© © Anne Archibald, Elizabeth Mahony 2014

Our group studies pulsars from young to old, slow to fast, and from LOFAR frequencies up to high-energy gamma-rays. Recent results include: the discovery of correlated X-ray and radio mode-switching in pulsar B0943+10 (Hermsen et al. 2013, Science); a detailed study of PSR B0943+10's mode switching properties with LOFAR (Bilous et al. 2014, A&A); the discovery and modeling of the first pulsar stellar triple system (Ransom et al. 2014, Nature); LOFAR studies of the interstellar medium using cyclic spectroscopy (Archibald et al. 2014, ApJ); observations of state transitions in millisecond pulsar XSS J12270-4859 (Bassa et al. 2014, MNRAS) and PSR J1023+0038 (Patruno et al. 2014, ApJ); an overview of the mJIVE-20 survey with the VLBA (Deller et al. 2014, ApJ); and a continuous 24-hour campaign to assess precision timing of the millisecond pulsar J1713+0747 (Dolch et al. 2014).

© JIVE

A new, shiny map will soon be installed. Just like its predecessor, it will highlight (by means of bright LEDs) the radio telescopes that are supplying our users with a steady stream of high-quality data, from all over the world.

The image shows Martin Leeuwinga installing the LEDs one by one. Here he is busy with one of the EVN's latest members, the Sardinia Radio Telescope.

© Cormac Purcell (Sydney Uni)

© ASTRON, 2014 - 2015

The image on the left shows the distribution of compound beams. During the actual measurement, the seven highlighted ones were measured. Their shape was determined by a single scan over a bright source.

The image on the right shows the result for 300 MHz for two of these compound beams (top and bottom panels). This illustrates the Phased Array Feed concept: it is possible to form multiple compound beams at the same time, by using just a single telescope.

For this measurement, the 61 APERTIF receiver chains (LNA, DCU, LOG, ADU and UniBoard) were used. Each compound beam was formed by adding the receiver signals in the beamformer with suitable weights. The beamformer input datarate is 61 x 800e6 x 8 = 390 Gbps. The total bandwidth of 300 MHz is built up from 384 subbands which also had to be weighted. So, for all 37 compound beams, 61 [receiver chains] x 384 [subbands] x 37 [compound beams] = 866688 weight values need to be determined. This was done by performing a so called 'hot/cold' measurement on the supernova remnant Cassiopeia A, followed by the max-snr algorithm.

© DESP

The correlator, which is implemented on 16 UniBoards, will receive dual polarized data from 12 WSRT telescopes. Each telescope has a local beamformer that generates 37 independent beams of 300 MHz for two polarizations. All beams are transported to the correlator via optical fiber links creating a total input data rate of 3.7 Terabit per second (which corresponds to 100 DVD's per second!). This is quite a lot, considering that the input data rate for the Cobalt correlator of Lofar is �only� 320 Gbit/s.

The blockdiagram gives an overview of the digital system of Apertif. The upper part shows the beamformer system, which is implemented near each telescope. The bottom part is the correlator, which is located in the central building.

Parts of the correlator functionality are physically implemented in the beamformer system. For example the delay tracker, which compensates for differences in path length in between the telescopes, is implemented directly after digitizing (ADC) the antenna signals. In the beamformer system, the filterbank splits the 400 MHz input signal into subbands, such that the beams can be formed by means of a phase shift. Within the beamformer, the subband data is copied in order to create 37 independent beams.

Finally, the transpose function reorders the output data of the beamformer in such a way that the correlator can easily integrate the visibilities for a second, without having to store intermediate correlation products. The transpose function will be integrated on the local beamformers based on 384 x 4 Gbyte DDR3 modules (= 1.5 Tbyte).

So yes, we are making progress. Each module contains a progress bar that shows the actual status. In the coming months we will finish the remaining modules, and by the end of May we will start the integration of the whole system. This will be very exciting. The functional modules in the lower part of the diagram will be explained in future daily images. Stay tuned!

