The prototype 4-meter telescope developed by the ASTRI/CTA project.
It is located at the observing station operated by INAF Catania Astrophysical Observatory,
at Serra La Nave, Mount Etna, where it was inaugurated in September 2014.
(credit: cta-observatory.org)

In October 2016, the ASTRI telescope prototype, pictured above, a novel dual-mirror Schwarzschild-Couder telescope design proposed for the Cherenkov Telescope Array (CTA), passed its biggest test yet by demonstrating a constant point-spread function of a few arc minutes over a large field of view of 10 degrees.

Three classes of telescope types are required to cover the full CTA very-high energy range (20 GeV to 300 TeV): Medium-Size Telescopes (MST) will cover CTA’s core energy range (100 GeV to 10 TeV) while the Large-Size Telescopes (LST) and Small-Size Telescopes (SST) are planned to extend the energy range below 100 GeV and above a few TeV, respectively.

The ASTRI telescope is one of three proposed SST designs being prototyped and tested for CTA’s southern hemisphere array. The ASTRI telescope uses an innovative dual-mirror Schwarzschild-Couder configuration with a 4.3 m diameter primary mirror and a 1.8 m monolithic secondary mirror. In 1905, the German physicist and astronomer Karl Schwarzschild proposed a design for a two-mirror telescope intended to eliminate much of the optical aberration across the field of view. This idea, enhanced in 1926 by André Couder, lay dormant for almost a century because it was considered too difficult and expensive to build. It was in 2007 that a study by Vladimir Vassiliev and colleagues at the University of California Los Angeles (UCLA) demonstrated the design’s usefulness for atmospheric Cherenkov telescopes.

The ASTRI prototype, the first Schwarzschild-Couder telescope to be built and tested, was inaugurated in September 2014 and has been undergoing testing at the Serra La Nave observing station on Mount Etna in Sicily ever since. The technical challenges of the design were overcome by recent advances, particularly in dual-mirror technology, making it a feasible implementation for the observation of Cherenkov light.

Polaris, the North Star, as observed by ASTRI with different offsets from the optical axis of the telescope.
(credit: Enrico Giro, Rodolfo Canestrari, Salvo Scuderi and Giorgia Sironi, INAF Padova, Brera and Catania)

Pictured above, Polaris, the North Star, as observed by ASTRI with different offsets from the optical axis of the telescope. The recorded images have approximately the same angular size, each one from a different observational direction in the field of view (from 0 to 4.5 degrees from each side with respect to the central optical axis). These images show that the optical point-spread function of the telescope is approximately constant across the full field of view. This information will allow scientists to reconstruct the direction of gamma-ray photons emitted from celestial sources.

"This is also the first time that a Cherenkov telescope with two focusing mirrors has been completely characterized from the opto-mechanical point of view," said Giovanni Pareschi, astronomer at the INAF-Brera Astronomical Observatory and principal investigator of the ASTRI project. "This is an important result because it allows us to move immediately to the next step: to mount a Cherenkov camera by December 2016 with the aim to observe the first gamma-ray light with ASTRI."

Left to right: Prof. Kraft, KIT vice-president; Prof. Drexlin, KATRIN spokesperson;
Prof. Blümer, head of division Physics and Mathematics at KIT; Prof. Otten, one of
KATRIN founding fathers; Prof. Robertson, head of the KATRIN group at Uni Washington
(credit: HAP/A.Chantelauze)

As the most precise scale in the world, the KATRIN experiment aims to determine the exact mass of the smallest particle in the universe, the so-called neutrino, the exact same one which has been awarded the Nobel Prize in Physics last year. An important step on the way to measuring the neutrino mass is the "First Light", i.e. when the detector first sees electrons that have passed through the entire 70-meter-long KATRIN line.

The final preparations of the KATRIN kick-off began at 15:30, with introductory words by Prof. Guido Drexlin - KATRIN spokesperson, Prof. Oliver Kraft - KIT vice-president, Prof. Ernst Otten - one of KATRIN founding fathers, Prof. Blümer - head of division Physics and Mathematics at KIT, and Professor Hamish Robertson - head of the KATRIN group at the University Washington.

At 15:50, it was finally time: all speakers pressed on the "red button" to start the electron source, which delivered with great applause the expected signal of the "First Light", as seen on the monitoring screen below.

"First Light"!
(credit: HAP/A.Chantelauze)

Neutrinos play an important role in the investigation of the origin of matter and in the design of the visible structures in the cosmos. Their mass, which must be more than a billion times smaller than that of a hydrogen atom, is an important but still imprecisely determined parameter. The international KATRIN experiment will put limit on the neutrino mass with an accuracy that is more than a whole order of magnitude better than before. But neutrinos cannot be directly measured as they interact very little with matter. The trick is to use the well-known beta decay from Tritium for which both neutrinos and electrons are created. These electrons will be precisely measured and will give information on the "missing" non-measurable neutrinos. The measurement campaign will start in Fall 2017.

Even if the "First Light" does not yet provide its full performance, this first beam is an important working test for the scientists and engineers. The KATRIN experiment is an assembly of numerous components and for the first time they are working together. On the 70-meter-long path of an electron through the entire experiment are superconducting magnets and cold traps, gas-filled areas and vacuum, zones with temperatures below 4 Kelvin and room temperature, each of them has to be carefully operated. For the "First Light", a switchable electron source is used, which uses a UV light source to strike suitable electrons from a gold-coated stainless steel plate, which strike the detector after a flying time of a few millions of a second. The detector of silicon semiconductor material has a diameter of around 125 millimetres and contains 148 pixels which are arranged similarly to a dartboard, thus providing a spatial "look" into the world of KATRIN.

In the past years, the KATRIN collaboration has solved numerous scientific challenges and entered new technological fields in order to cope with the century's task of measuring the neutrino mass. For example, to maintain a high voltage of 18,600 volts with an accuracy of 0.01 volts stable. Or the production of an ultra-high vacuum, which corresponds to the one on the moon surface, in a world-wide turn-around world record volume of 1,240 cubic meters. About 150 scientists from 6 countries and 18 renowned institutions are involved in the KATRIN experiment, whose budget is 60 million euros.

The measurement of the neutrino mass using Tritium is scheduled to start in Fall 2017. The final planned sensitivity for the KATRIN experiment will be reached after five calendar year measurement time.However, first interesting results are already expected for mid-2018. Then the measurement sensitivity of KATRIN will already be significantly better than this of all other Tritium decay experiments together over the last three decades.

