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A daily view of all the goings-on at ASTRON and JIVE.

older | 1 | .... | 19 | 20 | (Page 21) | 22 | 23 | .... | 71 | newer

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    © ASTRON

    Currently in AARTFAAC, data is correlated from all dipoles in six LOFAR stations. The total number of signals that are correlated is 576. In 2015 this will be extended by a factor of two, to 1176 signals, by using dipole data of six additional stations.

    The hardware required for that consists of URI boards (to tap off LOFAR data), UniBoards (to re-order data before correlation) and GPU machines (to correlate data). As is shown in the picture above, the URI boards (designed by Gijs Schoonderbeek) are mounted in their casings by Sjouke Zwier and Sieds Damstra to make them ready for deployment in the field. Furthermore Anne Koster modified ~ 200 cables for the additional stations. These cables replace the LOFAR ring cables in between the LOFAR RSP boards and are connected to the URI boards. On the URI boards the ring for LOFAR is re-constructed and the data is copied.

    Sieds is from the Radio group and Anne from the Technical Support group. We would like to thank them both for supporting the Digital and Embedded Signal Processing Group in these busy times!

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    © ASTRON

    ASTRON wishes you all a merry Christmas and a prosperous 2015!

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    © ...

    On December 18, 2014, the ERIC decision was published in all 23 official languages of the EC. This means that three days later the ERIC legal entity comes into existence. That very same date, 21 December, is also the 21st birthday of JIVE. Clearly, great occasions fit in well with this time of the year. And no, the imminent new fiscal year had nothing to do with starting these entities on winter solstice!

    The JIVE staff is very busy preparing the rites of passage for this new, but now fully mature organisation; a symposium, inaugural ceremony and council meeting will be held 20 - 22 April 2015 in Dwingeloo. Otherwise they would wish you a great holiday season and a very happy 2015 in all languages of the world!

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    © ATNF

    It is with great pleasure that we welcome the launch of the ATNF Daily Astronomy Picture (ADAP):

    We would of course like to flatter ourselves that our colleagues got the idea from our very own Astron/Jive Daily Image (AJDI), of which they might be keen followers(*). But since they seem to limit their scope to astronomy, it is more likely that they will model themselves after the NASA Astronomical Picture of the Day (APOD). That really is a tough act to follow.

    In any case, our friends down under will perhaps permit us a few words of friendly advice. When we started the AJDI, eight years ago, people predicted that we wouldn't last 6 months. Indeed, it is not easy to extract so many pictures from the workforce, even though there are amply enough worthy things going on. For that and other reasons, the scope of our AJDI also embraces technical subjects, and more personal ones. In fact, the AJDI aspires to be the family chronicle of ASTRON and JIVE (and NOVA and DOME). And, within the bounds of management propriety, we try to radiate a little fun.

    But even with this wider scope it requires editors that are posessed of a sustained irrational determination, bordering on love. We wish the ADAP a long life, and will keenly follow its adventures. It is definitely worth the trouble.

    (*) Puzzlingly, the external AJDI fanclub is more devoted than the locals.

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    © Laura Driessen

    The Crab Pulsar is one of the few pulsars still surrounded by the supernova remnant (SNR) from the explosion that created it. The SNR is composed of filaments of gas and dust which scatter the observed pulse profile properties of the pulsar.

    Generally, scattering effects on pulse profiles are attributed to the interstellar medium (ISM) and are seen as an extra exponential tail in the pulse profile. In this summer student project, we measure this effect by convolving an intrinsic (unscattered) profile, with one-sided exponentials. In the top panel we show the intrinsic profile of the Crab Pulsar at 350MHz (red), as well as the expected profile including scattering (blue).

    The Crab Pulsar is known to experience 'extreme scattering events' where the normal scattering is enhanced during a period of weeks to months. One of these scattering events was monitored in a high-cadence campaign using WSRT at 350MHz in late 2012 to early 2013, with regular monitoring observations continuing afterwards. One of the pulse profiles from December 2012 is shown in the lower plot (black). Surprisingly, we can see that there is an extra peak preceding both the main pulse and interpulse which is not visible in the upper plot.

    Since the scattering is normally quite small at 350MHz, we can only see this during the extreme scattering event of the Crab Pulsar, at favourable radio frequencies where the effect is large enough to be measurable but not too large to smear the profile completely.