© ASTRON

After presenting DSL at the ESA-CAS workshop in Copenhagen last year, the DSL team meets again this week to discuss the science cases for DSL and to discuss the technical concept in preparation for the proposal. The picture above shows colleagues from the Netherlands, China, France and Poland during a break on the first day of the DSL2015 workshop at ASTRON.

(*) http://sci.esa.int/jump.cfm?oid=55262

© AN

Before it reaches our telescopes, this emission has to propagate first through the pulsar magnetosphere, then through the interstellar medium, and then the Earth's ionosphere. These different media distort the intrinsic polarisation signal but the propagation effects that cause such distortion are measurable and provide invaluable information about the properties of those media.

We have used the LOFAR core stations (i.e. the inner 3km) to capture the polarisation properties of 20 pulsars at 150 MHz, with unprecedented detail for this frequency regime. By combining our LOFAR observations with archival data at higher frequencies, we were able to map the evolution of the polarisation between 1400 MHz and 150 MHz. This helped us investigate the observational effects of magnetospheric birefringence, wherein the ordinary (O) and extraordinary (X) modes of the linearly polarised emission are refracted away from each other as the radio waves propagate in the magnetosphere.

In addition, we studied the distorting effects of interstellar scattering on measurements of the amount of Faraday rotation towards pulsars. Surprisingly, these effects are more evident at higher frequencies than in LOFAR data. Finally, we have translated temporal delays between the arrival time of the total and the polarised emission at 150 MHz into emission heights, assuming that such delays are caused by relativistic aberration due to the pulsars' fast rotation. In all examined cases, we find that the low-frequency radio emission is generated within a few hundred km above the pulsar surface.

A paper describing these results has been accepted for publication in Astronomy & Astrophysics: Pulsar polarisation below 200 MHz: Average profiles and propagation effects by Noutsos et al. 2015, A&A in press

© V. Beskin, A. Philippov, M. Kramer, P. Jaroenjittichai

Editor's note: Due to a technical problem with the submitted image we use this jolly snowman to draw your esteemed attention to today's colloqium.

In the talk, we firstly include into consideration the transition from geometrical optics to vacuum propagation, the cyclotron absorption, and the wave refraction simultaneously. In addition, non-dipole magnetic field configuration, drift motion of plasma particles, and their realistic energy distribution were taken into account. The one-to-one correspondence between the signs of circular polarization and position angle derivative for both ordinary and extraordinary waves is predicted. It is shown that the p.a.curve could differ from the rotation vector model (RVM) prediction. We critically confront the theory with observational data on a large data sample and show that observations support the main prediction of propagation theory.

© Derek McKay-Bukowski

As our experience grows, we get ever better at packaging things in such a way that the new owners can roll out their station with a minimum of fuss. One of our Finnish friends was so impressed that he created these IKEA-like assembly instructions. His kind gesture was much appreciated by the ASTRON Family Service Division (AFSD).

The exported LOFAR stations can be used stand-alone, but also as part of an ever-growing fibre-linked array that is centered on the superterp in Exloo (NL). They represent long baselines that allow observations with very high spatial resolution (1" for a baseline of 400km, @150MHz). These baselines can be made extra sensitive by using the entire LOFAR core as a single station. Among other things, such long baselines give LOFAR an important advantage in the ability to remove the many foreground sources that obscure the elusive Epoch of Reionization (EoR).

© Astron

As the head of the workshop, he has over these years been responsible for the continuous innovation with respect to instruments, technologies, materials and processes. For that reason, we had cake with the image of Gyro Gearloose (Willie Wortel).

In his speech to Jan, Johan Pragt as head of the Mechanics group also mentioned the many apprentices that Jan has been coaching in his career. Many craftsman owe him for transferring his knowledge and enthusiasm. As a sign of our appreciation, Jan was awarded the title of "Master Craftsman".

