21/05/2021: Jacco Vink to use large grant to seek out the sources of gamma rays

Astrophysicists and GRAPPA member Jacco Vink has received an NWO Large Investment grant in the amount of 1.5 million euros from the Netherlands Organisation for Scientific Research (NWO). The grant money will enable the Netherlands to develop and manufacture highly specialised cameras for the future southern Cherenkov Telescope Array (CTA), which is scheduled for construction in northern Chile. Jacco Vink will implement this project together with astronomers and physicists from the University of Amsterdam (GRAPPA members Sera Markoff and Christoph Weniger), the University of Groningen (Manuela Vecchi and Andrey Baryshev) and the NOVA lab located at the latter.

The gamma radiation from the sources in the Milky Way, as observed by H.E.S.S. A predecessor of CTA, H.E.S.S. consists of 5 telescopes. Credit: H.E.S.S., F. Acero.

The array of 50 telescopes will indirectly measure the gamma-ray photons (particles of light) that enter our atmosphere and then generate a cloud composed mainly of electrons that are moving at close to the speed of light. This causes the electrons to emit a bluish light known as Cherenkov radiation. CTA’s telescopes register this nanosecond-long flashes of blue light and identify the direction in which the electron cloud is moving. In doing so, the CTA actually uses the atmosphere as a detector: the larger the portion of the atmosphere that is being looked at, the more sensitive the CTA is. And when the same flash is detected using multiple telescopes, it is possible to much more accurately determine the direction in which the electron cloud is moving. This, in turn, enables the scientists to more precisely identify the direction of the original gamma-ray photon as well. That is why they use so many telescopes.

Measuring the flash from multiple angles enables them to reconstruct the direction from which the gamma rays came and how much energy the gamma ray photon had. In this way, the location of a gamma ray source can be pinpointed to a precision of 0.1% of the lunar diameter, while images can be made with a pixel size of 10% of the lunar diameter. Known sources of gamma rays include jets created by black holes, the remnants of supernovas and neutron stars, though it might also be emitted when neutron stars merge and set off gravitational waves. Massive particles of dark matter can emit gamma rays as well. CTA hopes to shed new light on the composition of this mysterious and dark component of the universe.

CTA is a European project with international partners including the United States, Japan and Australia. While Dutch astronomers have been involved in the scientific plans for CTA for some time, countries are required to contribute to the array’s construction before they can participate in the CTA observation project. Thanks to the NWO Large grant, the Netherlands will now be able to make such a contribution. The Dutch project will be led from within the UvA, in close cooperation with the University of Groningen and the NOVA lab it hosts, which specialises in the development and construction of astronomical detectors via serial production. This NOVA team will build the detectors for the 50 telescopes that are meant to enable CTA to measure extremely high-energy gamma rays between 10 and 100 tera-electronvolts (TeV). 

More information can be found at IoP and NOVA.

08/03/2021: Samaya Nissanke receives Suffrage Science Award

Physicist Dr Samaya Nissanke of GRAPPA, the University of Amsterdam and Nikhef will receive the prestigious Suffrage Award for outstanding science, science communication and support of women in STEM. Dr Nissanke and eleven other awardees will be honored at an online celebration on Monday 8 March 2021, the tenth anniversary of the scheme. This will be the fifth Suffrage Science awards for the Engineering and Physical Sciences.

Professor Amina Helmi, University of Groningen, who nominated Dr Samaya Nissanke, University of Amsterdam and Nikhef, said: “Samaya is an excellent researcher focusing on a very hot topic in modern Astrophysics, namely on gravitational waves (Nobel Prize 2017) and how they tell us about the merger of pairs of neutron stars and black holes. She was part of the discovery of the first ever binary neutron star merger that was seen by gravitational waves by the international LIGO and Virgo collaboration in 2017. She was in fact part of the writing team for the discovery paper and was responsible for coordinating the follow-up from many different observatories. Not only is she an outstanding scientist, but she is also a fervent and devoted advocate of diversity and inclusion in science, and beyond. She is the chair of Netherlands Astronomy Equity and Inclusion Committee, which she helped set up.”

On International Woman’s Day in 2011, the inaugural cohort of eleven ‘Suffrage Scientists’ from the life sciences were recognised in a ceremony at the Institute of Contemporary Arts, London. Their awards were hand-crafted items of jewellery created by art students from Central Saint Martins-UAL, who worked with scientists to design pieces inspired by research and by the Suffragette movement. But rather than produce a new set of pieces for the next awards, each holder chose who they would like to pass their award onto, thus generating an extensive ‘family tree’ of incredible scientists and communicators.

Suffrage Science pioneer Professor Fisher said: “We dreamed up the awards scheme to celebrate the contribution that women have made to science, which often gets overlooked. This is as important now as it was ten years ago. This year’s awardees join a community of over 148 women scientists. I’m thrilled that since 2011, the awards have travelled from the UK, across Europe to the USA, Hong Kong, Iran and to Ghana, illustrating the international nature of science and engineering, and the global effort to improve the representation of women in STEM.”

More information:

The female group members of Samaya Nissankes research group. Credit: Martijn van Calmthout

01/03/2021: GRAPPA Postdoctoral Fellows: Lionel London and Oscar Macias

We are pleased to announce two GRAPPA Postdoctoral Fellows who will be starting from fall 2021. They are Lionel London (postdoc at MIT) and Oscar Macias (postdoc at Kavli IPMU). We are very excited and look forward to welcoming and working with them in the very near future!

Lionel London

Lionel London earned his PhD in Physics at the Georgia Institute of Technology under the supervision of Deirdre Shoemaker. There he specialized in using simulations of binary black hole collisions to model gravitational waves from binary black hole mergers. He has since gone on to work as a postdoc at Cardiff University, and at MIT. His research to date has been impactful in tests of gravity as well as the detection and parameter estimation of astrophysical black hole mergers. His work combines General Relativity theory and numerics with insights from computer science and pure maths. 

As a member of the LIGO-Virgo-KAGRA collaboration, London is involved in efforts at the intersection of signal modelling, parameter estimation, and tests of General Relativity.

