Limits on spin determination from disc spectral fitting in GX 339-4

Understand

We attempt to constrain the black hole spin in GX 339-4 from spectral fitting of disc dominated data using RXTE spectra from the three most recent outbursts.

• We use the best current models for the disc emission, including full radiative transfer through the photosphere rather than assuming that the intrinsic emission from each radius has a (colour temperature corrected) blackbody spectrum.
• The results strongly depend on the poorly known binary system parameters, but we find a strict upper limit of a<0.9 for any distance greater than 6 kpc, assuming that the orbital inclination is the same as that of the inner disc.
• By contrast, the higher spin of 0.935 +/- 0.01 (statistical) +/-0.01 (systematic) claimed from fitting the iron line profile in this object requires that the inner disc is misaligned by over 20 degrees from the orbital inclination.