We present a time-dependent disk instability model for short duration flare events observed in th... more We present a time-dependent disk instability model for short duration flare events observed in the black hole microquasar V4641 Sgr = SAX J1819.3-2525. To reproduce the observed short duration of the flare, a ring-like, truncated Keplerian disk is required to reproduce the observed flares of a few day duration. §1. Black hole microquasar V4641 Sagittari Outbursts in V4641 Sgr (= SAX J1819.3-2525) were observed twice in September 1999 and in May 2002 (e.g., Refs. 1)-3)). Its super-Eddington, multi-component jets, accompanied by an unusually short, few day outburst in 1999 observed in the radio, leads us to identify V4641 Sgr as a microquasar. 1) Optical spectroscopic observations show that V4641 Sgr is a high mass X-ray binary with a black hole and companion masses of ∼ 9.6 and ∼6.5 M , respectively, with a 2.81 day orbital period. 4) §2. Flares in V4641 Sgr and the disk instability model
The Bulletin of The Korean Astronomical Society, 2020
Korea Institute for Advanced Study (고등과학원) We examine how the mass assembly of central galaxies d... more Korea Institute for Advanced Study (고등과학원) We examine how the mass assembly of central galaxies depends on their location in the cosmic web. The HORIZON-AGN simulation is analysed at z ∼2 using the DISPERSE code to extract multi-scale cosmic filaments. We find that the dependency of galaxy properties on large-scale environment is mostly inherited from the (large-scale) environmental dependency of their host halo mass. When adopting a residual analysis that removes the host halo mass effect, we detect a direct and non-negligible influence of cosmic filaments. Proximity to filaments enhances the build-up of stellar mass, a result in agreement with previous studies. However, our multi-scale analysis also reveals that, at the edge of filaments, star formation is suppressed. In addition, we find clues for compaction of the stellar distribution at close proximity to filaments. We suggest that gas transfer from the outside to the inside of the haloes (where galaxies reside) becomes less efficient closer to filaments, due to high angular momentum supply at the vorticity-rich edge of filaments. This quenching mechanism may partly explain the larger fraction of passive galaxies in filaments, as inferred from observations at lower redshifts.
Monthly Notices of the Royal Astronomical Society, Jan 15, 2018
3C 84 (NGC 1275) is a well-studied mis-aligned Active Galactic Nucleus (AGN), which has been acti... more 3C 84 (NGC 1275) is a well-studied mis-aligned Active Galactic Nucleus (AGN), which has been active in γ rays since at least 2008. We have monitored the source at four wavelengths (14 mm, 7 mm, 3 mm and 2 mm) using the Korean VLBI network (KVN) since 2013 as part of the interferometric monitoring of γ-ray bright AGN (iMOGABA) program. 3C 84 exhibits bright radio emission both near the central supermassive black hole (SMBH) feature known as C1 and from a moving feature located to the south known as C3. Other facilities have also detected these short-term variations above a slowly rising trend at shorter wavelengths, such as in γ-ray and 1 mm total intensity light-curves. We find that the variations in the γ rays and 1 mm total intensity lightcurves are correlated, with the γ rays leading and lagging the radio emission. Analysis of the 2 mm KVN data shows that both the γ rays and 1 mm total intensity shortterm variations are better correlated with the SMBH region than C3, likely placing the short-term variations in C1. We interpret the emission as being due to the random alignment of spatially separated emission regions. We place the slowly rising trend in C3, consistent with previous results. Additionally, we report that since mid-2015, a large mm-wave radio flare has been occurring in C3, with a large γ-ray flare coincident with the onset of this flare at all radio wavelengths.
Journal of the Korean Astronomical Society, Oct 31, 2015
The Interferometric Monitoring of Gamma-ray Bright Active galactic nuclei (iMOGABA) program provi... more 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.
Proceedings of the International Astronomical Union, Sep 1, 2017
VLBI observation of masers is a powerful mean to understand the early evolutionary phase of massi... more VLBI observation of masers is a powerful mean to understand the early evolutionary phase of massive star formation. A few different scenarios of outflow evolution in the massive protostars have been proposed, and cannot be readily examined because the precise timing of appropriate maser phenomena is difficult. In particular, it has been a matter of debate whether a well-collimated or a less-collimated outflow comes first in the very early phase of the massive protostellar evolution. Long-term, multi-epoch VLBI monitoring is probably the most important method to trace the outflow evolution. Such a monitoring of a massive star-forming region W75N(B) has been very successful. Since the first detection of the expanding water maser shell associated with the star-forming region VLA 2 of W75N(B) in 1999, the observations in 2005 and 2007 displayed that the expanding water maser shell has been evolved to well-collimated from a less collimated morphology. Observations in 2012 also confirmed such a transition. It would be a major breakthrough in our knowledge of the formation and evolution of the first stages of massive protostars. We performed multi-epoch VLBI observations in mid-2014. On the contrary to its expansion for 13 years, the maser shell at VLA 2 observed in 2014 is comparable to the size observed in 2012. The quenching of the maser shell size indicates that the previously expanding outflow has been decelerated plausibly due to the interaction with surrounding interstellar medium.
