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RESOLVING THE GEOMETRY OF THE INNERMOST RELATIVISTIC JETS IN ACTIVE GALACTIC NUCLEI
Algaba, J. C.,Nakamura, M.,Asada, K.,Lee, S. S. American Astronomical Society 2017 The Astrophysical journal Vol.834 No.1
<P>In the current paradigm, it is believed that the compact VLBI radio core of radio-loud active galactic nuclei (AGNs) represents the innermost upstream regions of relativistic outflows. These regions of AGN jets have generally been modeled by a conical outflow with a. roughly constant opening angle and flow speed. Nonetheless, some works suggest that a parabolic geometry would be more appropriate to fit the high. energy spectral distribution properties and it has been recently found that, at least in some nearby radio galaxies, the geometry of the innermost regions of the jet is parabolic. We compile here multi-frequency core sizes of archival data to investigate the typically unresolved upstream regions of the jet geometry of a sample of 56 radio-loud AGNs. Data combined from the sources considered here are. not consistent with the classic picture of a conical jet starting in the vicinity of the super-massive black hole (SMBH), and may exclude a pure parabolic outflow solution, but rather suggest an intermediate solution with quasi-parabolic streams, which are frequently seen in numerical simulations. Inspection of the large opening angles near the SMBH and the range of the Lorentz factors derived from our results support our analyses. Our result suggests that the conical jet paradigm in AGNs needs to be re-examined by millimeter/sub-millimeter VLBI observations.</P>
Algaba, Juan-Carlos,Lee, Sang-Sung,Kim, Dae-Won,Rani, Bindu,Hodgson, Jeffrey,Kino, Motoki,Trippe, Sascha,Park, Jong-Ho,Zhao, Guang-Yao,Byun, Do-Young,Gurwell, Mark,Kang, Sin-Cheol,Kim, Jae-Young,Kim, American Astronomical Society 2018 The Astrophysical Journal Vol.852 No.1
<P>We present multi-frequency simultaneous VLBI radio observations of the flat spectrum radio quasar 1633+382 (4C 38.41) as part of the interferometric monitoring of gamma-ray-bright active galactic nuclei (iMOGABA) program combined with additional observations in the radio, optical, X-rays, and gamma-rays carried out during the period 2012 March-2015 August. The monitoring of this source reveals a significant long-lived increase in its activity for approximately two years in the radio bands, which correlates with a similar increase in all other bands from submillimeter to gamma-rays. A significant correlation is also found between radio fluxes and simultaneous spectral indices during this period. The study of the discrete correlation function indicates time lags smaller than the uncertainties of similar to 40 days among both radio bands and high-energy bands, and a time lag of similar to 70 days, with gamma-rays leading radio emission. We interpret this as showing that the high-energy and radio fluxes arise from different emitting regions, located at 1 +/- 13 and 40 +/- 13 pc from the central engine respectively.</P>
Exploring the Variability of the Flat-spectrum Radio Source 1633+382. II. Physical Properties
Algaba, Juan-Carlos,Lee, Sang-Sung,Rani, Bindu,Kim, Dae-Won,Kino, Motoki,Hodgson, Jeffrey,Zhao, Guang-Yao,Byun, Do-Young,Gurwell, Mark,Kang, Sin-Cheol,Kim, Jae-Young,Kim, Jeong-Sook,Kim, Soon-Wook,Par American Astronomical Society 2018 The Astrophysical journal Vol.859 No.2
<P>The flat-spectrum radio quasar 1633+382 (4C 38.41) showed a significant increase of its radio flux density during the period 2012 March-2015 August, which correlates with gamma-ray flaring activity. Multi-frequency simultaneous very long baseline interferometry (VLBI) observations were conducted as part of the interferometric monitoring of gamma-ray bright active galactic nuclei (iMOGABA) program and supplemented with additional radio monitoring observations with the OVRO 40. m telescope, the Boston University VLBI program, and the Submillimeter Array. The epochs of the maxima for the two largest gamma-ray flares coincide with the ejection of two respective new VLBI components. Analysis of the spectral energy distribution indicates a higher turnover frequency after the flaring events. The evolution of the flare in the turnover frequency-turnover flux density plane probes the adiabatic losses in agreement with the shock-in-jet model. The derived synchrotron self-absorption magnetic fields, of the order of 0.1. mG, do not seem to change dramatically during the flares, and are much weaker, by a factor 10(4), than the estimated equipartition magnetic fields, indicating that the source of the flare may be associated with a particle-dominated emitting region.</P>
RESOLVING THE ROTATION MEASURE OF THE M87 JET ON KILOPARSEC SCALES
Algaba, J. C.,Asada, K.,Nakamura, M. American Astronomical Society 2016 The Astrophysical journal Vol.823 No.2
<P>We investigate the distribution of Faraday rotation measure (RM) in the M87 jet at arcsecond scales by using archival polarimetric Very Large Array data at 8, 15, 22 and 43 GHz. We resolve the structure of the RM in several knots along the jet for the first time. We derive the power spectrum in the arcsecond-scale jet and find indications that the RM cannot be associated with a turbulent magnetic field with a 3D Kolmogorov spectrum. Our analysis indicates that the RM probed on jet scales has a significant contribution of a Faraday screen associated with the vicinity of the jet, in contrast with that on kiloparsec scales, typically assumed to be disconnected from the jet. Comparison with previous RM analyses suggests that the magnetic fields giving rise to the RMs observed in jet scales have different properties and are well less turbulent than those observed in the lobes.</P>
Core-Jet Blending Effects in AGN under the KVN View at 43 GHz
Juan-Carlos Algaba,Sincheol Kang,Dae-Won Kim,Jae-Young Kim,Jee Won Lee,Sang-Sung Lee,Sascha Trippe 한국천문학회 2019 Journal of The Korean Astronomical Society Vol.52 No.2
A long standing problem in the study of Active Galactic Nuclei (AGNs) is that the observed VLBI core is in fact a blending of the actual AGN core (classically defined by the tau=1 surface) and the upstream regions of the jet or optically thin flows. This blending may cause some biases in the observables of the core, such as its flux density, size or brightness temperature, which may lead to misleading interpretation of the derived quantities and physics. We study the effects of such blending under the view of the Korean VLBI Network (KVN) for a sample of AGNs at 43 GHz by comparing their observed properties with observations obtained using the Very Large Baseline Array (VLBA). Our results suggest that the observed core sizes are a factor 11 larger than these of VLBA, which is similar to the factor expected by considering the different resolutions of the two facilities. We suggest the use of this factor to consider blending effects in KVN measurements. Other parameters, such as flux density or brightness temperature, seem to possess a more complicated dependence.