    We model this effect by using two exponential tails, shifted relative to each other and with different scattering values. Our best fit model is shown in the lower plot (blue), as well as the two components (intrinsic profile scattered by the two different scattering tails; red, green) that combined make up the best-fit model.

    We attribute the extra scattering components to changes in the filaments of the SNR still surrounding the Crab Pulsar. With further analysis and more frequent observations during a new extreme scattering event we will investigate the filamentary structure of the Crab Nebula and confirm the scattering effect dependence on frequency.

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  • 01/04/15--16:00: ASTRON RFoF towards AAVS0.5

    The Low Frequency Aperture Array (LFAA) covers the lowest frequency band of the SKA, from 50MHz up to 350MHz, as described in the Baseline Design. To transport the dual polarised antenna signals over the required distance of up to 10 kilometres towards the processing facilities, RF-over-Fibre (RFoF) technology is used.

    For such long links, there are no real alternatives to RFoF technology, which offers the lowest cost and the lowest signal loss. Also, RFoF systems are highly immune to RFI pickup, static and lightning, as well as causing no electromagnetic interference themselves.

    Commercially available RFoF links typically feed remote wireless transceivers and other antenna-systems for communication and CATV. The value added to these systems is large, and only a few links are typically required, so that the link itself is not very cost-constrained. Therefore, the price for off-the-shelf RFoF solutions is too high for the large number of links required for the LFAA.

    Fortunately, the components required to build a customized, minimum-complexity and low-cost link for the LFAA are all commercially available. Furthermore, commodity applications like fibre-to-the-home drive volumes, and are bringing the cost of e.g. DFB lasers down to a few euros in volume. These devices work up to many GHz, so building a low frequency link is relatively straightforward.

    Using such components, we have successfully realized and tested our RFoF link, and showed that it can meet the LFAA requirements that are derived from the Baseline Design Level 2 specifications.

    The picture (top left) shows the most recent ASTRON deliverable (16 RFoF links) towards the Aperture Array Verification System0.5 (AAVS0.5), which is located on the Australian SKA site. The coaxial cables that are currently connecting all (16) antennas (bottom right) will soon be replaced by the RFoF links. The first field test is expected during January 2015.

    Many thanks for all the work done by the ASTRON and ICRAR team!

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  • 01/06/15--16:00: Mapping the LOFAR antennas
  • © Background (c) Microsoft

    Of course we know that we have the position coordinates of the LOFAR antennas right, to a tiny fraction of the wavelength. Otherwise, all kinds of funny effects would be visible in the images of the sky. But it is always nice to see it confirmed by other means. So, we converted the antenna coordinates from the International Terrestial Reference Frame (IRTF) into the WGS84 frame used by popular internet mapping systems, and overlaid the antennas on the map.

    The sequence of images gives an overview of the current array of LOFAR stations (soon to be extended), and some zoom-ins of various parts. All LOFAR stations actually consist of one station (array) of Low Band Antennas (LBA, 10-80 MHz) and one or two stations of High Band Antennas (HBA, 120-240 MHz). The central superterp has a diameter of 300m, and contains six LBA and 12 HBA stations.

    Here are some of the underlying considerations:

  • The individual antennas (dipole-pairs) of an LBA station have a random distribution, with a decreasing density towards the edge. This minimizes the primary beam sidelobes.

  • An HBA station consists of close-packed square units of 4x4 dipole-pairs. Note that all HBA stations are slightly rotated w.r.t. all others, again to minimize the far sidelobes of the primary beam.

  • NB: The individual dipole-pairs of all LOFAR stations have been de-rotated, so that they are all parallel to each other. This greatly simplifies wide-field polarization calibration.

  • The HBA stations in the 3km LOFAR core have been split in two, in order to increase the uv-coverage for short baselines, and thus the sensitivity for extended structures like the EoR. Obviously, the larger size of the more remote HBA stations has an impact on sensitivity and primary beam size. This has advantages and disadvantages.

    The existence and operation of LOFAR has a huge influence on the design of the Square Km Array (SKA).

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    © AH

    A renaissance is taking place in optical and radio astronomy, due to application of rapidly evolving commercial technology. Moreover, by all accounts (including Astro2010, The Astronomy and Astrophysics Decadal Survey) this decade is regarded as the decade of time domain astronomy. The dynamic radio sky is seen as a frontier area in astrophysics, ripe for discovery.