Journal of the Korean Physical Society, Oct 1, 2016
We present the intraday flaring activity of Cyg X-3 in the rise phase of a major flare observed i... more We present the intraday flaring activity of Cyg X-3 in the rise phase of a major flare observed in November-December 2008 at 22 GHz with the VLBI Exploration of Radio Astrometry (VERA). The two consecutive intraday flares with a time scale of ∼ 3 − 4 hours are similar in their light curve morphologies. The intraday flares observed in 2008 are compared to other interferometric observations of intraday flares in Cyg X-3, as well as to those observed in GRS 1915+105. The intraday flares observed in Cyg X-3 can be classified into three types: flares with a fast rise and exponential decay (type A), flares with an almost equal time scale in the rise and decay (type B), and asymmetric flares with a slow rise and rapid decay (type C). Such types of intraday flares have also been observed in another representative microquasar GRS 1915+105, and are, therefore, a common characteristic in both microquasars. The long-term multiwavelengh light curves in 2007 and 2008 are similar except during a few-day-long transition from an unltrasoft to harder states. In the transition state, the intraday flares of type A were observed in the 2008 VERA observation whereas, in the 2007 VERA observation, a type C flare was observed, opposite to the flares observed in 2008. The 2007 observation is the only observation of a type C flare in Cyg X-3. A possible cause of the different types of intraday flares is briefly discussed in the context of the jet propagation process.
We present Very Long Baseline Interferometry (VLBI) observations of 22 GHz H 2 O masers in the hi... more We present Very Long Baseline Interferometry (VLBI) observations of 22 GHz H 2 O masers in the high-mass star-forming region of W75N, carried out with VLBI Exploration of Radio Astrometry (VERA) for three-epochs in 2007 with an angular resolution of ∼ 1 mas. We detected H 2 O maser emission toward the radio jet in VLA 1 and the expanding shell-like structure in VLA 2. The spatial distribution of the H 2 O masers detected with VERA and measured proper motions around VLA 1 and VLA 2 are similar to those found with previous VLBI observations in epochs 1999 and 2005, with the masers in VLA 1 mainly distributed along a linear structure parallel to the radio jet and, on the other
Publications of the Astronomical Society of Japan, Dec 16, 2022
We aim to reveal the structure and kinematics of the Outer-Scutum-Centaurus (OSC) arm located on ... more We aim to reveal the structure and kinematics of the Outer-Scutum-Centaurus (OSC) arm located on the far side of the Milky Way through very long baseline interferometry (VLBI) astrometry using KaVA, which is composed of KVN (Korean VLBI Network) and VERA (VLBI Exploration of Radio Astrometry). We report the proper motion of a 22 GHz H 2 O maser source, which is associated with the star-forming region G034.84−00.95, to be (µ α cosδ, µ δ) = (−1.61±0.18, −4.29±0.16) mas yr −1 in equatorial coordinates (J2000). We estimate the 2D kinematic distance to the source to be 18.6±1.0 kpc, which is derived from the varianceweighted average of kinematic distances with LSR velocity and the Galactic-longitude component of the measured proper motion. Our result places the source in the OSC arm and
We aim to reveal the structure and kinematics of the Outer-Scutum-Centaurus (OSC) arm located on ... more We aim to reveal the structure and kinematics of the Outer-Scutum-Centaurus (OSC) arm located on the far side of the Milky Way through very long baseline interferometry (VLBI) astrometry using KaVA, which is composed of KVN (Korean VLBI Network) and VERA (VLBI Exploration of Radio Astrometry). We report the proper motion of a 22 GHz H 2 O maser source, which is associated with the star-forming region G034.84−00.95, to be (µ α cosδ, µ δ) = (−1.61±0.18, −4.29±0.16) mas yr −1 in equatorial coordinates (J2000). We estimate the 2D kinematic distance to the source to be 18.6±1.0 kpc, which is derived from the varianceweighted average of kinematic distances with LSR velocity and the Galactic-longitude component of the measured proper motion. Our result places the source in the OSC arm and
