Andreas; Eckert, Chris; Erickson, Neal R.; Everett, Wendeline B.; Faber, Aaron; Farah, Joseph R.; Fath, Vernon; Folkers, Thomas W.; Forbes, David C.; Freund, Robert; Gale, David M.; Feng, Gao; Geertsema, Gertie; Graham, David A.; Greer, Christopher H.; Grosslein, Ronald; Gueth, Frédéric; Haggard, Daryl; Halverson, Nils W.; Han, Chih Chiang; Han, Kuo Chang; Hao, Jinchi; Hasegawa, Yutaka; Henning, Jason W.; Hernández-Gómez, Antonio; Herrero-Illana, Rubén; Heyminck, Stefan; Hirota, Akihiko; Hoge, James; Huang, Yau De; Violette Impellizzeri, C. M.; Jiang, Homin; John, David; Kamble, Atish; Keisler, Ryan; Kimura, Kimihiro; Kono, Yusuke; Kubo, Derek; Kuroda, John; Lacasse, Richard; Laing, Robert A.; Leitch, Erik M.; Li, Chao Te; Lin, Lupin C.C.; Liu, Ching Tang; Liu, Kuan Yu; Lu, Li Ming; Marson, Ralph G.; Martin-Cocher, Pierre L.; Massingill, Kyle D.; Matulonis, Callie; Mccoll, Martin P.; Mcwhirter, Stephen R.; Messias, Hugo; Zheng, Meyer Zhao; Michalik, Daniel; Montaña, Alfredo; Montgomerie, William; Mora-Klein, Matias; Muders, Dirk; Nadolski, Andrew; Navarro, Santiago; Neilsen, Joseph; Nguyen, Chi H.; Nishioka, Hiroaki; Norton, Timothy; Nowak, Michael A.; Nystrom, George; Ogawa, Hideo; Oshiro, Peter; Parsons, Harriet; Juan, Peñalver; Phillips, Neil M.; Poirier, Michael; Pradel, Nicolas; Primiani, Rurik A.; Raffin, Philippe A.; Rahlin, Alexandra S.; Reiland, George; Risacher, Christopher; Ruiz, Ignacio; Sáez-Madaín, Alejandro F.; Sassella, Remi; Schellart, Pim; Shaw, Paul; Silva, Kevin M.; Shiokawa, Hotaka; Smith, David R.; Snow, William; Souccar, Kamal; Sousa, Don; Sridharan, Tirupati K.; Srinivasan, Ranjani; Stahm, William; Stark, Antony A.; Story, Kyle; Timmer, Sjoerd T.; Vertatschitsch, Laura; Walther, Craig; Wei, Ta Shun; Whitehorn, Nathan; Whitney, Alan R.; Woody, David P.; Wouterloot, Jan G.A.; Wright, Melvin; Yamaguchi, Paul; Yu, Chen Yu; Zeballos, Milagros; Zhang, Shuo; Ziurys, Lucy3C 279 is an archetypal blazar with a prominent radio jet that show broadband flux density variability across the entire electromagnetic spectrum. We use an ultra-high angular resolution technique - global Very Long Baseline Interferometry (VLBI) at 1.3mm (230 GHz) - to resolve the innermost jet of 3C 279 in order to study its fine-scale morphology close to the jet base where highly variable-ray emission is thought to originate, according to various models. The source was observed during four days in April 2017 with the Event Horizon Telescope at 230 GHz, including the phased Atacama Large Millimeter/submillimeter Array, at an angular resolution of ∼20 μas (at a redshift of z = 0:536 this corresponds to ∼0:13 pc ∼ 1700 Schwarzschild radii with a black hole mass MBH = 8 × 108 M⊙). Imaging and model-fitting techniques were applied to the data to parameterize the fine-scale source structure and its variation.We find a multicomponent inner jet morphology with the northernmost component elongated perpendicular to the direction of the jet, as imaged at longer wavelengths. The elongated nuclear structure is consistent on all four observing days and across diffierent imaging methods and model-fitting techniques, and therefore appears robust. Owing to its compactness and brightness, we associate the northern nuclear structure as the VLBI "core". This morphology can be interpreted as either a broad resolved jet base or a spatially bent jet.We also find significant day-to-day variations in the closure phases, which appear most pronounced on the triangles with the longest baselines. Our analysis shows that this variation is related to a systematic change of the source structure. Two inner jet components move non-radially at apparent speeds of ∼15 c and ∼20 c (∼1:3 and ∼1:7 μas day-1, respectively), which more strongly supports the scenario of traveling shocks or instabilities in a bent, possibly rotating jet. The observed apparent speeds are also coincident with the 3C 279 large-scale jet kinematics observed at longer (cm) wavelengths, suggesting no significant jet acceleration between the 1.3mm core and the outer jet. The intrinsic brightness temperature of the jet components are ≤1010 K, a magnitude or more lower than typical values seen at ≥7mm wavelengths. The low brightness temperature and morphological complexity suggest that the core region of 3C 279 becomes optically thin at short (mm) wavelengths. - Event Horizon Telescope observations of the jet launching and collimation in Centaurus A
A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä(2021-10) Savolainen, Tuomas; , Event Horizon Telescope CollaboratVery-long-baseline interferometry (VLBI) observations of active galactic nuclei at millimetre wavelengths have the power to reveal the launching and initial collimation region of extragalactic radio jets, down to 10-100 gravitational radii (r(g) equivalent to GM/c(2)) scales in nearby sources(1). Centaurus A is the closest radio-loud source to Earth(2). It bridges the gap in mass and accretion rate between the supermassive black holes (SMBHs) in Messier 87 and our Galactic Centre. A large southern declination of -43 degrees has, however, prevented VLBI imaging of Centaurus A below a wavelength of 1 cm thus far. Here we show the millimetre VLBI image of the source, which we obtained with the Event Horizon Telescope at 228 GHz. Compared with previous observations(3), we image the jet of Centaurus A at a tenfold higher frequency and sixteen times sharper resolution and thereby probe sub-lightday structures. We reveal a highly collimated, asymmetrically edge-brightened jet as well as the fainter counterjet. We find that the source structure of Centaurus A resembles the jet in Messier 87 on similar to 500 r(g) scales remarkably well. Furthermore, we identify the location of Centaurus A's SMBH with respect to its resolved jet core at a wavelength of 1.3 mm and conclude that the source's event horizon shadow(4) should be visible at terahertz frequencies. This location further supports the universal scale invariance of black holes over a wide range of masses(5,6). - Evidence of the Gaia-VLBI position differences being related to radio source structure