London says, “I am excited about solving puzzles related to black holes, general relativity, and the fundamental structure and story of the universe. My research aims to tackle some of the big questions in these topics. I want to know if it is possible to understand the nonlinear structure of black hole merger, and whether we use this understanding to learn more about the Universe from exotic theories of gravity to cosmology. During my time at GRAPPA I hope to make progress on these topics and thereby contribute to what we can learn from current Earth based experiments like LIGO, Virgo and KAGRA, as well as future detectors such as the Einstein Telescope and LISA.”

Oscar Macias

Occar Macias currently holds a postdoctoral position at Kavli IPMU. He was previously a postdoctoral researcher in the Center for Neutrino Physics at Virginia Tech for 2015-2018. Before that, he completed his PhD in Physics from University of Canterbury, where he worked with Chris Gordon on dark matter searches with gamma rays.

Macias says, “I study energetic astrophysical phenomena, with a focus on potential signals from dark matter emission. I apply my expertise in analyzing cosmic rays and multiwavelength electromagnetic signals, my experience with numerical simulations, and my rigorous statistical background to probe the inner workings of the Universe. My work informs the current limitations of dark matter models to explain the observations, improves the sensitivity to dark matter searches by producing better background models, and provides precise probes of the high-energy Universe.”

Macias received his undergraduate degree in Civil Engineering from the National University of Colombia, Medellin in 2008, where he also completed his master degree in Physics in 2011. Before moving to Medellin for his undergraduate studies, he lived in a small village — far away from Colombian cities — which was badly affected by the Colombian internal conflict.

Macias adds, “The challenges I faced in the early stages of my educational path have galvanized my passion for equity, diversity and justice in academia. I plan to design and run workshops aimed at increasing the representation of minorities in Physics and Astronomy.”

10/02/2021: Rare Blast’s Remains Discovered in Milky Way Center

Astronomers may have found our galaxy’s first example of an unusual kind of stellar explosion. This discovery, made with NASA’s Chandra X-ray Observatory, adds to the understanding of how some stars shatter and seed the universe with elements critical for life on Earth.

This intriguing object, located near the center of the Milky Way, is a supernova remnant called Sagittarius A East, or Sgr A East for short. Based on Chandra data, astronomers previously classified the object as the remains of a massive star that exploded as a supernova, one of many kinds of exploded stars that scientists have catalogued. Using longer Chandra observations, a team of astronomers has now instead concluded that the object is left over from a different type of supernova. It is the explosion of a white dwarf, a shrunken stellar ember from a fuel-depleted star like our Sun. When a white dwarf pulls too much material from a companion star or merges with another white dwarf, the white dwarf is destroyed, accompanied by a stunning flash of light.

Astronomers use these “Type Ia supernovae” because most of them mete out almost the same amount of light every time no matter where they are located. This allows scientists to use them to accurately measure distances across space and study the expansion of the universe. Data from Chandra have revealed that Sgr A East, however, did not come from an ordinary Type Ia. Instead, it appears that it belongs to a special group of supernovae that produce different relative amounts of elements than traditional Type Ias do, and less powerful explosions. This subset is referred to as “Type Iax,” a potentially important member of the supernova family.

“While we’ve found Type Iax supernovae in other galaxies, we haven’t identified evidence for one in the Milky Way until now,” said Ping Zhou of Nanjing University in China, who led the new study while at the University of Amsterdam. “This discovery is important for getting a handle of the myriad ways white dwarfs explode.”

The explosions of white dwarfs is one of the most important sources in the universe of elements like iron, nickel, and chromium. The only place that scientists know these elements can be created is inside the nuclear furnace of stars or when they explode.
Astronomers are still debating the cause of Type Iax supernova explosions, but the leading theory is that they involve thermonuclear reactions that travel much more slowly through the star than in Type Ia supernovae. This relatively slow walk of the blast leads to weaker explosions and, hence, different amounts of elements produced in the explosion. It is also possible that part of the white dwarf is left behind.

UvA researcher Ping Zhou and GRAPPA member Jacco Vink are co-authors of the paper, and more details can be found here and here.

X-ray: NASA/CXC/Nanjing Univ./P. Zhou et al. Radio: NSF/NRAO/VLA


GRAPPA member Jacco Vink released his textbook on Physics and Evolution of Supernova Remnants as part of Springer’s Astronomy and Astrophysics Library this December!

Apart from supernova remnants it also contains chapters on pulsar wind nebulae and cosmic rays. More information and pre-orders can be found here.

14/12/2020: Opening of GRAPPA Postdoctoral Fellowship!

We are happy to announce a new GRAPPA postdoctoral fellowship position. We invite applications for a GRAPPA Postdoctoral Fellowship position in the areas of Astroparticle Physics, Cosmology, and Gravitation. Established in 2012, GRAPPA Institute strives for revealing fundamental aspects of the universe and laws of physics. The GRAPPA Fellowship has been our flagship postdoctoral program, which fully ensures academic freedom to pursue any theoretical and experimental research subjects that are of relevance to the GRAPPA’s scientific focus.

Deadline for application is 20 December and we encourage applications from people of diverse backgrounds and underrepresented groups.

All details can be found here.

07/12/2020: NWA-ORC for neutrino observations

A consortium of knowledge institutions and public stakeholders, led by UvA physicist Auke Pieter Colijn, was awarded a 1.1 million euro NWA-ORC grant to study so-called ‘relic neutrinos’ that came into existence in the very first second after the Big Bang.

Every second Earth is bombarded with an enormous number of neutrinos from the cosmos. These neutrinos were created in the primordial soup one second after the Big Bang, but they have never been observed. In the proposed research, the consortium will develop an experiment to observe these relic neutrinos by investigating the decay of heavy-hydrogen tritium. The proposed project is called PTOLEMY, and is a new method to measure such low-energy particles. This is particularly difficult because neutrinos hardly interact with ordinary matter, such as a detector. If this is possible with the proposed detector, it will be a groundbreaking result.

Besides the University of Amsterdam, the consortium, for which Auke Pieter Colijn is the coordinator and which also involves UvA physicist Shin’ichiro Ando, consists of the Radboud Universiteit, De Haagse Hogeschool (The Hague University of Applied Sciences), TNO, the Princeton Physics Department, the Laboratorio Nazionale di Gran Sasso (LNGS), the Netherlands’ Physical Society (NNV), Ampulz, and the Karlsruhe Institute of Technology (KIT).