    The synergy between optical and radio astronomy, such as the joint VLA-PTF collaboration, have proved to be fruitful. This includes the earliest radio observation of a Type Ia SN, systematic measurements of circumstellar matter close to the progenitor of core-collapse SNe, and a possible discovery of a new type of relativistic explosion. Furthermore, radio observatories are now taking the role of discovering transients independently. A new generation of radio facilities is being built at decameter and centimeter wavelengths and all of them have identified the exploration of the time domain as Key Science. These include, for example, the LOFAR Transient KSP, ASKAP VAST, MeerKAT ThunderKAT, and WSRT APERTIF.

    In my talk I will discuss what we have learned in recent years from observations of the dynamic radio sky and will briefly present the future of time-domain radio astronomy.

    The picture above shows the imaging and radio spectra of the supernova PTF13bvn: the first type Ib SN with a progenitor identification.

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  • 01/08/15--16:00: The Sheep of Obi-Wan
  • © astron

    One of the charms of Astron is that it is situated in one of the most beautiful spots in the Netherlands: the National Park Dwingelderveld. Very characteristic of this Park are herds of sheep freely grazing the Dwingelderveld, often right next to Astron where the above picture was taken.

    These herds are actually of very special sheep. This is because they belong to the oldest breed of sheep of mainland Western Europe: the Drents Heathsheep (Drents Heideschaap) of which only a few thousand are left. Drents Heathsheep were kept in Drenthe as livestock already in prehistoric times and have been grazing the Dwingelderveld and places alike, exactly as in the picture, for about 6000 years. So the picture above is to some extent a time warp.

    But the Drents Heathsheep are also special another reason: they are the sheep of Obi-Wan. It is the wool of these sheep that was used to make the cloak of Obi-Wan and the other Jedi Knights in the movie Star Wars I (The Phantom Manace, broadcasted on TV in the Netherlands tomorrow). The wool of Drents Heathsheep is fairly sturdy. This is why it is preferred by artist Claudy Jongstra in nearby Friesland to make the felt she uses in her art (which you can see in places like the MOMA in New York, the Hammer Museum in Los Angeles and of course in the Stedelijk Museum in Amsterdam). And she used it for the felt she made for the producers of Stars Wars I who used it for the cloaks of the Jedi Knights. Next time you see the herd near Astron, think about this and imagine you feel a Disturbance in the Force...

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  • 01/11/15--16:00: The New Year's Speeches
  • © Madroon Community Consultants (MCC)

    Today, at 10:30, the Directors of ASTRON and JIVE will regale us with their traditional New Year's Speeches.

    Cognoscenti will recognize the Dwingeloo heath (*), and know what lies at the centre of the glorious double arc, in between the pots of gold.

    (*) Home of the Sheep of Obi-Wan.

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  • 01/12/15--16:00: AJDI of the Year (2014)
  • © ASTRON

    During his truly inspirational New Year's Speech, ASTRON Director Mike Garrett paid handsome tribute to the ASTRON/JIVE Daily Image (AJDI). He did this by using many of the iconic images published in the past year. He even honoured us by selecting an Image of the Year 2014, which will of course be a tradition from now on.

    Please refer to the original image for a brief explanation of its subject. It was probably selected because it most closely symbolizes the virile spirit of ASTRON, projecting itself around the globe.

    As pointed out in a recent piece of subtle self-promotion, the AJDI endeavours to be the family chronicle of ASTRON and JIVE (and NOVA and DOME). We are very proud of the archive of images that we have collected over the last eight years. Like Mike, you may gainfully use this resource in your presentations. In the meantime, its emotional value can only increase with age (you'll see).

    Our only regret is that we are still missing too many poignant items. To improve, we beseechingly rely on you, as AJDI benefactor and beneficiary, whether you are an employee or a friend. Remember, nous sommes tous AJDI!

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    In a recent paper, an international team of scientists, including several researchers from ASTRON and JIVE, discussed the exciting prospects for conducting searches for extraterrestrial intelligence (SETI) with the Square Kilometre Array (SKA). This image is a composite of several figures from this work.

    Absent of the actual detection of an artificial extraterrestrial radio transmitter, our best points of reference for the sensitivity of radio SETI experiments, and the luminosities of sources we might detect, come from our own terrestrial technology. Several terrestrial transmitters that produce emission in the bands probed by the SKA, along with their pseudo-luminosities as described by their equivalent isotropically radiated power (EIRP), are listed above the two plot panels.

