Science frontiers for SKA-VLBI
During this period the Critical Design Reviews for the different elements of the SKA telescope to assess the completeness, compliance and adequateness of the different designs have continued. Telescope Manager and Signal and Data Transport elements have successfully passed their reviews, in particular good shape with respect to the VLBI capability implementation.
This report outlines examples of recent and fascinating scientific results that demonstrates the relevance that the high-resolution VLBI component will have for the SKA telescope. In parallel, key technical advances, such as a new direction dependent calibration scheme to correct for systematic errors at lower frequencies, will contribute to the success of the SKA inclusion in the VLBI networks.
As part of the work package 10 “VLBI with the SKA” of the JUMPING JIVE project (“Joining up Users for Maximizing the Profile, the Innovation and Necessary Globalization of JIVE”), a workshop to discuss Key Science Projects that will be enabled by SKA-VLBI is being planned for fall 2019. Heads-up!
SHARP – V. Modelling gravitationally-lensed radio arcs imaged with global VLBI observations
Spingola et al. present one of the sharpest images of the gravitationally lensed radio source MG J0751+2716 (at z = 3.2) obtained with global Very Long Baseline Interferometry (VLBI) at 1.65 GHz (Fig. 1). Even fainter gravitationally lensed radio sources will be detected when the SKA telescope contributes to the global arrays (SKA-VLBI). In this study the background object is highly resolved in the tangential and radial directions, showing evidence of both compact and extended structure across several gravitational arcs. By identifying compact sub-components in the multiple images, the mass distribution of the foreground z = 0.35 gravitational lens is constrained using analytic models for the main deﬂector and for the members of the galaxy group, showing an inner mass-density slope steeper than isothermal for the main lensing galaxy. Additionally, at the milli-arcsecond level, the assumption of a smooth mass distribution fails, requiring additional structure in the model. However, given the environment of the lensing galaxy, 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 of the galaxy group.
Figure 1. Global VLBI image of radio source MG J0751+2716 at 1.65 GHz obtained by using uniform weights and multi-scale cleaning in wsclean. The European VLBI Network, the Very Long Baseline Array and the Green Bank Telescope from NRAO were part of the experiment to form a global array of telescopes (Spingola et al. 2018, MNRAS).
LEAP: An innovative direction-dependent ionosphere calibration method for low frequency arrays
The ambitious scientific goals of the SKA require a matching capabilityforcalibrationofthe systematic errors that contaminate the observed signals. M. Rioja and R. Dodson recently visited the SKA H/Q and presented a scheme, LEAP, foraddressingthe direction-dependent (DD) ionospheric and instrumental phase effects at the low frequencies and with fields of view planned for SKA-Low (Rioja et al. 2018, MNRAS, 478, 2337).
LEAP is an embarrassingly parallel process,that ismultiple directions can be processed independently and simultaneously, as it does not depend on a complete sky model. Using MWA Phase I observations at 88 and 154 MHz under various weather conditions,LEAPhas beenshownto be ableto measure and correct for temporal and direction-dependent spatial distortions on a wide range of scales, including those comparabletoor less than the array size.
Initialanalysis of MWA Phase IIobservations, with baselines up to 6 km, shows that the incidence of higher order spatial ionosphericdistortions, on scales comparable to or smaller than the array size and on short timescales, becomeeven more significant. Such ionospheric distortions can only be corrected for in the visibility domain, so a DD-correction scheme such as LEAP is imperative for MWA Phase II.Furthermore, LEAP it is expected to scale well(being embarrassingly parallel)to the requirements of SKA-Low.
Fig. 2 a-d: Sequence of four consecutive, 30-secondlong, Direction Dependent (DD) Ionospheric screens, measured with LEAP above the MWA-Phase 2 array, for a given direction within theFoV at150 MHz.The Z-axis represents the DD ionospheric phase errors (in degrees) and the X,Y-axisare the antenna coordinates in metres. They suggest a change in character compared to those from MWA Phase 1 observations with shorter baselines:significant higher order ionospheric phase structureis seenover the array, on a finer scale thanthe size of the array, along with significant temporal fluctuations.In these cases, thedominant image artefactisdefocusing, in addition tothewell-knownposition shifts,and calibration in the visibility domain is required.
