分类： 天文学 >> 天文学 提交时间： 2023-02-19
摘要： Up to now, jet kinematic studies of radio quasars have barely reached beyond the redshift range at $z>3.5$. This significantly limits our knowledge of high-redshift jets, which can provide key information for understanding the jet nature and the growth of the black holes in the early Universe. In this paper, we selected 9 radio-loud quasars at $z>3.5$ which display milliarcsec-scale jet morphology. We provided evidence on the source nature by presenting high-resolution very long baseline interferometry (VLBI) images of the sample at 8.4~GHz frequency and making spectral index maps. We also consider Gaia optical positions that are available for 7 out of the 9 quasars, for a better identification of the jet components within the radio structures. We find that 6 sources can be classified as core--jet blazars. The remaining 3 objects are more likely young, jetted radio sources, compact symmetric objects. By including multi-epoch archival VLBI data, we also obtained jet component proper motions of the sample and estimated the jet kinematic and geometric parameters (Doppler factor, Lorentz factor, viewing angle). Our results show that at $z>3.5$, the jet apparent transverse speeds do not exceed 20 times the speed of light ($c$). This is consistent with earlier high-redshift quasar measurements in the literature and the tendency derived from low-redshift blazars that fast jet speeds ($>40\,c$) only occur at low redshifts. The results from this paper contribute to the understanding of the cosmological evolution of radio AGN.
分类： 天文学 >> 天文学 提交时间： 2023-02-19
摘要： Up to now, jet kinematic studies of radio quasars have barely reached beyond the redshift range at $z>3.5$. This significantly limits our knowledge of high-redshift jets, which can provide key information for understanding the jet nature and the growth of the black holes in the early Universe. In this paper, we selected 9 radio-loud quasars at $z>3.5$ which display milliarcsec-scale jet morphology. We provided evidence on the source nature by presenting high-resolution very long baseline interferometry (VLBI) images of the sample at 8.4~GHz frequency and making spectral index maps. We also consider Gaia optical positions that are available for 7 out of the 9 quasars, for a better identification of the jet components within the radio structures. We find that 6 sources can be classified as core--jet blazars. The remaining 3 objects are more likely young, jetted radio sources, compact symmetric objects. By including multi-epoch archival VLBI data, we also obtained jet component proper motions of the sample and estimated the jet kinematic and geometric parameters (Doppler factor, Lorentz factor, viewing angle). Our results show that at $z>3.5$, the jet apparent transverse speeds do not exceed 20 times the speed of light ($c$). This is consistent with earlier high-redshift quasar measurements in the literature and the tendency derived from low-redshift blazars that fast jet speeds ($>40\,c$) only occur at low redshifts. The results from this paper contribute to the understanding of the cosmological evolution of radio AGN.
分类： 天文学 >> 天文学 提交时间： 2023-02-19
摘要： The Square Kilometre Array (SKA) project consists of delivering two largest radio telescope arrays being built by the SKA Observatory (SKAO), which is an intergovernmental organization bringing together nations from around the world with China being one of the major member countries. The computing resources needed to process, distribute, curate and use the vast amount of data that will be generated by the SKA telescopes are too large for the SKAO to manage on its own. To address this challenge, the SKAO is working with the international community to create a shared, distributed data, computing and networking capability called the SKA Regional Centre Alliance. In this model, the SKAO will be supported by a global network of SKA Regional Centres (SRCs) distributed around the world in its member countries to build an end-to-end science data system that will provide astronomers with high-quality science products. SRCs undertake deep processing, scientific analysis, and long-term storage of the SKA data, as well as user support. China has been actively participating in and promoting the construction of SRCs. This paper introduces the international cooperation and ongoing prototyping of the global SRC network, the construction plan of the China SRC and describes in detail the China SRC prototype. The paper also presents examples of scientific applications of SKA precursor and pathfinder telescopes completed using resources from the China SRC prototype. Finally, the future prospects of the China SRC are presented.