    The upper panel depicts the sensitivity of each component of the SKA to narrow-band transmitters at 15 pc, as compared with other facilities actively performing SETI searches over the same band. Search parameter assumptions here match roughly what might be expected for a significant fraction of commensal (or "piggy-back") observations, namely a maximum integration time of 10 minutes. In the upper panel, a transmitter is detectable if its EIRP is above the curve for a given telescope. Thus in the observing scenario presented, a transmitter with an EIRP of 2 x 10^20 ergs/sec (planetary radar) is detectable with all of the telescopes shown, while a transmitter with an EIRP of 1 x 10^17 ergs/sec (airport radar) is detectable only with SKA2.

    The lower plot panel depicts what sensitivities could be attained in a more optimistic scenario, in which SETI was the primary observing purpose or commensal observations were performed with another science case very well matched to SETI. Here we assume an integration time of 60 minutes, and the minimum channelization bandwidth permitted by ISM and IPM effects. As shown, with SKA1, radio transmitter luminosities similar to our high power radars will be detectable from tens of thousands of stars across the entire terrestrial microwave window, and with SKA2 these signals will be detectable from hundreds of thousands of stars. Further, with SKA2 we will for the first time have the sensitivity to detect radio emission similar in power to our own TV and radio stations from a few of our nearest neighbors.

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  • 01/14/15--16:00: Singing in the Rain
  • © Madroon Community Consultants (MCC)

    We have some wet but happy visitors to the Institute this week. Dan Stinebring (Oberlin College, USA) has brought three students with him for two weeks to work with the pulsar group. Here they are seen in front of the famous and historic Dwingeloo telescope, although in the stormy weather it was turned away from them in stow position. From left to right: Ben Izmirli, Dan, Keeley Hagenbuch, and Nora Rice.

    Dan follows in the tradition of many radio astronomy luminaries who "spent time in Leiden in the sixties". Like them, he insists on practicing his budding Dutch on indulgent locals, which is much appreciated. He visited us for a year recently, and continues to enjoy his frequent trips to the Netherlands - especially ASTRON.

    In the spirit of singing for his dinner, Dan will give a seminar for the pulsar group today: "Estimating Dispersion Delay in Pulsar Signals: Promises and Pitfalls". At 15:30 in the Auditorium. This is not a colloquium, but anyone is welcome to attend.

    Brief description of the seminar: "High precision pulsar timing requires accurate removal of interstellar propagation delays. The largest of these, due to the dispersive nature of the interstellar plasma, is normally estimated from multi-frequency observations, although proposals to use single broad-band receivers are also being considered. I will present an analysis from a forthcoming paper (Cordes, Shannons, and Stinebring) that quantitatively estimates the average error incurred in both the multi-frequency and the broad-band approaches. There will be some introductory material to set the context for this analysis, but this talk will be mainly of interest to pulsar astronomers and students".

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  • 01/15/15--16:00: The 26th Solvay Conference
  • © Ger de Bruyn

    Many people will remember the historic, iconic, photograph taken in 1911 at the first Solvay conference on physics: it hosted many of the key players in physics at the start of the 20th century (e.g. Planck, Lorentz, Einstein, Mme Curie, Kamerlingh Onnes, de Broglie, etc; see the inset). Many famous Solvay conferences followed, e.g. the one in 1927 which included Lorentz, Einstein, Born, Bohr, Dirac, Pauli, Heisenberg and Schrodinger to name a few. That was the 5th conference in the series and also the last one chaired by Lorentz who played a key role as the driving force and chairman during this whole period.

    Ever since, the Solvay conferences have had the name of being the most prestigious conferences in physics. They have addressed fundamental problems in (quantum) physics and chemistry. The theme of the 26th Solvay conference, which was held from 9-11 October 2014, was 'Astrophysics and Cosmology'. So when Roger Blandford invited me, as well as my colleague Saleem Zaroubi, to participate in this meeting I did not have to think long ! The meeting took place at the Metropole Hotel in Brussels, where all Solvay conferences were hosted. The 2014 meeting was only the 4th Solvay meeting on astrophysics, the last one being held in 1973, 41 years ago ! Only two persons (Ed van den Heuvel and Martin Rees) were present at both meetings and reminisced about it at the conference banquet.

    A total of five 'hot' themes were discussed: Neutron Stars, Black Holes, Cosmic Dawn, Dark Matter and the Microwave Background; for each half a day was set aside. The format of Solvay conferences is notably different from that of 'ordinary' conferences. All topics were introduced in 30 min presentations by two rapporteurs who each reviewed the developments in the field from their perspective, followed by discussion. After a coffee break about 5-6 short invited contributions, each 5 min maximum, were then addressing recent and special developments in that field. Only one slide could be shown during these short presentations !

