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evn:evn_science [2017/05/15 08:59] – [Introduction] michael.lindqvistevn:evn_science [2018/09/12 09:34] – [A dust-enshrouded tidal disruption event with a resolved radio jet in a galaxy merger] antonis
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-[[http://www.jive.eu/jivewiki/doku.php?id=evn:guidelines|The next deadline is June 1, 2017]]. +[[http://www.jive.eu/jivewiki/doku.php?id=evn:guidelines|The next deadline is October 1, 2018]]. 
  
  
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 +=====Gravitationally-lensed radio arcs observed with global VLBI=====
 +
 +
 +
 +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, showing the complex surface brightness structure of the background
 +source in unprecedented detail (see Figure from [[https://doi.org/10.1093/mnras/sty1326|Spingola et al (2018)]]). Because of the complexity of this system, the imaging was
 +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) gravitaTonal arcs been detected at an angular resoluTon of a few mas. The
 +excellent uv-coverage and surface brightness sensiTvity 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.
 +
 +{{:evn:grav_lens_spingola.jpeg?400|}}
 +
 +**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. 
 +
 +
 +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, the lens mass model suggests an inner density slope for the main lensing galaxy that is steeper than isothermal. This is consistent with studies of other low-mass early-type satellite galaxies in dense environments, and is in agreement with the two-phase galaxy formation scenario (Guo & White 2008).
 +
 +====A dust-enshrouded tidal disruption event with a resolved radio jet in a galaxy merger====
 +
 +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, as the inferred angle of the jet to the line-of-sight was in clear disagreement with expectations from a "normal" AGN jet, while in the case of a TDE this angle can have any value.
 +
 +{{ :evn:arp299_transient_science2018.jpg?400 |}}
 +
 +**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, near-IR 2.2 micron images [insets (B) and (C)] showing the brightening of the B1 nucleus. (D) Radio evolution of Arp 299-B AT1 as imaged with VLBI at 8.4 GHz [7×7 mas region with the 8.4-GHz peak position in 2005, RA= 11h28m30.9875529s, dec= 58°33ʹ40ʹʹ.783601 (J2000.0), indicated by the dotted lines]. The VLBI images are aligned with an astrometric precision better than 50 mas. The initially unresolved radio source develops into a resolved jet structure a few years a_er the explosion, with the centre of the radio emission moving westward with time at an average intrinsic speed of 0.22 >mes the speed of light. The radio beam size for each epoch is indicated in the lower-right corner.
 +
 +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, resulting in TDEs injecting large amounts of energy into their surroundings. However, events similar to Arp 299-B AT1 would have remained hidden within dusty and dense environments, and would thus not be detectable by optical, UV or so| X-ray observations. Such TDEs from relatively massive, newly formed stars might provide a large radiative feedback, especially at higher redshifts where galaxy mergers and luminous infrared galaxies like Arp 299 are more common.
 +
 +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://science.sciencemag.org/content/early/2018/06/13/science.aao4669|Science, 2018]]
 +
 +
 ===== 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://www.nature.com/nature/journal/v541/n7635/full/nature20797.html | Chatterjee et al. (2017) ]] has been able to locate (with a 100 mas precision) the repeating FRB 121102. This has led to its unambiguous association with persistent radio and optical counterparts, and to the identification of its host galaxy. However, an even more precise localization is needed in order to probe the direct physical relationship between the millisecond bursts themselves and the associated persistent emission. [[http://www.nature.com/nature/journal/v541/n7635/full/nature20797.html | Chatterjee et al. (2017) ]] has been able to locate (with a 100 mas precision) the repeating FRB 121102. This has led to its unambiguous association with persistent radio and optical counterparts, and to the identification of its host galaxy. However, an even more precise localization is needed in order to probe the direct physical relationship between the millisecond bursts themselves and the associated persistent emission.
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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? =====
-[[https://academic.oup.com/mnras/article-lookup/doi/10.1093/mnras/stx160|Cao et al. 2017]] report the +[[https://academic.oup.com/mnras/article-lookup/doi/10.1093/mnras/stx160|Cao et al. (2017)]] report the 
 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, due to its relativistically beamed radio emission. Another target, J0839+5112 shows a compact radio structure typical of quasars.  There is evidence for flux density variability and its radio ‘core’ has a flat spectrum. However, the EVN data suggest that its emission is not Doppler-boosted. The remaining two blazar candidates (J1420+1205 (see Fig. 4) and J2220+0025) show radio properties totally unexpected from radio AGN with small-inclination jet. Their emission extends to arcsec scales and the Doppler factors of the central components are well below 1. Their structures resemble that of double-lobed radio AGN with large inclination to the line of sight. Their work underlines the importance of high-resolution VLBI imaging in confirming the blazar nature of high-redshift radio sources. 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, due to its relativistically beamed radio emission. Another target, J0839+5112 shows a compact radio structure typical of quasars.  There is evidence for flux density variability and its radio ‘core’ has a flat spectrum. However, the EVN data suggest that its emission is not Doppler-boosted. The remaining two blazar candidates (J1420+1205 (see Fig. 4) and J2220+0025) show radio properties totally unexpected from radio AGN with small-inclination jet. Their emission extends to arcsec scales and the Doppler factors of the central components are well below 1. Their structures resemble that of double-lobed radio AGN with large inclination to the line of sight. Their work underlines the importance of high-resolution VLBI imaging in confirming the blazar nature of high-redshift radio sources.
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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.
 +
 +
 +\\
 +===== No AGN evidence in NGC 1614 from deep radio VLBI observations  =====
 +
 +[[https://academic.oup.com/mnrasl/article-lookup/doi/10.1093/mnrasl/slx085| Herrero-Illana et al. (2017)]] present deep dual-band 5.0- and 8.4 GHz EVN observations of NGC 1614, a local luminous infrared galaxy with a powerful circumnuclear starburst ring, and whose nuclear engine origin is still controversial. The aim was to detect and characterize any compact radio structures both in the nuclear region and in the circumnuclear ring. They did not find any compact source in the central 200 pc region. However, they do detect a compact source in the circumnuclear ring located about 0.6 arcsec (190 pc) to the north of the nucleus. Its luminosity and spectral shape are indicative of a SN origin, possibly a Type IIn or Ic. Their result favours the pure starburst scenario, even for the nucleus of NGC 1614. 
 +
 +{{ :evn:slx085fig2.jpeg?600 |}}
 +**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