Highlights

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Demonstrator detector assembly delivered for Wide Field X-ray Monitor

We are very happy that in June 2024 the delivery of 6 detector assemblies (DAs; 2 functionals, 4 dummies) was completed for a demonstration model of the Wide Field Monitor for X-ray astronomy. A DA consists of a silicon drift detector (from INFN Trieste), front-end electronics (designed by SRON with ASICs from CEA Saclay) and mechanical support structures (from ICE-CSIC in Barcelona). The FEE and DA assembly was carried out by Neways Electronics in Echt (Netherlands) under contract from INAF (Italy) and under supervision by SRON. All components, processes and materials are US free to make the assembly applicable for the enhanced X-ray Timing Polarization (eXTP) mission being developed under the leadership of the Chinese Academy of Sciences, as well as other missions such LEM-X (led by Italy) and STROBE-X (led by the USA). This delivery is a major milestone for the WFM with team members from the above institutes and the Eberhard Karls Universitaet of Tuebingen (Germany). The WFM collaboration further involves Sabanci University (Turkey) and the Danish Technical University. The functional DAs have been electronically tested at Neways Electronics and will undergo performance testing and assembly into a full WFM camera later in 2024 at INFN and ICE-CSIC.
(July 24, 2024; Image: dummy WFM detector assembly. The top surface shows the silicon drift detector)

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Large grant for Dutch contributions to LISA

The LISA space-borne gravitational wave detector, foreseen for launch in 2035, will have three Dutch candidate contributions: the Quadrant PhotoReceivers (QPR, led by SRON; 72 needed on LISA), the Mechanisms Control Unit (MCU; led by SRON; 6 needed) and the Point Ahead Angle Mechanism (PAAM; led by TNO; 6 needed). The Dutch funding agency NWO has granted 12 million euros to develop the first two contributions to flight level, as well as fund overlapping development of the QPR for the Einstein Telescope. This grant was provided as part of the 2021 NWO call for Large-scale Research Infrastructures. Matching funds are provided by SRON, Nikhef and 5 universities in the Netherlands.See SRON news flash.
(February 2, 2023; Image: schematic outline of Dutch contributions to LISA)

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Green light for continued development of LISA photoreceivers in the Netherlands

Since 2018, we (SRON, Nikhef, BRIGHT Photonics and SMART Photonics) are developing quadrant photodiodes for ESA's Laser Interferometer Space Antenna. First prototypes have been produced and tested. They show good promise that LISA's stringent requirements can be met (to ultimately measure distances of 2.5 million km with an accuracy of picometers). We are happy that NWO has now recognized this and granted funding for the continued development of this device in the next 2-3 years. We are also working on the design of the housing of the photodiodes, under strict alignment requirements, that will also host the readout electronics (developed elsewhere). All components together form the LISA 'photoreceiver' of which 72 will be needed 6 optical benches with each 3 interferometers. We look forward to the work ahead! See SRON news flash.
(September 28, 2021; Image: LISA constellation within our solar system in front of gravitational waves emitted by an active galaxy / © University of Florida / Simon Barke)

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Multi-INstrument Burst ARchive released

At last, the MINBAR database of 7083 thermonuclear shell flashes on neutron stars has been released. Collecting and streamlining the data was a major project for the past 12 years by 10 researchers from 5 countries. We are very proud the work is completed. The data have been collected through measurements with the Dutch-Italian BeppoSAX-WFC instrument (1996-2002), the NASA instrument RXTE-PCA (1995-2012) and ESA's INTEGRAL-JEMX (2003-2012). They present a unique and convenient data set for researchers in the future. Compiling these data has resulted in one large overview paper and many spin-off scientific results. We are convinced there are still plenty of gems to be found!