分类： 天文学 >> 天文学 提交时间： 2023-02-19
摘要： Large surveys using modern telescopes are producing images that are increasing exponentially in size and quality. Identifying objects in the generated images by visual recognition is time-consuming and labor-intensive, while classifying the extracted radio sources is even more challenging. To address these challenges, we develop a deep learning-based radio source detector, named \textsc{HeTu}, which is capable of rapidly identifying and classifying radio sources in an automated manner for both compact and extended radio sources. \textsc{HeTu} is based on a combination of a residual network (ResNet) and feature pyramid network (FPN). We classify radio sources into four classes based on their morphology. The training images are manually labeled and data augmentation methods are applied to solve the data imbalance between the different classes. \textsc{HeTu} automatically locates the radio sources in the images and assigns them to one of the four classes. The experiment on the testing dataset shows an average operation time of 5.4 millisecond per image and a precision of 99.4\% for compact point-like sources and 98.1\% for double-lobe sources. We applied \textsc{HeTu} to the images obtained from the GaLactic and the Galactic Extragalactic All-Object Murchison Wide-field Array (GLEAM) survey project. More than 96.9\% of the \textsc{HeTu}-detected compact sources are matched compared to the source finding software used in the GLEAM. We also detected and classified 2,298 extended sources (including Fanaroff-Riley type I and II sources, and core-jet sources) above $5\sigma$. The cross-matching rates of extended sources are higher than 97\%, showing excellent performance of \textsc{HeTu} in identifying extended radio sources. \textsc{HeTu} provides an efficient tool for radio source finding and classification and can be applied to other scientific fields.
分类： 天文学 >> 天文学 提交时间： 2023-02-19
摘要： The Square Kilometre Array (SKA) project consists of delivering two largest radio telescope arrays being built by the SKA Observatory (SKAO), which is an intergovernmental organization bringing together nations from around the world with China being one of the major member countries. The computing resources needed to process, distribute, curate and use the vast amount of data that will be generated by the SKA telescopes are too large for the SKAO to manage on its own. To address this challenge, the SKAO is working with the international community to create a shared, distributed data, computing and networking capability called the SKA Regional Centre Alliance. In this model, the SKAO will be supported by a global network of SKA Regional Centres (SRCs) distributed around the world in its member countries to build an end-to-end science data system that will provide astronomers with high-quality science products. SRCs undertake deep processing, scientific analysis, and long-term storage of the SKA data, as well as user support. China has been actively participating in and promoting the construction of SRCs. This paper introduces the international cooperation and ongoing prototyping of the global SRC network, the construction plan of the China SRC and describes in detail the China SRC prototype. The paper also presents examples of scientific applications of SKA precursor and pathfinder telescopes completed using resources from the China SRC prototype. Finally, the future prospects of the China SRC are presented.
分类： 天文学 >> 天文学 提交时间： 2023-02-19
Yuanqi Liu
Ran Wang
Emmanuel Momjian
Yingkang Zhang
Tao An
Xiaolong Yang
Jeff Wagg
Eduardo Banados
Alain Omont
摘要： High-resolution imaging is crucial for exploring the origin and mechanism of radio emission in quasars, especially at high redshifts. We present 1.5 GHz Very Long Baseline Array (VLBA) images of the radio continuum emission from the radio-intermediate quasar (RIQ) J2242+0334 at $z = 5.9$. This object was previously detected at both 1.5 GHz and 3 GHz with the Karl G. Jansky Very Large Array (VLA) as a point source. However, there is no clear detection in the VLBA images at both the full resolution of 10.7 milliarcsecond (mas) $\times$ 4.5 mas (61.7 pc $\times$ 26.0 pc) and a tapered resolution of 26 mas $\times$ 21 mas (150 pc $\times$ 121 pc). This suggests that the radio emission from the quasar is diffuse on mas scales with surface brightness fainter than the $3\sigma$ detection limit of 40.5 $\mu \rm Jy \ beam^{-1}$ in the full resolution image. The radio emission in the RIQ J2242+0334 is likely to be wind-like (i.e., diffuse) rather than in the form of collimated jets. This is different from the previous radio detections of the most luminous quasars at $z \sim$6 which are usually dominated by compact, high brightness temperature radio sources. Meanwhile, compared with RIQs at low redshifts, the case of J2242+0334 suggests that not all RIQs are beamed radio-quiet quasars. This optically faint RIQ provides an important and unique example to investigate the radio activity in the less powerful active galactic nuclei at the earliest cosmic epoch.