Press release: KIT/KCETA
More reading: KIT, Phys.org, New York Times

(credit: Juan Antonio Aguilar and Jamie Yang. IceCube/WIPAC)

Solving the mystery of the origin of cosmic rays will not happen with a "one-experiment show." High-energy neutrinos might be produced by galactic supernova remnants or by active galactic nuclei as well as other potential sources that are being sought. And, if our models are right, gamma rays at lower energies could also help identify neutrino sources and, thus, cosmic-ray sources. It’s sort of a “catch one, get them all” opportunity.

IceCube’s collaborative efforts with gamma-ray, X-ray, and optical telescopes started long ago. Now, the IceCube, MAGIC and VERITAS collaborations present updates to their follow-up programs that will allow the gamma-ray community to collect data from specific sources during periods when IceCube detects a higher number of neutrinos. Details of the very high energy gamma-ray follow-up program have been submitted to the Journal of Instrumentation.

From efforts begun by its predecessor AMANDA, IceCube initiated a gamma-ray follow-up program with MAGIC for sources of electromagnetic radiation emissions with large time variations. If we can identify periods of increased neutrino emission, then we can look for gamma-ray emission later on from the same direction.

For short transient sources, such as gamma-ray bursts and core-collapse supernovas, X-ray and optical wavelength telescopes might also detect the associated electromagnetic radiation. In this case, follow-up observations are much more time sensitive, with electromagnetic radiation expected only a few hours after neutrino emission from a GRB or a few weeks after a core-collapse supernova.

Updates to this transient follow-up system will use a multistep high-energy neutrino selection to send alerts to gamma-ray telescopes, such as MAGIC and VERITAS, if clusters of neutrinos are observed from a predefined list of potential sources. The combined observation of an increased neutrino and gamma-ray flux could point us to the first source of astrophysical neutrinos. Also, the information provided by both cosmic messengers will improve our understanding of the physical processes that power those sources.

The initial selection used simple cuts on a number of variables to discriminate between neutrinos and the atmospheric muon background. IceCube, MAGIC, and VERITAS are currently testing a new event selection that uses learning machines and other sophisticated discrimination algorithms to take into account the geometry and time evolution of the hit pattern in IceCube events. Preliminary studies show that this advanced event selection has a sensitivity comparable to offline point-source samples, with a 30-40% sensitivity increase in the Northern Hemisphere with respect to the old selection. The new technique does not rely only on catalogues of sources and allows observing neutrino flares in the Southern Hemisphere. Thus, those alerts will also be forwarded to the H.E.S.S. collaboration, expanding the gamma-ray follow-up program to the entire sky.

During the last few years, IceCube has sent several alerts to VERITAS and MAGIC that have not yet resulted in any significant correlation between neutrino and gamma-ray emission. For some of those, however, the source was not in the reach of the gamma-ray telescopes, either because it was out of the field of view or due to poor weather conditions. Follow-up studies have allowed setting new limits on high-energy gamma-ray emission.

With the increased sensitivity in the Northern Hemisphere and new alerts to telescopes in the Southern Hemisphere, the discovery potential of these joint searches for neutrino and gamma-ray sources is greatly enhanced. Stay tuned for new results!

Press release: IceCube
Reference: Very High-Energy Gamma-Ray Follow-Up Program Using Neutrino Triggers from IceCube, The IceCube Collaboration, the MAGIC Collaboration, the VERITAS Collaboration, submitted to Journal of Instrumentation

On Sep. 15, six Surface Scintillator Detector (SSD) stations of the Pierre Auger Observatory were deployed in a single trip to the engineering array site. Five more stations are planned to be deployed next week. The photograph shows the first detector set up in the field, which is part of the triplet at the so-called station Generalife.

The upgrade of the Pierre Auger Observatory consists of installing plastic scintillators on top of each existing Surface Detector station together with a new readout electronics (UUB). It will provide a complementary measurement of the showers allowing the reconstruction of muons and electromagnetic particles.

The six detectors in the field are not yet connected to the UUB. A seventh station Didi, located at the campus next to the Assembly Building, has also been equipped with scintillators, but connected to UUB for software debugging. And preliminary results from the upgraded Didi already show first muon signals. Very soon all detectors will be connected to new electronics and will record the first beautiful data. After many years of preparation, this is a true milestone for the Observatory!

Press release: Pierre Auger Obs.

Two black holes spiral in toward one another and merge, emitting a burst of gravitational waves that LIGO can detect.
(credit: LIGO/T. Pyl)

Scientists working at the twin LIGO instruments detected a second gravitational wave. The signal was recorded on December 26, 2015 the LIGO. It originates from a pair of merging black holes of about 14 and 8 solar masses – smaller than the ones detected on September 14 of last year. Researchers from the Max Planck Institute for Gravitational Physics in Potsdam and Hannover and the Leibniz Universität Hannover made significant contributions to the discovery in several key areas: the development of highly accurate gravitational-wave models, search methods to detect faint signals, determining their astrophysical parameters, and advanced detector technology. This second discovery proves that a new era of gravitational-wave astronomy has begun.

The signal was detected in LIGO's first observation run "O1" on December 26, 2015 at 4:38:54 Central European Time (CET) by both of the LIGO detectors, and was named GW151226. The wave arrived 1.1 ms earlier at the Livingston detector than at the Hanford detector

This illustration shows when the Ligo scientists have discovered gravitational waves.
(credit: LIGO)

The event was much weaker than the first detection on September 14 and was buried in the detector noise. A so-called "matched-filter" search was essential for the detection. In such searches, the data are compared to or filtered with many predicted signals in order to find the best match. The predicted signals are based on highly accurate gravitational-wave models developed by scientists at the Max Planck Institute for Gravitational Physics (Albert Einstein Institute, AEI). It was thanks to these models that the LIGO science team was able to show that the signal was caused by the merger of two black holes.

"It's fabulous that our waveform models have pulled out from the noise such a weak but incredibly valuable gravitational-wave signal," says Alessandra Buonanno, director at the AEI in Potsdam and professor at the University of Maryland. "GW151226 perfectly matches our theoretical predictions for how two black holes move around each other for several tens of orbits and ultimately merge. Remarkably, we could also infer that at least one of the two black holes in the binary was spinning!"

After the initial detection of the signal, subsequent analyses, for which the AEI provided half of the computing power, revealed the astrophysical properties of the observed binary system. Most of the data analysis was performed on the Atlas supercomputer, the most powerful computer cluster in the world designed for gravitational-wave data analysis, which has contributed significantly more than any other system used by the LIGO and Virgo Collaborations.

The results show that the observed system GW151226 consists of one black hole with about 14 times the mass of our Sun and one with about 8 solar masses. The two gravity traps merged at a distance of some 1.4 billion light-years from Earth. The researchers also found that at least one of the black holes spins on its axis. The merger emitted the equivalent of about 1 solar mass in gravitational wave energy and left behind a rotating 21 solar mass black hole.