We present a study of the inexplicit connection between radio jet activity and γ-ray emission of ... more We present a study of the inexplicit connection between radio jet activity and γ-ray emission of BL Lacertae (BL Lac; 2200+420). We analyze the long-term millimeter activity of BL Lac via interferometric observations with the Korean VLBI Network (KVN) obtained at 22, 43, 86, and 129 GHz simultaneously over three years (from January 2013 to March 2016); during this time, two γ-ray outbursts (in November 2013 and March 2015) can be seen in γ-ray light curves obtained from Fermi observations. The KVN radio core is optically thick at least up to 86 GHz; there is indication that it might be optically thin at higher frequencies. To first order, the radio light curves decay exponentially over the time span covered by our observations, with decay timescales of 411±85 days, 352±79 days, 310±57 days, and 283±55 days at 22, 43, 86, and 129 GHz, respectively. Assuming synchrotron cooling, a cooling time of around one year is consistent with magnetic field strengths B ∼ 2 µT and electron Lorentz factors γ ∼ 10 000. Taking into account that our formal measurement errors include intrinsic variability and thus overestimate the statistical uncertainties, we find that the decay timescale τ scales with frequency ν like τ ∝ ν −0.2. This relation is much shallower than the one expected from opacity effects (core shift), but in agreement with the (sub-)mm radio core being a standing recollimation shock. We do not find convincing radio flux counterparts to the γ-ray outbursts. The spectral evolution is consistent with the 'generalized shock model' of Valtaoja et al. (1992). A temporary increase in the core opacity and the emergence of a knot around the time of the second γ-ray event indicate that this γ-ray outburst might be an 'orphan' flare powered by the 'ring of fire' mechanism.
The Korean Very Long Baseline Interferometry Network (KVN) is a recently proposed millimeter very... more The Korean Very Long Baseline Interferometry Network (KVN) is a recently proposed millimeter very long baseline interferometry (VLBI) with multi-channel receivers of 2.5−129 GHz. We propose microquasar observations with KVN, Korean mm arrays, and a collaboration with Japanese VLBI system, VERA. §1. Korean very long baseline interferometry and arrays The Korean Very Long Baseline Interferometry Network (KVN) is a VLBI project for millimeter astronomy proposed by KAO in 2000. * * *) Three 21 m antennas will be built at Seoul, Ulsan and Jeju Island, by 2006−2007 (see the left-hand side of Fig. 1). The baselines for Seoul−Ulsan, Ulsan−Jeju Island, and Seoul−Jeju Island are 305.2, 358.5, and 477.7 km, respectively. KVN is unique since the phase
Journal of the Korean Astronomical Society, Dec 1, 2017
We present a study of the inexplicit connection between radio jet activity and γ-ray emission of ... more We present a study of the inexplicit connection between radio jet activity and γ-ray emission of BL Lacertae (BL Lac; 2200+420). We analyze the long-term millimeter activity of BL Lac via interferometric observations with the Korean VLBI Network (KVN) obtained at 22, 43, 86, and 129 GHz simultaneously over three years (from January 2013 to March 2016); during this time, two γ-ray outbursts (in November 2013 and March 2015) can be seen in γ-ray light curves obtained from Fermi observations. The KVN radio core is optically thick at least up to 86 GHz; there is indication that it might be optically thin at higher frequencies. To first order, the radio light curves decay exponentially over the time span covered by our observations, with decay timescales of 411±85 days, 352±79 days, 310±57 days, and 283±55 days at 22, 43, 86, and 129 GHz, respectively. Assuming synchrotron cooling, a cooling time of around one year is consistent with magnetic field strengths B ∼ 2 µT and electron Lorentz factors γ ∼ 10 000. Taking into account that our formal measurement errors include intrinsic variability and thus overestimate the statistical uncertainties, we find that the decay timescale τ scales with frequency ν like τ ∝ ν −0.2. This relation is much shallower than the one expected from opacity effects (core shift), but in agreement with the (sub-)mm radio core being a standing recollimation shock. We do not find convincing radio flux counterparts to the γ-ray outbursts. The spectral evolution is consistent with the 'generalized shock model' of Valtaoja et al. (1992). A temporary increase in the core opacity and the emergence of a knot around the time of the second γ-ray event indicate that this γ-ray outburst might be an 'orphan' flare powered by the 'ring of fire' mechanism.