分类： 天文学 >> 天文学 提交时间： 2023-02-19
Yuanqi Liu
Ran Wang
Emmanuel Momjian
Yingkang Zhang
Tao An
Xiaolong Yang
Jeff Wagg
Eduardo Banados
Alain Omont
摘要： High-resolution imaging is crucial for exploring the origin and mechanism of radio emission in quasars, especially at high redshifts. We present 1.5 GHz Very Long Baseline Array (VLBA) images of the radio continuum emission from the radio-intermediate quasar (RIQ) J2242+0334 at $z = 5.9$. This object was previously detected at both 1.5 GHz and 3 GHz with the Karl G. Jansky Very Large Array (VLA) as a point source. However, there is no clear detection in the VLBA images at both the full resolution of 10.7 milliarcsecond (mas) $\times$ 4.5 mas (61.7 pc $\times$ 26.0 pc) and a tapered resolution of 26 mas $\times$ 21 mas (150 pc $\times$ 121 pc). This suggests that the radio emission from the quasar is diffuse on mas scales with surface brightness fainter than the $3\sigma$ detection limit of 40.5 $\mu \rm Jy \ beam^{-1}$ in the full resolution image. The radio emission in the RIQ J2242+0334 is likely to be wind-like (i.e., diffuse) rather than in the form of collimated jets. This is different from the previous radio detections of the most luminous quasars at $z \sim$6 which are usually dominated by compact, high brightness temperature radio sources. Meanwhile, compared with RIQs at low redshifts, the case of J2242+0334 suggests that not all RIQs are beamed radio-quiet quasars. This optically faint RIQ provides an important and unique example to investigate the radio activity in the less powerful active galactic nuclei at the earliest cosmic epoch.
分类： 天文学 >> 天文学 提交时间： 2023-02-19
摘要： Studies of high redshift radio galaxies can shed light on the activity of active galactic nuclei (AGN) in massive elliptical galaxies, and on the assembly and evolution of galaxy clusters in the Universe. J1606+3124 has been tentatively identified as a radio galaxy at a redshift of 4.56, at an era of one-tenth of the current age of the Universe. Very long baseline interferometry (VLBI) images show a compact triple structure with a size of 68 parsecs. The radio properties of J1606+3124, including the edge-brightening morphology, peaked GHz radio spectrum, slow variability, and low jet speed, consistently indicate that it is a compact symmetric object (CSO). The radio source size and expansion rate of the hotspots suggest that J1606+3124 is a young (kinematic age of ~3600 years) radio source. Infrared observations reveal a gas- and dust-rich host galaxy environment, which may hinder the growth of the jet; however, the ultra-high jet power of J1606+3124 gives it an excellent chance to grow into a large-scale double-lobe radio galaxy. If its redshift and galaxy classification can be confirmed by further optical spectroscopic observations, J1606+3124 will be the highest redshift CSO galaxy known to date.
