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Articles on this Page
- 01/20/14--16:00: _Discovery of Carbon...
- 01/19/14--16:00: _In memoriam: Peter ...
- 01/21/14--16:00: _Visit from Hoogesch...
- 01/22/14--16:00: _Tracking tied-array...
- 01/23/14--16:00: _The power of wimpy ...
- 01/27/14--16:00: _UvA 2nd year resear...
- 01/26/14--16:00: _A First for LFAA
- 01/28/14--16:00: _What does the prese...
- 01/30/14--16:00: _StefStefCal edging ...
- 01/29/14--16:00: _Supernova 2014J in M82
- 02/02/14--16:00: _Winter Wildlife in ...
- 02/03/14--16:00: _Crippled by unenlig...
- 02/04/14--16:00: _What does the prese...
- 02/05/14--16:00: _Today's Colloquium:...
- 02/06/14--16:00: _Low Noise Tile
- 02/09/14--16:00: _Storm-damaged HBA t...
- 02/10/14--16:00: _First Results from ...
- 02/11/14--16:00: _Extra Colloquium To...
- 02/12/14--16:00: _Today's Colloquium:...
- 02/13/14--16:00: _Cobalt sees a Pulsar
- 01/19/14--16:00: In memoriam: Peter Fridman (1940-2014)
- 01/21/14--16:00: Visit from Hoogeschool Windesheim
- 01/22/14--16:00: Tracking tied-array beam wobble
- 01/23/14--16:00: The power of wimpy radio sources
- 01/27/14--16:00: UvA 2nd year research practicum
- 01/26/14--16:00: A First for LFAA
- 01/30/14--16:00: StefStefCal edging into the Mainstream
- 01/29/14--16:00: Supernova 2014J in M82
- 02/02/14--16:00: Winter Wildlife in the LOFAR Core
- 02/03/14--16:00: Crippled by unenlightened Nature
- 02/06/14--16:00: Low Noise Tile
- 02/09/14--16:00: Storm-damaged HBA tile in transit
- 02/10/14--16:00: First Results from the APERTIF ALPHA-3 Correlator
- 02/12/14--16:00: Today's Colloquium: The Use of the Moon as a Particle Detector
- 02/13/14--16:00: Cobalt sees a Pulsar
© J.B.R. OonkA study led by ASTRON astronomer J.B.R. Oonk has presented the first detection of Carbon radio recombination lines along the line of sight to Cygnus A. The detection of these very weak lines is made possible by the superior sensitivity and resolution of the LOFAR telescope as compared to previous low-frequency telescopes.
The observed Carbon absorption feature at 45 MHz towards Cygnus A is associated with foreground gas in our own Galaxy. Its velocity is consistent with the cold neutral medium in the Orion spiral arm. From the observed line width and optical depth we determine that the gas has an electron temperature of about 110 K and an electron density of about 0.06 cm-3, albeit with large uncertainties. Observations at higher frequencies will help us to further constrain the properties of this gas.
Radio recombination lines associated with the extragalactic source Cygnus A itself were also searched for, but not detected. To the best of our knowledge this is the first time that foreground Milky Way radio recombination line absorption has been observed against a bright extragalactic source. This study paves the way for future detailed pinhole studies of the interstellar matter in our Galaxy.
The results are published in J.B.R. Oonk, R.J. van Weeren, F. Salgado, L.K. Morabito, A.G.G.M. Tielens, H.J.A. Rottgering, A. Asgekar, G.J. White, et al. ( 2014 MNRAS 437 3506, http://arxiv.org/abs/1401.2876 ). The above figure shows the foreground Carbon absorption detected along the line of sight to Cygnus A as observed with LOFAR LBA around 45 MHz.
© A. NeedhamOne of the side-effects of the demise of the Soviet Union was that many talented scientists found their way to knowledge-institutes in the West. ASTRON and JIVE have been fortunate in their share of clever "Russians". One of them was Dr Peter Aleksandrovich Fridman, who recently passed away.
Peter was born in Odessa (now Ukraine), the undisputable "capital of humour" of the USSR. For three quarters of his life, he lived in Leningrad/St.Petersburg. He worked at the Special Astrophysical Observatory of the USSR/Russian Academy of Sciences, where he played a key role in developing advanced radio astronomy techniques for the RATAN-600 radio telescope in North Caucasus. In the 1970s, Peter was an active member of a small group of enthusiasts who proposed the national VLBI network now known as KVAZAR, a member of the European VLBI Network.
