evn:evn_science
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evn:evn_science [2017/05/02 11:38] – michael.lindqvist | evn:evn_science [2018/09/13 13:39] – [A selection of recent EVN Science] antonis | ||
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===== Introduction ===== | ===== Introduction ===== | ||
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+ | =====Gravitationally-lensed radio arcs observed with global VLBI===== | ||
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+ | The Strong lensing at High Angular ResoluTon Program (SHARP) led by McKean (ASTRON) has | ||
+ | obtained a deep global VLBI observaTon of the gravitaTonal lens MG J0751+2716 at 1.65 GHz | ||
+ | (project GM070; PI: McKean). The 18.5 hours observation used 24 antennas from the EVN and | ||
+ | the VLBA, including the large apertures (> 50 m) of the Lovell, Effelsberg, Robledo and Green Bank | ||
+ | telescopes. Data were recorded at 512 Mbps and correlated at JIVE to produce 8 spectral | ||
+ | windows (IFs) with 8 MHz bandwidth and 32 channels each, through both circular polarisaTons. | ||
+ | MG J0751+2716 is one of the few quadruply imaged radio-loud gravitationally lensed quasars that | ||
+ | show extended arcs on VLBI-scales. The global VLBI L-band deep imaging detects all of the extended | ||
+ | arcs at high significance, | ||
+ | source in unprecedented detail (see Figure from [[https:// | ||
+ | performed using multi-scale cleaning within the wsclean algorithm (Offringa et al. 2014). The total | ||
+ | flux density of the target is 350 mJy and the off-source rms is 41 μJy/beam. Never before have such | ||
+ | extended (200-600 mas) gravitational arcs been detected at an angular resoluTon of a few mas. The | ||
+ | excellent uv-coverage and surface brightness sensitivity provided by the global VLBI array have been | ||
+ | fundamental for a precise study of the structure of the extended arcs on mas-scales from | ||
+ | MG J0751+2716. | ||
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+ | **Figure 1.**Global VLBI imaging of MG J0751+2716 at 1.65 GHz (Spingola et al. (2018)). The off-source | ||
+ | rms is 41 μJy/beam and the peak surface brightness is 2.9 mJy/beam. The restored beam is 5.5x1.8 | ||
+ | mas, and is shown within the white box in the bottom-hand corner. | ||
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+ | Spingola et al. (2018) analysed these observations and identified lensed emission corresponding to the same source component, providing a very large number of constraints on the mass model that also sampled a large radial and tangential extent. When performing the mass modelling of this system, they found a discrepancy between the observed and predicted positions of the lensed images, with an average position rms of the order of 3 mas, which is much larger that the measurement errors (40 μas on average). A possible explanation for the offset between the observed and model-predicted positions is the presence of some additional mass structure (e.g. Metcalf & Madau 2001). However, since the lensing galaxy lies in a small group of galaxies, it is not clear whether this extra mass is in the form of sub-haloes within the lens or along the line of sight, or from a more complex halo for the galaxy group. Furthermore, | ||
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+ | ==== A dust-enshrouded tidal disruption event with a resolved radio jet in a galaxy merger ==== | ||
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+ | Tidal disruption events (TDEs) are transient flares produced when a star is ripped apart by the gravitational field of a supermassive black hole (SMBH). In a TDE, roughly half of the star’s mass is ejected, whereas the other half is accreted onto the SMBH, generating a bright flare that is normally detected at X-ray, ultraviolet (UV), and optical wavelengths. TDEs are also expected to produce radio transients, lasting from months to years and including the formation of a relativistic jet, if a fraction of the accretion power is channelled into a relativistic outflow. | ||
+ | An international team of astronomers have, for the first time, directly imaged the formation and expansion of a fast-moving jet of material ejected when the powerful gravity of the SMBH in the nucleus of Arp 299-B (D=45 Mpc) ripped apart a star that wandered too close to the cosmic monster in Arp 299-B. It is one of the two merging galaxies (Arp 299-A and Arp 299-B) forming the Arp 299 system, which hosts prolific supernova factories in its nuclear regions. | ||
+ | The team tracked the event with radio and infrared telescopes, including the EVN, for over a decade. The patient, continued observations with the EVN and other radio telescopes around the world, eventually showed the source of radio emission expanding in one direction, just as expected for a jet (Fig. 2). The measured expansion indicated that the material in the jet moved at an average of about one-fourth the speed of light. The crucial piece of information solving the puzzle of this event was provided by VLBI observations, | ||
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+ | **Figure:** The tidal disruption event Arp 299-B AT1 and its expanding radio jet. (A) A color-composite optical image from the HST, with high-resolution, | ||