Jan: congratulations with this anniversary, and we hope that the continuation of your career at Astron will be as inspiring as the previous 40 years!

© ASTRON

The winner for 2014 is ...(drrumm rroll)...Pieter Benthem who submitted an impressive image of ASTRON staff having recently deployed the MFAA environmental prototypes at the SKA site in South Africa.

By a tradition of sorts, the winner of this award receives a bottle of the most excellent Irish Whiskey (see image above*). The Jameson distillery is very well connected to the world of radio science - born in Scotland, the famous distiller Andrew Jameson (1783 -1856) was the father of Annie Jameson (1840-1920), and Annie was the mother of one Guglielmo Marconi, Nobel prize winner in Physics (1909), and the father of wireless (radio) communication!

(*) Editor's note: As an extra incentive, the winner(s) will be ushered into the Presence, and treated to some motivational words. In addition, the winning picture will be on prominent display in the Director's office for a year. Of course the decent thing to do would be to share the bottle with the long-suffering AJDI editors, and perhaps with our greatest fan Himself.

© R. F. Pizzo

Since this date, significant effort was devoted by various people working at and in collaboration with ASTRON to make sure that the new station would be ready for production observing as soon as possible at the beginning of the new year.

The observing campaign to record calibration data has already delivered good calibration tables for the supported HBA modes. In the meantime, the new station has been included in test observations that helped to highlight a few remaining network issues preventing proper data recording when adopting 9 international stations in the observing setup. Thanks to the intensive testing performed by the Radio Observatory personnel, these network problems have been identified and solved on February 4th 2015. On the same day, DE609 participated successfully to the first record LOFAR observation correlating 71 antenna fields, of which 48 are core stations, 14 remote stations and 9 international stations! The plot above shows the visibility amplitude of the various stations included in this observing run and it highlights that signal was successfully recorded from all 9 LOFAR international stations. DE609 successfully participated to the first Cycle production run the same evening.

Each LOFAR station adds valuable collecting area to the ILT, thus allowing astronomers to detect ever-fainter signals. The placement of the new station is such that it provides critical intermediate baselines, thus improving the imaging capabilities of the LOFAR array. 3 more international LOFAR stations are being built in Poland and will become part of the array by the end of the year.

© Alexander Wagner (University of Tsukuba)

© E. Varenius/Onsala Space Observatory/LOFAR collaboration

The image is a composite of two observed radio bands, 2.5 m/118 MHz [orange] and 1.9 m/154 MHz [blue], and covers about 3 kiloparsecs at the distance of M 82. The multi scale image shows compact unresolved supernova remnants embedded in diffuse emission (sampled by the 100 km Dutch remote baselines). This image demonstrates that, despite ionospheric propagation effects, high-resolution high-sensitivity images can be made at metre-wavelengths; both using the baseline lengths envisioned for SKA1-low and also using longer many thousand kilometre baselines that could be deployed in later stages of the SKA project.

In the image we see the effects of strong free-free absorption in the star-forming disk of M82, with brighter emission found below and above this disk. The radio continuum spectra of the compact sources show a variety of shapes implying patchy free-free absorption. We expect detailed analysis of the spectra of both the diffuse and compact components to be able to constrain the distribution of ionised gas in the centre of this starburst galaxy, and whether or not this ionised gas is well mixed with the synchrotron emitting gas.

Read the full press-release about this image at Chalmers. The research is described in the paper "Subarcsecond international LOFAR radio images of the M82 nucleus at 118 MHz and 154 MHz" by E. Varenius et al., published in Astronomy & Astrophysics. A version of the paper is available online at Arxiv.

© ASTRON/JWTH

This year's students, Bastiaan, Koen, Xanthe and Zazo, analyzed 14 LOFAR pulsar observations in order to blindly discover the pulsar's rotational period and dispersion measure, refine their parameter estimation and then track the changes of the rotational period with time. Thereafter they used this information to model the pulsar's binary orbit and the properties of the eclipsing gas in this system.