    There were some interesting rules for the conference: all participants had to sign up for the full three days and laptops were not allowed to be open during the session, to assure the undivided attention of all participants. Very refreshing and stimulating indeed ! The conference room was also unusual: the 60-odd participants were seated around a large rectangular open space where 7 high quality LED screens and a rotatable camera were located to give a close up view for all participants. There were lots of high level and occasionally exciting, discussions in all sessions and I personally learned a lot about the state of the various fields and where they are heading.

    From the Netherlands the following people were present: Ed van den Heuvel (chair of the Neutron Stars session), Ralph Wijers, Conny Aerts, Saleem Zaroubi and myself. In addition Robbert Dijkgraaf, former president of the KNAW, was present (he is also a member of the Solvay Board).

    My 1-slide, 5-min contribution in the Cosmic Dawn session included a brief report on the status and first results of the LOFAR Epoch of Reionization project. Much to our regret, we could not yet report that we had detected the feeble signals from redshifted hydrogen. However, we could tell the assembled audience that we are getting close.

    The conference summary was remarkable too: each participant was requested to summarise her or his impressions, in just 15 s. A lot of people looked forward to results from the Cosmic Dawn period, and many trusted that the next conference on astrophysics would not take another 41 years. The proceedings, including the recorded discussions, will be published.

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    © Todd Mason, Mason Productions Inc. / LSST Corporation

    The Large Synoptic Survey Telescope (LSST) is a large aperture, wide-field, ground-based optical telescope designed to provide a deep time-domain survey of the entire southern hemisphere in six color bands covering the wavelength range 320-1050 nm.

    Every piece of the southern sky will be 'visited' by LSST approximately 1000 times over the ten-year duration of the mission. The resulting database will enable a diverse array of in-depth investigations ranging from studies of moving small bodies in the solar system to the structure and evolution of the universe as a whole.

    I will review the basic design of the LSST, and provide a brief tour of some of the exciting science that we expect to come from this major new Facility.

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  • 01/18/15--16:00: Panta Rhei, also @ R&D

    According to the Greek philosopher Heraclitus everything is in motion. This was ever so true for ASTRON R&D in 2014.

    2014 was the first full year under the new leadership of Gert Kruithof. Andre Gunst became the new lead for the DESP group, taking over from Albert-Jan Boonstra who, together with Jan Geralt bij de Vaate, joined the NL-SKA Office while also being the ASTRON Technical Director for the DOME Project. Dion Kant left ASTRON and was replaced by Koos Kegel as head of the System Design & Integration group. Gert, Andre, Koos, together with Wim, Johan, Nico, Ronald and Ronald now form the new R&D Management Team. In August Monique Sluiman took over from Patricia Breman as Office Manager for the R&D department, while Patricia took up the position of Project Assistant for the POLFAR project.

    In order to celebrate these changes (or was it because the CGL's complained too much???) Gert provided cake at the last R&D MT meeting of 2014. R&D is ready for 2015!

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    © astron

    Active galactic nuclei (AGN) are known to influence the evolution of their host galaxy through the injection of energy into the surrounding interstellar medium (ISM). Radio-loud AGN do this by their twin jets that are launched into the ISM and interact with the gas in the ISM.

    Centaurus A is the closest radio galaxy (d = 3.8 Mpc, or 12 million light year) in which an interaction between the radio jet and the surrounding ISM is thought to occur. This is because in the northern part of this galaxy, several filaments of highly ionised gas are found that are well aligned with the radio jet. Particularly complex is the situation in the so-called outer filament (shown in the background figure), which is found about 15 kpc (or almost 50,000 light year) from the centre of Cen A: this filament op ionised gas is located close to the axis of radio jet and, interestingly, also close to a large cloud of cold, atomic hydrogen gas (HI). Gas with anomalous velocities has been found at the southern tip of this HI cloud which could be an indication that the radio jet is hitting the HI cloud, blowing off and heating some of its gas and so creating the filaments of ionised gas.

