Stratos Boutloukos (University of Tuebingen, Germany)

Neutron star mass and radius constraints from high-precision burst spectra

Thermonuclear X-ray bursts that have photospheric radius expansion have long been proposed to provide
constraints on neutron star masses and radii. However, these methods have potentially significant
systematic errors due to their assumption of uniform emission from the entire stellar surface and due to the
particular spectral models used, as well as statistical uncertainties in the source distance and peak flux.
Here we describe a different method to constrain neutron star masses and radii, based on detailed fits to the
most precise burst spectra.

We focus on the RXTE PCA data for a superburst from 4U 1820-30. The data we analyze span roughly 1600
seconds, starting after photospheric touchdown, and have nearly 20 million counts. The surface radiative
flux varies over this interval, but we find that joint fits performed with the assumption that the surface
composition, surface gravity, and surface redshift are all constant and using the model atmospheres of
Suleimanov et al. 2011 (improved to include fully relativistic scattering) closely match the data and are
strongly preferred over simple Bose-Einstein thermal spectra, which are in turn dramatically preferred over
the Madej et al. and Majczyna et al. spectra commonly used to constain masses and radii in the standard
method. We note that data from more typical bursts that last just a few seconds are insufficient to distinguish
between any of these models, or even significantly different models such as Wien spectra.

As pointed out by Majczyna and Madej (2005), precise constraints through spectral fits on both the redshift
and surface gravity also constrain the stellar mass and radius. We will discuss the prospects for such
constraints using current data, as well as the improvements to be expected using the much larger collecting
area of LOFT. 
A particular advantage of this method is that it is independent of absolute flux calibration of the
instrument.