For the paper, click here.
(August 17, 2020; picture from press release Monash University)

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XMM-Newton and Chandra 20 years in operation

This month, celebrations are going on in Madrid and Boston because it is 20 years ago that ESA's cornerstone mission XMM-Newton and NASA's flagship mission Chandra were launched and put into operation. For SRON, this is also reason for celebration, because it contributed high-resolution spectrometers to both missions - instruments that are still functioning very well. At the SRON Science Days on November 18 and 19, there was a special session with contributions by some of the important SRON players in these missions at SRON then and now, from left to right: Jelle Kaastra, Frits Paerels, Johan Bleeker, Fred Jansen and Piet de Korte. They gave us insight in how the missions were conceived, designed and built, and what impressive science results came out so far. The missions are still going strong and are expected to do so for another 10 years at least.

(December 6, 2019)

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Lorents workshop on thermonuclear X-ray bursts

(June 19, 2019)

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The legacy of Giacconi

Last Sunday, sadly the great pioneer in X-ray astronomy Riccardo Giacconi passed away. In 1962, he discovered the first extra-solar X-ray emitting object (Sco X-1), against all contemporaneous beliefs about such a possibility. Appropriately, this won him the Nobel Prize. Now, 56 years and 30 X-ray observatories later (11 in operation now!), the community wants to push further and further with larger and larger telescopes. The large European-led Athena mission will launch in 2031 to uncover for example the massive voids (really) between clusters of galaxies.

China also has big plans, to launch in 2025 the largest X-ray detectors ever on the 'eXTP' observatory. A major European contribution is foreseen for this observatory. Today, a series of five white papers was published on arXiV to explain the mission and show what new science can be done with eXTP. I had the privilege to write one of these papers together with 195 other scientists that are just as excited about eXTP as I am. The opportunities that eXTP provides in the fields of the densest matter, strongest gravity, strongest magnetic fields, flare stars, thermonuclear burning, black holes, etc., are fantastic. The legacy of Giacconi continues.
For the paper, click here.
(December 12, 2018; pictures Giacconi et al. 1962 and eXTP website)

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Swift Observatory -
good for Gamma Ray Bursts and X-ray Bursts

'Everybody' knows that The Neil Gehrels Swift Observatory is a highly successful NASA mission for the study of gamma-ray bursts. Hardly anybody knows that it is also very useful for the study of X-ray bursts. Both phenomena may seem very similar in name, but are totally different in origin. Gamma-ray bursts are stellar implosions yielding black holes in far-away galaxies, while X-ray bursts are nuclear explosions on nearby neutron stars in the Milky Way. The difference in wavelength is about a factor of 100. X-ray bursts emit at around 10 Angstrom and gamma-ray bursts at about 0.1 Angstrom. Swift detected about as many X-ray as gamma-ray bursts. We studied the subset of these for which Swift downloaded abundant data to Earth. We find some pretty extraordinary cases that are very powerful. So powerful that it seems that the explosion disrupts the surroundings of the neutron star.
For the paper, click here.
(November 19, 2018; picture Swift website)

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The incomprehensible simplicity
of cooling in X-ray bursts

We have concluded a study of the the decay phase of 501 X-ray bursts, both to get better statistics than a previous study of 37 bursts and to, for the first time, obtain a quantitative overview of the extent of the nuclear rapid-proton capture process - a nuclear fusion process that occurs at extremely high temperatures of around one billion degrees Kelvin. For the decays we find an elegantly simple model composed of a power law function (due to neutron star cooling) and a Gaussian (due to the rp process). The power law decay indices are between 1.0 and 2.5, confirming earlier measurements of the 37 burst sample. Neutron star cooling seems unaffected by the nature of the fuel. The simplicity of these cooling functions is as yet unexplained by theory. The rp-process fluence fraction peaks at 60%, which implies a hydrogen abundance remaining after the helium flash that is at least five times sub-solar. This is the first time such a measurement has been carried out for this unusual nuclear process.
For the paper, click here. For an online table and fit plots, click here
(September 7, 2017; picture Katalin Szarvas)

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Discovery of a far away burster

While investigating RXTE data for the MINBAR project, we discovered a previously unknown burster in an X-ray transient that was active nine years ago - XTE J1812-182. This was probably not noticed before because the bursts are an order of magnitude fainter than usually. The faintness suggests a far away location, at the far end of the Milky Way, possibly as far away as 50-100 thousand light years. This testifies to the brilliance of these events, orginating on objects (neutron stars) as small as 20 km and but far away as hundreds of quadrillion km (a number equal to 1 or a few plus 17 zeros).
For the initial discovery report, see here. A paper is in preparation.
(September 7, 2017; picture is time profile of one burst from XTE J1812-182)