分类： 天文学 >> 天文学 提交时间： 2023-02-19
Ailing Wang
Tao An
Xiaopeng Cheng
Luis C. Ho
Kenneth I. Kellermann
Willem A. Baan
Jun Yang
Yingkang Zhang
摘要： We observed 20 Palomar-Green (PG) quasars at low redshift ($z 1 mJy, including 4 radio-loud quasars (RLQs) and 16 radio-quiet quasars (RQQs), using the Very Long Baseline Array (VLBA) at 5 GHz. Ten RQQs are clearly detected in the VLBA images, and a compact radio core is identified in eight of them, indicating the prevalence of active galactic nucleus (AGN)-related radio emission in this flux-density-limited RQQ sample. The RQQs and RLQs in our sample have a division at $\sim$30~mJy. The radio emission from RQQs appears to be the result of a combination of star formation and AGN-associated activities. All RQQs in our sample have a 5 GHz flux density ratio of Very Large Array (VLA) A-array to D-array $f_{\rm c} = S_{\rm A}^{\rm VLA}/S_{\rm D}^{\rm VLA}$ above 0.2. The RQQs with $f_a$ (VLBA and VLA flux density ratio $S^{\rm VLBA}/S_{\rm A}^{\rm VLA}) > 0.2$ versus $f_a < 0.2$ show significant differences in morphology, compactness and total flux density. $f_{\rm a}$ of RQQs is systematically lower than that of RLQs, probably due to the extended jets or relic jets of RQQs on 10s to 100s parsecs which are resolved out in VLBA images. Future larger samples, especially with the addition of milli-arcsec resolution radio images of RQQs with total flux densities below 1 mJy, can test the conclusions of this paper and contributes to the understanding of the radio emission mechanism of RQQs, and the dichotomy and physical connection between RQQs and RLQs.
分类： 天文学 >> 天文学 提交时间： 2023-02-19
Ailing Wang
Tao An
Xiaopeng Cheng
Luis C. Ho
Kenneth I. Kellermann
Willem A. Baan
Jun Yang
Yingkang Zhang
摘要： We observed 20 Palomar-Green (PG) quasars at low redshift ($z 1 mJy, including 4 radio-loud quasars (RLQs) and 16 radio-quiet quasars (RQQs), using the Very Long Baseline Array (VLBA) at 5 GHz. Ten RQQs are clearly detected in the VLBA images, and a compact radio core is identified in eight of them, indicating the prevalence of active galactic nucleus (AGN)-related radio emission in this flux-density-limited RQQ sample. The RQQs and RLQs in our sample have a division at $\sim$30~mJy. The radio emission from RQQs appears to be the result of a combination of star formation and AGN-associated activities. All RQQs in our sample have a 5 GHz flux density ratio of Very Large Array (VLA) A-array to D-array $f_{\rm c} = S_{\rm A}^{\rm VLA}/S_{\rm D}^{\rm VLA}$ above 0.2. The RQQs with $f_a$ (VLBA and VLA flux density ratio $S^{\rm VLBA}/S_{\rm A}^{\rm VLA}) > 0.2$ versus $f_a < 0.2$ show significant differences in morphology, compactness and total flux density. $f_{\rm a}$ of RQQs is systematically lower than that of RLQs, probably due to the extended jets or relic jets of RQQs on 10s to 100s parsecs which are resolved out in VLBA images. Future larger samples, especially with the addition of milli-arcsec resolution radio images of RQQs with total flux densities below 1 mJy, can test the conclusions of this paper and contributes to the understanding of the radio emission mechanism of RQQs, and the dichotomy and physical connection between RQQs and RLQs.
分类： 天文学 >> 天文学 提交时间： 2023-02-19
Yingkang Zhang
Tao An
Sándor Frey
Xiaolong Yang
Máté Krezinger
Oleg Titov
Alexey Melnikov
Pablo de Vicente
Fengchun Shu
Ailing Wang
摘要： Jets of high-redshift active galactic nuclei (AGNs) can be used to directly probe the activity of the black holes in the early Universe. Radio sources with jets misaligned with respect to the line of sight are expected to dominate the high-redshift AGN population. In this paper, we present the high-resolution imaging results of a z=4.57 AGN J2102+6015 by analyzing its multi-epoch dual-frequency very long baseline interferometry (VLBI) data. The 8.4-GHz VLBI images reveal two major features along the east-west direction separated by $\sim$ 10 milli-arcsec (mas). From the spectral index map, both features show flat/inverted spectra. The separation between the two features remains almost unchanged over an observation period of $\sim$ 13 years, placing an upper limit of the separation speed as about 0.04 mas year$^{-1}$. Previous studies have classified the source as a GHz-peaked spectrum quasar. Our results indicate that J2102+6015 is most likely a young, compact symmetric object rather than a blazar-type core-jet source.
