© Sarrvesh Sridhar

Since the first version was published a few years ago, the LOFAR imaging cookbook has undergone significant changes. These reflect the significant changes that have been made to the telescope and the post-processing pipelines used to calibrate and image LOFAR data.

With this version, the Imaging Cookbook is available as a PDF and as a separate web site: https://support.astron.nl/LOFARImagingCookbook

The LOFAR Imaging Cookbook is edited by Sarrvesh Sridhar with contributions to individual chapters by many LOFAR users including G. van Diepen, T. J. Dijkema, F. de Gasperin, M. Iacobelli, A. Offringa, E. Orru, D. Rafferty, B. van der Tol, V. Vacca, R. van Weeren, W. Williams and S. Yatawatta.

© Colloquium

Alongside the mammoth efforts to reduce the LOFAR data, there has been a parallel effort to combine these unprecedented radio samples with available panchromatic datasets to provide robust optical counterparts, redshift estimates and extract physical properties through the latest techniques for modelling spectral energy distributions (e.g. Calistro Rivera et al. 2017).

© JWTH/ASTRON

Given that the radio bursts apparently have no multi-wavelength counterparts, we were strongly motivated to investigate the properties of the radio waves at higher radio frequencies - in the hopes of finding more clues as to the nature of this enigmatic source.

Using high-frequency (4-8GHz) observations with Arecibo and the Green Bank Telescope, we have recently discovered that the radio bursts from FRB 121102 are highly linearly polarized and show an extreme Faraday rotation (about 146000 rad/m2 in the source's reference frame; about 500x larger than measured in any other FRB to date). Furthermore, one of the bursts we detected has a very short duration of only about 30 microseconds, thereby strongly constraining the size of the emission region and suggesting a neutron star origin for the bursts (as also previously suspected).

The extreme Faraday rotation implies an extreme magneto-ionic environment for the source, which we hypothesize could be in the immediate environment of an accreting massive black hole or within a surrounding nebula and supernova remnant.

On Jan 10th, 2018 we presented these results at the 231st meeting of the American Astronomical Society in Washington, during a dedicated press conference. This picture shows the presenters in front of a poster of the most recent cover of Nature, in which the results appear. This beautiful cover artwork was made by Danielle Futselaar.

Presenters from left to right: Andrew Seymour (USRA, Arecibo), Jason Hessels (ASTRON / UvA), Betsey Adams (ASTRON / Kapteyn), Daniele Michilli (ASTRON / UvA) and Vishal Gajjar (Breakthrough Listen, Berkeley).

© Carole Jackson

© ASTRON

Designing a significant upgrade in hardware, algorithms, and software will allow to create, at a fraction of the cost of building a new facility, a large scale cutting-edge research facility providing simultaneous independent access to both the radio astronomy and the space weather research communities. LOFAR4SW will in particular address capabilities to monitor Solar dynamic spectra, interstellar scintillation measurements of densities and velocities in the inner heliosphere, and high-resolution measurements of TEC variations in the Earth�s ionosphere. A strong point of the LOFAR4SW is to uniquely enable provision of the missing link of global measurements of the interplanetary magnetic field � a key parameter in forecasting the severity of geomagnetic storms.

Today and tomorrow, the project kickoff meeting is hosted at ASTRON. Besides a general project overview, all workpackages will present their progress and there will be plenty of room for discussion. Finally, focused breakout sessions are scheduled, to make sure all interconnects between workpackages are secured and worked upon.

© Rik ter Horst

For the BlackGEM/MeerLICHT telescope we built here in Dwingeloo, we've designed a corrector for this dispersion and I decided to integrate a similar solution in my new telescope, a 400 mm F/13 Modified Dall Kirkham. The system has a three-lens field corrector, of which one element is moved laterally in order to introduce dispersion opposite to the dispersion of the atmosphere. First tests show that it works flawlessly, and although the optics still need further correction I'm looking forward to observe these "low planets" in the next couple of years. I hope to show some first images soon!

(*) Due to the tilt of the Earth-axis with respect to the plane of the ecliptic (i.e. the plane in which the planets orbit the Sun), the declination of Mars will be negative this year, and even longer for the two Gas-Giants Jupiter and Saturn. Therefore, from our 52 degrees Northern latitude, Saturn and Mars will not rise more than 18 degrees above the southern horizon this year, and Jupiter will follow next year with comparable numbers.

© Colloquium

The figure presents the parameter space for the strength and coherence length of the intergalactic magnetic field, showing regions excluded by past limits (light shaded) and new work (dark shaded). Theoretical constraints are set by magnetohydrodynamic turbulence, which causes the decay of short-scale structure, and by the Hubble radius, which places an upper limit to the size of any observable structure. An upper limit is set by observations of the cosmic microwave background, and a lower limit is set by the non-detection of gamma-ray cascades. The detection of anisotropy in the arrival directions of ultra-high-energy cosmic rays imposes a new limit (Bray & Scaife, submitted). The solid and dotted lines show the possible range of the limit, depending on the unknown composition of cosmic rays.