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Another clocked burster

In the spring of 2015, we carried out a careful observing campaign on the binary star system 'the Rapid Burster'. The purpose was to catch the onset of one of its transient outbursts with the large X-ray telescope on NASA's Chandra observatory. But first, we monitored for a month the Rapid Burster with the smaller telescope on NASA's Swift observatory. We were careful not to trigger Chandra on small brightenings that fail to develop to a full-blown outburst. On May 20th, however, we detected a fast flux increase and triggered Chandra. The turn-around of Chandra was quick: the telescope was on target only 1.3 days after our trigger. The observation lasted 22 hours and served our purposes excellently. Our intention was to detect 20 thermonuclear X-ray bursts and that's just what it did! The regular bursting provided us with another clocked burster for 22 hours. Unfortunately, we did not detect the sought-after narrow features in the spectrum of these bursts. Nevertheless, this observation shows that careful campaigns can be succesful in catching as many burst as possible in a short time. Now we only need to target the one kind of bursts not targeted yet: 'superexpansion' bursts.
For the paper, click here.
(April 12, 2017; picture from paper)

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Two decades of superbursts

At the beginning of this millennium, superbursts were discovered by Remon Cornelisse and the WFC team at SRON in data from 1996, see picture above. They are day-long flashes from neutron stars. Theoreticians jumped up and quickly found an explanation: the nuclear ignition of a thick layer of carbon on the surface of a neutron star. One could say superbursts are the non-destructive cousins of the type Ia supernovae. Superbursts are a rare phenomenon: they happen only once every few years on an accreting neutron star. The current discovery machine for superbursts is the Japanese MAXI all-sky X-ray monitor, active since 2009 on the International Space Station. At the workshop '7 years MAXI' in Tokyo last December, I gave a status report of the research on superbursts. Sofar, 26 cases have been detected from 15 neutron stars. However, the understanding has not been made as much progress: the conditions seem insufficient for carbon ignition and it is hard to accumulate sufficient amounts of carbon even over a time scale of years. The community looks forward to more detailed observations of superbursts in the future with for instance high-duty cycle all-sky monitors. For a concise review paper, click here.
(March 30, 2017; picture Remon Cornelisse)

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High-throughput X-ray astronomy

The enhanced X-ray Timing Polarimetry project changed gears. It is now endorsed by the Chinese Academy of Sciences (CAS) and the Chinese Space Agency (CNSA). CAS and CNSA ranked it at the top of science space missions of the next 5-year plan and gave it a Chinese go-ahead for phase B with a tentative launch date of 2024, pending commitments by European national space agencies and ESA. There was great enthousiasm at an international conference 'High-throughput X-ray astronomy in the eXTP era' in Rome this week that was attended by 180 participants from China, Europe and the USA. The conference highlighted many interesting science questions that can be addressed by eXTP, particularly testing General Relativity in the strong field regime around black holes great and small, testing QCD in the dense and cool regime of neutron stars and mapping out magnetic fields around neutron stars. Interesting times lay ahead!
(February 10, 2017; picture Yuri Evangelista)

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From LOFT to eXTP

The LOFT Coordination Team has decided to concentrate its technical and programmatic efforts to further consolidate a coordinated European participation to eXTP. It was decided not to submit a LOFT proposal to the ESA M5 call that is due in October 2016. On a longer time scale, the LOFT Consortium will continue its support to the study of a LOFT-like mission (10 m² effective area) in the US context - LOFT-P - and possibly consider future ESA calls. There is unique science that can only be addressed with an instrument in that (10 m²) class. The eXTP mission has the following basic specs: 4 m² photon collectin area at 6 keV, soft-energy coverage down to 0.1 keV with 0.6 m² at 1 keV and no pile up, polarimetric capability similar to what is proposed for other ESA and NASA-proposed missions, wide-field monitoring capability of 30% of prompt sky coverage and envisioned launch date 2025. For further details, see soon at this web site.
(summary of LOFT communication, June 27, 2016)

