Suzaku Observations of 4U 1957+11: Potentially the Most Rapidly Spinning Black Hole in (the Halo of) the Galaxy

TL;DR

This study analyzes Suzaku X-ray observations of 4U 1957+11, suggesting it may host a rapidly spinning black hole with near-maximal spin, possibly due to its unique location in the Galactic halo.

Contribution

It provides the first detailed spectral analysis indicating a high black hole spin in 4U 1957+11, linking spectral properties to black hole spin and location.

Findings

Black hole in 4U 1957+11 likely has spin parameter > 0.9

Spectral data suggest a small inner disk radius and high spin

The system's location in the halo may relate to its high spin

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_sun, 10 kpc), the dimensionless spin parameter a* = Jc/GM^2 >~ 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.