A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä(2021-03-01) Xu, Minghui; Lunz, Susanne; Anderson, James M.; Savolainen, Tuomas; Zubko, Nataliya; Schuh, HaraldWe report the relationship between the Gaia--VLBI position differences and the magnitudes of source structure effects in VLBI observations. Because the Gaia--VLBI position differences are statistically significant for a considerable number of common sources, we attempt to discuss and explain these position differences based on VLBI observations and available source images at cm-wavelengths. Based on the derived closure amplitude root-mean-square (CARMS), which quantifies the magnitudes of source structure effects in the VLBI observations used for building the third realization of the International Celestial Reference Frame, the arc lengths and normalized arc lengths of the position differences are examined in detail. The radio jet directions and the directions of the Gaia--VLBI position differences are investigated for a small sample of sources. Both the arc lengths and normalized arc lengths of the Gaia and VLBI positions are found to increase with the CARMS values. The majority of the sources with statistically significant position differences are associated with the sources having extended structure. Radio source structure is the one of the major factors of these position differences, and it can be the dominate factor for a number of sources. The vectors of the Gaia and VLBI position differences are parallel to the radio-jet directions, which is confirmed with stronger evidence. - Fast scale and illumination invariant method for region labeling
Perustieteiden korkeakoulu | Master's thesis(2019-01-28) Savolainen, TuomasThis work describes how to find 3D objects in 2D images. The images may contain various illumination conditions and backgrounds. Furthermore the distance and the rotation of the camera with respect to the object can be arbitrary. The method described in this work provides a way to reduce computation time of the 3D object localization problem by searching only from the regions of the image that include a combination of the most common colors of the object. The accuracy and speed of the implementation is tested on images taken under various illuminations and backgrounds. - First M87 Event Horizon Telescope Results. I. the Shadow of the Supermassive Black Hole
A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä(2019-04-10) Savolainen, Tuomas; , Event Horizon Telescope CollaboratWhen surrounded by a transparent emission region, black holes are expected to reveal a dark shadow caused by gravitational light bending and photon capture at the event horizon. To image and study this phenomenon, we have assembled the Event Horizon Telescope, a global very long baseline interferometry array observing at a wavelength of 1.3 mm. This allows us to reconstruct event-horizon-scale images of the supermassive black hole candidate in the center of the giant elliptical galaxy M87. We have resolved the central compact radio source as an asymmetric bright emission ring with a diameter of 42 ±3 μas, which is circular and encompasses a central depression in brightness with a flux ratio ≈10:1. The emission ring is recovered using different calibration and imaging schemes, with its diameter and width remaining stable over four different observations carried out in different days. Overall, the observed image is consistent with expectations for the shadow of a Kerr black hole as predicted by general relativity. The asymmetry in brightness in the ring can be explained in terms of relativistic beaming of the emission from a plasma rotating close to the speed of light around a black hole. We compare our images to an extensive library of ray-traced general-relativistic magnetohydrodynamic simulations of black holes and derive a central mass of M =(6.5 ±0.7) ×10 9 M o . Our radio-wave observations thus provide powerful evidence for the presence of supermassive black holes in centers of galaxies and as the central engines of active galactic nuclei. They also present a new tool to explore gravity in its most extreme limit and on a mass scale that was so far not accessible. - First M87 Event Horizon Telescope Results. II. Array and Instrumentation
A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä(2019-04-10) Savolainen, Tuomas; , Event Horizon Telescope CollaboratThe Event Horizon Telescope (EHT) is a very long baseline interferometry (VLBI) array that comprises millimeter- and submillimeter-wavelength telescopes separated by distances comparable to the diameter of the Earth. At a nominal operating wavelength of ∼1.3 mm, EHT angular resolution (λ/D) is ∼25 μas, which is sufficient to resolve nearby supermassive black hole candidates on spatial and temporal scales that correspond to their event horizons. With this capability, the EHT scientific goals are to probe general relativistic effects in the strong-field regime and to study accretion and relativistic jet formation near the black hole boundary. In this Letter we describe the system design of the EHT, detail the technology and instrumentation that enable observations, and provide measures of its performance. Meeting the EHT science objectives has required several key developments that have facilitated the robust extension of the VLBI technique to EHT observing wavelengths and the production of instrumentation that can be deployed on a heterogeneous array of existing telescopes and facilities. To meet sensitivity requirements, high-bandwidth digital systems were developed that process data at rates of 64 gigabit s -1 , exceeding those of currently operating cm-wavelength VLBI arrays by more than an order of magnitude. Associated improvements include the development of phasing systems at array facilities, new receiver installation at several sites, and the deployment of hydrogen maser frequency standards to ensure coherent data capture across the array. These efforts led to the coordination and execution of the first Global EHT observations in 2017 April, and to event-horizon-scale imaging of the supermassive black hole candidate in M87.