Michael Zamfir, Andrew Cumming, Duncan K. Galloway
We investigate the constraints on neutron star mass and radius in GS 1826-24
from models of lightcurves and spectral evolution of type I X-ray bursts. This
source shows remarkable agreement with theoretical calculations of burst
energies, recurrence times, and lightcurves. We first exploit this agreement to
set the overall luminosity scale of the observed bursts. When combined with a
measured blackbody normalization, this leads to a distance and anisotropy
independent measurement of the ratio between the redshift 1+z and color
correction factor f_c. We find 1+z=1.25-1.34 for f_c=1.4-1.5. We then compare
the evolution of the blackbody normalization with flux in the cooling tail of
bursts with predictions from spectral models of Suleimanov et al. (2011b). The
observations are well described by the models at luminosities greater than
about one third of the peak luminosity, with deviations emerging at
luminosities below that. We show that this comparison leads to distance
independent upper limits on R_\infty and neutron star mass of 10.8-14.2 km and
1.4-1.8 M_\odot, respectively, for solar abundance of hydrogen at the
photosphere and a range of metallicity and surface gravity. The radius limits
are low in comparison to previous measurements. This may be indicative of a
subsolar hydrogen fraction in the GS 1826-24 photosphere, or of larger colour
corrections than that predicted by spetral models. Our analysis also gives an
upper limit on the distance to GS 1826-24 of d<4.8-6.0 kpc \xi_b^{-1/2}, where
\xi_b is the degree of anisotropy of the burst emission.
View original:
http://arxiv.org/abs/1111.0347
No comments:
Post a Comment