(credit: Numerical - relativistic Simulation: S. Ossokine , A. Buonanno (Max Planck Institute for Gravitational Physics) and the Simulating eXtreme Spacetime project Scientific Visualization : T. Dietrich, R. Haas (Max Planck Institute for Gravitational Physics))
Cosmic Dance of Death I: The simulation shows two black holes of 14 and 8 solar masses orbiting each other.	Cosmic Dance of Death II: The black holes are getting closer and closer to each other.	Cosmic Dance of Death III: The two black holes have merged into a single hole with 21 solar masses. The colours reflect different gravitational fields, with cyan signifying weak, and orange signifying strong fields.

Scientists within the Simulating eXtreme Spacetime collaboration have corroborated this scenario by running numerical-relativity simulations with parameters close to GW151226. These were in excellent agreement over the entire signal with the waveform models mentioned above used to infer the astrophysical properties of the source, further verifying that GW151226 was generated by the collision of two stellar-mass black holes in General Relativity.

The signal, extracted from the detector noise, differs in several key aspects from the first detected signal. Because of the smaller masses, the signal was registered by the instruments for a longer time, about 1 second - 27 orbits of the black holes before merger. For the first detection on September 14, 2015 (GW150914), only the last five orbits were observable. During the one second mentioned, the gravitational wave increased in frequency from 35 Hz to 430 Hz. The signal's peak strain amplitude of about 3×10^-22 made it about three times weaker than the first detection.

Mapping LIGO’s Detections During First Observing Run (credit: LIGO (Leo Singer) /Milky Way image (Axel Mellinger)	Black Holes with Known Masses (credit: LIGO)

"Now even the skeptics have to admit that our first detection was not a fluke," says Bruce Allen, Managing Director of the Max Planck Institute for Gravitational Physics in Hannover."I am now totally confident that in the next few years we will detect dozens of similar black hole mergers, and learn a lot about the universe. It is very satisfying to see that the data analysis methods we have invented in the past twenty years work as well as we had hoped."

"With this second observation we truly are on the path to genuine gravitational-wave astronomy. We can start to explore the variety of sources on the unknown dark side of the Universe," says Karsten Danzmann, director at the AEI in Hannover and director of the Institute for Gravitational Physics at LUH. "After so many years of research, development, and preparation it is very satisfying to see our vision finally come true."

Advanced LIGO’s (aLIGO) next data-taking run "O2" will begin this fall and is expected to last for about six months. By then, further improvements in detector sensitivity should allow LIGO to reach as much as 1.5 to 2 times more of the volume of the universe. The GEO600 gravitational-wave detector will also take part in the observation run. The Virgo detector is expected to join in the latter half of the O2 run.

Press release: Max-Planck Society, LIGO Caltech
More reading: LIGO/VIRGO, APS Physics, Nature, Scientific American, New-York Times, The Guardian
Reference: GW151226: Observation of Gravitational Waves from a 22 Solar-mass Binary Black Hole Coalescence, B.P. Abbott et al. (LIGO Scientific Collaboration and Virgo Collaboration), Phys. Rev. Lett. 116, 241103 (arXiv:1606.04855)

	Computer rendering of CTA Headquarters Building, Bologna, Italy (credit: Bologna University Project Office)		Architectural rendering of the building for the scientific centre of CTA on the DESY campus in Zeuthen (credit:Dahm Architekten & Ingenieure, Berlin)

A major international project in gamma-ray astronomy, the Cherenkov Telescope Array (CTA), has taken another important step towards becoming a reality. At today’s meeting in Munich, the shareholders’ meeting of the company CTAO GmbH decided that the Science Data Management Centre and the seat of the CTA Scientific Director should be located at the DESY research centre in Zeuthen. At the same time, it decided that the administrative headquarters of the CTA organisation would be in Bologna, Italy. "We are very pleased that we have won the international bid and managed to bring the scientific coordination of CTA to Germany," says Beatrix Vierkorn-Rudolph of Germany’s Ministry of Education and Research, who is the deputy chairperson of the CTAO’s shareholders’ meeting.

The Cherenkov Telescope Array is a project that seeks to build an observatory for conducting gamma-ray astronomy, unlike any other in the world. The observatory will consist of more than 100 individual telescopes located at a site in the southern hemisphere and another site in the northern hemisphere. Over 1,000 scientists and engineers from more than 30 different countries have joined forces to set up the facility over the next five years and to operate it for at least 20 years. Negotiations are currently underway concerning the sites in Chile and on La Palma and are due to be completed by the end of this year. The project will cost some 400 million euros and is part of the Federal Ministry of Education and Research’s national roadmap for future research infrastructures, as well as its European counterpart, the ESFRI roadmap.

"Germany has a long and successful tradition in the field of gamma-ray astronomy, which we can put to excellent use in the scientific coordination of CTA," explains Werner Hofmann of the Max Planck Institute for Nuclear Physics in Heidelberg, who is spokesman for the international CTA Consortium and founding director of CTAO GmbH. "CTA is going to revolutionise this field of astronomy. We are expecting CTA to provide profound insights into the role of high-energy processes in the development of the universe and many scientific surprises."

Artist's view of two particle showers hitting the array of CTA telescopes
(credit: DESY/Milde Science Comm.)

The proposals for observations put forward by scientists from all over the world will be collected in Zeuthen, under the management of the CTA Scientific Director, and prepared for future measurement campaigns of the telescope array, and the data from the observations will be processed here and then made available to the research community. Scientists at DESY have for many years been carrying out research in the field of gamma-ray and neutrino astronomy, collaborating closely with the surrounding universities and research institutions in the Berlin-Brandenburg region. This makes it the ideal scientific setting for the Science Data Management Centre of CTA.

"This is a great day for German astroparticle physics, and also for the Zeuthen region as a whole," says Christian Stegmann, head of DESY’s Zeuthen site. "The decision will be a long-term incentive for the Berlin-Brandenburg region, which is already firmly established in the field of astronomy and astrophysics in Germany. We would like to thank CTAO GmbH for the trust it is placing in DESY."

A new building is to be erected for the Science Data Management Centre at the DESY Campus on Lake Zeuthen. It will operate in close coordination with the existing centre, making use of available synergies. "Before long, the excavators will be arriving at the DESY Campus in Zeuthen, and further down the line the CTA researchers at Lake Zeuthen will use gamma radiation to study how, deep inside the universe, shock waves from huge stellar explosions plough their way through our Milky Way," remarks Stegmann, "or how matter is carried off by huge maelstroms in the neighbourhood of black holes."

