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 = 1.19 – 1.28 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 R _ { \infty } \lesssim 9.0 – 13.2 { km } and M < 1.2 – 1.7 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 color corrections than that predicted by spetral models .
Our analysis also gives an upper limit on the distance to GS 1826 - 24 of d < 4.0 – 5.5 { kpc } \xi _ { b } ^ { -1 / 2 } , where \xi _ { b } is the degree of anisotropy of the burst emission .