He joined JIVE in 1997 as EVN System Scientist. In 1999 he transferred to NFRA (now ASTRON) as Telescope Physicist for the WSRT. He retired in 2005, but was retained for special projects for several years.
One of his chief interests was Radio Frequency Interference, a very important subject for radio telescopes operating in densely populated Western Europe. He developed an RFI mitigation system for the WSRT, which was also used in Effelsberg (Germany). He applied his profound knowledge of mathematics and signal processing to radio interferometry (even with a single telescope!), correlators, radiometers, VLBI and transients.
In his personal life, there was music (a huge collection of records, from classics to jazz), poetry (including ancient French), history and love of nature in all its manifestations. Perhaps not surprisingly, several papers on Peter�s desk through his very last days were dedicated to SETI.
Only a month ago he paid a last visit to his daughter Marina in Israel. Peter has been laid to rest by his friends in the Jewish cemetry in Nijmegen, where he has been living happily with his partner Angela Needham for too few years.
© Madroon Community Consultants (MCC)On January 9th, a group of students and teachers from Hoogeschool Windesheim (Zwolle) visited ASTRON. The group consisted of first and second year students studying to become teachers. They were particularly interested in ICT-related topics, which naturally included the work of the ASTRON/IBM DOME group.
The program consisted of two presentations introducing both radio astronomy and big-data issues. Afterwards, a tour of the ASTRON site showed them the laboratories and various projects. The picture catches them in the picturesque LOFAR/WSRT control room, as they are being instructed by telescope operator Jur Sluman.
The group showed a keen interest in the work that is done at ASTRON, and actively participated in the tour program. We hope they enjoyed it as much as we did showing them around. Perhaps we will see some of them again in the future, with their own students.
© W. FrieswijkLOFAR's many-beam tied-array mode is not only great for surveys, but also very useful for mapping the array beam shape using pulsars. This movie shows such an experiment, in which 169 simultaneous beams are used to map the LOFAR full-core tied-array HBA beam at 126-161 MHz.
Each circle represents an individual beam, and its color represents the detected signal-to-noise ratio of the bright pulsar B1919+21. Note that the pulsar is intrinsically variable with time, so only the relative colors matter. The pulsar itself is at the phase center, in the direction of beam 1. The expected beam shape is shown in the bottom-left corner.
This sequence of 2-minute observations, each separated by 20-minute gaps shows that, though the measured beam shape is close to the model prediction, its centroid is also bouncing around due to uncalibrated ionospheric phase delays.
Such effects are also detectable in LOFAR imaging data. Here we're interested in determining how large an effect tied-array beam wobble will have on the raw sensitivity of beam-formed observations that use the full core. Further tests are ongoing to determine how typical these observations, which we obtained in the afternoon of January 10th, 2014, really are.
© Kristina Nyland, ApJIn recent years, a lot of interest and observing time has been devoted to the study of gas outflows generated by active galactic nuclei (AGN). These outflows are thought to have a major impact on the evolution of the host galaxy, by clearing the galaxy from gas and, in particular, cold gas. By getting rid of this potential fuel for star formation, the outflows represent the most likely mechanism to produce so-called "red and dead" galaxies, galaxies where both the star formation and the growth of the central black hole have stopped.
However, if only a very powerful active nucleus can produce such outflows, their occurrence would turn out to be too rare to be interesting and useful to explain the widespread presence of "red and dead" galaxies. Thus, a lot of effort is now invested to understand whether also active nuclei considered to be "wimpy" release enough energy to be able to power massive gas outflows.
In a recent paper led by Kristina Nyland (New Mexico Tech/NRAO), and co-authored by Raffaella Morganti, VLBA observations have been used to investigate whether such wimpy radio sources can also drive outflows. Indeed, one such "wimpy" radio source is at the centre of the galaxy NGC 1266. In this object, outflows of molecular gas and HI are observed (see Figure), depleting the galaxy of 13 solar masses of molecular gas every year. This is higher than the star formation rate in the galaxy, therefore suggesting that in the not-too-far-away-future (85 Myr) this galaxy will be completely devoid of gas.
