AMSP X-RAY BURSTS · Localization of thermonuclear burning on accreting millisecond X-ray pulsars

Localization of thermonuclear burning on accreting millisecond X-ray pulsars

Analysis of observations of thermonuclear X-ray bursts has long been considered a possible method for constraining the masses and radii of some neutron stars. The standard approach is based on a number of assumptions about the burst emission, including that the emitting area is constant and equal to the surface area of the entire star and that the spectra are accurately described by a particular, detailed stellar atmosphere spectral model or even, in some cases, that the spectra can be adequately described by blackbody spectra. We tested these assumptions for the first time using the highest quality X-ray spectra that were available and explored what would be possible using a future instrument with the capabilities planned for LOFT. We find that RXTE PCA spectra of the 4U 1820-30 superburst obtained during an interval of 1600 seconds following the peak of the burst are clearly better fit by the most recent, detailed atmospheric spectral models of Suleimanov et al. than by simple thermal spectral models or other available detailed atmospheric spectral models. Our analysis of this superburst is the first analysis that makes such a comparison meaningful, because PCA observations of ordinary bursts that last only a few seconds do not provide enough counts to discriminate between even very different spectral models. We also find that that the emitting area of the superburst decreased by 20% during the interval we studied, and may never have included the entire surface. This result is inconsistent with a basic assumption of previous analyses, which is that the entire stellar surface emits uniformly during tails of bursts. Finally, we explored the constraints that would be possible if a superburst is observed using an instrument with the planned capabilities of the LOFT LAD. We do this by synthesizing a sequence of superburst spectra, folding them through the LAD response matrix, and then fitting model spectra to them. Accurate fits of detailed spectral models that are sufficiently precise to determine accurately both the surface gravity and the surface redshift would allow estimates of the stellar mass and radius that are independent of the distance to the source and the absolute flux calibration of the instrument. Use of this method would require accurate knowledge of the energy dependence of the effective area and would be affected by other possible systematic effects. Constraints obtained in this way would provide independent checks on constraints obtained using complementary methods. In the limited time of the subproject that were attacked, the CIG contributed towards european excellence and competitiveness. Through outreach activities like the ones the researcher got engaged to, a broader group of people got interested in the physical processes and every-day appliances that may otherwise appear distant to them, creating thus a technologically more advanced population. Such activities are also beneficial for the european citizens to be well informed about where research funds end up to and for what they are used. The involvement of astronomical observations, general relativity, unknown states of matter proved to be attractive to students and people outside academia, contributing to the spreading of scientific knowledge and interest in the society. This is expected to enhance technological development and competitiveness of the European Union. The contribution of the project to the development of the LOFT mission has also long-term prospects of supporting European competitiveness because of the increased dependence of large projects (e.g. GPS mapping) on satellites; the more involved members at the European Union are in remote astronomical observer facilities, the more knowledge is gained relevant to satellite functioning.

Data: CORDIS, © European Union