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X-ray bursts and burst oscillations
Though generally understood, there are many aspects of thermonuclear shell flashes on neutron stars which are not well understood. This varies from understanding the ignition conditions of carbon flashes, through the relationship between nuclear burning stability and accretion rate, to the origin of the millisecond oscillations seen in many flashes.
Thermonuclear flashes from neutron stars are detected as bright bursts of X-rays that usually last one minute. They were discovered in 1975 with the Dutch-built Astronomische Nederlandse Satelliet (ANS). Recent missions such as NASA's Rossi X-ray Timing Explorer, the Dutch-Italian Satellite per Astronomia X ('BeppoSAX'), and ESA's INTEGRAL provided a large growth of data on this phenomenon and enabled the detection of rare but important types of X-ray bursts and burst oscillations.
35 years after the discovery, it was time for a first workshop dedicated to X-ray bursts and it was very fitting to have it in Holland, at the Lorentz Center. 47 researchers participated from 12 countries, most from the USA (15) and the Netherlands (11). Except for a handful, all participated in the whole workshop. The program consisted of 22 plenary invited talks during the mornings, 11 contributed posters, a plenary flash poster session with 5-min oral presentations of the posters, 6 organized/chaired discussion meetings and 3 end-of-the day 'wrap-up' sessions where the chairs of the discussion meetings reported back to the whole group. There was 8 hours of schedule-free 'office time' spread over 3 days. The social program consisted of a wine-and-cheese party on the first day and a boat trip with dinner on the third day.
The workshop was received very well by the participants. They enjoyed the stimulating atmosphere induced by the abundant time for
discussion, the flexibility of the program, the well-balanced variety in expertise contained in the group and program, and the pleasant environment provided by the Lorentz Center staff and locality. The workshop appeared to initiate various new collaborations in the field. The most important result of the workshop is a status overview of the field. Where do we stand on observations and models, what are the unanswered questions and how do they need to be addressed. This brought about a list of 'homework assignments'.
Most of the discussions centered on three issues. First, X-ray bursts hold the promise of a profound new insight into the fundamental behavior of matter. Inside neutron stars, matter is compressed to super-nuclear densities. Therefore, their mass and size depends on ill-understood details of the strong nuclear force. Measuring mass and size therefore yields constraints on this fundamental force. While neutron star masses are well measured in a few tens of cases, radius measurements have been insufficiently accurate so far. X-ray bursts, being bright surface phenomena, might bring resolution. Currently, measurements are seriously hampered by calibration issues. A lively workshop discussion succeeded in clarifying what those issues are and how they can be tackled to make further progress. A homework assignment is that agreement needs to be achieved on the dependence of the color correction on the gravitational acceleration.
Another focus of discussions was burst modeling, involving questions such as how high the fuel heating from the crust is, what breakout reactions are important in mixed hydrogen-helium burning, and what can be learned from models of classical novae. A positive conclusion from recent work is that consensus arises on the nature of the upper crust. Homework assignments involve modeling of convection and photospheric expansion, including rotation in burst models, move on to 2-D modeling, and self-consistently model the crust/envelope interaction. Furthermore, the different codes for burst models need to be mutually verified and validated.
The third focal point of discussion was the origin of burst oscillations. It is generally accepted that their (asymptotic) frequency is equal to the spin frequency of the neutron star, but the mechanism by which they are caused is unclear. During burst rise they are thought to be caused by local ignition spreading out with a time of order 1 s. This time scale is supported by models involving flame spreading at high rotation rates. However, the discussion made clear that the evidence for this explanation is based on just a few bursts and the data on all burst rises need to be investigated more thoroughly. During burst decay this explanation is, a priori, not applicable. The discussion yielded that it is likely related to the non-dipolar component of the NS magnetic field, but there are many questions on the generation, destruction and morphology of such fields in bursting neutron stars. A suggestion is that the magnetic field is locally so strong (~1010 G) that it may contain the fuel. The homework assignment is the further development of the theory of such magnetic fields - how are they generated/changing (through processes like convection, winding up by differential rotation, the Tayler-Spruit dynamo and thermomagnetic drift) and is it possible to get pulsations out of disordered fields? The further exploitation of existing data was considered important as well, for instance to investigate the relationship between 'type II' bursts (powered by accretion, possibly gated by the magnetic field) and thermonuclear 'type I' bursts.
There is room for improvements through future measurements. Many burst oscillations are measured with moderate accuracy and there is need for more sensitive observations, particularly to increase the time resolution of frequency drifts in burst oscillations and search the oscillation profile for higher harmonics, detect more bursts with oscillations to enable population studies to test flame spreading models, and measure the spectrum below 2 keV (which is below the bandpass of most current detectors) and with higher spectral resolution. Several missions are being designed that meet these challenges, particularly IXO (ESA/NASA) and AXTAR (NASA). In the mean time, several niches still exist for current missions (RXTE, XMM-Newton, Chandra) and soon-to-be-launched missions (Astrosat, Astro-H, Nustar, GEMS). The homework assignment is to bring these future observations to a success by advocating them and writing observation and instrumental proposals.
The organizers thank the Lorentz Center, NOVA, NWO, ESA, Stichting Physica and SRON for their generous financial and infrastructural support, and the staff of the Lorentz Center for their work which enabled a very smooth organization. Lastly, we are indebted to all participants. Their involvement and enthusiasm were essential to the success of the workshop.
A. Watts (Amsterdam, the Netherlands)
J. in 't Zand (Utrecht, the Netherlands)
E. Kuulkers (Madrid, Spain)
A. Cumming (Montreal, Canada)