We present a time-dependent disk instability model for short duration flare events observed in th... more We present a time-dependent disk instability model for short duration flare events observed in the black hole microquasar V4641 Sgr = SAX J1819.3-2525. To reproduce the observed short duration of the flare, a ring-like, truncated Keplerian disk is required to reproduce the observed flares of a few day duration. §1. Black hole microquasar V4641 Sagittari Outbursts in V4641 Sgr (= SAX J1819.3-2525) were observed twice in September 1999 and in May 2002 (e.g., Refs. 1)-3)). Its super-Eddington, multi-component jets, accompanied by an unusually short, few day outburst in 1999 observed in the radio, leads us to identify V4641 Sgr as a microquasar. 1) Optical spectroscopic observations show that V4641 Sgr is a high mass X-ray binary with a black hole and companion masses of ∼ 9.6 and ∼6.5 M , respectively, with a 2.81 day orbital period. 4) §2. Flares in V4641 Sgr and the disk instability model
The Bulletin of The Korean Astronomical Society, 2020
Korea Institute for Advanced Study (고등과학원) We examine how the mass assembly of central galaxies d... more Korea Institute for Advanced Study (고등과학원) We examine how the mass assembly of central galaxies depends on their location in the cosmic web. The HORIZON-AGN simulation is analysed at z ∼2 using the DISPERSE code to extract multi-scale cosmic filaments. We find that the dependency of galaxy properties on large-scale environment is mostly inherited from the (large-scale) environmental dependency of their host halo mass. When adopting a residual analysis that removes the host halo mass effect, we detect a direct and non-negligible influence of cosmic filaments. Proximity to filaments enhances the build-up of stellar mass, a result in agreement with previous studies. However, our multi-scale analysis also reveals that, at the edge of filaments, star formation is suppressed. In addition, we find clues for compaction of the stellar distribution at close proximity to filaments. We suggest that gas transfer from the outside to the inside of the haloes (where galaxies reside) becomes less efficient closer to filaments, due to high angular momentum supply at the vorticity-rich edge of filaments. This quenching mechanism may partly explain the larger fraction of passive galaxies in filaments, as inferred from observations at lower redshifts.
Monthly Notices of the Royal Astronomical Society, Jan 15, 2018
3C 84 (NGC 1275) is a well-studied mis-aligned Active Galactic Nucleus (AGN), which has been acti... more 3C 84 (NGC 1275) is a well-studied mis-aligned Active Galactic Nucleus (AGN), which has been active in γ rays since at least 2008. We have monitored the source at four wavelengths (14 mm, 7 mm, 3 mm and 2 mm) using the Korean VLBI network (KVN) since 2013 as part of the interferometric monitoring of γ-ray bright AGN (iMOGABA) program. 3C 84 exhibits bright radio emission both near the central supermassive black hole (SMBH) feature known as C1 and from a moving feature located to the south known as C3. Other facilities have also detected these short-term variations above a slowly rising trend at shorter wavelengths, such as in γ-ray and 1 mm total intensity light-curves. We find that the variations in the γ rays and 1 mm total intensity lightcurves are correlated, with the γ rays leading and lagging the radio emission. Analysis of the 2 mm KVN data shows that both the γ rays and 1 mm total intensity shortterm variations are better correlated with the SMBH region than C3, likely placing the short-term variations in C1. We interpret the emission as being due to the random alignment of spatially separated emission regions. We place the slowly rising trend in C3, consistent with previous results. Additionally, we report that since mid-2015, a large mm-wave radio flare has been occurring in C3, with a large γ-ray flare coincident with the onset of this flare at all radio wavelengths.
Journal of the Korean Astronomical Society, Oct 31, 2015
The Interferometric Monitoring of Gamma-ray Bright Active galactic nuclei (iMOGABA) program provi... more 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.
Proceedings of the International Astronomical Union, Sep 1, 2017
VLBI observation of masers is a powerful mean to understand the early evolutionary phase of massi... more VLBI observation of masers is a powerful mean to understand the early evolutionary phase of massive star formation. A few different scenarios of outflow evolution in the massive protostars have been proposed, and cannot be readily examined because the precise timing of appropriate maser phenomena is difficult. In particular, it has been a matter of debate whether a well-collimated or a less-collimated outflow comes first in the very early phase of the massive protostellar evolution. Long-term, multi-epoch VLBI monitoring is probably the most important method to trace the outflow evolution. Such a monitoring of a massive star-forming region W75N(B) has been very successful. Since the first detection of the expanding water maser shell associated with the star-forming region VLA 2 of W75N(B) in 1999, the observations in 2005 and 2007 displayed that the expanding water maser shell has been evolved to well-collimated from a less collimated morphology. Observations in 2012 also confirmed such a transition. It would be a major breakthrough in our knowledge of the formation and evolution of the first stages of massive protostars. We performed multi-epoch VLBI observations in mid-2014. On the contrary to its expansion for 13 years, the maser shell at VLA 2 observed in 2014 is comparable to the size observed in 2012. The quenching of the maser shell size indicates that the previously expanding outflow has been decelerated plausibly due to the interaction with surrounding interstellar medium.