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Doctor Bagnoli

Rapid Burster expert Tullio Bagnoli succesfully defended his PhD thesis today at the University of Amsterdam. The opposition, consisting of Mariano Mendez, Alessandro Patruno, Wim Hermsen, Ralph Wijers, Phil Uttley and Rudy Wijnands, questioned him on a large variety of subjects: convexity, mHz QPOs, type I and type II bursts, magnetic fields, pulsar quenching by plasmas, dead accretion disks, and why the weird Rapid Burster is worth the sole subject of a thesis. Promotor Michiel van der Klis and co-promotors Anna Watts and Jean in 't Zand are very pleased with Tullio's success. Tullio is eager to continue his research elsewhere! Picture right: Tullio with paranimphs Rik van Lieshout and Marianne Heida (thanks to Twitter account of Abigail Stevens). Thesis copies are still available.
(December 4, 2015)

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Population study reveals new Rapid Burster features

Tullio Bagnoli, Caroline d'Angelo (of Leiden University) and myself have finished a massive population study of almost 8000 type-II bursts from the Rapid Burster that we discovered in 1.0 Msec of RXTE observations during which the Rapid Burster (RB) was active. This is the richest data set available of the RB, in burst number, burst diversity and data quality. Our study resulted in the discovery of a new kind of type-II burst, one which lasts a couple of orders of magnitude shorter than any other known sofar. These bursts were found in only two instances and sometimes have weird time profiles: slow-rise fast-decay instead of the other way around. Furthermore, we find that type-II bursts are Eddington limited. Peculiarly, none show any sign of photospheric expansion, so radiation pressure does not seem to be at play in the RB. Finally, we have found two instances where the RB shows behavior that is very similar to the enigmatic black hole system GRS 1915+105 and is thought to be related to strong outflow (jet) events. We attempt to explain the type-II burst behavior in the frame of the 'dead disk' model which explain spasmodic accretion by magnetic channeling of the accretion. These results have been published in two papers for Monthly Notices of the Royal Astronomical Society, see paper 1 and paper 2.
(April 2015; picture from paper 2)

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The future of thermonuclear X-ray burst research

The Rossi X-ray Timing Explorer (RXTE) between 1995 and 2012 revealed a lot of interesting detail in X-ray bursts, thanks to its unprecedented 0.6 square meters of effective detector area in the classical X-ray band. This resulted in many discoveries, like burst oscillations. Still, one wonders how thermonuclear flames spread over neutron star surfaces and whether burst oscillations are due to vibrations of the neutron star surface brought on by those flames. Also, it would be truly insightful to view the effects of curved space-time around the neutron star and be able to take measurements of fundamental physics (QCD, GR) at the extremes of dense matter and space-time curvature. Therefore, a group of scientists and engineers have conceived the idea of the Large Observatory for X-ray Timing ('LOFT'). Making use of innovative light-weight detector technology (silicon drift detectors combined with micro-channel plates) and joining the potential of many European countries, China, Japan and the USA, they found a way to build and launch 9 square meters of detector area, 15 times more than RXTE. This will revolutionize a number of research fields, not in the least that of X-ray bursts. To support the LOFT idea and show the potential of LOFT for X-ray burst measurements, 34 researchers have written this 'White Paper'. The ball is now for ESA to pick up..
(January 2015; picture from the front cover of the LOFT proposal to the ESA M4 call)

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Relativistic expansion due to thermonuclear X-ray bursts

Thermonuclear X-ray bursts are usually not explosions. Gravity is so strong on the neutron star that any flung-out debris is firmly held tight to the surface. Only when a burst is powerful enough (in 20% of all cases), the pressure exerted by the radiation may be able to compensate for gravity. In such a case the atmosphere is briefly lifted off the star and then pulled back again. A new study by us has identified two bursts, out of more than ten thousand thus far detected, that are so powerful that a shell, visible for only a few tens of milliseconds, is flung loose from the star at 10 to 30% the speed of light. This is also the minimum velocity required to escape the strong gravity. Furthermore, the thermonuclear flame has to engulf the neutron star surface in merely 1 ms. This puts stringent constraints on theoretical models for such flames and suggests that the fuel burns in the so-called auto-ignition regime. For more details, see our paper
(July 2014; picture from paper: the blue part the initial 4-s stage of the main thermonuclear burst; the red spike is the unprecedently short (30 ms) precursor event arriving from the relativistically expanding shell)

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Now you see me, now you don't..

