ORCID Profile
0000-0001-6558-9053
Current Organisation
Kyung Hee University
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Publisher: American Astronomical Society
Date: 06-2021
Publisher: Oxford University Press (OUP)
Date: 15-01-2018
Publisher: Oxford University Press (OUP)
Date: 08-04-2019
DOI: 10.1093/MNRAS/STZ970
Abstract: We present the jet kinematics of the flat spectrum radio quasar (FSRQ) 4C+21.35 using time-resolved KaVA very long baseline interferometry array radio maps obtained from 2014 September to 2016 July. During two out of three observing c aigns, observations were performed bi-weekly at 22 and 43 GHz quasi-simultaneously. At 22 GHz, we identified three jet components near the core with apparent speeds up to (14.4 ± 2.1)c. The timing of the ejection of a new component detected in 2016 is consistent with a γ-ray flare in 2014 November. At 43 GHz, we found four inner jet (& mas) components with speeds from (3.5 ± 1.4)c to (6.8 ± 1.5)c. Jet component speeds tend to be higher with increasing distances from the core. We compared our data with archival Very Long Baseline Array (VLBA) data from the Boston University (BU) 43 GHz and the Monitoring Of Jets in Active galactic nuclei with VLBA Experiments (MOJAVE) 15.4 GHz monitoring programmes. Whereas MOJAVE data and our data are in good agreement, jet speeds obtained from the BU programme data in the same time period are about twice as high as the ones we obtain from the KaVA data. The discrepancy at 43 GHz indicates that radio arrays with different angular resolution identify and trace different jet features even when the data are obtained at the same frequency and at the same time. The flux densities of jet components decay exponentially, in agreement with a synchrotron cooling time-scale of ∼1 yr. Using known electron Lorentz factor values (∼9000), we estimate the magnetic field strength to be ∼1–3 $\\mu$T. When adopting a jet viewing angle of 5°, the intrinsic jet speed is of order 0.99c.
Publisher: American Astronomical Society
Date: 16-10-2020
Publisher: EDP Sciences
Date: 05-2023
DOI: 10.1051/0004-6361/202142988
Abstract: Context. Because of its proximity and the large size of its black hole, M 87 is one of the best targets for studying the launching mechanism of active galactic nucleus jets. Currently, magnetic fields are considered to be an essential factor in the launching and accelerating of the jet. However, current observational estimates of the magnetic field strength of the M 87 jet are limited to the innermost part of the jet (≲100 r s ) or to HST-1 (∼10 5 r s ). No attempt has yet been made to measure the magnetic field strength in between. Aims. We aim to infer the magnetic field strength of the M 87 jet out to a distance of several thousand r s by tracking the distance-dependent changes in the synchrotron spectrum of the jet from high-resolution very long baseline interferometry observations. Methods. In order to obtain high-quality spectral index maps, quasi-simultaneous observations at 22 and 43 GHz were conducted using the KVN and VERA Array (KaVA) and the Very Long Baseline Array (VLBA). We compared the spectral index distributions obtained from the observations with a model and placed limits on the magnetic field strengths as a function of distance. Results. The overall spectral morphology is broadly consistent over the course of these observations. The observed synchrotron spectrum rapidly steepens from α 22 − 43 GHz ∼ −0.7 at ∼2 mas to α 22 − 43 GHz ∼ −2.5 at ∼6 mas. In the KaVA observations, the spectral index remains unchanged until ∼10 mas, but this trend is unclear in the VLBA observations. A spectral index model in which nonthermal electron injections inside the jet decrease with distance can adequately reproduce the observed trend. This suggests the magnetic field strength of the jet at a distance of 2−10 mas (∼900 r s − ∼4500 r s in the deprojected distance) has a range of B = (0.3−1.0 G)( z /2mas) −0.73 . Extrapolating to the Event Horizon Telescope scale yields consistent results, suggesting that the majority of the magnetic flux of the jet near the black hole is preserved out to ∼4500 r s without significant dissipation.
Publisher: Springer Science and Business Media LLC
Date: 26-04-2023
DOI: 10.1038/S41586-023-05843-W
Abstract: The nearby radio galaxy M87 is a prime target for studying black hole accretion and jet formation 1,2 . Event Horizon Telescope observations of M87 in 2017, at a wavelength of 1.3 mm, revealed a ring-like structure, which was interpreted as gravitationally lensed emission around a central black hole 3 . Here we report images of M87 obtained in 2018, at a wavelength of 3.5 mm, showing that the compact radio core is spatially resolved. High-resolution imaging shows a ring-like structure of $${8.4}_{-1.1}^{+0.5}$$ 8.4 − 1.1 + 0.5 Schwarzschild radii in diameter, approximately 50% larger than that seen at 1.3 mm. The outer edge at 3.5 mm is also larger than that at 1.3 mm. This larger and thicker ring indicates a substantial contribution from the accretion flow with absorption effects, in addition to the gravitationally lensed ring-like emission. The images show that the edge-brightened jet connects to the accretion flow of the black hole. Close to the black hole, the emission profile of the jet-launching region is wider than the expected profile of a black-hole-driven jet, suggesting the possible presence of a wind associated with the accretion flow.
Publisher: Oxford University Press (OUP)
Date: 25-07-2018
Publisher: Springer Science and Business Media LLC
Date: 19-07-2021
DOI: 10.1038/S41550-021-01417-W
Abstract: Very-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 ≡ G M / 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° 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 ~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 .
Publisher: EDP Sciences
Date: 07-2021
DOI: 10.1051/0004-6361/202040198
Abstract: Aims. We present the results of multi-epoch, multifrequency monitoring of blazar 4C +29.45, which was regularly monitored as part of the Interferometric Monitoring of GAmma-ray Bright Active Galactic Nuclei (iMOGABA) program – a key science program of the Korean Very long baseline interferometry Network (KVN). Methods. Observations were conducted simultaneously at 22, 43, 86, and 129 GHz over the 4 years from 5 December 2012 to 28 December 2016. We also used additional data from the 15 GHz Owens Valley Radio Observatory (OVRO) monitoring program. Results. From the 15 GHz light curve, we estimated the variability timescales of the source during several radio flux enhancements. We found that the source experienced six radio flux enhancements with variability timescales of 9–187 days during the observing period, yielding corresponding variability Doppler factors of 9–27. From the simultaneous multifrequency KVN observations, we were able to obtain accurate radio spectra of the source and hence to more precisely measure the turnover frequencies ν r , of synchrotron self-absorption (SSA) emission with a mean value of ν r ̅ = 28.9 GHz. Using jet geometry assumptions, we estimated the size of the emitting region at the turnover frequency. We found that the equipartition magnetic field strength is up to two orders of magnitude higher than the SSA magnetic field strength (0.001–0.1 G). This is consistent with the source being particle dominated. We performed a careful analysis of the systematic errors related to the making of these estimations. Conclusions. From the results, we concluded that the equipartition region is located upstream from the SSA region.
Publisher: American Astronomical Society
Date: 15-12-2017
Location: Korea, Republic of
Location: Taiwan, Province of China
No related grants have been discovered for Jongho Park.