The VLBA observations at 1.65 GHz reveal a continuum source within the densest portion of the molecular gas, with a diameter d 1.5 × 10^7 K, which is most consistent with an AGN origin. The radio continuum energetics implied by the compact VLBA source, as well as archival VLA continuum observations at lower spatial resolution, further support the possibility that the AGN in NGC 1266 could be driving the molecular outflow. These findings suggest that even low-level AGNs may be able to launch massive outflows in their host galaxies. These results have been presented in a paper by Nyland, Alatalo, Wrobel, Young, Morganti et al. Astrophysical Journal 779, 173 (http://arxiv.org/abs/1310.7588 ).
Figure caption: (Top left) HST image of NGC1266, (Top right) CO contours superimposed to the zoomed-in HST image, showing the distribution and kinematics of the molecular gas (red and blue contours tracing the extended outflow). (Bottom) continuum emission from VLA data (left) with dashed contours marking the location of the HI absorption and from the 1.65 GHz VLBA (middle). (Bottom left) HI absorption profile from the VLA showing the blueshifted wing.
© JWTH/ASTRONAs in past years, from Jan 14-16th, 2014, the ASTRON Astronomy Group hosted four 2nd year undergraduate students from the University of Amsterdam. The students visited ASTRON as part of a bachelors course in which they undertake a short "real life" research project.
This year's students, Anouk, Ramon, Rico, and Sjoerd, analyzed 12 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; and then finally use 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.
© Andre van EsOn Thursday January 23rd 2014 the Aperture Array Design Consortium (AADC), designing the Low Frequency Aperture Array element in SKA phase 1 was the first to have the MoU with SKA Organisation signed. The AADC is a truly global consortium with partners from the United Kingdom (Universities of Cambridge and Oxford), Italy (INAF), Australia (ICRAR) and China (KLAASA). The consortium is completed with contributions from JIVE, the German Glow consortium, University of Malta and MIT. ASTRON is leading the consortium that will invest around 20 Million euro in the design of the Low Frequency telescope in the next three years.
The Low Frequency Element of the SKA Phase 1 will cover the lowest frequency band of the SKA. It will consist of almost 260.000 antennae that will be stationed in the desert of Western Australia. The current design proposes log periodic antennae. The configuration places 75% of the antennae in a 2 km diameter core with three spiral arms out to a radius of 50 km. The combined system will have a data-rate of 10 Tb/s. The phased array concept, already in use by LOFAR, will provide great flexiblity and optimisation for very different experiments.
The first major goal for the AADC team is to pass a succesful Preliminary Design Review by the end of 2014, leading up to a Critical Design Review in 2016.
For more information on the consortium’s activities and the partners involved on the map you can have look at: (https://www.skatelescope.org/skadesign/wp/lfaa/
© ASTRONThe application of sparse regular arrays raises the question whether the presence of grating lobes reduces the effective area of the array in the scan direction. We have investigated this problem by considering the array in transmit situation as this allows for a more intuitive explanation. It does not matter for the results, since the array pattern is the same in transmit and in receive situation due to the reciprocity theorem. This theorem means that if in transmit situation the antenna radiates a lot of power in a certain direction, the effective area in that direction is large if you use the same antenna in receive situation.
The picture on the left shows the radiation pattern of 20x20 uniform rectangular array with isotropic elements if it is scanned 29 degrees from zenith. The gain of the array found to be 32.1 dB. The width of the main beam is found to be 10.5 degrees. The picture on right shows what happens if the array is scanned 43 degrees from zenith. The grating lobe at -60 degrees is clearly visible. The gain of the main lobe has decreased to 27.9 dB. The beam width has increased to 12.6 degrees. Normally, the gain of an antenna is roughly inversely proportional to the main beam width. If the main beam width increases from 10.5 to 12.6 degrees one expects a decrease in gain by a factor of 1.2 (17%). However, the gain decreases with 4.2 dB, corresponding to a factor 2.6, in this example. This result can be explained very intuitively by considering that in the transmit situation, the total amount of energy distributed over the sky following the array beam pattern is the same in the two cases discussed here. Since the energy is distributed over two lobes when the grating lobe is present (the main lobe and the grating lobe), this results in less energy for each direction and hence in a lower gain. The latter is proportional to the effective area of the array. This means that if the array is considered in receive situation, the effective area also drops with a factor of 2.6 if the scan angle is increased from 29 to 43 degrees.
