Carbon neutron star atmospheres

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The physical properties of super-dense matter in neutron-star (NS) cores is one of the most intriguing problems of modern physics. Also known as the NS equation-of-state problem, it cannot be resolved using theoretical considerations or laboratory experiments with heavy-ion collisions alone. A straightforward way for attacking this problem is the simultaneous mass and radius determination of NSs from X-ray spectroscopy. Despite of great progress in the last decade, this kind of measurements is still not sufficiently accurate to draw firm conclusions. Two of the main obstacles are generally unknown distances to NSs and unknown chemical compositions of NS atmospheres, however, accurate emergent spectra of NS model atmospheres (with appropriate composition) are crucially necessary for the measurement of basic neutron star parameters. We propose to use a particular subclass of NSs that allow to overcome this obstacles.NSs in supernova remnants (so-called Central Compact Objects, CCOs) are particularly interesting because for some of them distances (and ages) are sufficiently well known. In addition, there is no evidence for on-going accretion onto these objects so their gaseous upper layers are undisturbed and can be described by model stellar atmosphere methods. Recently, evidence was found from X-ray spectroscopy that some of the CCOs have atmospheres composed of carbon. Knowing distance and envelope composition of CCOs enables us to derive more precise NS parameters, provided the model atmospheres and emergent spectra can be computed with sufficiently high accuracy. The immediate aim of this project proposal is to investigate the potentially most important physical processes that can significantly affect existing theoretical carbon NS atmosphere spectra. As a final result we plan to compute new grids of accurate model spectra and use them for analyses of archival X-ray spectra of NSs with possible carbon atmospheres. These grids will be published so that future observations can be analysed with them. New, tight constraints on the equation-of-state in NS cores and also on neutrino-cooling physics will be obtained.