More information can be found at the IOP news as well as at the Nikhef press release.

04/12/2020: Sera Markoff receives Diversity Initiative Award

Altair, a project launched by GRAPPA member astronomer Sera Markoff, has been awarded the NWO Diversity Initiative Award. The project introduces primary school children to astronomy and physics. The Diversity Initiative Award is awarded to initiatives that improve diversity in the field. Initiator Prof. Sera Markoff (University of Amsterdam) will receive € 50,000 to help further develop the project.

Markoff wants to use the award to launch Altair+. ‘The idea is to work together with a team of students and volunteers to develop new activities for secondary school pupils, also at the Science Park in Amsterdam. We want to keep in touch with the Altair “graduates”from the primary school by organising annual activities, which also involve other pupils from nearby schools and their parents. This is a step towards a “pre-academic” programme. This will hopefully result in more students with a non-Western background studying at the Science Park!’

The jury was impressed with how the project successfully promotes the sciences to a wide range of communities. Children from various ethnic backgrounds are taught by UvA academics, and their parents are also involved with a range of activities on the university campus. This project reaches 140 children a year. The jury noted that it is remarkable that Markoff launched the initiative alongside her regular work as Professor of Theoretical Astrophysics.

More information can be found here, as well as at the IOP news.

04/12/2020: The Dutch Black Hole Consortium receives €4.9 million in funding

The Dutch Black Hole Consortium (DBHC), which includes several GRAPPA members, has been awarded a grant of € 4.9 million by the Dutch Research Council (NWO) as part of the National Science Agenda. This new interdisciplinary consortium aims to further unravel the mysteries surrounding black holes and the wider universe.

Astronomers and physicists are joining forces to make new discoveries about black holes, using the Event Horizon Telescope, the international project that took the first direct image of a black hole last year, and via gravitational waves, where geologists are also ‘delving into the ground’ for the potential underground construction of the planned gravitational wave detector Einstein Telescope. The consortium will work with universities of applied sciences to develop new educational materials aimed at sparking interest in science among people of all backgrounds and especially children. Lead applicant Stefan Vandoren from Utrecht University: ‘This is an excellent project in which we are combining research, technology and the interests of society.’

The Dutch Black Hole Consortium is made up of researchers from the fields of theoretical physics, astronomy and geology, technicians working on the next generation of gravitational wave detectors and researchers of science communication and science history, combined with teacher training courses and museums. The UvA researchers who are part of the consortium are Sera Markoff, Jan de Boer, Franke Linde, Ralph Wijers, Alejandra Castro, Jeroen van Dongen, Erik Verlinde, Alessandro Bertollini, Sarah Caudill and Samaya Nissanke from the Anton Pannekoek Institute (API), Institute of Physics (IOP) and GRAPPA.

Sera Markoff, who sits on the Consortium Board: ‘We are excited to launch this interdisciplinary project in order to enhance our understanding of the nature of black holes, while also conveying our enthusiasm about our discoveries to the public through museums and outreach projects.’

The consortium is made up of 31 applicants from universities and universities of applied sciences and six partners from industry, museums and government. More information can be found here.

18/09/2020: Dark matter even more elusive than previously thought

Dark matter is even more elusive than thought before. This is the main message of a new study by a group of scientists that includes GRAPPA member Shin’ichiro Ando. The results of the new study were published in Physical Review D Rapid Communications this week.

Revealing the nature of dark matter might be achieved by looking for faint signatures coming from pairs of dark matter particles. These pairs may annihilate each other and turn into gamma-ray light that could be observed. Some of the best targets for such a search strategy, using gamma-ray telescopes such as the Fermi satellite, are dwarf spheroidal galaxies – small satellite galaxies that are trapped in the gravitational pull of our own Milky Way. Dwarf galaxies are full of dark matter but little else, meaning they are nearly pristine laboratories to look for the tell-tale signatures of dark matter annihilation without the pesky complications of other astronomical phenomena.

In order to perform the search, the scientists combined theoretical and observational methods to put together the first ‘end to end’ analysis that models the dwarf galaxies in their cosmic context. Shin’ichiro Ando said: “Our method starts at the big bang, follows the evolution of dark matter and the growth of galaxies across cosmic time, and uses this as the basis to understand how dark matter might be distributed within the individual dwarf galaxies we see in the sky today. This understanding is then used as the basis to search for the gamma-ray signal which reveals information about the microscopic properties of dark matter particles.”

In particular, the team concluded that the expected signal of the dark matter annihilation is much weaker than earlier estimates. Simply put: dark matter is harder to detect using dwarf galaxies than previously expected. Adopting the more holistic analysis therefore implies, counter-intuitively, that the gamma-ray data actually tells scientists less about dark matter than they thought it did previously. One immediate consequence is that there is still a possibility that dark matter annihilation explains one of the great current astronomical mysteries — the anomalous and unexplained gamma-ray glow that we see emerging from the center of our own galaxy. Until now, it was difficult to see how dark matter annihilation could power the galactic center anomaly but avoid generating a similar signal from the dwarf galaxies.

The results strongly call for revising one of the key search strategies towards discovery of dark matter particles and the quest for physics beyond the Standard Model. More information can be found at the IOP news, and the paper can be found here.

28/08/2020: An international research team analysis rules out dark matter destruction as origin of extra radiation in galaxy center

Exhaustive emissions modeling by global physicists narrows down particle candidates

This representation of data from the Fermi Gamma Ray Space Telescope after its launch in 2008 shows an excess of high-energy radiation in the Milky Way’s Galactic Center. Many physicists attributed this to the annihilation of weakly interacting dark matter particles, but a UCI-led study has excluded this possibility through a range of particle masses. Oscar Macias for UCI

The detection more than a decade ago by the Fermi Gamma Ray Space Telescope of an excess of high-energy radiation in the center of the Milky Way convinced some physicists that they were seeing evidence of the annihilation of dark matter particles, but a team led by researchers at the University of California, Irvine has ruled out that interpretation.
In a paper published recently in the journal Physical Review D, the UCI scientists and colleagues at the University of Amsterdam and other institutions report that – through an analysis of the Fermi data and an exhaustive series of modeling exercises – they were able to determine that the observed gamma rays could not have been produced by what are called weakly interacting massive particles, most popularly theorized as the stuff of dark matter.
By eliminating these particles, the destruction of which could generate energies of up to 300 giga-electron volts, the paper’s authors say, they have put the strongest constraints yet on dark matter properties.