Next time, we will show a picture where the effect of decreasing effective area due to the presence of grating lobes is investigated further.
© Madroon Community Consultants (MCC)During the 70's, Hans van Someren Greve used a simple but effective technique to calibrate the WSRT antenna phases on a bright point source, prior to a 12-hour observation. The invention of selfcal around 1980 saw the rise of the least-squares fit through matrix inversion. The latter proved general enough to solve for the extra parameters of ever more sophisticated Measurement Equations over the next few decades. Inevitably, all this required more and more processing, which we could not really afford with the ever increasing data volumes.
A few years ago, Stef Salvini from Oxford independently hit upon the old WSRT technique in his search for a more efficient solver(*). But he improved on it by introducing a little averaging trick that considerably speeded up its convergence. This "StefCal trick" is illustrated in the picture, which features Stef on the left, and Stefan Wijnholds on the right.
The importance of StefCal was first recognised by Oleg Smirnov, who also coined the name(**). It has been available in the MeqTrees package for some time. In the meantime, Stefan has implemented it for LOFAR station calibration, and he and Oleg have teamed up with Stef to make the technique robust and even faster, and to put it on a firm mathematical basis.
It is not yet clear whether StefCal alone will be sufficient for the most demanding calibration. But it certainly comes close, especially since it works for full polarization, and for direction-dependent effects (DDE). The next step is to take account of the known continuity of instrumental parameters, in frequency and time. This might yet require a little matrix inversion.
StefCal is definitely moving into the mainstream(***). Because of its simplicity and speed, it promises to become the workhorse of radio astronomical calibration. It will soon be made routinely available in LOFAR data reduction. In addition, the emergence of StefCal solves important problems for the LOFAR AARTFAAC project, and should do the same for the giant SKA.
(*) At the behest of the late Steve Rawlings, and closely associated with the MeqTrees project and the European SKADS program.
(**) The official name is a clumsy acronym, StEFCal, which is generally ignored.
(***) The picture was taken at a recent 2-day workshop in Dwingeloo that was devoted entirely to the practicalities of StefCal.
© Albert van DuinOn January 21st, a type Ia supernova was discovered in M82, by a group of astronomy students from London during an introductory demonstration of how to use the CCD camera on one of the University of London Observatory's automated 0.35-metre telescopes.
Read more on http://www.universetoday.com/108673/cloudy-weather-led-to-fluke-m82-supernova-discovery/#ixzz2rjAz3uh5 for this interesting story.
Since then, it has brightened to about magnitude 10.7 and it may continue to brighten still.
The colour of the supernova seems to be shifted to orange due to obscuring dust in M82.
These two images were made with a 0.4 metre telescope from downtown Beilen. North is at the bottom of the images.
© ASTRONThe present Dutch winter is very mild, without any snow or skateable ice. As a result, many geese have chosen the LOFAR core area to waggle around and feed on the nice green grass. In contrast, there are very few geese in the surrounding nature; they apparently prefer their grass to be properly cut.
The same applies to roe-deer. The wholesome LOFAR grass is very attractive to them also, and one may often see them roaming between the antennae.
In the meantime, it is difficult to tear oneself away from this magnificent image. It is positively dripping with the majesty of the Dutch landscape, which has proved such a fertile perch for probing the unseen Universe with radio waves and radiant minds.
© n.v.t.Business trips are not always without risk. Especially when the pristine Nature around ASTROJIVE decides to strike back. We really should explain to the various non-human species that we are there for them too. That we do a lot of good work, trying to understand more of the world, our space, which we all share.
As it is, some sadly uninformed martens nibbled through the vital tubes of an ASTRON car, causing us to arrive rather late (albeit in some style) at the ESTEC SpaceMatch in Noordwijk. We trust that, after reading this, these rodents and their ilk will behave more responsibly in future.
© ASTRONOn January 29th we showed a picture of the array pattern of a 20x20 uniform rectangular array with isotropic elements (see http://www.astron.nl/dailyimage/main.php?date=20140129 ). We showed that that the gain (and therefore the effective area) reduces when a grating lobe appears. Today we investigate this effect further. Instead of using the gain of the array we consider the effective area. Two pictures are shown.
