A systematic look at the Very High and Low/Hard state of GX 339-4: Constraining the black hole spin with a new reflection model

We present a systematic study of GX 339-4 in both its very high and low hard states from simultaneous observations made with XMM-Newton and RXTE in 2004 and 2006. The X-ray spectra of both these extreme states exhibit strong reflection signatures, with a broad, skewed Fe-Kα line clearly visible above the continuum. Using a newly developed, self-consistent reflection model which implicitly includes the blackbody radiation of the disc as well as the effect of Comptonisation, blurred with a relativistic line function, we were able to infer the spin parameter of GX 339-4 to be 0.92± 0.01 (statistical) ± 0.03 (systematic) at 90 per cent confidence. We find that both states are consistent with an ionised thin accretion disc extending to the innermost stable circular orbit around the rapidly spinning black hole.