“For 40 years or so, the leading candidate for dark matter among particle physicists was a thermal, weakly interacting and weak-scale particle, and this result for the first time rules out that candidate up to very high-mass particles,” said co-author Kevork Abazajian, UCI professor of physics & astronomy.
“In many models, this particle ranges from 10 to 1,000 times the mass of a proton, with more massive particles being less attractive theoretically as a dark matter particle,” added co-author Manoj Kaplinghat, also a UCI professor of physics & astronomy. “In this paper, we’re eliminating dark matter candidates over the favored range, which is a huge improvement in the constraints we put on the possibilities that these are representative of dark matter.”
Abazajian said that dark matter signals could be crowded out by other astrophysical phenomena in the Galactic Center – such as star formation, cosmic ray deflection off molecular gas and, most notably, neutron stars and millisecond pulsars – as sources of excess gamma rays detected by the Fermi space telescope.
“We looked at all of the different modeling that goes on in the Galactic Center, including molecular gas, stellar emissions and high-energy electrons that scatter low-energy photons,” said co-author Oscar Macias, a postdoctoral scholar in physics and astronomy at the Kavli Institute for the Physics and Mathematics of the Universe at the University of Tokyo whose visit to UCI in 2017 initiated this project. “We took over three years to pull all of these new, better models together and examine the emissions, finding that there is little room left for dark matter.”
Macias, who is also a postdoctoral researcher with the GRAPPA Centre at the University of Amsterdam, added that this result would not have been possible without data and software provided by the Fermi Large Area Telescope collaboration.
The group tested all classes of models used in the Galactic Center region for excess emission analyses, and its conclusions remained unchanged. “One would have to craft a diffuse emission model that leaves a big ‘hole’ in them to relax our constraints, and science doesn’t work that way,” Macias said.
Kaplinghat noted that physicists have predicted that radiation from dark matter annihilation would be represented in a neat spherical or elliptical shape emanating from the Galactic Center, but the gamma ray excess detected by the Fermi space telescope after its June 2008 deployment shows up as a triaxial, bar-like structure.
“If you peer at the Galactic Center, you see that the stars are distributed in a boxy way,” he said. “There’s a disk of stars, and right in the center, there’s a bulge that’s about 10 degrees on the sky, and it’s actually a very specific shape – sort of an asymmetric box – and this shape leaves very little room for additional dark matter.”
Does this research rule out the existence of dark matter in the galaxy? “No,” Kaplinghat said. “Our study constrains the kind of particle that dark matter could be. The multiple lines of evidence for dark matter in the galaxy are robust and unaffected by our work.”
Far from considering the team’s findings to be discouraging, Abazajian said they should encourage physicists to focus on concepts other than the most popular ones.
“There are a lot of alternative dark matter candidates out there,” he said. “The search is going to be more like a fishing expedition where you don’t already know where the fish are.”

GRAPPA member Oscar Macias is one of the co-authors on the paper. More information can be found here and the paper can be found here.

23/06/2020: Nature paper: Resolving acceleration to very high energies along the jet of Centaurus A

We know now that most, if not all, galaxies have supermassive black holes in their centers. Under the right circumstances these black holes swallow mass from their surroundings. If this is the case they often eject part of the mass they gathered in a pair of very fast jet streams. In the jets  gas  has velocities close to the speed of light. These jets have been known  for decades and were discovered initially due to the bright radiation emitted in the radio bands.  Galaxies with these jets are called radio galaxies.
The H.E.S.S. Imaging Atmospheric Cherenkov Telescope system in Namibia has now found evidence that inside jets in a nearby galaxy, Centaurus A, electrons are being accelerated to very-high energies, around 50 Tera-electronVolt; higher than the energies protons can obtain at the CERN LHC facility! Scientists of the University of Amsterdam (GRAPPA & Anton Pannekoek Institute) were involved in this new study that just appeared in the Nature scientific journal.
Centaurus A is a radio galaxy at a distance of 12 million light years from Earth. The jets launched by its black hole are also detected in X-rays. But a question has been for a long time, what causes this radiation. For radio emission the mechanism is electrons that are rotating in the magnetic fields of the jets. This is called synchrotron radiation. In X-rays the same radiation mechanism could be happening, but only if the electrons causing it have much more energy than the electrons causing radio radiation. The energies needed are of the order of 50 Tera-electronVolt. However, since these electrons radiate so much, they also lose energy very fast. So the presence of Tera-electronVolt electrons throughout the jet is only possible if somehow the electrons keep on gaining energy within the jet through some acceleration mechanism.
An alternative explanation is that the X-ray emission is from electrons with much less energy, which can scatter light from the surroundings. The scattering process itself can boost the energy of the scattered light, making it visible in X-rays. This process is called Compton scattering.
One way to test which mechanism is responsible for X-rays is to observe the jets in gamma-rays. These gamma-rays can also be caused by Compton scattering, but in this case by bouncing off electrons with 10-100 Tera-electronVolt energies, causing gamma-ray radiation with energies of 1-10 Tera-electronVolt.

Shown are the observed and modelled spectral energy distribution (SED) from radio to gamma-ray energies for the kiloparsec-scale jet of Centaurus A. The VHE emission is dominated by relativistic electrons with energies above 10 TeV inverse Compton up-scattering dust photons to high energies (solid blue curve, ‘IC total’). This emission from the kiloparsec-scale jet makes a major contribution to the unexpected spectral hardening above a few GeV as seen by Fermi-LAT (red points). The lower-energy part of the gamma-ray spectrum (red points) is attributed to emission from the core (grey dashed line referring to a core model. The green curve (‘Sync.’) designates the synchrotron emission of the inferred broken power-law electron distribution in a magnetic field of characteristic strength B = 23 μG. The blue ‘butterfly’ corresponds to the H.E.S.S. spectra, while green data points mark radio, infrared and X-ray measurements and reported uncertainties (error bars) from the inner region of the Centaurus A jet. A breakdown is provided of the full IC contribution, from the scattering of: the cosmic microwave background (CMB), the starlight emission of the host galaxy, infrared emission from dust, and the low-energy synchrotron jet emission (synchrotron self Compton, SSC). 

