© JvL

The original data, from WSRT plus PuMaI, are Fig 1. in Janssen & van Leeuwen (2004)

The poster was one of the entries in the Astronomy Pretty Poster Pageant (APPP) 2014. All entries are available for editing and outreach purposes within ASTRON.

© Madroon Community Consultants (MCC)

In addition to presentations about the LOFAR experience, Stefan Wijnholds discussed the more general topic of calibratability of the new generation of giant radio telescopes. He stressed the danger that, even with perfect imaging and calibration software, the instrument will not allow us to reach the thermal noise level if the wrong design decisions are made. For instance if the PSF sidelobes are too high.

The picture shows Mark Waterson, Tim Cornwell and Maria Grazia Labate of the SKA Office, being shown one of the latest LOFAR images(*) by Ger de Bruyn, the leader of the LOFAR EoR group. All under the benign gaze of Jan Geralt bij de Vaate, the leader of the SKA LFAA and MFAA (Aperture Array) consortia, who hosted the visit.

(*) The deep LOFAR image of the North Polar Cap (NCP) was made by Sarod Yatawatta. It emphasizes the many faint foreground sources that have to be subtracted before the elusive Epoch of Reionization (EoR) can be studied.

© Stas Shabala (University of Tasmania)

While we understand the consequences of AGN feedback reasonably well, teasing out the physical mechanisms driving the feedback has proved tricky. In particular, quantifying the efficiency of AGN feedback is difficult. A promising way forward is to combine dynamical models of radio AGN with semi-analytic galaxy formation models. I will describe these two techniques, and explain how they can be used together to (i) interpret AGN observables; and (ii) study the mechanisms responsible for AGN triggering and feedback.

© JvL

In it, we report on the first low-frequency fast-radio burst (FRB) limit. The detection of such bright, highly dispersed radio bursts of apparent extra-galactic origin was first reported by Lorimer and team in 2007 and further detections have since been reported from Parkes and Arecibo.

As the LPPS survey provides both long dwell times and large field of view, the survey data can be used to either detect or limit FRBs at low radio frequencies. LPPS covered a large fraction of the northern sky, ~1.4x10^4 sq.deg., with 1-hr dwell times. Each observation covered ~75 sq.deg. using 7 independent fields formed by incoherently summing the high-band antenna fields.

The left panel in the image above shows this large on-sky time. The Galactic plane and Center are shown with a grey line and cross, respectively. The grayscale of the individual beams shows the usable observation length, where white is 0 min, and black the full 57 min. LPPS data was commissioning data and a significant fraction needed to be discarded because of early LOFAR teething problems. Still LPPS provided the equivalent of 9.7 minutes of all-sky integration, one of the highest coverages of any survey to date.

We searched all LPPS single-pulse data for bursts at dispersion measures (DMs) between 2--3000 pc/cc. This is a much larger DM than predicted for any typical line-of-sight through our Galaxy away from the Galactic center. All found bursts were associated with either a known pulsar or RFI. Thus, no FRBs were detected in LPPS.

From this, we derive a limit on the occurrence, at 142 MHz, of bright (S > 107 Jy for the narrowest) dispersed radio bursts of

In ongoing LOFAR transient searches, we are continuing to improve on this limit and better constrain the spectra and scattering properties of such bursts. These searches will either soon detect such signals or show that the higher-frequency (~1.4 GHz) window is ideal for their detection.

© ASTRON

This year, in Toronto, Mike Garrett was elected with 5 others as new IAA Corresponding Members of the Basic Sciences section - from Dwingeloo he joins Leonid Gurvits who was elected in 2012 - see http://www.astron.nl/dailyimage/index.html?main.php?date=20120928

A few days later, Mike was also elected to the position of vice-chair of the SETI PC. The image above shows his induction into the IAA at the members dinner with the presentation being made by the IAA president, Dr. G. Madhavan Nair, former chair of India's Space Research Organisation (ISRO). The dinner was attended by many space aficionados, including Apollo 11 astronaut, Buzz Aldrin.

© ASTRON

The relevant electrical properties of these materials are the dielectric constant and the loss tangent. These are measured using a rectangular cavity. To investigate a material, the resonance frequency and quality factor of the cavity are measured first while empty, and then while filled with the material under test.

The shift of the resonance frequency is a measure for the dielectric constant of the material. Since the quality factor is a measure for the conductive losses of the cavity, the "empty" and "filled" values can be used to calculate the loss tangent of the material under test, which is a measure for the loss of the material.

The pictures show a cavity, together with the measurement setup, and a more detailed view of the cavity. The latter consists of an aluminum box with a removable lid. To avoid RF-leakage from the (narrow) slot between box and lid, copper tape is used to close this slot.