Press release: CTA, DESY

In other CTA-related news:

• [19/05/2016] Jürgen Knödlseder (IRAP) was elected to a second three-year term as Chair of the Consortium Board.
• [17/05/2016] Stefano Vercellone (Istituto Nazionale di Astrofisica) was elected as Science Coordinator and
  Stefan Funk (Friedrich-Alexander University Erlangen-Nürnberg) as Deputy Science Coordinator by the Consortium Board at the CTA Consortium Meeting in Kashiwanoha, Japan. In these positions they will jointly coordinate the science activities in the CTA Consortium and its science work groups.

The Selection Committee of the Breakthrough Prize in Fundamental Physics today announced a Special Breakthrough Prize in Fundamental Physics recognizing scientists and engineers contributing to the momentous detection of gravitational waves – a detection announced on February 11, 2016.

The Special Breakthrough Prize can be conferred at any time in recognition of an extraordinary scientific achievement. The $3 million award will be shared between two groups of laureates: the three founders of the Laser Interferometer Gravitational-Wave Observatory (LIGO), Ronald W. P. Drever (Caltech), Kip S. Thorne (Caltech) and Rainer Weiss (MIT), who will each equally share $1 million; and 1012 contributors to the experiment, who will each equally share $2 million.

The contributors sharing the prize include 1005 authors of the paper describing the discovery of gravitational waves from the numerous institutions involved in LIGO and its sister experiment, the Virgo Collaboration. Also sharing the prize are seven scientists who made important contributions to the success of LIGO.

Stephen Hawking, who won the Special Breakthrough Prize in 2013, said, "This discovery has huge significance: firstly, as evidence for general relativity and its predictions of black hole interactions, and secondly as the beginning of a new astronomy that will reveal the universe through a different medium. The LIGO team richly deserves the Special Breakthrough Prize."

Edward Witten, the chair of the Selection Committee, commented, "This amazing achievement lets us observe for the first time some of the remarkable workings of Einstein’s theory. Theoretical ideas about black holes which were close to being science fiction when I was a student are now reality.”

The laureates will be recognized at the 2017 Breakthrough Prize ceremony in the fall of 2016, where the annual Breakthrough Prize in Fundamental Physics (distinct from the special prize) will also be presented, along with the Breakthrough Prizes in Life Sciences and Mathematics. 

	Before...	... and after.
	Left: An average of LAT data centered on July 8, 2011 covering 300 days when the blazar was inactive.	Right: An average of 300 active days centered on Feb. 27, 2013, when PKS B1424-418 was the brightest blazar in this part of the sky.
	Fermi LAT images showing the gamma-ray sky around the blazar PKS B1424-418. Brighter colors indicate greater numbers of gamma rays. The dashed arc marks part of the source region established by IceCube for the Big Bird neutrino (50-percent confidence level). (Credit: NASA/DOE/LAT Collaboration)

Nearly 10 billion years ago in a galaxy known as PKS B1424-418, a dramatic explosion occurred. Light from this blast began arriving at Earth in 2012. Now, an international team of astronomers, led by Prof. Matthias Kadler, professor for astrophysics at the university of Würzburg, and including other scientists from the new research cluster for astronomy and astroparticle physics at the universities of Würzburg and Erlangen-Nürnberg, have shown that a record-breaking neutrino seen around the same time likely was born in the same event. The results are published in Nature Physics.

Neutrinos are the fastest, lightest, most unsociable and least understood fundamental particles, and scientists are just now capable of detecting high-energy ones arriving from deep space. The present work provides the first plausible association between a single extragalactic object and one of these cosmic neutrinos.

Although neutrinos far outnumber all the atoms in the universe, they rarely interact with matter, which makes detecting them quite a challenge. But this same property lets neutrinos make a fast exit from places where light cannot easily escape -- such as the core of a collapsing star - and zip across the universe almost completely unimpeded. Neutrinos can provide information about processes and environments that simply aren't available through a study of light alone.

Recently, the IceCube Neutrino Observatory at the South Pole found first evidence for a flux of extraterrestrial neutrinos, which was named the Physics World breakthrough of the year 2013. To date, the science team of IceCube Neutrino has announced about a hundred very high-energy neutrinos and nicknamed the most extreme events after characters on the children's TV series "Sesame Street." On Dec. 4, 2012, IceCube detected an event known as Big Bird, a neutrino with an energy exceeding 2 quadrillion electron volts (PeV). To put that in perspective, it's more than a million million times greater than the energy of a dental X-ray packed into a single particle thought to possess less than a millionth the mass of an electron. Big Bird was the highest-energy neutrino ever detected at the time and still ranks second.

Where did it come from? The best IceCube position only narrowed the source to a patch of the southern sky about 32 degrees across, equivalent to the apparent size of 64 full moons. "It’s like a crime scene investigation", says lead author Matthias Kadler, a professor of astrophysics at the University of Würzburg in Germany, "The case involves an explosion, a suspect, and various pieces of circumstantial evidence."

Starting in the summer of 2012, NASA’s Fermi satellite witnessed a dramatic brightening of PKS B1424-418, an active galaxy classified as a gamma-ray blazar. An active galaxy is an otherwise typical galaxy with a compact and unusually bright core. The excess luminosity of the central region is produced by matter falling toward a supermassive black hole weighing millions of times the mass of our sun. As it approaches the black hole, some of the material becomes channeled into particle jets moving outward in opposite directions at nearly the speed of light. In blazars one of these jets happens to point almost directly toward Earth.

During the year-long outburst, PKS B1424-418 shone between 15 and 30 times brighter in gamma rays than its average before the eruption. The blazar is located within the Big Bird source region, but then so are many other active galaxies detected by Fermi.

These radio images from the TANAMI project reveal the 2012-2013
eruption of PKS B1424-418 at a radio frequency of 8.4 GHz. The core of
the blazar’s jetbrightened by four times, producing the most dramatic
blazar outburst  TANAMI has observed to date
(Credit: TANAMI Collaboration)

The scientists searching for the neutrino source then turned to data from a long-term observing program named
TANAMI. Since 2007, TANAMI has routinely monitored nearly 100 active galaxies in the southern sky, including many flaring sources detected by Fermi. Three radio observations between 2011 and 2013 cover the period of the Fermi outburst. They reveal that the core of the galaxy's jet had been brightening by about four times. No other galaxy observed by TANAMI over the life of the program has exhibited such a dramatic change.

"Within their jets, blazars are capable of accelerating protons to relativistic energies. Interactions of these protons with light in the central regions of the blazar can create pions. When these pions decay, both gamma rays and neutrinos are produced," explains Karl Mannheim, a coauthor of the study and astronomy professor in Würzburg, Germany. "We combed through the field where Big Bird must have originated looking for astrophysical objects capable of producing high-energy particles and light," adds coauthor Felicia Krauß, a doctoral student at the University of Erlangen-Nürnberg in Germany. "There was a moment of wonder and awe when we realized that the most dramatic outburst we had ever seen in a blazar happened in just the right place at just the right time."