H.E.S.S. already detected gamma-ray radiation from Centaurus A, but it was not clear whether the radiation originated from the jet, or from near the black hole. But in the new publication in Nature the H.E.S.S. collaboration announced that they have been able to make an image of the Centaurus A gamma-ray emission, which clearly shows that the gamma-ray radiation is coming from the jets. The jet system detected in gamma-rays has a length of several thousands of light years. Among the authors of the new publication are also scientists from the University of Amsterdam, D. Prokhorov, R. Simoni, and local H.E.S.S. group leader Jacco Vink. Dr. Dmitry Prokhorov was actively involved in the analysis of the observations and and led the efforts of a spectral analysis of H.E.S.S. and Fermi-LAT data presented in Figure 2 of this paper. He is very enthusiastic about the results, saying that the results shows that “the surprising implication is that ultrarelativistic electrons may be commonplace in the large-scale jets of radio-loud active galaxies!“

H.E.S.S. is a system of Imaging Atmospheric Cherenkov Telescopes that investigates cosmic gamma rays in the energy range from 10s of GeV to 10s of TeV. The instrument allows scientists to explore gamma-ray sources with intensities at a level of a few thousandths of the flux of the Crab nebula. H.E.S.S. is located in in the Khomas Highland in Namibia, a country in Southern Africa. The area is well known for its excellent optical quality. The four telescopes of Phase I of the H.E.S.S. project were operational in December 2003, while a much larger fifth telescope – H.E.S.S. II – is operational since July 2012, extending the energy coverage towards lower energies and further improving sensitivity. The H.E.S.S. data reported in this paper were accumulated during the Phase I. Scientists from Jacco Vink’s group at University of Amsterdam are leading a new H.E.S.S. observation campaign on Centaurus A with participation of all five telescopes of H.E.S.S.. and collaborating on their task with Namibian and Swedish colleagues. The new study will result in an even better understanding of the spectral characteristics of the Centaurus A jet especially below a few hundred GeV energy, given the participation of a fifth telescope outfitted with a new camera.

05/03/2020: LOFAR images cosmic radio monsters

Pareidolia is a tendency that pushes humans to see shapes in clouds or faces in inanimate objects. The picture shown here is a composition of four cosmic radio sources that can in fact look like a scary monster. To obtain this effect, the sources have been rearranged compared to their original position in the sky but their apparent sizes were preserved.

However, in some sense, these sources are real monsters. Their names are: Cassiopeia A (top left), Taurus A (top right), Cygnus A (center), and Virgo A (bottom). These are the four most powerful radio sources in the northern hemisphere. Historically, the brightest radio sources in the sky were named after the constellation in which they were found followed by a letter starting with an “A”. They were then grouped in the so-called A-team, like the famous TV series from the 80s.

The nature of these four monsters is very diverse. The eyes of the monster (Cassiopeia A and Taurus A) are two supernova remnants: the leftovers of the explosions of two stars in our own Galaxy. The evil pupil that stares at you in Taurus A is the Crab pulsar. The nose of the monster, Cygnus A, is an extremely powerful radio galaxy 600 million light years away, whose two lobes are powered by jets of energetic particles formed near a supermassive black hole. The mouth of the monster (Virgo A) is the extended structure (larger than an entire galaxy) that surrounds the famous supermassive black hole at the centre of the galaxy M87, the same black hole recently imaged by the Event Horizon Telescope.

These four sources are well known to radio astronomers, but this is the first time that they were able to see them in such great detail at the extremely long wavelengths of 5 meters, close to the longest wavelength we can observe with ground instruments. The images used to make the radio monster were obtained with the Low Frequency Array (LOFAR), a pan-European radio telescope made by 52 stations spread across 8 different countries (The Netherlands, Germany, France, UK, Poland, Sweden, Latvia, Ireland, and soon Italy) and coordinated by a supercomputer in Groningen (NL).

GRAPPA member Jacco Vink is one of the co-authors on the paper. Read the full paper here.

31/01/2020: Top prize in high-energy awarded to Event Horizon Telescope

The top prize in high-energy astrophysics, the Bruno Rossi Prize 2020, has been awarded to the Event Horizon Telescope team for the landmark image and analysis of the first shadow of a black hole. The UvA-researchers Sera Markoff, Oliver Porth, Doosoon Yoon and Koushik Chatterjee are part of this team.

The American Astronomical Society awards the Rossi Prize in recognition of significant contributions as well as recent and original work in high-energy astrophysics.

The Event Horizon Telescope, is a planet-scale array of eight ground-based radio telescopes that was designed to capture images of black holes. In April 2019, the EHT collaboration announced that it had successfully taken the first picture of the black hole at the center of the Messier 87 galaxy, with the results published in a series of six papers in the Astrophysical Journal Letters. The image, a bright ring formed by light bending around the black hole, quickly circled the globe, appearing across the front pages of newspapers and throughout social media. The 2020 Rossi Prize recognizes this historic scientific achievement.

The prize is in honor of Professor Bruno Rossi, an authority on cosmic ray physics and a pioneer in the field of X-ray astronomy. The Rossi Prize includes an engraved certificate and a 1,500 dollar award. Dr. Doeleman will give a lecture at the 237th AAS meeting in Phoenix, Arizona, in January 2021.


More information can be found here.


20/12/2019: Prestigious ERC consolidator grant awarded to Christoph Weniger

The European Research Council (ERC) has awarded a €2 million consolidator grant to GRAPPA researcher Christoph Weniger for the project “UnDark – New Approaches to Uncover Dark Matter in the post-WIMP Era”.

The goal of the ERC project UnDark is to discover particle dark matter in the post-WIMP era. Weniger will search for deviations from the cold dark matter paradigm by studying the dark matter distribution in the Universe at the smallest scales, and search for anomalous emission from dark matter particles across the electromagnetic spectrum. He will develop a ‘principled lampposts’ approach to systematically identify gaps in searches for a large number of dark matter models, and probe astrophysical hints for sterile neutrino and axion dark matter, which are some of the most promising dark matter candidates next to the WIMP. To this end, Weniger will leverage recent breakthroughs in computer science that enabled the deep learning revolution.