分类： 天文学 >> 天文学 提交时间： 2023-02-19
Krisztina Perger
Yingkang Zhang
Sándor Frey
Tao An
Krisztina É. Gabányi
Leonid I. Gurvits
Chorng-Yuan Hwang
Ekaterina Koptelova
Zsolt Paragi
Ailing Wang
摘要： There are approximately 250 quasars discovered at redshift $z\geq6$, of which only a handful were detected in radio bands, and even fewer were imaged with the highest resolution very long baseline interferometry (VLBI) technique. Here we report the results of our dual-frequency observations with the Very Long Baseline Array (VLBA) of two such recently discovered quasars, VIKING J231818.35$-$311346.3 at $z=6.44$ and FIRST J233153.20$+$112952.11 at $z=6.57$. Both extremely distant sources were imaged with VLBI for the first time. The radio properties of the former are consistent with those of quasars with young radio jets. The latter has an UV/optical spectrum characteristic of BL Lac objects, of which no others have been found beyond redshift 4 so far. Our VLBA observations revealed a flat-spectrum compact radio source.
分类： 天文学 >> 天文学 提交时间： 2023-02-19
Leonid I. Gurvits
Sándor Frey
Máté Krezinger
Oleg Titov
Tao An
Yingkang Zhang
Alexander G. Polnarev
Krisztina É. Gabányi
Krisztina Perger
Alexey Melnikov
摘要： We present and briefly discuss results of several studies of the source J2102+6015 with tentatively defined redshift z=4:575 which demonstrates unusual properties in imaging and astrometric VLBI observations. Its properties might be considered as indications on the supermassive black hole binary which can be considered as a so far rare example of a high-redshift source of known electromagnetic and, possibly, predictable gravitational wave emissions.
分类： 天文学 >> 天文学 提交时间： 2023-02-19
Tao An
Ailing Wang
Yuanqi Liu
Yulia Sotnikova
Yingkang Zhang
J. N. H. S. Aditya
Sumit Jaiswal
George Khorunzhev
Baoqiang Lao
Ruqiu Lin
Alexander Mikhailov
Marat Mingaliev
Timur Mufakharov
Sergey Sazonov
摘要： Jets may have contributed to promoting the growth of seed black holes in the early Universe, and thus observations of radio-loud high-redshift quasars are crucial to understanding the growth and evolution of the early supermassive black holes. Here we report the radio properties of an X-ray bright $z=5.5$ quasar, SRGE J170245.3+130104 (J1702+1301). Our high-resolution radio images reveal the radio counterpart at the optical position of J1702+1301, while another radio component is also detected at $\sim$23.5\arcsec\ to the southwest. Our analysis suggests that this southwest component is associated with a foreground galaxy at $z\approx 0.677$, which is mixed with J1702+1301 in low-frequency low-resolution radio images. After removing the contamination from this foreground source, we recalculated the radio loudness of J1702+1301 to be $R>$1100, consistent with those of blazars. J1702+1301 exhibits a flat radio spectrum ($\alpha = -0.17 \pm 0.05$, $S \propto \nu^\alpha$) between 0.15 and 5 GHz; above 5 GHz, it shows a rising spectrum shape, and the spectral index $\alpha^{8.2}_{4.7}$ appears to be correlated with the variation of the flux density: in burst states, $\alpha^{8.2}_{4.7}$ becomes larger. J1702+1301 displays distinct radio variability on timescales from weeks to years in the source's rest frame. These radio properties, including high radio loudness, rising spectrum, and rapid variability, tend to support it as a blazar.