In a paper published Monday, April 18, in Nature Physics, the team suggests the PKS B1424-418 outburst and Big Bird are linked, calculating only a 5-percent probability the two events occurred by chance alone. Using data from Fermi, NASA’s Swift and WISE satellites, the LBA and other facilities, the researchers determined how the energy of the eruption was distributed across the electromagnetic spectrum and showed that it was sufficiently powerful to produce a neutrino at PeV energies.

"Taking into account all of the observations, the blazar seems to have had means, motive and opportunity to fire off the Big Bird neutrino, which makes it our prime suspect," explains Matthias Kadler.

Francis Halzen, the principal investigator of IceCube at the University of Wisconsin–Madison, and not involved in this study, thinks the result is an exciting hint of things to come. "IceCube is about to send out real-time alerts when it records a neutrino that can be localized to an area a little more than half a degree across, or slightly larger than the apparent size of a full moon," he concludes. "We're slowly opening a neutrino window onto the cosmos."

But this study also demonstrates the vital importance of classical astronomical observations in an era when new detection methods like neutrino observatories and gravitational-wave detectors open new but unknown skies.

 
Press release: Uni Würzburg, Phys.org
Reference: Coincidence of a high-fluence blazar outburst with a PeV-energy neutrino event, M. Kadler et al., Nature Physics (doi:10.1038/nphys3715, arXiv:1602.02012)

All at a glance: H.E.S.S. (High Energy Stereoscopic System)
(credit: HAP / A.Chantelauze)

Scientists of the H.E.S.S. Observatory have identified an area around the black hole in the centre of the Milky Way that emits intense gamma radiation of extremely high energy. The source of the radiation is an astrophysical accelerator speeding up protons to energies of up to one peta electronvolts (PeV) – more than 100 times higher than the largest and most powerful man-made particle accelerator, the Large Hadron Collider LHC at CERN. The scientists have now published their detailed analysis of recent H.E.S.S. data in the scientific journal Nature. The analysis shows the first identification of a source of cosmic rays with peta-electronvolt energy within the Milky Way: it is very likely the supermassive black hole at the centre of our galaxy itself. 

For more than 10 years, H.E.S.S. (High Energy Stereoscopic System), a gamma ray telescope in Namibia which is operated by 150 scientists from 12 countries, has mapped the centre of the Milky Way in highest energy gamma rays. The gamma rays observed by the researchers are produced by so-called cosmic radiation – high-energy protons, electrons and atomic nuclei, which are accelerated in different places of the universe. Scientists have wondered about the astrophysical sources of this cosmic radiation since its discovery more than a century ago. The problem is that the particles are electrically charged and are therefore deflected in interstellar magnetic fields from their straight path. For this reason, their flight does not point back to its place of production. However, the particles of cosmic radiation often encounter interstellar gas or photons close to their source, producing high-energy gamma rays which reach the earth on a straight path. These gamma rays are used by the scientists of H.E.S.S. Observatory to make the sources of cosmic rays in the sky visible. 

 
Artist's view of the high energy accelerator in the centre of our galaxy:
accelerated protons (blue) interact in dense molecule clouds around the centre and
produce neutral particles which decay immediately emitting gamma rays (yellow waves).
In the background a photo ov the Milky Way
(credi: M.A. Garlick, H.E.S.S. Collaboration)

When gamma rays hit the Earth's atmosphere, they produce short bluish flashes of light that can be detected by large mirror telescopes with fast light sensors at night. With this technique, more than 100 sources of high-energy gamma rays have been discovered in the sky over the past decades. Currently, H.E.S.S. is the most sensitive tool for their detection.

It is known that cosmic radiation with energies up to about 100 tera electronvolts (TeV) is generated in the Milky Way. However theoretical arguments and the direct measurement of the cosmic radiation suggest that these particles should be accelerated in our galaxy up to energies of at least one peta electronvolt (PeV). In recent years, many extragalactic sources have been discovered that accelerate cosmic rays to multi-TeV energies, but the search for accelerators of the highest-energy cosmic rays in our galaxy remains unsuccessful so far.

Detailed observations of the centre of the Milky Way, which were carried out with the H.E.S.S. telescopes during the past 10 years, now provide the first answers. "We have located an astrophysical accelerator accelerating protons to energies of up to one peta electronvolts, and that continuously over at least 1,000 years," says Prof. Christian Stegmann, head of DESY in Zeuthen and former spokesperson of the H.E.S.S. Collaboration.

Very high-energetic gamma-ray image of the Galactic Centre region.
(credit: H.E.S.S. Collaboration)

Already during the first years of observation since 2002 H.E.S.S. had detected a strong compact source and an extended band of diffuse highest-energy gamma rays in the galactic centre. Evidence of this diffuse radiation, which covers an area of about 500 light years across, was already a clear indication of a source of cosmic rays in this region; proof of the source itself remained unfulfilled for the researchers. A significantly larger amount of observational data together with advances in analytical techniques have made it now possible to measure for the first time both the spatial distribution as well as the energy of the cosmic rays.

Although the central region of our Milky Way hosts many objects that can generate high-energy cosmic rays, for instance a supernova remnant, a pulsar wind nebulae and a compact star clusters, the measurement of gamma rays from the galactic centre provides strong evidence that the supermassive black hole at the galactic centre itself accelerates protons to an energy of up to one PeV. "Our data show that the observed glow of gamma rays around the galactic centre is symmetrical," says H.E.S.S. researcher Stefan Klepser from Zeuthen. "The gamma rays are of a high energy and concentrated towards the centre, which suggests that they must be the echo of a huge particle accelerator which is located in the centre of this glow." Prof. Stegmann adds that "several possible acceleration regions can be considered, either in the immediate vicinity of the black hole, or further away, where a fraction of the material falling into the black hole is ejected back into the environment, potentially initiating the acceleration of particles."

However, the analysis of the measurements also shows that this source alone cannot account for the total flux of cosmic rays detected on Earth. "If, however, our central black hole has been more active in the past", the researchers argue, "then it might actually be responsible for the entire bulk of today's galactic cosmic rays". If their assumption is correct, the 100-year-old mystery of the origin of cosmic rays would be solved.

On March 10, at its launch event at the Royal Netherlands Academy of Arts and Sciences in Amsterdam, the European Strategy Forum for Research Infrastructures (ESFRI) announced that KM3NeT 2.0 is selected for the 2016 ESFRI Roadmap. KM3NeT is a distributed research infrastructure with deep-sea sites planned in the Mediterranean Sea near Toulon (France), Sicily (Italy) and Pylos (Greece). The scientific goals of the KM3NeT Collaboration are the discovery of astrophysical sources of cosmic neutrinos, the determination of the neutrino mass ordering and synergetic research opportunities for marine and environmental studies.