17/12/2019: Christoph Weniger receives ‘Big Science’ grant for dark matter research

The ‘DarkGenerators’ proposal by GRAPPA researcher Christoph Weniger was awarded an Innovative eScience Technologies for ‘Big Science’ grant from the Netherlands eScience Center and SURFsara. With the grant, Weniger will use advanced data science methods to enhance and accelerate the interpretation of astrophysical and collider data in the search for signals of dark matter.

Weniger’s grant-winning proposal was titled ‘DarkGenerators – Interpretable Large Scale Deep Generative Models for Dark Matter Searches’. Dark matter is five times more abundant in the universe than visible matter. Yet, its nature remains unknown and constitutes one of the most exciting and complex research questions today. The project will use advanced data science methods to enhance and accelerate the interpretation of astrophysical and collider data in the search for signals of dark matter. Deep generative models and differentiable probabilistic programming will be used to construct a framework for the fast and precise inference of high-dimensional data models.

09/12/2019: Nature paper: A wide star–black-hole binary system from radial-velocity measurements

Exciting new results from observations of a star orbiting a massive black hole, published in a recent Nature paper.
Observation of the radial velocity variations of a Galactic star suggest it is orbiting a black hole, in a first taste of how the Galactic black-hole population could be revealed by future time-domain spectroscopic surveys. The visible star is inferred to be a B-type star with a mass roughly 8 times greater than that of the Sun; this star is in a circular orbit with a period of 78.9 days around an unseen mass. Together this indicates a system containing a black hole of at least 6 solar masses, and one which is wider than any previously known Galactic black-hole binary.

The most exciting possibility comes If a H-alpha emission line from the system is interpreted as indicating the motion of the black hole. In that case then the black hole is inferred to have a mass of 68 solar masses. This is highly unexpected. Even though gravitational wave experiments have detected similarly massive black holes, forming black holes as massive as that in high-metallicity environment would be extremely challenging to current evolution theories.

GRAPPA & API member Stephen Justham is one of the co-authors on the paper. Read the full paper.

02/12/2019: Philipp Moesta joined GRAPPA

We are delighted to welcome Philipp Moesta as a faculty member at GRAPPA, shared between IHEF and API. Philipp’s research focuses on high-energy general-relativistic astrophysics.



Philipp expressed his excitement by stating, I am very excited to be joining GRAPPA and building up a multimessenger astrophysics group here. It is transformative time to be working in general-relativistic astrophysics with the routine detection of gravitational waves, electromagnetic signals, and neutrinos from the most powerful explosions in the universe. I am particularly looking forward to exploring the connections between my expertise in simulating these explosions with the already existing gravitation and astroparticle groups at GRAPPA and am thrilled to be working together with the outstanding students, postdocs and staff members in Physics and Astronomy at UvA and Nikhef across the road.”


Philipp has started working with GRAPPA in July 2019. We look forward to his contributions to our intellectual community for many years to come.

21/11/2019: First gamma-ray burst detection in very-high energy gamma light

Today two Nature papers appeared on the detection of a gamma-ray bursts in the 100 GeV domain, observed by MAGIC and HESS.
After a decade-long search, scientist have for the first time detected a gamma-ray burst in very-high energy gamma light, discovered by the H.E.S.S. collaboration in July 2018. This gamma-ray burst, an extremely energetic flash following a cosmological cataclysm, was found to emit very-high-energy gamma-rays long after the initial explosion. Extremely energetic cosmic explosions generate gamma-ray bursts, typically lasting for only a few tens of seconds. They are the most luminous explosions in the universe, The burst is followed by a longer lasting afterglow mostly in the optical and X-ray spectral regions whose intensity decreases rapidly. The prompt high energy gamma-ray emission is mostly composed of photons several hundred-thousands to millions of times more energetic than visible light, that can only be observed by satellite-based instruments. Whilst these space-borne observatories have detected a few photons with even higher energies, the question if very-high-energy gamma radiation is emitted, has remained unanswered until now.

GRAPPA member, Jacco Vink, is one of the co-authors on the H.E.S.S. paper. He calls it a great success, which testifies to the power of these type of gamma-ray telescopes for finding distant explosions. A big surprise was, according to him, that the emission was picked up so long after the initial explosion occurred.

Read the full paper

29/10/2019: Self-learning machines hunt for explosions in the universe

The National Science Agenda has awarded a 5 million euro grant to CORTEX – the Center for Optimal, Real-Time Machine Studies of the Explosive Universe. The CORTEX consortium of 12 partners from academia, industry and society will make self-learning machines faster, to figure out how massive cosmic explosions work, and to innovative systems that benefit our society.

Machine learning has rapidly become an integral part of our lives. It is now commonly used for speech recognition and information retrieval. This is also true in science, for detecting patterns in nature and the Universe. But the need is growing rapidly for such machines to respond quickly, for example in self-driving cars and for responsive manufacturing. On a more fundamental level, self-learning machines help us unveil a dynamical Universe we did not know existed up until recently. Bright explosions appear all over the radio and gravitational-wave sky. Many citizens and scientists are curious to understand where these come from.

The University of Amsterdam GRAPPA, Anton Pannekoek Institute for Astronomy and Institute of Physics led efforts focus on the numerical modelling, joint characterisation and interpretation of neutron star binary mergers using gravitational wave and radio observations. The effort comprises the participation of the groups led by Samaya Nissanke (UvA work package contact and lead), Antonia Rowlinson (co-project lead), Philipp Moesta, Oliver Porth, and Sera Markoff.

More information can be found at the IOP news

29/10/2019: Camila Correa to join GRAPPA as a Veni fellow

We will be welcoming Camila Correa at GRAPPA, who, together with 166 young researchers have been granted a Veni grant. The Dutch Research Council (NWO) has awarded a Veni grant worth up to 250,000 euros to 166 highly promising young scientists. The grant provides the laureates with the opportunity to further elaborate their own ideas during a period of three years.