The first picture shows the effective area of a 20x20 uniform rectangular array with an element spacing of 1.25 meter and an isotropic element pattern as a function of frequency and scan angle w.r.t. zenith. It is assumed that the array is scanned along one of its primary axes. The black line shows the scan angle where the grating lobe appears as a function of frequency (the grating lobe is present for scan angles larger than that angle). One can see that the effective area drops dramatically as soon as the grating lobe appears.
The second picture shows the effective area of the same array but with a cos^2 element pattern instead of an isotropic pattern. It is clearly visible that the decrease of the effective area due to the grating lobe is less than in the case of an isotropic pattern. The reduced effect of the appearance of the grating lobe is caused by the suppression of the grating lobe by the cos^2 element pattern.
From these pictures, we can conclude that the effect of the appearance of grating lobes on the effective area strongly depends on the element pattern. For broad element patterns, grating lobes cause a significant reduction of the effective area. For narrow element patterns, the effect of grating lobes on the effective area is less.
Next time we will show a comparison of the measured and simulated sensitivity (Aeff/Tsys) of a LOFAR high band tile to illustrate the effect of grating lobes on effective area (and therefore on sensitivity) of the LOFAR high band antenna system.
© Emma RigbyProtoclusters, the high-redshift ancestors of local galaxy clusters, are powerful laboratories for tracing the emergence of large-scale structure, and studying the evolution of galaxies in dense environments. Observations of protoclusters over a range of wavelengths can map both the star forming and evolved galaxy populations, facilitating studies of the build-up of galaxies and clusters during a crucial epoch of their evolution. Targeting high-redshift radio galaxies is an efficient tool for selecting these overdense regions which will eventually grow into today's massive clusters. In this talk I will review the progress made over the past years using this technique, and present the results of the first far-infrared wide-field survey of these objects. This survey was carried out using the SPIRE instrument on-board the Herschel Space Observatory, over the key redshift range 2
Above is a Hubble ACS image of the core of a protocluster at z = 2.16 (known as the Spiderweb; Miley et al. 2006), overlaid with 250 micron contours from the Herschel Space Observatory's SPIRE instrument. The central radio galaxy, MRC1138-262, is surrounded by many galaxy 'flies' which are in the process of merging with it. This structure will evolve into a massive cluster by z = 0.
© copyright ASTRONThe Low Noise Tile (LNT) aims to demonstrate state-of-the-art noise performance of aperture arrays for the Mid Frequency Aperture Array (MFAA) of the SKA. An earlier AJDI of December 18, 2013 showed the first results of the noise temperature of the new frontend design based on the SkyWorks LNA, to be used in the LNT, giving a considerable improvement with respect to the Apertif design, shown in the AJDI of August 6, 2010.
Today's image, as a next step, shows a similar improvement for the new LNA, now integrated with the Vivaldi antenna element, for which the noise temperature was measured in a special single antenna test fixture in THACO ( for THACO see the AJDI of July 11, 2011). While the Apertif design gave a noise temperature of approximately 60 K in the test fixture, this has decreased to approximately 40 K for the new antenna element. The improvement gives a good indication of what may be achieved for the noise temperature of an array, using these new elements.
We are now in the process of producing enough elements to make an array, so we can directly compare the array performance with the Apertif elements with that of an array of improved LNT elements. The pictures show the single antenna test fixture used for the measurements, and the noise temperatures for the Apertif and the LNT single antenna elements.
© ASTRONBuilding a cutting-edge radio telescope is one thing, but operating it offers many unexpected challenges.
One of the spare LOFAR HBA antenna tiles was severely damaged by the storms of December. It was not even possible anymore to transport it in the normal way. But a local farmer came to the rescue. At his suggestion, we put two layers of pallets (5m wide) on a trailer, and manually pulled the wrecked tile on top by means of a telehandler. This wide and unwieldy load was then slowly driven over local agricultural roads to a large barn with 6m wide doors.
We will now repair the tile on the trailer in the barn where we are protected against rain, sleet, snow and wind. After that, we can unload the tile in the normal way, and put it back onto the pile of spares.