The ESFRI Roadmap identifies new Research Infrastructures of pan-European interest corresponding to the long-term needs of the European research communities. Its mission is to ensure that scientists in Europe have access to world-class facilities for cutting-edge research.

To be eligible for the roadmap a research infrastructure should have at least three countries with funding commitment and political support.

Dr. S. Harissopulos, director of the Institute of Nuclear and Particle Physics of NCSR "Demokritos": "The Greek Minister of Research encouraged us to actively participate in KM3NeT 2.0."

After a rigorous selection process in which projects were assessed for scientific excellence, pan-European relevance, socio-economic impact, e-needs and maturity level, KM3NeT 2.0 is amongst the 21 chosen projects.

"Full steam ahead for KM3NeT 2.0. The ESFRI review was maybe not easy, but certainly beneficial." said Prof. Dr. Maarten de Jong, spokesperson of the KM3NeT Collaboration, during his talk at the launch event, in which he explained the scientific goals of the research infrastructure and highlighted the recent progress.

Prof. Dr. Michel Spiro, chairperson of the KM3NeT Scientific and Technical Advisory Committee confirms: "This is a new step towards neutrino astronomy and further deciphering the Universe and neutrino mysteries."

Prof. Dr. Antonio Masiero, the chairperson of the KM3NeT Resources Review Board, chairperson of the APPEC Scientific Advisory Committee and vice-president of INFN notes, "This is excellent news, KM3NeT continues to be considered by EU as an important project and an innovative research infrastructure at the continental level. This vote of confidence will be instrumental as KM3NeT rapidly moves forward on the realisation of the envisaged research facility."

Press release: KM3Net, Physik Journal
Reference: Strategy Report on Research Infrastructures, ESFRI Roadmap 2016

Image of air showers, simulated with CORSIKA,
mounted onto a photo of the central station ("superterp")
of the LOFAR telescope network near Exloo/Netherlands.
(credit: ASTRON/KIT)

LOFAR, the low-frequency array radio telescope, normally receives weak radio waves from the distant universe. But now and then an ultra-short, bright radio pulse is observed somewhere in between AM and FM radio frequencies. This radio blast would appear as a short cracking sound in your car radio. While usually ignored, this radio signal is actually the last SOS of an elementary particle entering the Earth atmosphere at almost the speed of light. The particles were fired off by a cosmic accelerator Millions of year ago. 

An international team of astronomers including a number of scientists from the German Long Wavelength consortium (
GLOW) have now unravelled the radio code of these intruders to determine their nature and constrain their origin.

Supernova explosions, dying stars, black holes. All these phenomena have been named as sources of cosmic ray particles. But until now nobody really knows the origin. Cosmic ray particles are elementary particles that travel through the universe with an energy that is a million times bigger than in the largest particle accelerator on earth. With almost the speed of light, they collide like bullets with the atmosphere, before falling apart into a cascade of secondary particles. Their interaction with the Earth’s magnetic field leads to an extremely short radio signal, no longer than one billionth of a second. Thousands of LOFAR antennas help to find the signal and measure it accurately.

Finding the signal is one thing, knowing what caused it is another. For the first time astronomers now succeeded in calculating and modelling what kind of particle came in. "We can now identify the bullet," says Heino Falcke from Radboud University in the Netherlands, the chair of the International LOFAR Telescope board who also pioneered this new technique. "In most cases the bullet turns out to be a single proton or the light nucleus of a helium atom."

"Because of the enormous energy, most astrophysicists assume that cosmic particles originate deep in the universe, like black holes in other galaxies", adds Stijn Buitink from the Vrije Universiteit Brussel, the first author of the
Nature paper. "But we think they come from a nearby source and get their energy from a cosmic accelerator in the Milky Way – perhaps a very massive star."

The sources of cosmic particles are cosmic accelerators, up to a million times stronger than the Large Hadron Collider (LHC) in Geneva or any conceivable man-made accelerator for that matter. “These particles come to Earth anyway, so we only have to find them”, says Heino Falcke who is affiliated with the Max Planck Institute for Radio Astronomy (MPIfR) in Bonn, Germany. “We can now do high energy physics with simple FM radio antennas.”  This opens up a new window to the high-energy universe and high-precision measurement of cosmic particles.

"The main difference to ordinary FM radios is in the digital electronics and the broad-band receivers which allow us to measure a large number of frequencies simultaneously at high speed", explains Andreas Horneffer from MPIfR who built the antennas of a pre-cursor of the present experiment, LOPES ("LOFAR Prototype Experimental Station") as part of his PhD project.

The particle identification from the radio measurements relies on exact knowledge of the radio emission physics. The LOFAR data are compared with simulations made with the CoREAS code developed by Tim Huege and his colleagues at Karlsruhe Institute of Technology (KIT) in the framework of the CORSIKA air shower simulation program. "When we started the radio signal simulations ten years ago and compared with data of our LOPES experiment, the physics of the radio emission was a big puzzle. Today, the simulations can reproduce even the high-quality LOFAR data in great detail, and could therefore be used to interpret the measurements with confidence." says Tim Huege.

"It is a remarkable experience having particle physicists and radio astronomers working together to realize such a successful experiment in the rising new field of astroparticle physics", concludes Ralf-Jürgen Dettmar from Ruhr-Universität Bochum, the chairman of the German GLOW consortium.

 
Press release: Max Planck Association, Radboud Uni
Reference: A large light-mass component of cosmic rays at 10¹⁷–10^17.5 electronvolts from radio observations, S. Buitink, A. Corstanje, H. Falcke, J. R. Hörandel, T. Huege, A. Nelles et al., Nature 531, 70–73, 061102 (arXiv:1603.01594)

Upper limit on the high-energy muon neutrino spectral fluence from GW150914 as a function of source direction. For comparison, the contours of the GW sky map are also shown. See more details about the figure in the paper.
(credit:The ANTARES Collaboration, the IceCube Collaboration, the LIGO Scientific Collaboration and the Virgo Collaboration)

The detection of gravitational waves is an almost unprecedented success of fundamental science. It not only yields the one yet missing proof of Einstein’s Theory of General Relativity, but also opens a totally new observational window to the Universe.

 

This discovery may remind of another breakthrough which was achieved three years ago – the identification of cosmic neutrinos of highest energy with IceCube, also opening a new window to the cosmos.  Therefore it appeared suggestive that LIGO scientists shared their data with the neutrino telescope collaborations, so that physicists from
IceCube and ANTARES had the opportunity to check whether these two detectors had registered an excess of neutrinos coinciding in time with GW150914, i.e. with the signal reported by LIGO.  A possible joint detection would have conveyed exciting astrophysical insight and could have been be used in targeted electromagnetic follow-up observations, given the significantly better angular resolution of neutrino events compared to gravitational waves.