Dr Camila Correa (Leiden University) will conduct her research within GRAPPA on: Do dark matter particles interact with each other? The nature of dark matter is a great unsolved mystery. The proposed research project will use state-of-the-art simulations to analyse signatures of forces between dark matter particles on galaxies colliding, filling a major gap in our understanding of dark matter.

25/10/2019: Bachelor students publish research on dwarf galaxies

This year, twelve bachelor students completed the workshop that was organized by Shin’ichiro Ando, Bradley Kavanagh and Oscar Macias. The topic of this year’s workshop was the potential discovery of dwarf galaxies and the role these may play in searches for dark matter. The students were responsible for the lion’s share of the research that went into the paper, which was published in the Journal of Cosmology and Astroparticle Physics last week.


More information can be found at the IOP news, and the paper can be found here.

14/10/2019: Breakthrough Prize for EHT collaboration, New Horizons Prize for Nissanke

The Breakthrough Prize Foundation announced the recipients of the prestigious 2020 Breakthrough Prizes and 2020 New Horizons Prizes in recognition of important achievements in the Life Sciences, Fundamental Physics, and Mathematics. The Event Horizon Telescope collaboration, including several UvA scientists, was awarded the Breakthrough Prize, while UvA astrophysicist Samaya Nissanke received the New Horizons in Physics Prize.

The annual Breakthrough Prize in Fundamental Physics, for a total amount of $3 million, goes to the 347 scientists that co-authored the six papers published by the Event Horizon Telescope collaboration in April 2019. The papers announced the first image of a supermassive black hole, taken by means of an Earth-sized alliance of telescopes. Among the laureates are UvA scientists Sera Markoff, Oliver Porth and Koushik Chatterjee. UvA postdoc Doosoo Yoon also contributed to the EHT results.

The $100,000 New Horizons Prize recognizes early-career achievements in physics and mathematics. Samaya Nissanke, an assistant professor at the GRAPPA center of excellence at the University of Amsterdam, received the prize together with her colleagues Jo Dunkley (Princeton University) and Kendrick Smith (Perimeter Institute). The prize was awarded for their development of novel techniques to extract fundamental physics from astronomical data. In 2016, Nissanke was also a member of the team receiving a Breakthrough Prize for the detection of gravitational waves.

The new laureates will be recognized at the eighth annual Breakthrough Prize gala awards ceremony on Sunday, November 3, at NASA Ames Research Center in Mountain View, California, and broadcast live on National Geographic. Each year, the program has a theme, and this year’s topic – “Seeing the Invisible” – is inspired by the Event Horizon Telescope collaboration.

28/07/2019: Samaya Nissanke elected GRAPPA spokesperson

Samaya Nissanke has been elected as the new spokesperson for the Gravitation and AstroParticle Physics Amsterdam (GRAPPA) centre of excellence at the University of Amsterdam. Nissanke is the successor of Gianfranco Bertone, who was recently appointed director of EuCAPT.

Nissanke is currently an assistant professor at the University of Amsterdam. She is a joint faculty member at the Anton Pannekoek Institute for Astronomy and the Institute for High Energy Physics, and is already a member of GRAPPA.

Nissanke’s predecessor, Gianfranco Bertone, said: ‘I am sure that thanks to her scientific excellence, enthusiasm and leadership, Samaya will shine in this role. I look forward to seeing GRAPPA further strengthening its scientific impact and establishing its presence at the local, national and international level under her leadership.’


16/07/2019: Gianfranco Bertone appointed founding director of EuCAPT

On July 10, 2019, a group of leading theoretical physicists active in fields of astroparticle physics and cosmology met with representatives of APPEC and CERN to kick-off EuCAPT (European Consortium for AstroParticle Theory), a new initiative that aims to coordinate and favour theoretical astroparticle and cosmology activities in European centres and institutions.

The steering committee of EuCAPT includes internationally renowned scientists affiliated to leading European institutions: Gianfranco Bertone (U. of Amsterdam), Philippe Brax (CEA Saclay), Vitor Cardoso (IST Lisbon), Gian Giudice (CERN), David Langlois (APC Paris), Silvia Pascoli (U. of Durham), Hiranya Peiris (UCL London & OKC Stockholm) Antonio Riotto (U. of Geneva), Subir Sarkar (U. of Oxford), Piero Ullio (SISSA Trieste), Andrew Taylor (DESY Hamburg), Licia Verde (U. of Barcelona). CERN has agreed to act as central hub for EuCAPT, and committed to provide financial and administrative support to its activities.

GRAPPA Spokesperson Gianfranco Bertone, who has been appointed EuCAPT director, said: “EuCAPT is a bottom-up initiative that aims to be open and inclusive. We will soon reach out to to the scientific community and to the broader society, with an invitation to participate in our exciting programme of scientific and outreach activities. Stay tuned!”.

The first EuCAPT activities will include one annual meeting of the entire theoretical astroparticle community that will take place at CERN starting in 2020, and two topical workshops at other participating institutions. More information on EuCAPT activities will be released in September, when the website of the initiative will be be launched.

06/03/2019: Kenny Ng to join GRAPPA on a Marie Curie Individual Fellowship

We will welcome Kenny Ng to GRAPPA later this year, who has been awarded, together with ten talented researchers, a Marie Skłodowska-Curie Individual Fellowship to conduct research at the University of Amsterdam (UvA). The awarded projects include research on public support for different interpretations of ‘Social Europe’, the transmission of pathogens from animals to humans and metaphorical narratives in discourses on dementia. The awarded fellowships range from 175,000 to 280,000 euros. The grants are awarded as part of the Marie Skłodowska-Curie Actions, which forms part of the European Union’s Horizon 2020 research and innovation programme. The aim of the fellowship is to equip researchers with the necessary skills and international experience to pursue a successful career.

Kenny Ng’s research will be on understanding the peculiar gamma-ray emission of the sun.
With space telescopes, we only recently gained the sensitivity to precisely study the gamma rays from the Sun, presumably caused by energetic cosmic particles (cosmic rays) bombarding the Sun. Surprisingly, many properties of these gamma rays are not yet explained. Kenny Chun Yu Ng plans to understand them through new observational and theoretical investigations, which will ultimately help us better understand the Sun itself and the solar system space environment.