© ASTRON

Katinka was supervised by Raffaella Morganti and Tom Oosterloo and a copy of her thesis can be found on http://dissertations.ub.rug.nl/faculties/science/2014/k.gereb/?pLanguage=en&pFullItemRecord=ON

The thesis focused on a large amount of data taken with the Westerbork Synthesis Radio Telescope to carry out a spectral stacking analysis of the HI properties of galaxies. Stacking is the process of adding the spectra of many individual galaxies, with the goal of recovering the faint average HI signal in the combined spectrum. This method is particularly useful to obtain a sensitivity below that of the instrument, not only for sources in the nearby Universe, but also at large distances where the flux emitted by individual sources falls well below the sensitivity limit of current radio telescopes.

Neutral hydrogen gas is thought to play an important role in the evolution of galaxies, as it provides the gas from which stars eventually form. Furthermore, it is thought to be partly responsible for the triggering of one of the most energetic phenomena in the Universe, i.e. the active galactic nuclei (AGN), by feeding supermassive black holes in the centres of galaxies. Katinka has investigated the HI properties of hundreds of galaxies to get a better understanding of the role of HI in the evolution of galaxies over the past 1.5 billion year. She also looked at the gas properties of accreting supermassive black holes to investigate the interplay between AGN activity and the surrounding gas.

She obtained a number of interesting results. For example that the presence of HI is well correlated with the star formation (SF) history. In galaxies where cold gas is present, conditions are favorable for (residual) SF to be seen. Furthermore, galaxies with more gas have a higher star formation rate. And finally, she found that young AGN are rich in unsettled HI gas, supporting the hypothesis that nuclear activity in radio galaxies is triggered through feeding of cold gas.

The work done by Katinka has been highly relevant in preparation for the HI surveys that will be done with Apertif. Some of the results have already been published in refereed papers, and they appeared on the Daily Images of August 20, 2014 and November 26, 2013. We expect more to come!

Katinka has now left for a PostDoc position in Melbourne: good luck for your future carrier from your friends and colleagues at ASTRON!

© Chomiuk et al. (2014), Nature

Novae are stellar explosions in binary stellar systems where a dense white dwarf accretes material from a companion star, and this eventually triggers a thermonuclear explosion. These explosions have been known to astronomers for a long time, but it was not expected that they would produce very high energy gamma ray emission. It came as a surprise when Nova Monoceros (V959 Mon) was detected by NASA's Fermi spacecraft in June 2012. This event was followed-up by a number of radio telescopes including the European VLBI Network (EVN), e-MERLIN in the UK, as well as the Karl G. Jansky Very Large Array (VLA), and the Very Long Baseline Array (VLBA) in the US. These produced very high resolution images of the exploding ejecta that helped solve the mystery of where the gamma ray emission was produced. The results were published in the prestigious journal Nature, by a group led by Laura Chomiuk (Michigan State University). For more details, see the JIVE press release.

The upper panel of the figure shows radio images of V959 Mon. The EVN images on 91 days and 113 days after the nova explosion are shown in contours and colour scale, respectively. The compact ejecta are shocks that produced relativistic electrons as well as the observed gamma rays. In the middle, the VLA image on day 126 (colour scale) shows east-west expansion of hot gas (apparently offset from the orientation of the structure observed by the EVN, overlaid in contours). The VLA image to the right on day 615 (colour scale) shows a structure dominated by a slow outflow of thermal gas expanding in north-south direction. The lower panel explains the various stages of the explosion: a) immediately following the thermonuclear runaway, the nova envelope expands and interacts with the binary system, yielding dense material in the equatorial plane. b) the white dwarf powers a fast wind that is funnelled towards the low-density poles, causing shocks. c) once the white dwarf wind ceases, the polar outflow will detach from the binary and quickly drop in density as it expands, while the slower-expanding equatorial material will remain dense.

© ASTRON/SHAO

Prior to this workshop, there were three partially overlapping mission concepts in the ultra-long wavelength radio regime: a three-dimensional satellite array in the Sun-Earth L2 Lagrangian point, a linear array in a low Moon-orbit, and a set of a few satellites also in Moon orbit. These three were merged into one concept, with acronym DSL and a tentatively chosen expanded name:

Discovering the Sky at the Longest wavelengths

This mission would open up the last virtually unexplored frequency range, which is inaccessible from Earth due to the Earth's ionospheric cut-off. Its science would include a full-sky continuum survey of discrete sources and of continuum diffuse emission, transients, and search for signatures of the Dark Ages, to name but a few.

The picture shows part of the DSL team posing at the quay close to the workshop venue, the Eigtveds Pakhus, which is a historical building complex located next to the Ministry of foreign affairs in Copenhagen. The inset upper left shows Leonid Gurvits (JIVE/TUDelft) outlining the science that can be done with the DSL. The upper right inset shows Jingye Yan (NSSC) explaining the DSL concept and technologies.