INTERFEROMETRIC MONITORING OF GAMMA–RAY BRIGHT ACTIVE GALACTIC NUCLEI II: FREQUENCY PHASE TRANSFER
Juan-Carlos Algaba,Guang-Yao Zhao,이상성,변도영,강신철,김대원,김재영,김정숙,김순욱,KINOMOTOKI,Atsushi Miyazaki,박종호,Sascha Trippe,Kiyoaki Wajima 한국천문학회 2015 Journal of The Korean Astronomical Society Vol.48 No.5
The Interferometric Monitoring of Gamma--ray Bright Active galactic nuclei (iMOGABA) program provides not only simultaneous multifrequency observations of bright gamma--ray detected active galactic nuclei (AGN), but also covers the highest Very Large Baseline Interferometry (VLBI) frequencies ever being systematically monitored, up to 129~GHz. However, observation and imaging of weak sources at the highest observed frequencies is very challenging. In the second paper in this series, we evaluate the viability of the frequency phase transfer technique to iMOGABA in order to obtain larger coherence time at the higher frequencies of this program (86 and 129 GHz) and image additional sources that were not detected using standard techniques. We find that this method is applicable to the iMOGABA program even under non--optimal weather conditions.
Riva, A.,Algaba, I.,Pepio, M. The Korean Fiber Society 2007 Fibers and polymers Vol.8 No.2
The ultraviolet protection factor of a fabric is a quantitative measurement of the effectiveness of the fabric to protect the human skin against ultraviolet radiation. The protection provided by uncoloured cellulosic fabrics is, in general, too low, but can be improved by the finishing treatment with UV-absorbers. In the present paper Modal and Modal Sun fabrics with different compactness, and hence with different initial Ultraviolet Protection Factor values, are treated with several concentrations of an UV-absorber, according to a predefined experimental plan. The influence of each variable as well as their interaction on the response ultraviolet protection factor is analysed and a statistical model for predictions is proposed.
The Power of Simultaneous Multi-frequency Observations for mm-VLBI: Beyond Frequency Phase Transfer
Zhao, Guang-Yao,Algaba, Juan Carlos,Lee, Sang Sung,Jung, Taehyun,Dodson, Richard,Rioja, Marí,a,Byun, Do-Young,Hodgson, Jeffrey,Kang, Sincheol,Kim, Dae-Won,Kim, Jae-Young,Kim, Jeong-Sook,Kim, Soo American Institute of Physics 2018 The Astronomical journal Vol.155 No.1
<P>Atmospheric propagation effects at millimeter wavelengths can significantly alter the phases of radio signals and reduce the coherence time, putting tight constraints on high-frequency Very Long Baseline Interferometry (VLBI) observations. In previous works, it has been shown that non-dispersive (e.g., tropospheric) effects can be calibrated with the frequency phase transfer (FPT) technique. The coherence time can thus be significantly extended. Ionospheric effects, which can still be significant, remain however uncalibrated after FPT as well as the instrumental effects. In this work, we implement a further phase transfer between two FPT residuals (i.e., so-called FPT-square) to calibrate the ionospheric effects based on their frequency dependence. We show that after FPT-square, the coherence time at 3 mm can be further extended beyond 8 hr and the residual phase errors can be sufficiently canceled by applying the calibration of another source, which can have a large angular separation from the target (> 20 degrees) and significant temporal gaps. Calibrations for all-sky distributed sources with a few calibrators are also possible after FPT-square. One of the strengths and uniqueness of this calibration strategy is the suitability for high-frequency all-sky survey observations including very weak sources. We discuss the introduction of a pulse calibration system in the future to calibrate the remaining instrumental effects, allowing the possibility of imaging the source structure at high frequencies with FPT-square, where all phases are fully calibrated without involving any additional sources.</P>
The Power of Simultaneous Multi-frequency Observations for mm-VLBI: Beyond Frequency Phase Transfer
Guang-Yao Zhao,Juan Carlos Algaba,Sang Sung Lee,Taehyun Jung,Richard Dodson,Maria Rioja,Do-Young Byun,Jeffrey Hodgson,Sincheol Kang,Dae-Won Kim,Jae-Young Kim,Jeong-Sook Kim,Soon-Wook Kim,Motoki Kino,A 한국천문학회 2017 天文學會報 Vol.42 No.1