Kenny Ng will collaborate with Dr Shin’ichiro Ando here at GRAPPA.

15/11/2018: Mass or interaction, but not both

If upcoming direct searches discover the elusive dark matter in the universe, they may be able to measure the mass of the particle or the way it interacts with ordinary matter, but can only do both if we’re lucky, researchers from the UvA Institute of Physics argue.

It is one of the great open questions of modern-day physics and astronomy: what is the mysterious dark matter in the universe? Astronomers suspect that the universe contains much more matter than they can see through there telescopes – simply because there is much more gravity than visible matter can account for. So far, however, nobody has been able to find particles that this dark matter can be made of.

Rare interactions

Yet, many astronomers and particle physicists are optimistic that a dark matter candidate particle can be found soon, in the next generation of experiments and observations. While this may be true, Amsterdam researchers now point out that even if such a new dark matter particle is found, this does not automatically mean that we will immediately know all of its properties.

In their investigation, the researchers looked at planned underground detectors which aim to detect dark matter by looking for its rare interactions with ordinary atomic nuclei. The as yet undiscovered dark matter particle has an unknown mass (how heavy it is) and an unknown cross section (how strongly it interacts with nuclei).

Using new statistical tools, inspired by a concept known as ‘information geometry’ that the same researchers developed earlier this year, the physicists mapped out what a discovery would look like, without assuming a particular dark matter particle mass or cross section – allowing to explore a wide range of these dark matter properties.

New materials and techniques

It was found that a discovery using multiple different target materials that the dark matter particle can interact with, will substantially improve the measurement of the dark matter properties. However, even with multiple detectors, it will be hard to measure the mass of heavy dark matter particles and at the same time understand their precise interactions. Only over a narrow range of properties can both of these things be measured simultaneously.

This result, which was accepted for publication in Physical Review Letters last week, should spur the dark matter community to explore new detector materials and techniques which will improve the prospects for pinning down the properties of the dark matter particle in the event of a future discovery.


Assessing near-future direct dark matter searches with benchmark-free forecasting, Thomas D. P. Edwards, Bradley J. Kavanagh, and Christoph Weniger, Physical Review Letters 2018. (arXiv preprint)

05/10/2018: A new era in the quest for dark matter

In a Nature review article published this week, physicists Gianfranco Bertone (GRAPPA, UvA) and Tim Tait (UvA and UC Irvine) discuss the past accomplishments and the current status of dark matter search experiments and how they will shape the future efforts for dark matter searches.

More information can be found at the IOP news.
The nature article can be found here and the pre-print version can be found here.

06/08/2018: Another blow for the dark matter interpretation of the Galactic Center Excess

For almost ten years, astronomers have been studying a mysterious diffuse radiation coming from the center of our Galaxy. Originally, it was thought that this radiation could originate from the elusive dark matter particles that many researchers are hoping to find. However, physicists from the University of Amsterdam and the Laboratoire d’Annecy-le-Vieux de Physique Théorique have now found further evidence that rapidly spinning neutron stars are a much more likely source for this radiation.

Observations of the gamma-ray radiation from the Galactic center region with the Fermi Large Area Telescope have revealed a mysterious diffuse and extended emission. Discovered almost 10 years ago, this emission generated a lot of excitement in the particle physics community, since it had all the characteristics of a long-sought-after signal from the self-annihilation of dark matter particles in the inner Galaxy. Finding such a signal would confirm that dark matter, a substance that so far has only been observed through its gravitational effects on other objects, is made out of new fundamental particles. Moreover, it would help to determine the mass and other properties of these elusive dark matter particles. However, recent studies show that arguably the best astrophysical interpretation of the excess emission is a new population in the Galactic bulge of thousands of rapidly spinning neutron stars called millisecond pulsars, which have escaped observations at other frequencies up to now.

Figure 1. Observed gamma-ray emission from the Galactic disk, with the bulge region indicated. The insets show the expected profiles of excess radiation coming from dark matter and stars respectively. The researchers were able to show that the stars profile matches the measurements much better than the dark matter profile.

Where there are stars, there is radiation
‘Understanding in detail the morphology [the location and shape] and spectrum [the combined frequencies] of the excess emission is of vital importance for discriminating between the dark matter and astrophysical interpretations of the Galactic Center excess radiation.’, says Christoph Weniger, one of the researchers that conducted the study. A new study by researchers at the University of Amsterdam and the Laboratoire d’Annecy-le-Vieux de Physique Théorique, a research unit of the French Centre National de la Recherche Scientifique, found strong evidence that the emission actually seems to come from regions where there is also a large amount of stellar mass in the Galactic bulge (the ‘boxy bulge’) and center (the ‘nuclear bulge’). Furthermore, the researchers found that the light-to-mass ratio in the Galactic bulge and center are mutually consistent, so that the gamma-ray GeV emission is a surprisingly accurate tracer of stellar mass in the inner Galaxy – see figure 2. This study was based on a new analysis tool, SkyFACT (Sky Factorization with Adaptive Constrained Templates), developed by the researchers themselves, which combines physical modeling with image analysis.

Figure 2. Comparison of the stellar mass (horizontal axis) and gamma-ray luminosity (vertical axis) for the “boxy bulge” (in blue) and for the “nuclear bulge” (in green). The prediction for a population of millisecond pulsars in the Galactic disk (in red) and the bulge of the nearby Andromeda galaxy (M31, in pink) are also shown. Stellar mass and luminosity are proportional to each other in the inner Galaxy (dashed line), which shows that the mysterious excess radiation very likely has a stellar origin and is not coming from dark matter.

The findings support the millisecond pulsar interpretation of the excess emission, since neither a dark matter signal nor other astrophysical interpretations are expected to show such a correlation. ‘The results will help guide upcoming radio searches for this hidden population of millisecond pulsars in the Galactic bulge with MeerKAT and the future Square Kilometre Array’, said Francesca Calore, another of the paper’s authors. ‘This makes these upcoming searches even more promising.’

R. Bartels, E. Storm, C. Weniger and F. Calore, The Fermi-LAT GeV excess traces stellar mass in the Galactic bulge, Nature Astronomy 2018.