Journal of the Korean Physical Society, Oct 1, 2016
We present the intraday flaring activity of Cyg X-3 in the rise phase of a major flare observed i... more We present the intraday flaring activity of Cyg X-3 in the rise phase of a major flare observed in November-December 2008 at 22 GHz with the VLBI Exploration of Radio Astrometry (VERA). The two consecutive intraday flares with a time scale of ∼ 3 − 4 hours are similar in their light curve morphologies. The intraday flares observed in 2008 are compared to other interferometric observations of intraday flares in Cyg X-3, as well as to those observed in GRS 1915+105. The intraday flares observed in Cyg X-3 can be classified into three types: flares with a fast rise and exponential decay (type A), flares with an almost equal time scale in the rise and decay (type B), and asymmetric flares with a slow rise and rapid decay (type C). Such types of intraday flares have also been observed in another representative microquasar GRS 1915+105, and are, therefore, a common characteristic in both microquasars. The long-term multiwavelengh light curves in 2007 and 2008 are similar except during a few-day-long transition from an unltrasoft to harder states. In the transition state, the intraday flares of type A were observed in the 2008 VERA observation whereas, in the 2007 VERA observation, a type C flare was observed, opposite to the flares observed in 2008. The 2007 observation is the only observation of a type C flare in Cyg X-3. A possible cause of the different types of intraday flares is briefly discussed in the context of the jet propagation process.
We present Very Long Baseline Interferometry (VLBI) observations of 22 GHz H 2 O masers in the hi... more We present Very Long Baseline Interferometry (VLBI) observations of 22 GHz H 2 O masers in the high-mass star-forming region of W75N, carried out with VLBI Exploration of Radio Astrometry (VERA) for three-epochs in 2007 with an angular resolution of ∼ 1 mas. We detected H 2 O maser emission toward the radio jet in VLA 1 and the expanding shell-like structure in VLA 2. The spatial distribution of the H 2 O masers detected with VERA and measured proper motions around VLA 1 and VLA 2 are similar to those found with previous VLBI observations in epochs 1999 and 2005, with the masers in VLA 1 mainly distributed along a linear structure parallel to the radio jet and, on the other
Publications of the Astronomical Society of Japan, Dec 16, 2022
We aim to reveal the structure and kinematics of the Outer-Scutum-Centaurus (OSC) arm located on ... more We aim to reveal the structure and kinematics of the Outer-Scutum-Centaurus (OSC) arm located on the far side of the Milky Way through very long baseline interferometry (VLBI) astrometry using KaVA, which is composed of KVN (Korean VLBI Network) and VERA (VLBI Exploration of Radio Astrometry). We report the proper motion of a 22 GHz H 2 O maser source, which is associated with the star-forming region G034.84−00.95, to be (µ α cosδ, µ δ) = (−1.61±0.18, −4.29±0.16) mas yr −1 in equatorial coordinates (J2000). We estimate the 2D kinematic distance to the source to be 18.6±1.0 kpc, which is derived from the varianceweighted average of kinematic distances with LSR velocity and the Galactic-longitude component of the measured proper motion. Our result places the source in the OSC arm and
We aim to reveal the structure and kinematics of the Outer-Scutum-Centaurus (OSC) arm located on ... more We aim to reveal the structure and kinematics of the Outer-Scutum-Centaurus (OSC) arm located on the far side of the Milky Way through very long baseline interferometry (VLBI) astrometry using KaVA, which is composed of KVN (Korean VLBI Network) and VERA (VLBI Exploration of Radio Astrometry). We report the proper motion of a 22 GHz H 2 O maser source, which is associated with the star-forming region G034.84−00.95, to be (µ α cosδ, µ δ) = (−1.61±0.18, −4.29±0.16) mas yr −1 in equatorial coordinates (J2000). We estimate the 2D kinematic distance to the source to be 18.6±1.0 kpc, which is derived from the varianceweighted average of kinematic distances with LSR velocity and the Galactic-longitude component of the measured proper motion. Our result places the source in the OSC arm and