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+ | The gravitational field of the SMBH in Arp 299-B, with a mass 20 million Tmes that of the Sun, shredded a star with a mass more than twice that of the Sun. This resulted in a TDE that was not seen in the optical or X-rays because of the very dense medium surrounding the SMBH, but was detected in the near-infrared and radio. The so| X-ray photons produced by the event were efficiently reprocessed into UV and optical photons by the dense gas, and further to infrared wavelengths by dust in the nuclear environment. Efficient reprocessing of the energy might thus resolve the outstanding problem of observed luminosities of optically detected TDEs being generally lower than predicted. | ||
+ | The case of Arp 299-B AT1 suggests that recently formed massive stars are being accreted onto the SMBH in such environments, | ||
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+ | Published in: Makla S., Perez-Torres M., et al.: A dust enshrouded tidal disruption event with a resolved radio jet in a galaxy merger. [[http:// | ||
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+ | ==== Space-VLBI observations resolve the edge-brightened jet in 3C 84 (NGC1275) at 30 microarcseconds from the core ==== | ||
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+ | An international team of researchers from eight different countries has imaged with unprecedented accuracy the newly forming jets of plasma from the core of NGC1275, the central galaxy of the Perseus cluster, identified with radio source 3C 84. Radio images made with an array including the RadioAstron Space Radio Telescope (SRT) and a global array of ground radio telescopes resolve the jet structure ten times closer to the central engine than what has been possible in previous ground- based observations. | ||
+ | These space-VLBI observations were obtained within the RadioAstron Nearby AGN Key Science Project coordinated by Tuomas Savolainen. 3C 84 was observed on September 21-22, 2013. In addition to SRT, more than two dozen ground radio telescopes, including the EVN, the KVN, Kalyazin and the NRAO VLBA, GBT and the phased JVLA participated in the experiment. First results are now published in Giovannini et al. (2018). | ||
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+ | **Figure:** Radio image of the central parsec in 3C 84 obtained with the space-VLBI array. The half- power-beam-width (HPBW) is 0.10 x 0.05 mas at PA 0o. The noise level is 1.4 mJy/beam and the peak intensity is 0.75 Jy/beam. The radio core and emission features C2 and C3 are indicated in the image. | ||
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+ | The 22 GHz space-VLBI image shows that the edge-brightened jet in 3C 84 is surprisingly wide, with a transverse radius greater than 250 gravitational radii at a de-projected distance of 350 gravitational radii from the core. If the bright outer jet layer is launched by the black hole ergosphere, it has to rapidly expand laterally closer to the central engine. If this is not the case, then this jet sheath is likely launched from the accretion disk. | ||
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+ | Another major result discussed in the paper is that the previously known, almost cylindrical jet collimation profile on the scales larger than a few thousand gravitational radii extends down to a scale of a few hundred gravitational radii. It indicates a flat density profile of the external confining medium. This result is in contrast with the M87 jet collimation profile. One obvious difference between M87 and 3C 84 jets is the young age of the latter. The dynamical age of the C3 feature (the head of the restarted jet in 3C 84) at the time of the space VLBI observation is only about 10 years. The dynamical age of the jet is less than what is likely needed for the relaxation of the system, and we may not be seeing the final structure of the jet. | ||
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===== The Repeating Fast Radio Burst FRB 121102 as seen on milliarcsecond angular scales ===== | ===== The Repeating Fast Radio Burst FRB 121102 as seen on milliarcsecond angular scales ===== | ||
[[http:// | [[http:// | ||
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===== VLBI observations of four radio quasars at z > 4: blazars or not? ===== | ===== VLBI observations of four radio quasars at z > 4: blazars or not? ===== | ||
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observations of four blazar candidates at redshift z > 4 with the European VLBI Network (EVN) at 1.7 and 5 GHz. | observations of four blazar candidates at redshift z > 4 with the European VLBI Network (EVN) at 1.7 and 5 GHz. | ||
These objects were previously classified as blazar candidates based on X-ray observations. One of them, J2134−0419 is firmly confirmed as a blazar with our VLBI observations, | These objects were previously classified as blazar candidates based on X-ray observations. One of them, J2134−0419 is firmly confirmed as a blazar with our VLBI observations, | ||
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**Figure 6.** | **Figure 6.** | ||
The 5 GHz EVN image of GRB 151027A taken on 2015 November 18. | The 5 GHz EVN image of GRB 151027A taken on 2015 November 18. | ||
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+ | ===== No AGN evidence in NGC 1614 from deep radio VLBI observations | ||
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+ | **Figure 7.** | ||
+ | EVN maps of the detected compact source at the C band (5.0 GHz; left-hand panel) and the X band (8.4 GHz; right-hand panel). | ||
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evn/evn_science.txt · Last modified: 2021/05/03 07:30 by kazi