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A selection of recent EVN Science







Introduction

The European VLBI Network (EVN) is a VLBI network of radio telescopes in Europe and beyond, operated by an international Consortium of institutes. The EVN operates an “open sky” policy and observing proposals are accepted (and peer-reviewed) from astronomers from all over the world.

The next deadline is October 1, 2018.

VLBI continuum observations generally observe high brightness temperature radio emission from synchrotron or gyro-synchrotron processes with thermal emission perhaps detectable on short baselines. Radio spectral lines are observable in maser emission (mainly OH, H2O, CH3OH, and SiO) and in absorption against bright continuum sources (mainly in HI and OH).

VLBI observations with full-Stokes correlation allow investigation of polarization structure within your continuum or spectral-line sources. This in turn would allow detection and estimation of magnetic fields and ionised gas densities via Zeeman splitting, Faraday rotation, and Faraday depolarisation mechanisms.

VLBI has the extreme angular resolution to follow changes in source structure on timescales of hours (for stars), months (for many Galactic objects), and years (for many high-redshift systems).

Absolute and relative astrometry of compact radio structure is possible at the (sub)milliarsecond level, which for Galactic objects may allow the measurement of both parallax and proper motion to be made.

A selection of recent refereed EVN publications is presented here.


The Repeating Fast Radio Burst FRB 121102 as seen on milliarcsecond angular scales

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.

Marcote et al. (2017) report EVN observations, which simultaneously detect both the bursts and the persistent radio emission at milliarcsecond angular scales and show that they are co-located to within a projected linear separation of 40 pc (12 mas angular separation, at 95 % confidence). They detect consistent angular broadening of the bursts and persistent radio source (∼2–4 mas at 1.7 GHz), which are both similar to the expected Milky Way scattering contribution. They argue that a burst source associated with a low-luminosity active galactic nucleus or a young neutron star energizing a supernova remnant are the two scenarios for FRB121102 that best match the observed data.

Figure 1. EVN image of the persistent source at 1.7 GHz (white contours) together with the localization of the strongest burst (red cross), the other three observed bursts (gray crosses), and the position obtained after averaging all four bursts detected on 2016 September 20 (black cross).


Physical properties and astrometry of radio-emitting brown dwarf TVLM513-46546 revisited

Gawronski et al. 2017 present multi-epoch astrometric observations of the M9 ultra-cool dwarf TVLM513-46546 that is placed at the brown dwarf boundary. The new observations have been performed with the EVN at 6 cm. The target has been detected at 7 epochs spanning three years, with measured quiescent emission flux in the range 180-300 muJy. Combined with Very Long Baseline Array (VLBA) earlier data, their detections make it possible to refine the absolute parallax pi=93.27(+0.18,-0.17) mas. The data rule out TVLM513-46546 companions more massive than Jupiter in orbits with periods longer than ~1 yr.

Figure 2. Sky-projected, 5-parameter astrocentric model of TVLM 513 positions (grey curve) over-plotted with the VLBA (red filled circles) and with all new EVN detections (blue filled circles), relative to the first observation in (Forbrich et al. 2013).


Extended CH3OH maser flare excited by a bursting massive YSO

Moscadelli et al. (2017) followed an outburst in the 6.7 GHz methanol maser line (and in the IR) in the high-mass young stellar object S255. The goal was to study the change in the properties of the between the pre- and outburst phases. Both the maser flare and the extraordinarily large extent of the maser structure can be a natural consequence of the burst in the accretion luminosity of the high-mass YSO. Their results strongly support models that predict IR radiative pumping for the 6.7 GHz CH3 OH masers.

Figure 3. Distribution of the 6.7 GHz CH3OH masers toward NIRS 3. Circles and triangles represent maser spots before and after the outburst, respectively. Relative maser positions between the two epochs are accurate within a few mas. For more details, see Moscadelli et al. (2017).


VLBI observations of four radio quasars at z > 4: blazars or not?

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. 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.

Figure 4. Naturally weighted VLBI images of J1420+1205 at 1.7 and 5 GHz. At 5 GHz, only the south-eastern component was detected, which appears brighter and more compact in the 1.7-GHz image. The first contours are drawn at ±5σ image noise level. The positive contours increase by a factor of 2. The synthesized beam (FWHM) is shown at the lower-left corner of each image.


A multiwavelength study of GRB 151027A: the 999th Swift GRB

Nappo et al. (2017) study the X-ray, optical and radio emission of the bright Gamma Ray Burst GRB151027A. The radio late time curve proved essential to constrain the burst environment and jet energy. EVN observations, performed 22 and 138 days after the GRB, coupled with VLBA data taken in between (89 days) and an early-time (3 days) SRT upper limit, provide compelling evidence of a jet decelerated by a stratified density environment. This is what is expected if the progenitor were a massive W-R which underwent intense mass loss prior to its explosion. EVN observations (138 days after the burst) modelled together with early time X-ray and Optical data, lead to estimate a jet opening angle of 6.3 degrees and a jet radiative energy of 2.4×10^50 erg.

Figure 5. Radio observations (star symbols) performed with the EVN (at 22 and 138 days) and with the VLBA at 89 days. The SRT upper limit is also shown. The model curves show the case of a jet expanding in a constant density interstellar medium (solid line) and in a stratified density profile (where the density scales as the distance from the source as r^-2). These curves are obtained reproducing also the X-ray and optical data, not shown here for clarity.

Figure 6. The 5 GHz EVN image of GRB 151027A taken on 2015 November 18.


No AGN evidence in NGC 1614 from deep radio VLBI observations

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.

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).


evn/evn_science.1536741950.txt.gz · Last modified: 2018/09/12 08:45 by antonis