© ASTRON

Although the number of FPGAs is halved compared with version of UniBoard, the complexity is severe due to a large number of transceivers that need to be routed. The total number of transceiver lines to be connected is 696. The data rate of each line is at least 10 Gbps. Beside these transceiver lines, the board also needs connections to accommodate DDR4 memory modules. These devices will also be available just in time.

The total number of connections on the UniBoard^2 is ~10,000. To route all those connections, multiple PCB layers are necessary. The total amount of layers for UniBoard^2 will be 18.

In order to meet the deadline of the UniBoard^2 project, Gijs and Sjouke have volunteered to work in shifts. One week Gijs works during the day and Sjouke from 16:00 to midnight, while the other week the schedule is flipped. Another magnificent example of their prewar level of commitment.

UniBoard^2 is a European collaboration led by JIVE, and funded by Radionet3. ASTRON is responsible for the board design and, in cooperation with JIVE, for the development of the testing firmware.

© Zhang, Astron

The first image is an optical image with the HI contours overlaid. The lowest contour is about 2x10^19 cm^-2 . From the data cube we obtained we can see a bright ring-like structure that appears to have a fainter, more inclined outer extension. The intensity distribution also shows that there is a central HI component and there is a gap between the central component and the bright ring. We modelled the HI emission using the GIPSY task GALMOD. The HI data cube can be well modelled by using very simple parameters in the model. According to the model, the central component is nearly perpendicular to the bright ring. The second image we show here is the velocity field from the model (black contours) overlaid on the observed velocity field (white contours). From the velocity field we can see that the central HI component is rotating in a very different plane than the bright ring.

When we put HI contours on the optical image, we find that star formation occurs exactly in those places where the HI column density is above 3 x 10^20 cm^-2. This means that, despite the very different galaxy setting, the star formation in UGC 9515 behaves exactly as it does in spiral galaxies.

Maolin, Zhang, worked on this project as a summer student supervised by Brad Frank and Tom Oosterloo at ASTRON. He is currently doing his Master degree in department of Engineering physics at Tsinghua University in Beijing, China.

© Madroon Community Consultants (MCC)

The school is meant to broaden the knowledge of astronomy graduate students. Courses are taught dealing with topics that were missed during the undergraduate studies. Every graduate student in The Netherlands must participate in a NOVA Fall school at least once; participation in two schools is recommended.

Judging by their names, the participants hail from a fascinating range of countries all over the world, with only a small minority of recognizably Dutch ones. This is a powerful statement about the international position of Dutch astronomy. Of course the common language is English, and there is a satisfying gender balance.

The students are lodged in a nearby hotel, where they indulge in wholesome activities in the evening. And of course they get around on bicycles, which are neatly parked in the designated area.

© David Cseh

I will show our recent discovery of the jet of the ULX Holmberg II X-1. The energetics of the jet, the associated radio and optical bubble may imply the presence of a different kind of stellar-mass black hole and accretion history. After that I will focus on the origin of the radio emission and its evolution from an intermediate-mass black hole candidate, HLX-1.

These recent findings point towards a promising era of future surveys capable to address whether the black holes are evenly populated, whether jets are indeed scale-invariant, or whether accretion can be unified.

© John McKean / EVN / VLBA-GBT (NRAO)

Gravitational lensing is the deflection of light from a distant background object (the source) by an intervening mass distribution (the lens). If the surface mass density of the lens is sufficiently high, then multiple images of the background source, which are often highly magnified and distorted, are produced. The gravitational lensing phenomena is beautifully illustrated in this global very long baseline interferometry image of MG J0751+2716 at redshift 3.2. Here, the extended background radio source is highly distorted into several images, some of which are stretched to form large gravitational arcs. Never before has such high angular resolution of extended arcs been seen before, which highlights the excellent sensitivity that can be achieved with VLBI arrays today (a collecting area that is about 10 per cent of the proposed SKA).

The image was made from a 18 hour observation with an array that included 10 stations from the EVN, 10 stations from the VLBA and the GBT. The image noise is about 10 uJy / beam and the resolution is about 2 x 7 mas. The maximum image separation of the arcs is about 1 arcsecond.

© Gianni Alvito, INAF - Osservatorio Astronomico di Cagliari

The attendees of the symposium all contributed to the excellent scientific program, and in return they could enjoy the hospitality of Cagliari and the great Sardinian food. The symposium was held at the Hotel Regina Margherita located on the board of one of the central historical quarter of the city called "La Marina", just 10 minutes bus ride from the famous 8-km beach called "Poetto". "La Marina" was founded by Pisans in the XIII century and nowadays hosts several restaurants where it is possible to taste a large variety of Sardinian cuisine. The lunch buffet at Is Alinos (about 50 km from Cagliari) and the conference dinner on Thursday (never-ending food!) were also great opportunities to sample that.

