Suzaku observations of 4U 1957+11: The most rapidly spinning black hole in the galaxy?

We present three Suzaku observations of the black hole candidate 4U 1957+11-a source that exhibits some of the simplest and cleanest examples of soft, disk-dominated spectra. 4U 1957+11 also presents among the highest peak temperatures found fromdisk-dominated spectra. Such temperatures may be associated with rapid black hole spin. The 4U 1957+11 spectra also require a very low normalization, which can be explained by a combination of small inner disk radius and a large distance (> 10 kpc) which places 4U 1957+11 well into the Galactic halo. We perform joint fits to the Suzaku spectra with relativistic disk models. Assuming a low mass black hole and the nearest distance (3M☉, 10 kpc), the dimensionless spin parameter a* ≡ Jc/GM2≳0.9. Higher masses and farther distances yield a* ≈1. Low spin cannot be recovered unless 4U 1957+11 is a low mass black hole that is at the unusually large distance of ≳ 40 kpc