© ASTRONALPHA-3 is the final prototyping stage of APERTIF. It involves 3 WSRT dishes equiped with Phased Array Feeds (PAF) and a small-scale correlator, a.k.a. "QDR" (Quick and Dirty correlator) or "De Peppi&Kokki correlator". Last week saw the first correlated data!
ALPHA-3 should confirm that the APERTIF system is ready for production. Three WSRT dishes (RT2, 4 and 5) are being upgraded, offering three different baselines (144, 288 and 432 m). In each, a PAF is installed and a UniBoard-based beamformer combines the signals from the individual PAF elements into compound beams. The compound beam data is then transported to the QDR for correlation.
To demonstrate that the correlator is working, a test set-up was made in the lab. Three UniBoards are running as beamformers and a fourth is running the QDR firmware. Each beamformer outputs 4x10 Gbps of data, so the total input data-rate into the correlator is 120 Gbps. The QDR will correlate the data over the full 300 MHz bandwidth for 7 compound beams. But in this lab setup, only 1 UniBoard is used for each beamformer, so only 16 PAF elements can be used per compound beam. As a result, only 75 MHz per beam is correlated for 28 compound beams on a single UniBoard.
The plots show the 3 cross-correlations (greenish) between 3 dishes, and the 3 auto-correlations (reddish). An analog band-limited noise signal centered on 700 MHz was used as input. This result clearly shows that the ALPHA-3 correlator is beginning to work (and yes, there are still some details to be sorted out).
But this is a nice result in itself. It is also good to see that the re-use of firmware blocks worked out very well for this system. Only the 10 GbE send and receive firmware had to be written specially for the QDR. All other blocks have been re-used from the existing beamformer, which greatly reduced the development time.
© Markus DemleitnerIn the most technical words, the Virtual Observatory (VO) is an effort to enable uniform and efficient access to astronomical data. With more glitz, it is like the Web and Google, only for data. In this talk I will try to convince you that what sounds tedious and boring in reality is exciting and useful to your research. Thus, after some motivation filling in the gaps in the above definitions, I will go on describing some of the key VO technologies and ways to use them - right now, from your desktop. I will close with an invitation to publish your data to the VO as well.
© Justin BrayThe ultra-high-energy end of the spectrum of cosmic rays is critical for understanding the nature and origin of these particles. However, the low flux of cosmic rays at these energies necessitates the use of extremely large detectors. The largest potential aperture is offered by the use of the moon as the detector, with ground-based radio telescopes used to detect the nanosecond-scale pulse produced when an ultra-high-energy particle interacts in the lunar regolith. I will review the motivation and history of these experiments, covering the specialised techniques required to detect pulses on such short timescales and the evaluation of the resulting limits on the fluxes of ultra-high-energy cosmic rays and neutrinos.
Image: A photo taken during the first lunar-radio-transient experiment in 1995 (image credit: Seth Shostak).
© (c) Astron 2014As of the end of March, the loyal IBM BlueGene/P will no longer be LOFAR's digital back-end. It will be replaced by Cobalt, a cluster of 8 PCs (background image) with four NVIDIA graphics processors (GPUs) each. Although the correlator application of Cobalt approaches production readiness, the beamformer application started testing only very recently. On Thursday Feb 6, after an intense couple of days of systematic debugging by a team spanning R&D, the Radio Observatory, and the Astronomy Group, Cobalt produced its first pulsar detection.
We used 5 minutes of 20 sub-bands of prerecorded data from LOFAR core station CS302, taken on December 2013. COBALT produced beamformed data with 16 channels per sub-band, at a time resolution of 1.3 ms. The top graph shows the signal of the pulsar formerly known as LGM-1, after adding the two HBA "ears" CS302HBA0 and CS302HBA1 incoherently.
Emboldened by this success, we tried incoherently adding the signals from all stations of the LOFAR core (except CS013), which led to the middle graph. It has a much improved signal-to-noise ratio, demonstrating that incoherent addition works.
The last graph shows coherent addition. Initially, this led to a signal-to-noise ratio of infinity/0, but that was fortunately quickly solved. So on Friday Feb 7, we detected the pulsar with the full sensitivity of the coherent LOFAR core!
Background picture by Chris Broekema, pulsar analysis by Vlad Kondratiev.