© ASTRON

A few years ago, we observed the atomic hydrogen in this galaxy (Daily Image 10 September 2014) as well as the warm molecular hydrogen (Daily Image 29 March 2016), both of which revealed signs of accretion occurring in the object.

We have now made the next step, using ALMA observations of this source, which allowed us to trace the cold molecular gas (using carbon monoxide, CO), with unprecedented spatial resolution, on large scales but also down to the very centre of the galaxy, less than 100 light-years from the black hole. We found that on large scales (a thousand light-years), the gas is distributed in a circumnuclear disk (Fig.1, black contours). The position-velocity diagram taken along the major axis of this disk (Fig. 2) shows that the disk is regularly rotating, and the predicted rotation curve (white dashed contours) fits the kinematics of the molecular gas very well (solid contours). Against the compact radio emission (only 6 light-years in size!), we detect CO gas in absorption (dashed black contours in Fig. 2). The key thing is that this absorbing gas does not follow the regular rotation of the disk, and is moving towards the black hole with a velocity of more than 300 km s^-1. This is very likely a clear and direct detection of gas feeding the activity of the black hole.

Interestingly, gas with similar in-falling velocities was also found in the earlier observations of the warm molecular (cyan contours in Fig. 1 and 2). The combination of these studies is telling us that clouds with gas in different conditions (warm and cold) must be present close to the black hole (r

These ALMA observations of PKS 1718-649 provide one of the best indications a population of cold clouds is falling towards a radio AGN, likely fuelling its activity.

These results are presented by F.M. Maccagni, R. Morganti, T.A. Oosterloo, J.B.R. Oonk, and B.H.C. Emonts in a recently accepted A&A paper

© ASTRON

COBALT2.0 is a next-generation correlator and beam-former, which will turn LOFAR into an unparalleled multitasking radio telescope and thereby greatly boost the science return per observing hour. The project is based on a successful NWO-M grant written by Jason Hessels.

The COBALT2.0 5x greater computational power will enable the LOFAR Mega Mode: a parallel observing mode that will serve half a dozen scientific surveys and space weather applications at the same time. For example, we will double the speed at which LOFAR legacy all-sky imaging survey can be observed, which is critical to completing it on a short-enough timescale that it can inform upcoming multi-wavelength surveys. The LOFAR Mega Mode also offers high-cadence monitoring observations with space weather applications. At the same time, COBALT2.0 will strengthen collaboration between the diverse set of LOFAR Key Science Project teams because of the commensal nature of the LOFAR Mega Mode and the cross-fertilization that this implies.

On Thursday January 11, we had a very fruitful and successful kick-off meeting with the project team of the COBALT2.0 Phase 1 project here at ASTRON. The specific goal of Phase 1 is to replace the COBALT cluster and its current functionality. This means designing and acquiring the COBALT2.0 hardware, connecting it in parallel to the current COBALT, tuning and making the necessary software adaptations, testing and commissioning, and finally replacing the old cluster. Phase 2 of the project focuses on fully implementing and commissioning the LOFAR Mega Mode.

Thanks everybody for participating in the kick-off meeting and let us make this project a great success!

© Image credit: Wolfgang Steffen/Boy Lankhaar et al. (molecules: Wikimedia Commons/Ben Mills)

The factors were derived through theoretical molecular physics calculations, which leading author Boy Lankhaar started with Gerrit Groenenboom and Ad van der Avoird at Radboud University, Nijmegen. In the mean time Lankhaar has transferred to the group of Wouter Vlemmings at Onsala Space Observatory and Chalmers University, Gothenborg, Sweden. The other two authors on the paper are Huib van Langevelde from JIVE/Leiden and Gabriele Surcis, once at JIVE and now at INAF Cagliari, Sardinia.

© CAMRAS

The current setup consists of two VHF antennas connected to the internet, via webSDR software designed by Pieter Tjerk de Boer(CAMRAS). Its aim is to receive meteor echoes from the french GRAVES space radar, and the belgian BRAMS meteor beacons located at Dourbes and Ieper. Besides meteor scatter, also airplanes, the International Space Station (ISS) and other satellites are visible. And occasionally the receiver even shows moon echoes of the GRAVES radar.

A more detailed description, and a manual for building your own meteor scatter station, including how to use the Spectrum Lab software, is on the KNVWS website: http://werkgroepmeteoren.nl/radio/

The screenshot above has captured a beautiful 'overdense' ionized meteor trail of a random or possible early Orionid meteor on October 4th last year. The receiver used by Frans de Jong(CAMRAS) is the forementioned webSDR at http://websdr.camras.nl:8901/ in conjunction with Spectrum Lab FFT software for analysis. At the time it was tuned to the frequency of the meteor beacon at Ieper(B).

© astropix.nl

But the (now well-rested) amateur is still not satisfied, and turns his attention to the final frontier: deconvolution. An imaging instrument like a telescope does not image a point source (like a star) as a point, but as a sizeable blob with a diameter proportional to the brightness of the source. The size of the blob is increased by atmospheric turbulence. Obviously, the image quality will be improved if these blobs can be removed somehow, i.e. deconvolved.

