Disk Dominated States of 4U 1957+11: Chandra, XMM, and RXTE Observations of Ostensibly the Most Rapidly Spinning Galactic Black Hole

TL;DR

This study analyzes multi-epoch X-ray observations of 4U 1957+11 to investigate its black hole spin and disk properties, revealing that the data are consistent with a rapidly spinning black hole but do not definitively constrain the spin due to observational limitations.

Contribution

The paper demonstrates that current X-ray spectral data cannot definitively determine the black hole spin in 4U 1957+11, highlighting the need for broader bandpass observations and considering disk atmosphere effects.

Findings

Data are consistent with a rapidly spinning black hole (a*~1).

Spectral models can fit both high and low spin scenarios with different assumptions.

Limited observational bandpass and unknown system parameters hinder precise spin measurement.

Abstract

We present simultaneous Chandra-HETG and RXTE observations of a moderate flux `soft state' of the black hole candidate 4U1957+11. These spectra, having a minimally discernible hard X-ray excess, are an excellent test of modern disk atmosphere models that include the effects of black hole spin. The HETG data show that the soft disk spectrum is only very mildly absorbed with N_H =1-2 X 10^{21} cm^-2. These data additionally reveal 13.449 A NeIX absorption consistent with the warm/hot phase of the interstellar medium. The fitted disk model implies a highly inclined disk around a low mass black hole rapidly rotating with normalized spin a*~1. We show, however, that pure Schwarzschild black hole models describe the data extremely well, albeit with large disk atmosphere ``color-correction'' factors. Standard color-correction factors can be attained if one additionally incorporates mild Comptonization. We find that the Chandra observations do not uniquely determine spin. Similarly, XMM/RXTE observations, taken only six weeks later, are equally unconstraining. This lack of constraint is partly driven by the unknown mass and unknown distance of 4U1957+11; however, it is also driven by the limited bandpass of Chandra and XMM. We therefore present a series of 48 RXTE observations taken over the span of several years and at different brightness/hardness levels. These data prefer a spin of a*~1, even when including a mild Comptonization component; however, they also show evolution of the disk atmosphere color-correction factors. If the rapid spin models with standard atmosphere color-correction factors of h_d=1.7 are to be believed, then the RXTE observations predict that 4U1957+11 can range from a 3 M_sun black hole at 10 kpc with a*~0.83 to a 16 M_sun black hole at 22 kpc with a* ~ 1, with the latter being statistically preferred.