    We have observed two fields in the outer filament using the Visible MultiObject Spectrograph (VIMOS) at the Very Large Telescope (VLT) to study the kinematics of the ionised gas and to further study the signs of interaction. We detect two kinematical components in the ionised gas in the outer filament (see velocity maps on the left side of the figure). Interestingly, the velocities of these two components match those of the nearby HI cloud. Both the regular, undisturbed HI structure, as well as that part of the HI which appears disturbed by the jet, have a kinematical counterpart in the ionised gas. This is very suggestive that the ionised filament is indeed caused by a radio jet hitting as gas cloud.

    A paper describing these results has been accepted for publication in Astronomy & Astrophysics: The jet-ISM interaction in the Outer Filament of Centaurus A by Santoro, F.; Oonk, J. B. R.; Morganti, R.; Oosterloo, T. (arXiv:1411.4639)

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    © JvL

    Two movies accompany the press release for the presentation at AAS225, Seattle, Jan 4-8 of our results on pulsar J1906+0746.

    Within a few days these have done amazingly well, together drawing more than 100,000 views from all over the world.

    The animation above shows the effect of "geodetic precession" in the observer pulsar. Two neutron stars orbit one another. The star visible as a pulsar shows rotating beams. The companion is frozen at the frame center. In a flat space-time, where the companion is massless but the pulsar does orbit it for illustrative purpose, the pulsar rotation axis (represented by the arrow) is unchanged after one orbit. Once the companion mass increases to the measured 1.32 solar mass (about half a million Earth masses, but in a sphere only 10 kilometer across), space-time curves. Within one orbit, the pulsar axis now slants (the effect is exaggerated 1 million times here). Because of that change, the pulsar is now all but invisible from Earth!

    This animation shows the binary edge-on; the other animation depicts the system as seen from Earth.

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    © Image Credits: NASA, ESA, S. Baum and C. O'Dea (RIT), R. Perley and W. Cotton (NRAO/AUI/NSF), and the Hubble Heritage Team (STScI/AURA)

    There is increasing speculation that AGN are intimately linked to the evolution of their host galaxies. Not only are they triggered as galaxies build up mass through gas accretion, but they also have the potential to drive massive outflows that can directly affect galaxy evolution by heating the gas and expelling it from galaxy bulges. However, there remain considerable uncertainties about how, when and where AGN are triggered as galaxies evolve, and what the observed diversity of the AGN population can tell us about the accretion processes close the central black holes.

    In this talk I will present new Gemini, VLT, Spitzer and Herschel results for the 2Jy sample of luminous, radio-loud AGN which provide key information on the triggering mechanisms and accretion processes.

    The image shows a JVLA radio map of the radio galaxy Hercules A (pink) overlaid on an optical HST/WFPC3 image of the host galaxy and surrounding field (greyscale). The bubble-like structures in the south-western radio lobe suggest that the radio source activity is intermittent, and that the radio source has gone through several cycles of recent activity. Such intermittency is important in understanding the observed diversity in the radio galaxy population.

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    © Madroon Community Consultants (MCC)

    The largest remaining uncertainty in GNSS (GPS, Galileo, etc) position measurements is the ionosphere. For the most common type of receiver it is typically in the order of a few meters. Since LOFAR is very good at measuring the ionosphere, it seems logical to investigate whether LOFAR calibration data might be used to improve GNSS positioning. This is exactly what we are doing together with the Dutch Aerospace Laboratory (NLR), in a contract issued by the European Space Agency (ESA).

    For this purpose, we have recently installed an advanced dual-frequency GNSS receiver (the blue thingy) in the concentrator hut near the LOFAR superterp. The image shows our own Menno Norden with Hein Zelle(*) and Arnoud van Kleef(**) of NLR.

    The idea is to observe a sequence of bright radio sources with LOFAR, along the path of one or more GNSS satellites as they move across the sky. Thanks to the selfcal technique, the LOFAR data represent very accurate relative measurements of the ionosphere in the direction of the source (and thus approximately the satellite), as seen from the various LOFAR stations. These can be compared to the absolute measurements of the ionosphere by the GNSS receiver, which has an accurately known position.

    Of course there are some complications caused by the troposphere, multipath and receiver noise, and by the difficulty to measure the absolute ionosphere with LOFAR(***). Nevertheless, in view of the obvious importance of accurate GNSS positioning, it is worth exploring all available avenues.

    (*) It is a small world. Hein happens to be the partner of our own Agnes Mikis.

    (**) Arnoud also happens to have grown up in the same village (Middelstum) as Menno.

    (***) LOFAR only needs the absolute ionosphere for polarization observations. For imaging, relative measurements are sufficient.

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