Nature never ceases to surprise us. When we think we've seen it all, something new pops up. Take the Rapid Burster, arguably the most enigmatic accreting neutron star there is. Whenever the Rapid Burster turns on, it keeps firing thermonuclear and accretion-powered bursts at a rate that is tremendous - sometimes thousands per hour. That's already known for tens of years, but now a study by Tullio Bagnoli identified something new among the thermonuclear bursts: in a particular state these show a phenomenon that is seen in no other bursts - from the Rapid Burster nor any other of the 100 or so X-ray bursters known in the Galaxy. As soon as the burst peaks, it fades very quickly until oblivion after which it returns again in full glory. Bagnoli et al. think it may point to a particular geometry of the fuel line. Eventually, they hope this will give a clue to why the Rapid Burster is the only neutron star operating like a machine gun. Have a look at the paper
(Feb. 2014; picture snapshot from paper)

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Cooling of neutron stars simpler than thought

Two college students (Thomas Triemstra and Roger Mateijsen) helped map out the cooling process of a representative collection of neutron stars after the stars experienced a thermonuclear flash in their uppermost one meter thick atmosphere. In a toy model, one would expect the thermal radiation to die out according to a simple power law with index -4/3. A more complicated model, involving the effects of degenerate electrons, predicts a power law with an index changing with time, so not a power law nor any other simple decay law. Our research now shows that the decay is nevertheless a simple power law. Nature is kind to us. But not unconditional: the index is not -4/3 but somewhere between -1.3 and -2.4, and not the same after every flash. See our new paper, written in collaboration with Andrew Cumming of McGill University (Montreal) and Tullio Bagnoli at SRON.
(Jan. 2014; picture snapshot from paper)

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A new perspective of an old phenomenon

In November 2011 we observed, simultaneously with Chandra and RXTE, the accreting neutron star SAX J1808.4-3658. We were very lucky to detect one of the brightest X-ray bursts ever. Although that was exactly our intention, bursts are so unpredictable that help from nature is always welcome. The detection provided a unique chance to measure a burst at high spectral resolution and at longer wavelengths than usual. No narrow features were detected, but the continuum below 3 keV (a new perspective provided by Chandra) shows remarkable excess emission. Like an iceberg seen under water. Probably this is radiation scattered by the polluted surrounding of the neutron star, but more exotic effects may apply as well. See our 2013 paper.
(May 2013; picture by Uwe Kils)

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Rapid and slow?

One of the most exotic X-ray binaries is the Rapid Burster. It is unique in exhibiting both thermonuclear (type I) and rapid accretion-powered (II) X-ray bursts. We have carried out the first systematic study of the type I bursting behavior as observed with RXTE. Surprisingly, it shows the same dependence on accretion rate as the slow burster in Terzan 5. We speculate that this special dependency is again due to a slow rotation of the neutron star. So, rapid type II bursts needing slow neutron stars? See Tullio Bagnoli's 2013 paper.
(Mar. 2013; picture by Museum Boerhave)

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Superexpansion from the most dramatic thermonuclear
explosions on neutron stars

Thermonuclear explosions on neutron stars can, in rare cases, blow off a shell that is able to travel up against the walls of the deep gravitational well and disturb the surrounding accretion disk. Our 2011 paper shows a dramatic example of the impact on the time profile of the X-rays coming of such an explosion. Our 2012 paper takes this a little further and shows how this effect can be used to diagnose the geometry of the accretion flow.
(Nov. 2012; picture by icomei)