分类： 天文学 >> 天文学 提交时间： 2023-02-19
Tao An
Ailing Wang
Yuanqi Liu
Yulia Sotnikova
Yingkang Zhang
J. N. H. S. Aditya
Sumit Jaiswal
George Khorunzhev
Baoqiang Lao
Ruqiu Lin
Alexander Mikhailov
Marat Mingaliev
Timur Mufakharov
Sergey Sazonov
摘要： Jets may have contributed to promoting the growth of seed black holes in the early Universe, and thus observations of radio-loud high-redshift quasars are crucial to understanding the growth and evolution of the early supermassive black holes. Here we report the radio properties of an X-ray bright $z=5.5$ quasar, SRGE J170245.3+130104 (J1702+1301). Our high-resolution radio images reveal the radio counterpart at the optical position of J1702+1301, while another radio component is also detected at $\sim$23.5\arcsec\ to the southwest. Our analysis suggests that this southwest component is associated with a foreground galaxy at $z\approx 0.677$, which is mixed with J1702+1301 in low-frequency low-resolution radio images. After removing the contamination from this foreground source, we recalculated the radio loudness of J1702+1301 to be $R>$1100, consistent with those of blazars. J1702+1301 exhibits a flat radio spectrum ($\alpha = -0.17 \pm 0.05$, $S \propto \nu^\alpha$) between 0.15 and 5 GHz; above 5 GHz, it shows a rising spectrum shape, and the spectral index $\alpha^{8.2}_{4.7}$ appears to be correlated with the variation of the flux density: in burst states, $\alpha^{8.2}_{4.7}$ becomes larger. J1702+1301 displays distinct radio variability on timescales from weeks to years in the source's rest frame. These radio properties, including high radio loudness, rising spectrum, and rapid variability, tend to support it as a blazar.
分类： 天文学 >> 天文学 提交时间： 2023-02-19
Ailing Wang
Tao An
Shaoguang Guo
Prashanth Mohan
Wara Chamani
Willem A. Baan
Talvikki Hovatta
Heino Falcke
Tim J. Galvin
Natasha Hurley-Walker
Sumit Jaiswal
Anne Lahteenmaki
Baoqiang Lao
Weijia Lv
Merja Tornikoski
Yingkang Zhang
摘要： Disk winds and jets are ubiquitous in active galactic nuclei (AGN), and how these two components interact remains an open question. We study the radio properties of a radio-intermediate quasar III Zw 2. We detect two jet knots J1 and J2 on parsec scales, which move at a mildly apparent superluminal speed of $1.35\,c$. Two $\gamma$-ray flares were detected in III Zw 2 in 2009--2010, corresponding to the primary radio flare in late 2009 and the secondary radio flare in early 2010. The primary 2009 flare was found to be associated with the ejection of J2. The secondary 2010 flare occurred at a distance of $\sim$0.3 parsec from the central engine, probably resulting from the collision of the jet with the accretion disk wind. The variability characteristics of III Zw 2 (periodic radio flares, unstable periodicity, multiple quasi-periodic signals and possible harmonic relations between them) can be explained by the global instabilities of the accretion disk. These instabilities originating from the outer part of the warped disk propagate inwards and can lead to modulation of the accretion rate and consequent jet ejection. At the same time, the wobbling of the outer disk may also lead to oscillations of the boundary between the disk wind and the jet tunnel, resulting in changes in the jet-wind collision site. III Zw 2 is one of the few cases observed with jet-wind interactions, and the study in this paper is of general interest for gaining insight into the dynamic processes in the nuclear regions of AGN.
