Science frontiers for SKA-VLBI
Since last reported, a series of fascinating scientific results have demonstrated the relevance that the high-resolution component for the SKA telescope will have. Additionally, key technical advances that have taken place recently will pave the way towards a successful collaboration of the SKA with the VLBI networks. This report outlines the most important achievements during 2017.
Since November 1st 2017, the SKA VLBI working group leadership has changed. After two very productive years, leading the SKA design towards a successful VLBI component integration, Zsolt Paragi (JIVE) gives way to Tao An (ShAO) that will from now one co-chair the group together with the already appointed co-chair Cormac Reynolds (Curtin University).
EVN telescopes zoom in on gravitational wave detection
For the first time, scientists have directly detected gravitational waves in addition to light from the collision of two neutron stars. The discovery was made using the U.S.-based Laser Interferometer Gravitational-Wave Observatory (LIGO); the Europe-based Virgo detector; and some 70 ground- and space-based observatories.
Neutron stars are the smallest, densest stars known to exist and are formed when massive stars explode in supernovas. As these neutron stars spiralled together, they emitted gravitational waves that were detectable for about 100 seconds; when they collided, a flash of light in the form of gamma rays was emitted and seen on Earth about two seconds after the gravitational waves. In the days and weeks following the collision, other forms of light, or electromagnetic radiation – including X-ray, ultraviolet, optical, infrared, and radio waves – were detected.
While still too faint to be detected on baselines of thousands of km, these observations are part of ongoing efforts to localise the source of transient radio emission in the sky as it becomes brighter, with the highest precision to date, and with improve sensitivities, like the ones that will be provided by the SKA.
The collaboration between LIGO/Virgo and EVN/e-Merlin is part of the ASTERICS project that has received funding from the European Union’s Horizon 2020 research and innovation programme under grant agreement No 653477.
EVN localisation of recurrent Fast Radio Burst FRB121102
Fast Radio Bursts (FRBs) are very short duration (a few milliseconds) dispersed radio signals of unknown origin. The high dispersion measure indicated that FRBs are most likely extragalactic in origin. If true, these signals could be used to probe the distribution of baryonic matter in the Universe, fundamental to cosmological models. The only way to reliably measure the position of an FRB is direct, interferometric detection of a burst. At JIVE they have developed a technique to localize ms-duration signals with the EVN (see Paragi 2016, arXiv 1612.00508). The discovery of repeated bursts from FRB 121102 (Spitler et al. 2016, Nature, 531, 202) provided an excellent opportunity to demonstrate this technique. EVN and Arecibo observations performed in September 2016 resulted in the detection of four bursts, the brightest of which clearly showing that the source of the bursts and the persistent radio source are co-located within about 10 mas (see Fig. 2, from Marcote et al. 2017, ApJL, 834, L8). The persistent radio source itself is located in a dwarf galaxy at a redshift of z=0.1927 (Tendulkar et al. 2017, ApJL, 834, L7).
This mode of observations and the special processing of the VLBI data at JIVE (dedispersion, high time/frequency resolution correlation, gating) is available to the whole EVN community.
Successful combined use of linear and circular polarizers in the e-EVN.
Pioneering e-EVN observations at C-band demonstrated how SKA will be able to contribute to the VLBI networks with its linear polarization receivers, without additional signal processing. The observation EO014 was carried out on December 2016, in a “mixed polarisation” basis (i.e., where stations with linear and circular polarisers are observing simultaneously). Effelsberg antenna used a receiver with linear polarisers, whereas the other 6 participating e-EVN stations were using circular-polarisation receivers. Fringes were successfully correlated in this “mixed polarisation” basis and the program PolConvert (Marti-Vidal et al. 2016) was used to calibrate and transform these fringes into a pure circular basis (see Fig. 3).
Towards the African VLBI Network of radio telescopes.
The African VLBI Network (AVN) will play a key role for the successful inclusion of the SKA telescope in VLBI observations, providing intermediate length baselines to the SKA to improve the uv-coverage for observations with the EVN.
A first milestone in the development of the AVN has been the demonstration of fringes during a VLBI test experiment with the EVN and a 32-m converted telecommunications antenna at Kuntunse, Ghana (Fig. 4). The fringes were found by combining data from Kuntunse and other participating EVN stations using the SFXC software correlator, designed by the Joint Institute for VLBI ERIC (JIVE). This experiment is one of three positive detections, with the other two successes including methanol maser detection and pulsar observations, showing that the Kuntunse antenna can be used as a radio telescope for single dish observations and as part of a VLBI network.
Recent VLBI meetings and workshops with relevance for the SKA
e-MERLIN and EVN in the SKA era (JBO, 11-12 September 2017)
This meeting has been the second of a series of open meetings and workshops to allow the astronomical community to discuss and contribute to the future developments and operations of the UK’s National Radio Astronomy Facility, e-MERLIN and the European VLBI Network (EVN). The main purpose of the meeting was to inform the community of the existing and upgraded capabilities of e-MERLIN and the EVN and discuss the scientific and technical priorities for these facilities in the coming years. The meeting took place at Jodrell Bank Observatory, 11-12 September 2017. For further details, see http://www.jb.man.ac.uk/meeWngs/JBOinSKAeraII
6th International VLBI Technology workshop (9-11 October 2017)
Rapid advances in technologies relevant to VLBI are foreseen in many fields: data recording, transmission, correlation and data analysis. It has, in some cases, brought about a major re-thinking of the traditional ways of the VLBI observing technique and how we make prominent scientific progresses in both astronomy and geodesy. In addition to reports on current and near-term VLBI technology plans and achievements, an important focus of this meeting will be the opportunity for forward-looking views of VLBI technology in the decade of the 2020s.
The meeting took place in Bologna, Italy, 9-11 October 2017. For further details see: https://indico.ira.inaf.it/event/2/overview
JUMPING JIVE Project
The EC Horizon2020 project “Joining up Users for Maximizing the Profile, the Innovation and Necessary Globalization of JIVE” (“JUMPING JIVE”) took off at the beginning of 2017. This project aims to prepare and position European VLBI in the SKA era, with JIVE and the EVN as globally recognized centres of excellence in radio astronomy. Detailed information on the project can be found at:
The project work packages cover a number of topics, two of them are especially relevant for SKA:
- ’Integrating new elements’ work package has already demonstrated fringes with the Kuntunse telescope in Ghana, being the first operational element of the future African VLBI Network (AVN) and,
- ‘VLBI with SKA’ work package to support the SKA VLBI scientific working group, thanks to a liaison scientist, Cristina Garcia-Miro, that works at the SKA Organisation to discuss the technical and operational issues for doing VLBI experiments involving the SKA telescope.
The kick-off meeting of the project took place on February 20-21 2017 in Leiden, the Netherlands, Fig. 5. Rene Vermeulen (ASTRON) was elected as chair of the JJ board, and John Conway (Onsala Space Observatory) as vicechair.