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5
  • Taehyun Jung
  • il je Cho
  • G.-Y. Zhao, M. Kino, I.-J. Cho, K. Akiyama, B. W. Sohn, T. Jung, J. C. Algaba, K. Hada, Y. Hagiwara, J. Hodgson, M. Honma, N. Kawaguchi, S. Koyama, J. A. Lee, T. Lee, K. Niinuma, J. Oh, J.-H. Park, H. Ro, S. Sawada-Satoh, F. Tazaki, S. Trippe, K. Wajima, H. Yoo
  • 2017
We present the results of a broad-band radio-to-GeV observing campaign organized to get a better understanding of the radiation processes responsible for the γ-ray flares observed in 3C 279. The total intensity and polarization observations of the source were carried out between 2013 December 28 and 2014 January 03 using the Fermi-Large Area Telescope, Swift-XRT, Swift-UVOT, and Korean VLBI Network telescopes. A prominent flare observed in the optical/near-UV passbands was found to be correlated with a concurrent γ-ray flare at a confidence level >95 percent, which suggests a co-spatial origin of the two. Moreover, the flaring activity in the two regimes was accompanied by no significant spectral variations. A peak in the X-ray light curve coincides with the peaks of the fractional polarization curves at 43 and 86 GHz radio bands. No prominent variation was noticed for the total intensity and the electric vector position angle observations at radio bands during this period. We noticed a possible hint of steepening of the radio spectrum with an increase in percentage polarization, which suggests that the radio polarization variations could be simply due to a spectral change. In a simple scenario, the correlated optical/γ-ray flares could be caused by the same population of emitting particles. The coincidence of the increase in radio polarization with the X-ray flux supports the picture that X-rays are produced via inverse-Compton scattering of radio photons. The observed fractional variability for the γ-ray flare ∼0.23 does not exceed that in the optical regime, which is inconsistent with what we usually observe for 3C 279; it could be due to different dependencies of the magnetic field and the external radiation field energy densWe present recent observation results of Sgr A* at millimeter obtained with VLBI arrays in Korea and Japan. 7 mm monitoring of Sgr A* is part of our AGN large project. The results at 7 epochs during 2013-2014, including high resolution maps, flux density and two-dimensional size measurements are presented. The source shows no significant variation in flux and structure related to the G2 encounter in 2014. According to recent MHD simulations by kawashima et al., flux and magnetic field energy can be expected to increase several years after the encounter; We will keep our monitoring in order to test this prediction. Astrometric observations of Sgr A* were performed in 2015 at 7 and 3.5 millimeter simultaneously. Source-frequency phase referencing was applied and a combined ”core-shift” of Sgr A* and a nearby calibrator was measured. Future observations and analysis are necessary to determine the core-shift in each source.ity profiles along the jet.
4
  • Sang-Sung Lee
  • Sincheol Kang
  • B. Rani, T. P. Krichbaum, S.-S. Lee, K. Sokolovsky, S. Kang, D.-Y. Byun, D. Mosunova, J. A. Zensus
  • 2017
  • MNRAS, 464 1 : 418~427
We present the results of a broad-band radio-to-GeV observing campaign organized to get a better understanding of the radiation processes responsible for the γ-ray flares observed in 3C 279. The total intensity and polarization observations of the source were carried out between 2013 December 28 and 2014 January 03 using the Fermi-Large Area Telescope, Swift-XRT, Swift-UVOT, and Korean VLBI Network telescopes. A prominent flare observed in the optical/near-UV passbands was found to be correlated with a concurrent γ-ray flare at a confidence level >95 percent, which suggests a co-spatial origin of the two. Moreover, the flaring activity in the two regimes was accompanied by no significant spectral variations. A peak in the X-ray light curve coincides with the peaks of the fractional polarization curves at 43 and 86 GHz radio bands. No prominent variation was noticed for the total intensity and the electric vector position angle observations at radio bands during this period. We noticed a possible hint of steepening of the radio spectrum with an increase in percentage polarization, which suggests that the radio polarization variations could be simply due to a spectral change. In a simple scenario, the correlated optical/γ-ray flares could be caused by the same population of emitting particles. The coincidence of the increase in radio polarization with the X-ray flux supports the picture that X-rays are produced via inverse-Compton scattering of radio photons. The observed fractional variability for the γ-ray flare ∼0.23 does not exceed that in the optical regime, which is inconsistent with what we usually observe for 3C 279; it could be due to different dependencies of the magnetic field and the external radiation field energy density profiles along the jet.