As part of the project, the students also write-up a report on their findings and present them to their peers in Amsterdam. Part of their report and the python-based data reduction pipeline they wrote is shown here.

Thanks also to Roy Smits for giving them a tour of Westerbork!

© Astron

Radio-loud active galactic nuclei (AGN) are known to inject kinetic energy into the surrounding interstellar medium of their host galaxy via plasma jets. Understanding the impact these flows can have on the host galaxy helps to characterise a crucial phase in their evolution.

Being the closest radio galaxy, Centaurus A is an excellent laboratory in which the physics of the coupling of jet mechanical energy to the surrounding medium may be investigated. About 15 kpc northeast of this galaxy, a particularly complex region is found: the so-called outer filament, where jet-cloud interactions have been thought to occur. In the Daily Image of

© Morganti

Aleksandar has completed a thesis focused on LOFAR data of radio galaxies, analysing their spectral properties to derive parameters about their active life. His PhD has been a shared project between ASTRON and the Kapteyn Institute, with Raffaella Morganti and Peter Barthel as supervisors.

As John McKean pointed out in his question to the candidate during the ceremony, the thesis nicely shows the evolution, in the last four years, of LOFAR, with the improvement of both the quality of the data as well as the calibration techniques.

The work of Aleksandar has initially been limited by the quality of the LOFAR images and he had to focus on the study of single (known) objects. However, the thesis shows the importance of low frequencies for deriving the time-scale of the active and quiescent phase of radio sources.

Aleksandar has two papers based on LOFAR data already submitted to Astronomy & Astrophysics and one about to be. Hopefully he will have the chance to use and expand what he has done for his thesis during his postDoc in the Support group of the RO.

Good luck, Aleksandar!

© Spitler et al. 2014

The image above shows the 0.7 seconds during which the first FRB discovered using the Arecibo Observatory swept across the frequency band. The inset panel shows the average pulse profile, after correcting for dispersion due to the ionised component of the intervening medium that causes a pulse emitted at lower frequencies to arrive later than the same pulse emitted at higher frequencies.

See Spitler et al. 2014 for further information.

© JvL

The four 10-minute episodes will cover the inner planets, outer planets, Milky Way, and finally the Universe.

Klokhuis is an educational show on national channel 3, that aims to inform kids from 9 to 15 on a wide variety of topics (and is fun to watch for their parents, too). It's on every week day at 18.26, and generally draws ~300,000 viewers.

ASTRON has previously featured on Klokhuis, most recently in 2012.

Part of the 2015 astronomy series will feature presenters Nienke and I flying through space. We first visit most of the planets in the Solar System, then venture out to other Galaxies, and even get close, but not too close!, to a black hole.

For this we recorded in front of a chroma key screen for one long day in Amsterdam Noord (see image above). Turns out it's not so easy to casually deliver your lines, over and over, while trying to keep your 90 kgs "flying" and balanced for hours.

Pretending to be weightless sure hurts.

© ASTRON

Participants presented their latest results and methods for analysis and data reduction, with ample time allowed for feedback and discussion on the best way to proceed with future work. This lead to many fruitful discussions on possible solutions to outstanding problems and the best way to efficiently produce the highest quality science results.

The picture above shows the participants of the meeting (top) and a composite LOFAR and 2MASS image of the the powerful FR-II radio galaxy 3C452 (bottom). Pictured from right to left: Huub Rottgering, Jeremy Harwood (chair), Raffaella Morganti (co-chair), Francesco de Gasperin, Marisa Brienza, Volker Heesen, Therese Cantwell, Judith Croston, Emanuela Orru, Leith Godfrey, Elizabeth Mahony, Javier Moldon, Kristina Nyland, Tim Shimwell, Nicolas Vilchez (not pictured: Leah Morabito, Pepe Sabater, Martin Hardcastle, Aleksandar Shulevski, Magdalena Kunert-Bajraszewska)