We present a study of the inexplicit connection between radio jet activity and γ-ray emission of ... more We present a study of the inexplicit connection between radio jet activity and γ-ray emission of BL Lacertae (BL Lac; 2200+420). We analyze the long-term millimeter activity of BL Lac via interferometric observations with the Korean VLBI Network (KVN) obtained at 22, 43, 86, and 129 GHz simultaneously over three years (from January 2013 to March 2016); during this time, two γ-ray outbursts (in November 2013 and March 2015) can be seen in γ-ray light curves obtained from Fermi observations. The KVN radio core is optically thick at least up to 86 GHz; there is indication that it might be optically thin at higher frequencies. To first order, the radio light curves decay exponentially over the time span covered by our observations, with decay timescales of 411±85 days, 352±79 days, 310±57 days, and 283±55 days at 22, 43, 86, and 129 GHz, respectively. Assuming synchrotron cooling, a cooling time of around one year is consistent with magnetic field strengths B ∼ 2 µT and electron Lorentz factors γ ∼ 10 000. Taking into account that our formal measurement errors include intrinsic variability and thus overestimate the statistical uncertainties, we find that the decay timescale τ scales with frequency ν like τ ∝ ν −0.2. This relation is much shallower than the one expected from opacity effects (core shift), but in agreement with the (sub-)mm radio core being a standing recollimation shock. We do not find convincing radio flux counterparts to the γ-ray outbursts. The spectral evolution is consistent with the 'generalized shock model' of Valtaoja et al. (1992). A temporary increase in the core opacity and the emergence of a knot around the time of the second γ-ray event indicate that this γ-ray outburst might be an 'orphan' flare powered by the 'ring of fire' mechanism.
The Korean Very Long Baseline Interferometry Network (KVN) is a recently proposed millimeter very... more The Korean Very Long Baseline Interferometry Network (KVN) is a recently proposed millimeter very long baseline interferometry (VLBI) with multi-channel receivers of 2.5−129 GHz. We propose microquasar observations with KVN, Korean mm arrays, and a collaboration with Japanese VLBI system, VERA. §1. Korean very long baseline interferometry and arrays The Korean Very Long Baseline Interferometry Network (KVN) is a VLBI project for millimeter astronomy proposed by KAO in 2000. * * *) Three 21 m antennas will be built at Seoul, Ulsan and Jeju Island, by 2006−2007 (see the left-hand side of Fig. 1). The baselines for Seoul−Ulsan, Ulsan−Jeju Island, and Seoul−Jeju Island are 305.2, 358.5, and 477.7 km, respectively. KVN is unique since the phase
Journal of the Korean Astronomical Society, Dec 1, 2017
We present a study of the inexplicit connection between radio jet activity and γ-ray emission of ... more We present a study of the inexplicit connection between radio jet activity and γ-ray emission of BL Lacertae (BL Lac; 2200+420). We analyze the long-term millimeter activity of BL Lac via interferometric observations with the Korean VLBI Network (KVN) obtained at 22, 43, 86, and 129 GHz simultaneously over three years (from January 2013 to March 2016); during this time, two γ-ray outbursts (in November 2013 and March 2015) can be seen in γ-ray light curves obtained from Fermi observations. The KVN radio core is optically thick at least up to 86 GHz; there is indication that it might be optically thin at higher frequencies. To first order, the radio light curves decay exponentially over the time span covered by our observations, with decay timescales of 411±85 days, 352±79 days, 310±57 days, and 283±55 days at 22, 43, 86, and 129 GHz, respectively. Assuming synchrotron cooling, a cooling time of around one year is consistent with magnetic field strengths B ∼ 2 µT and electron Lorentz factors γ ∼ 10 000. Taking into account that our formal measurement errors include intrinsic variability and thus overestimate the statistical uncertainties, we find that the decay timescale τ scales with frequency ν like τ ∝ ν −0.2. This relation is much shallower than the one expected from opacity effects (core shift), but in agreement with the (sub-)mm radio core being a standing recollimation shock. We do not find convincing radio flux counterparts to the γ-ray outbursts. The spectral evolution is consistent with the 'generalized shock model' of Valtaoja et al. (1992). A temporary increase in the core opacity and the emergence of a knot around the time of the second γ-ray event indicate that this γ-ray outburst might be an 'orphan' flare powered by the 'ring of fire' mechanism.
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Papers by Soon-Wook Kim