The PixInsight image processing software https://pixinsight.com/ does this by sampling the blobs of unsaturated stars in the image, and creating a blob model from that information. This model is then removed from the image at the position of the various blobs. This is a laborious task, but absolutely worth the time. During deconvolution, the sky background is protected by a mask and after the process has finished one is rewarded with a better looking image (*).

The above image of a part of the Hercules galaxy cluster was deconvolved, at left the original, at right the final version. The improvements are most noticeable in the extended objects. The full size deconvolved image can be viewed here:

https://www.astrobin.com/full/329643/0/?nc=user

(*) The difference between the two images is fairly small, because the Point Spread Function (blob shape) is small in the case of the fully sampled aperture plane of a typical optical telescope. The effects of (de)convolution are much more dramatic in radio interferometry (ASTRON's core business), since the PSF of a radio aperture synthesis telescope is much more extended due to sparse sampling of the aperture plane.

© PB

© ASTRON

After the plenary sessions, focused breakout sessions were scheduled and all the teams got a lot of work done.

The group picture shows the attendees, representing the project partners:

- ASTRON, The Netherlands

- UNIBI, Germany

- CBK PAN, Poland

- ILT, The Netherlands

- OBSPARIS, France

- Chalmers, Sweden

- STFC, UK

- TCD, Ireland

Thanks everybody for participating in the kick-off meeting!

© Cees Bassa and Scott Tilley

Enter Scott Tilley, a Canadian electrical engineer and radio amateur, and myself. Scott and I have the unusual hobby of tracking satellites. Since 2011 we've been working together to develop hardware (Scott's expertise, see the omni-directional antenna in the bottom right) and software (my expertise) to track satellites from their radio signals transmitted at S-band between 2.2 and 2.3 GHz. We regularly perform scans of the band aimed at identifying newly launched satellites from their Doppler curves.

On January 20th, Scott, while searching for signals from a recently launched classified US satellite, noticed a Doppler curve consistent with an object in a high Earth orbit. Identifying the curve against orbits of known satellites, the signals were consistent with the known orbit of IMAGE. Scott realized that IMAGE was transmitting again, and had come back from the dead. I was able to confirm Scott's discovery, and IMAGE was also found to be transmitting in data I obtained in October 2016, though it was silent in January/February 2014. The plot above shows the Doppler curve (diagonal line) I obtained October 2016, with the green line the prediction from orbital elements.

Scott blogged his discovery and contacted the NASA PI of the project at the time. The responses were very encouraging, and on January 30th, NASA obtained telemetry of the spacecraft, confirming that the signals received by Scott are indeed from IMAGE. NASA is now trying to regain contact and determine which scientific instruments are still in operation. No small feat, given that the tracking software and hardware had evolved over the 12 years it last had contact with the space probe.

The blog post was picked up by social media (reddit), and later also by main stream media (Washington Post, CNN, BBC, Science Magazine). This generated a tremendous amount of interest, in particular due to the excitement that shows that citizen scientists can make serendipituous discoveries that went unnoticed by NASA. Both Scott and I have had a crazy week given all the attention.

© ASTRON

Also, we looked forward and made a list of items we would like to pass on to the new group: the Systems Engineering & Project Management group. They have a running start with this SD&I legacy and can begin a new chapter within R&D. The story continues.

© NRAO/AUI/NSF: D. Berry

After explaining the importance of this issue, I will show how late-time radio observations helped. I end with a discussion of the future, with an emphasis on the role of radio observations in finding and studying EM counterparts.

© Sieds Damstra and Monique Sluiman

In the weeks/months prior to the event, teams from several schools from the Netherlands and Belgium worked on designing, programming and building a robot from LEGO building bricks with reference to the theme 'hydro dynamics'.

That Saturday, at 10 o'clock in the morning, the competition started with the robots the teams had built. The winning team, consisting of only girls, from a school in Noordhorn eventually became the winner of this Benelux final. They will represent the Netherlands in the FIRST World LEGO Festival in Europe and possibly the United States later this year.

Sieds was accompanied by the ASTRON meccanoid robot, which attracted a lot of attention. The children (and not to forget their parents) enthusiastically tried to give a command to the robot, like 'give me a hug' or 'do a dance'.

Thank you, Sieds, for being an ASTRON ambassador at this event full of technology and science, though in a rather different way than we usually do.

© Annemieke Janssen

We first did some experiments with a light bulb (the Sun) and an apple (the Earth), to find out why we have day and night, and where the Sun goes during the night. After that, we compared the outfit of an astronaut to that of a skier: both wear helmets, sunglasses, gloves and warm clothes, but the astronaut additionally needs a supply of air to be able to breathe.

There were certainly some children interested in travelling to the Moon: about half the class would like to walk on the Moon. Some, however, don't want to because they think it too dangerous....

© JIVE