 

The investigations were performed on the basis of mutual agreements between the gravitational collaborations LIGO and Virgo on the one side and the two neutrino projects on the other. These agreements include strict confidentiality of the exchanged information. Knowing neither the significance of the LIGO signal nor its approximate position, the IceCube and Antares physicists selected data from a time window of ±500 seconds around GW150914. For IceCube, Thomas Kintscher from DESY in Zeuthen/Germany extracted the data from the stream of data reconstructed online directly at the South Pole and forwarded them to his colleagues Chad Finley from the University Stockholm and Imre Bartos from Columbia University, New York. These two performed the analysis, and only they did know about the approximate position derived from the gravitational data.  Similarly, a small group of people acted on the ANTARES side, with Bruny Baret and Alexis Coleiro (both APC Paris) on the front line. Given the character of the source, the result was not totally unexpected: No coincident neutrinos were observed, neither by IceCube nor by ANTARES. The corresponding joint publication can be found at https://dcc.ligo.org/LIGO-P1500271/public.

 

Resumé: A centennial success for gravitational physics, and a detective story of neutrino physics, which just started and will be continued for the following gravitational wave detections.

Press release: GNN
More reading: IceCube, ANTARES
Reference: High-energy Neutrino follow-up search of Gravitational Wave Event GW150914 with ANTARES and IceCube,The ANTARES Collaboration, the IceCube Collaboration, the LIGO Scientific Collaboration and the Virgo Collaboration.

 
The gravitational wave signals detected by the twin LIGO stations.
(credit: LIGO Collaboration)

LIGO opens new window on the universe with observation of gravitational waves from colliding black holes /key contributions from Max Planck Society and Leibniz Universität Hannover researchers.

For the first time, scientists have observed ripples in the fabric of spacetime called gravitational waves, arriving at the earth from a cataclysmic event in the distant universe. This confirms a major prediction of Albert Einstein’s 1915 general theory of relativity and opens an unprecedented new window onto the cosmos.

Gravitational waves carry information about their dramatic origins and about the nature of gravity that cannot otherwise be obtained. Physicists have concluded that the detected gravitational waves were produced during the final fraction of a second of the merger of two black holes to produce a single, more massive spinning black hole. This collision of two black holes had been predicted but never observed.

The gravitational waves were detected on September 14, 2015 at 5:51 a.m. Eastern Daylight Time (9:51 a.m. UTC) by both of the twin Laser Interferometer Gravitational-wave Observatory (LIGO) detectors, located in Livingston, Louisiana, and Hanford, Washington, USA. The LIGO Observatories are funded by the National Science Foundation (NSF), and were conceived, built, and are operated by Caltech and MIT.

The discovery, accepted for publication in the journal Physical Review Letters, was made by the LIGO Scientific Collaboration (which includes the GEO Collaboration and the Australian Consortium for Interferometric Gravitational Astronomy) and the Virgo Collaboration using data from the two LIGO detectors.

Researchers at the Max Planck Institute for Gravitational Physics (Albert Einstein Institute, AEI) in Hannover and Potsdam, Germany, and from the Institute for Gravitational Physics at Leibniz Universität Hannover (LUH) have made crucial contributions to the discovery in several key areas: development and operation of extremely sensitive detectors pushed to the limits of physics, efficient data analysis methods running on powerful computer clusters, and highly accurate waveform models to detect the signal and infer astrophysical information from it.

(credit: Ligo Scientific Collaboration, Scientific American)

Advanced detector techniques from GEO600

The GEO collaboration includes Max Planck and Leibniz Universität researchers together with UK colleagues. They designed and operate the GEO600 gravitational-wave detector near Hannover, Germany. It is used as a think tank and testbed for advanced detector techniques.

Most of the key technologies that contributed to the unprecedented sensitivity of Advanced LIGO (aLIGO) and enabled the discovery have been developed and tested within the GEO collaboration. Examples of these are signal recycling, resonant sideband extraction, and monolithic mirror suspensions. AEI researchers together with the Laser Zentrum Hannover e.V. also developed and installed the aLIGO high-power laser systems, which are crucial for the high-precision measurements.

"Scientists have been looking for gravitational waves for decades, but we’ve only now been able to achieve the incredibly precise technologies needed to pick up these very, very faint echoes from across the Universe," says Karsten Danzmann, director at the Max Planck Institute for Gravitational Physics in Hannover and director of the Institute for Gravitational Physics at Leibniz Universität Hannover. "This discovery would not have been possible without the efforts and the technologies developed by the Max Planck, Leibniz Universität, and UK scientists working in the GEO collaboration."

(credit: Ligo Scientific Collaboration, Scientific American)

Computing power and analysis methods for the discovery

Max Planck scientists developed and implemented advanced and efficient data analysis methods to search for weak gravitational-wave signals in the aLIGO detector data streams and carried out most of the production analysis. In addition, the majority of the computational resources for the discovery and analysis of the Advanced LIGO data were provided by Atlas, the most powerful computer cluster in the world designed for gravitational-wave data analysis, operated by the AEI. Atlas has provided more than 24 million CPU core hours for the analysis of Advanced LIGO data.

"I am proud that the first two scientists to look at the signal were at the Max Planck Institute for Gravitational Physics and that our institute played a leading role in this exciting discovery," says Bruce Allen, director at the Max Planck Institute for Gravitational Physics in Hannover. "Einstein himself thought gravitational waves were too weak to detect, and didn’t believe in black holes. But I don’t think he’d have minded being wrong!"

Accurate models of gravitational waves pave the way

Max Planck researchers developed highly accurate models of gravitational waves that black holes would generate in the final process of orbiting and colliding with each other. These waveform models were implemented and employed in the continuing search for binary coalescences in LIGO data. It is this search that observed the black-hole merger known as GW150914 with greater than 5-sigma confidence.

Max Planck scientists also used the same waveform models to infer the astrophysical parameters of the source, such as the masses and spins of the two black holes, the binary’s orientation and distance from Earth, and the mass and spin of the enormous black hole that the merger produced. The waveform models were also employed to test whether GW150914 is consistent with predictions from general relativity.

"We spent years modeling the gravitational-wave emission from one of the most extreme events in the Universe: pairs of massive black holes orbiting with each other and then merging. And that’s exactly the kind of signal we detected!" says Alessandra Buonanno, director at the Max Planck Institute for Gravitational Physics in Potsdam. "It is overwhelming to see how exactly Einstein’s theory of relativity describes reality. GW150914 gives us a remarkable opportunity to see how gravity operates under some of the most extreme conditions possible."