3
  • Yeon-Han Kim, Kyung-Suk Cho
  • Il-Hyun Cho
  • I.-H. Cho, K.-S. Cho, S.-C. Bong, Y.-J. Moon, V. M. Nakariakov, J. Park, J.-H. Baek, S. Choi, Y.-H. Kim, and J. Lee
  • 2017
  • The Astrophysical Journal Letters, 837:L11 (7pp)
For 478 centrally located sunspots observed in the optical continuum with Solar Dynamics Observatory/Helioseismic Magnetic Imager, we perform seismological diagnostics of the physical parameters of umbral photospheres. The new technique is based on the theory of slow magnetoacoustic waves in a non-isothermally stratified photosphere with a uniform vertical magnetic field. We construct a map of the weighted frequency of three-minute oscillations inside the umbra and use it for the estimation of the Alfven speed, plasma-beta, and mass density within the umbra. We find the umbral mean Alfven speed ranges between 10.5 and 7.5 km s-1 and is negatively correlated with magnetic field strength. The umbral mean plasma-beta is found to range approximately between 0.65 and 1.15 and does not vary significantly from pores to mature sunspots. The mean density ranges between (1-6) × 10-4 kg m-3 and shows a strong positive correlation with magnetic field strength.
2
  • Bong Won Sohn
  • Jeong Ae Lee
  • Jeong Ae Lee, Bong Won Sohn, Taehyun Jung, Do-Young Byun, and Jee Won Lee
  • 2017
  • The Astrophysical Journal Supplement Series, 228:22 (9pp)
We present the catalog of the KVN Calibrator Survey (KVNCS). This first part of the KVNCS is a single-dish radio survey simultaneously conducted at 22 (K band) and 43 GHz (Q band) using the Korean VLBI Network (KVN) from 2009 to 2011. A total of 2045 sources are selected from the VLBA Calibrator Survey with an extrapolated flux density limit of 100 mJy at the K band. The KVNCS contains 1533 sources in the K band with a flux density limit of 70 mJy and 553 sources in the Q band with a flux density limit of 120 mJy; it covers the whole sky down to -32.°5 in decl. We detected 513 sources simultaneously in the K and Q bands; ~76% of them are flat-spectrum sources (-0.5 ≤ α ≤ 0.5). From the flux-flux relationship, we anticipated that most of the radiation of many of the sources comes from the compact components. The sources listed in the KVNCS therefore are strong candidates for high-frequency VLBI calibrators.
1
  • Chang Won Lee
  • Gwanjeong Kim
  • A. Soam, Chang Won Lee, G. Maheswar, Gwanjeong Kim, S. Neha and Mi-Ryang Kim
  • 2017
  • MNRAS 464, 2403-2418
We present the R-band polarimetric results towards two nebulae L1415 and L1389 containing low-luminosity stars. Aim of this study is to understand the role played by magnetic fields in formation of low-luminosity objects. Linear polarization arises due to dichroism of the background starlight projected on the cloud providing the plane-of-the sky magnetic field orientation. The offsets between mean magnetic field directions obtained towards L1415 and L1389 and the projected outflow axes are found to be 35° and 12°, respectively. The offset between cloud minor axes and mean envelope magnetic field direction in L1415 and L1389 are 50° and 87°, respectively. To estimate the magnetic field strength by using the updated Chandrasekhar-Fermi (CF) relation, we obtained the 12CO(J = 1-0) line velocity dispersion value towards L1415 cloud using the Taeduk Radio Astronomical Observatory single dish observations. The values of Bpos in L1415 and L1389 are found to be 28 and 149 μG using CF technique and 23 and 140 μG using structure function analysis, respectively. The values of Bpos in these clouds are found to be consistent using both the techniques. By combining the present results with those obtained from our previous study of magnetic fields in cores with Very Low Luminosity Objects (VeLLOs), we attempt to improve the sample of cores with low-luminosity protostars and bridge the gap between the understanding of importance of magnetic fields in cores with VeLLOs and low-luminosity protostars. The results of this work and that of our previous work show that the outflow directions are aligned with envelope magnetic fields of the clouds.