Before the conference dinner, we played the traditional symposium football match. The two teams were the "Italian Antennas" and the "Rest of the Array". The final score was 1-1 which is quite uncommon. A possible reason is the crowded pitch (for the first time the match was played 13 vs 13), alternatively, the participants may have gained too much weight during the week (see above).

At the end it was a great symposium both scientifically and socially speaking. The location was perfect with its almost 30 degrees (have I already mentioned that?) and with its beauty. Many participants spent the weekend after the symposium to visit other parts of the island as well (e.g., Nora, Barumini). But in my humble opinion the screen in the conference room showed the best what the EVN community could expect from the 12th EVN symposium. Future astronomers will look at those slides as we are now doing by looking at the giants sculpture discovered in these days in the ancient Sardinia ( http://www.independent.co.uk/news/science/archaeology/prehistoric-cybermen-sardinias-lost-warriors-rise-from-the-dust-6988952.html ).

© John McKean / EVN / VLBA-GBT (NRAO)

The image was made from a 22 hour observation with an array that included 11 stations from the EVN, 7 stations from the VLBA and the GBT. The image noise is about 30 uJy / beam and the resolution is about 4 x 2 mas. The maximum separation of the images is about 1 arcsecond.

© Rob Millenaar

The exhibition brings together works of art by indigenous artists from both areas in a magnificent collection, equally breathtaking as the sky itself. The exhibition was opened last month in Perth, Australia and will ultimately travel to all corners of the globe. Next stop South Africa.

The Yamaji and other aboriginal artists have contributed paintings that are done in the typical colourful pointillistic style on phenomena in the sky, like the stars, planets, sun and moon and the Emu in the sky: the Milky Way.

The descendants of San people of the Karoo in South Africa have contributed artworks done in textiles, depicting stories of creation and ancient celestial culture.

More background information can be found on the website of the SKA:

https://www.skatelescope.org/shared-sky/

© Hubble: NASA, ESA, and Q.D. Wang (University of Massachusetts, Amherst); Spitzer: NASA, Jet Propulsion Laboratory, and S. Stolovy (Spitzer Science Center/Caltech)

However, the recent discovery of voluminous plumes of gamma-ray and radio emission seemingly emanating from the central regions of the Galaxy have begun to question this view. Much work has been done showing that these objects are perhaps due to the central black hole, either as an outburst or a sustained period of outflow similar to other 'radio-loud' galaxies in the Universe. The contra-point to this is that these Galactic-scale emissions come from the regular star-forming dust and gas residing in the Galactic centre, emanating as a wind and working to 'inflate' these large bubble-like objects.

In this talk, I will describe recent observational and theoretical work being done by myself and colleagues that is beginning to test these two models that gives key insights into the past of the centre of our Galaxy and answer the question of whether the Galactic centre black hole can dominate large parts of the Galaxy. The answer to this question has important ramifications about not only our own Galaxy and its past and future, but also galaxy evolution as a whole.

© JvL

The first survey, the LOFAR Pilot Pulsar Survey (LPPS), observed a large fraction of the northern sky and re-detected 65 previously known pulsars (background profiles in the image above). 54 of these were blind re-detections, a further 11 were found by folding at known pulsar periods. On completion LPPS was the deepest large-area survey in the 100 MHz regime to date. The 54 blind detections constitute the first low-frequency sample that is adequate in size to serve as input for a pulsar population model. We thus populated the modeled Galaxy with a population of pulsars that in van Leeuwen & Stappers (2010) best reproduced the survey results of 6 large surveys.

In our simulation, 2.7 million pulsars formed throughout the Galaxy. Of these, 50,000 were above the death line at the present day and beamed toward Earth. 10% of those are shown as the white dots in the foreground panel of the image above; 9,000 are in the LPPS Field of View. For 1,200 of these, the scatter and dispersion broadening exceeds their rotational period, making them undetectable. Of the remaining 8,000 pulsars, 80+-20 are bright enough to be detected in LPPS (circles in the foreground panel in the image). That is in reasonable agreement with the actual number of blind detections of 54. Some of the difference between these numbers could be in either the modeled survey (e.g., remaining incomplete understanding of the incoherent-addition of these commissioning era data) or the modeled population (e.g., the low-frequency spectrum turn-over behaving differently than simulated). Overall, these simulations confirm that our best pulsar population models (van Leeuwen & Stappers 2010) can accurately predict low-frequency surveys.

© ASTRON