 
Press release: Max-Planck Society, LIGO Caltech
More reading: LIGO/VIRGO, APS Physics, Nature (special), Scientific American, New-York Times, The Guardian,
Reference: Observation of Gravitational Waves from a Binary Black Hole Merger, B. P. Abbott et al. (LIGO Scientific Collaboration and Virgo Collaboration), Phys. Rev. Lett. 116, 061102 (arXiv:1602.03837)

The scientific potential of the ORCA detector of KM3NeT. On the x-axis, the year of operation of ORCA starting in 2020. On the y-axis, the significance to determine the neutrino mass hierarchy using ORCA. (credit: KM3Net Collaboration, picture from the Executive summary of the Letter of Intent for KM3NeT 2.0)	The scientific potential of the ARCA detector of KM3NeT. On the x-axis the year of operation of ARCA starting in 2020. On the y-axis the significance for observing the cosmic neutrino flux reported by the IceCube Collaboration. (credit: KM3Net Collaboration, picture from the Executive summary of the Letter of Intent for KM3NeT 2.0)

Today, scientists of the KM3NeT Collaboration have publicly announced KM3NeT 2.0, their ambition for the immediate future to further exploit the clear waters of the deep Mediterranean Sea for the detection of cosmic and atmospheric neutrinos. The published Letter of Intent details the science performance as well as the technical design of the KM3NeT 2.0 infrastructure.

The two major scientific goals of KM3NeT 2.0 are the discovery of astrophysical sources of neutrinos in the Universe with the KM3Net/ARCA detector and the measurement of the neutrino mass hierarchy using atmospheric neutrinos with the KM3NeT/ORCA detector. Thanks to the flexible KM3NeT design, efficient detection of neutrinos is possible over a wide energy range (GeV to PeV) with an almost identical implementation.

"The combination of the cost effective design of the ARCA detector of KM3NeT and state-of-the-art reconstruction software allows for efficient detection of all three neutrino flavours from cosmic origin in a few years," explains Rosa Coniglione, KM3NeT Workgroup leader HE Astrophysics. The KM3NeT scientists estimate that with the ARCA detector installed at the KM3NeT-It site south of Sicily, Italy, the observation of the cosmic neutrino flux reported by the IceCube Collaboration will be possible within one year of operation.

"With the densely instrumented ORCA detector of KM3NeT we will be able to determine the relative ordering of the neutrino masses, also referred to as the neutrino mass hierarchy," explains Antoine Kouchner, KM3NeT Workgroup leader LE Physics. With the ORCA detector installed at the KM3NeT-Fr site south of Toulon, France, the collaboration expects to determine neutrino mass hierarchy with at least 3-sigma significance after three years of operation.

The Letter of Intent is now open for scrutiny by the neutrino scientific community and will serve as the reference document for requests for funding by the various stakeholders in Europe and abroad. Pending funding, KM3NeT 2.0 could become reality as early as in 2020. Uli Katz, KM3NeT Physics and Software manager concludes, "The modular design of KM3NeT with detector blocks for the telescope makes it possible to swiftly react on new scientific developments. With KM3NeT 2.0 we are able to not only perform all-flavour neutrino astroparticle physics but also advance fundamental neutrino particle physics".

 
Press release: KM3Net
Reference: Letter of Intent for KM3NeT 2.0 (arXiv:1601.07459)

Scientists working with the Major Atmospheric Gamma-ray Imaging Cherenkov (MAGIC) observatory have reported the discovery of the most energetic pulsed emission radiation ever detected from the neutron star in the center of the supernova of 1,054 A.D. , known as the Crab pulsar.

The Crab pulsar is the corpse left over when the star that created the Crab nebula exploded as a supernova. It has a mass of 1.5 the mass of the Sun concentrated in a bout 10 kilometer s diameter object, rotates 30 times per second, and is surrounded by a region of intense magnetic field ten thousand billion times stronger than that of the Sun. This field is strong enough to dominate the motion of charges and forces them to rotate at the same rate as the stellar surface. This region is called the magnetosphere. The rotation of the magnetic field also generates intense electric fields that literally tear electrons from the surface. As these accelerated electrons stream outward, they produce beams of radiation that we receive every time the beam crosses our line of sight, like a lighthouse.

In 2011, the MAGIC and VERITAS observatories discovered unexpected very energetic photons. Emma de Oña Wilhelmi from the Institute of Space Sciences (IEEC - CSIC, Barcelona) and Principal Investigator of this observation program says : "We performed deep observation of the Crab pulsar with MAGIC to understand this phenomenon, expecting to measure the maximum energy of the pulsating photons". Roberta Zanin (ICCUB - IEEC, Barcelona) continues: "The new observations extend this tail to much higher, above TeV energies, that is, several times more energetic than the previous measurement, violating all the theory models believed to be at work in neutron stars."

The photons arrive in two precise beams which should be created far from the neutron star surface: on the far end of the magnetosphere or outside it, in the ultra-relativistic wind of particles around the pulsar, to be able to accelerate electrons to such energies and to escape the large absorption in the magnetised atmosphere. But very surprisingly, the TeV beams arrive at the same time as the radio and X-ray beams, which are very likely produced within the magnetosphere. This tight synchronization of the beams at different energies implies that the bright radiation observed in the multi-wavelength spectrum is produced altogether in a rather small region. Alternatively one can say that the electrons responsible from the TeV radiation keep somehow memory of the low-energy beams. Daniel Galindo Fernandez (ICCUB - IEEC, Barcelona) says: "Where and how this TeV emission is created remains still unknown and difficult to reconcile with the standard theories." And David Carreto Fidalgo from Complutense University of Madrid adds: "But how and where this effect is achieved in such a small region challenges our knowledge of physics".

MAGIC Spokeperson Razmik Mirzoyan from the – HAP partner – Max Planck Institute for Physics (MPP) in Munich, Germany says: "This is another very important result achieved by MAGIC on the puzzling celestial object, which incidentally besides the Sun is the most investigated oranges. Hence from the beginning of operation of the MAGIC experiment in 2004, we have been intensively observing the Crab Nebula and the Crab pulsar. And that has really paid-off – in the mean time we revealed significant features of this enigmatic object thus providing substantial input to our theory colleagues – now it is their move to explain how the things are at work. MAGIC has been designed to be the mo st suitable instrument among imaging atmospheric Cherenkov telescopes to perform this kind of observations."

Press release: MAGIC
Publication: Teraelectronvolt pulsed emission from the Crab Pulsar detected by MAGIC, MAGIC Collaboration:
S. Ansoldi et al., Astronomy&Astrophysics 585 (2016) A133 (A&A, arXiv:1510.07048)