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/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.
- Publication:
-
The Astrophysical Journal
- Pub Date:
- January 2012
- DOI:
- 10.1088/0004-637X/744/2/107
- arXiv:
- arXiv:1109.6008
- Bibcode:
- 2012ApJ...744..107N
- Keywords:
-
- accretion;
- accretion disks;
- black hole physics;
- radiation mechanisms: thermal;
- X-rays: binaries;
- Astrophysics - High Energy Astrophysical Phenomena
- E-Print:
- To be published in the Astrophysical Journal