分类： 天文学 >> 天文学 提交时间： 2023-02-19
Ailing Wang
Tao An
Shaoguang Guo
Prashanth Mohan
Wara Chamani
Willem A. Baan
Talvikki Hovatta
Heino Falcke
Tim J. Galvin
Natasha Hurley-Walker
Sumit Jaiswal
Anne Lahteenmaki
Baoqiang Lao
Weijia Lv
Merja Tornikoski
Yingkang Zhang
摘要： Disk winds and jets are ubiquitous in active galactic nuclei (AGN), and how these two components interact remains an open question. We study the radio properties of a radio-intermediate quasar III Zw 2. We detect two jet knots J1 and J2 on parsec scales, which move at a mildly apparent superluminal speed of $1.35\,c$. Two $\gamma$-ray flares were detected in III Zw 2 in 2009--2010, corresponding to the primary radio flare in late 2009 and the secondary radio flare in early 2010. The primary 2009 flare was found to be associated with the ejection of J2. The secondary 2010 flare occurred at a distance of $\sim$0.3 parsec from the central engine, probably resulting from the collision of the jet with the accretion disk wind. The variability characteristics of III Zw 2 (periodic radio flares, unstable periodicity, multiple quasi-periodic signals and possible harmonic relations between them) can be explained by the global instabilities of the accretion disk. These instabilities originating from the outer part of the warped disk propagate inwards and can lead to modulation of the accretion rate and consequent jet ejection. At the same time, the wobbling of the outer disk may also lead to oscillations of the boundary between the disk wind and the jet tunnel, resulting in changes in the jet-wind collision site. III Zw 2 is one of the few cases observed with jet-wind interactions, and the study in this paper is of general interest for gaining insight into the dynamic processes in the nuclear regions of AGN.
分类： 天文学 >> 天文学 提交时间： 2023-02-19
Hyunwook Ro
Motoki Kino
Bong Won Sohn
Kazuhiro Hada
Jongho Park
Masanori Nakamura
Yuzhu Cui
Kunwoo Yi
Aeree Chung
Jeffrey Hodgson
Tomohisa Kawashima
Tao An
Sascha Trippe
Juan-Carlos Algaba
Jae-Young Kim
Satoko Sawada-Satoh
Kiyoaki Wajima
Zhiqiang Shen
Xiaopeng Cheng
Ilje Cho
摘要： Because of its proximity and the large size of its black hole, M87 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 M87 jet are limited to the innermost part of the jet or to HST-1. No attempt has yet been made to measure the magnetic field strength in between. We aim to infer the magnetic field strength of the M87 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. 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 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. The overall spectral morphology is broadly consistent over the course of these observations. The observed synchrotron spectrum rapidly steepens from $\alpha_{22-43 GHz}$ ~ -0.7 at ~ 2 mas to $\alpha_{22-43 GHz}$ ~ -2.5 at ~ 6 mas. 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 $B=(0.3 - 1.0 G)(z/2 mas)^{-0.73}$. Extrapolating to the EHT 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.
分类： 天文学 >> 天文学 提交时间： 2023-02-19
Kazunori Akiyama
Juan-Carlos Algaba
Tao An
Keiichi Asada
Kitiyanee Asanok
Do-Young Byun
Thanapol Chanapote
Wen Chen
Zhong Chen
Xiaopeng Cheng
James O. Chibueze
Ilje Cho
Se-Hyung Cho
Hyun-Soo Chung
Lang Cui
Yuzhu Cui
Akihiro Doi
Jian Dong
Kenta Fujisawa
Wei Gou
摘要： The East Asian VLBI Network (EAVN) is an international VLBI facility in East Asia and is operated under mutual collaboration between East Asian countries, as well as part of Southeast Asian and European countries. EAVN currently consists of 16 radio telescopes and three correlators located in China, Japan, and Korea, and is operated mainly at three frequency bands, 6.7, 22, and 43 GHz with the longest baseline length of 5078 km, resulting in the highest angular resolution of 0.28 milliarcseconds at 43 GHz. One of distinct capabilities of EAVN is multi-frequency simultaneous data reception at nine telescopes, which enable us to employ the frequency phase transfer technique to obtain better sensitivity at higher observing frequencies. EAVN started its open-use program in the second half of 2018, providing a total observing time of more than 1100 hours in a year. EAVN fills geographical gap in global VLBI array, resulting in enabling us to conduct contiguous high-resolution VLBI observations. EAVN has produced various scientific accomplishments especially in observations toward active galactic nuclei, evolved stars, and star-forming regions. These activities motivate us to initiate launch of the 'Global VLBI Alliance' to provide an opportunity of VLBI observation with the longest baselines on the earth.
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