Fig. 2 e-f: Images of the same source before (Fig. 2-e, left) and after (Fig. 2-f, right) DD LEAP calibration using theionospheric screens shown to correct for the DD distortions. The LEAP calibration results in a 25% peak flux increase in the image.
A tidal disruption event with a resolved radio jet in a galaxy merger
SKA-VLBI will allow the detection of a great number of tidal disruption events (TDEs), that are transient flares produced when a star is ripped apart by the gravitational field of a supermassive black hole. In this study the authors have observed a transient source in the western nucleus of the merging galaxy pair Arp 299 that radiated >1.5×1052 erg in the infrared and radio but was not luminous at optical or X-ray wavelengths. They interpret this as a TDE with much of its emission re-radiated at infrared wavelengths by dust. The radio observations resolved an expanding and decelerating jet, probing the jet formation and evolution around a SMBH (Mattila et al. 2018, Science, DOI: 10.1126/science.aao4669).
Figure 3. The transient Arp 299-B AT1 and its host galaxy Arp 299. (A) A color-composite optical image from the Hubble Space Telescope (HST), with high resolution near-infrared 2.2 μm images (insets B and C) showing the brightening of the B1 nucleus. (D) The evolution of the radio morphology as imaged with Very Long Baseline Interferometry (VLBI) at 8.4 GHz in four epochs. The VLBI images are aligned with an astrometric precision better than 50 μas.
Early science observations with the East Asia VLBI Network
The East Asia VLBI Network (EAVN, Fig. 4) is a collaborative effort in the East Asian region. It currently consists of 21 telescopes with diverse equipment configurations and frequency setups, allowing flexible subarrays for specific science projects (An et al. 2018, Nature Astronomy, 2, 118). The EAVN plays an important role in fostering the fast-growing Asian VLBI community, offering easy access to the start-of-the-art VLBI facilities, and strengthening the regional science and technique collaboration in radio astronomy. The EAVN will also make significant contribution to the global VLBI network and the future SKA-VLBI, filling the gap between the EVN and the VLBA in the northern hemisphere, and linking to the Australian VLBI network. The first Call for Proposals was announced in April 2018 with official operations starting from September 2018. The 11th East Asia VLBI Workshop 2018 will take place in PyeongChang, Korea, on 4-7 September 2018.
Recent and future meetings and workshops with relevance for the SKA-VLBI
European Week of Astronomy and Space Science 2018, Liverpool, April 4, 2018
VLBI had a relevant presence in the latest edition of the EWASS in Liverpool. VLBI experts presented their scientific results in different sessions devoted to Cosmology, Strong Gravity, Novae and Astrometry, showing that VLBI is the only means of achieving sub-milliarcsecond scale angular resolution imaging in astronomy. Particularly successful was the Special Session SS11 Exploring the Universe: a European vision for the future of VLBI, where it was discussed the role of VLBI in the context of the challenges and open questions of astrophysics in the next decade. For a grand finale, in Dr. Phil Diamond’s plenary talk a special mention to VLBI and its contribution to the Square Kilometre Array was made. A dedicated SKA-VLBI poster was presented in the Special Session SS11 summarizing the VLBI capabilities with the SKA and the sharpest and deepest science that it will allow (Fig. 5).
The 14th European VLBI Network Symposium and Users Meeting, Granada, October 8-11, 2018.
This biennial meeting is the main forum for discussion of the latest VLBI scientific results and technical and technological developments within the EVN member countries. Apart from a wide range of science topics, the meeting will also focus on the role of the EVN on:
- Very high-sensitivity VLBI with the SKA
- Future multi wavelength and multi messenger astronomy including high angular-resolution astronomy at other wavelengths
Italy – South Africa Radio Astronomy Workshop “Enhancing the collaboration in radio-astronomy between Italy and South Africa”, Pretoria, October 24-25, 2018.
Radio astronomy is a key area of investment in Italy and South Africa. The aim of the workshop is to further strengthen and expand the existing successful collaborations between the two countries, whose main purpose is to pave the way to the full exploitation of the Square Kilometre Array’s breakthrough in both countries.
The meeting will focus on three key areas:
- SKA and Very Long Baseline Interferometry operations: African VLBI Network and Italian VLBI Network, VLBI with MeerKAT.
- SKA challenges: Big data and cloud computing; big data and data analytics; artificial intelligence for astronomy.
- The added value of industrial partnership – Joint human capital development initiatives in radio astronomy.