SUZAKU OBSERVATIONS OF 4U 1957+11: POTENTIALLY THE MOST RAPIDLY SPINNING BLACK HOLE IN (THE HALO OF) THE GALAXY

Abstract

We present three Suzaku observations of the black hole candidate 4U 1957+11 (V1408 Aql)—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 from disk-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 both relativistic and Comptonized disk models. Assuming a low-mass black hole and the nearest distance (3 M☉, 10 kpc), the dimensionless spin parameter a* ≡ Jc/GM² ≳ 0.9. Higher masses and farther distances yield a* ≈ 1. Similar conclusions are reached with Comptonization models; they imply a combination of small inner disk radii (or, equivalently, rapid spin) and large distance. 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. We speculate whether the suggested maximal spin is related to how the system came to reside in the halo.