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Jean in 't Zand
Astrophysicist at
SRON, Utrecht, The Netherlands
Publications Astronomical Telegrams GCN arXiv (HE) ADS A&A (ref,aut)

Data links
RXTE ASM (weather) MAXI Swift-BAT monitoring PCA bulge scans SIMBAD HEASARC Browse IBAS

Science collaborators David Ballantyne Jérôme Chenevez Andrew Cumming Duncan Galloway Peter Jonker Laurens Keek Erik Kuulkers Frits Paerels Anna Watts Nevin Weinberg

Research highlights

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 decays 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)

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)

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)

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)

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)

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)

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)

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)

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)

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)

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)

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)

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)

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)

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)

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