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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 2002 and 2004. 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 Comptonization, blurred with a relativistic line function, we were able to infer the spin parameter of GX 339−4 to be  0.935 ± 0.01  (statistical) ±0.01 (systematic) at 90 per cent confidence. We find that both states are consistent with an ionized thin accretion disc extending to the innermost stable circular orbit around the rapidly spinning black hole.

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