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We present simultaneous Chandra-HETG and RXTE observations of a moderate flux`soft state' of the black hole candidate 4U1957+11. These spectra, having aminimally discernible hard X-ray excess, are an excellent test of modern diskatmosphere models that include the effects of black hole spin. The HETG datashow that the soft disk spectrum is only very mildly absorbed with N_H =1-2 X10^{21} cm^-2. These data additionally reveal 13.449 A NeIX absorptionconsistent with the warm/hot phase of the interstellar medium. The fitted diskmodel implies a highly inclined disk around a low mass black hole rapidlyrotating with normalized spin a*~1. We show, however, that pure Schwarzschildblack hole models describe the data extremely well, albeit with large diskatmosphere ``color-correction'' factors. Standard color-correction factors canbe attained if one additionally incorporates mild Comptonization. We find thatthe Chandra observations do not uniquely determine spin. Similarly, XMM/RXTEobservations, taken only six weeks later, are equally unconstraining. This lackof constraint is partly driven by the unknown mass and unknown distance of4U1957+11; however, it is also driven by the limited bandpass of Chandra andXMM. We therefore present a series of 48 RXTE observations taken over the spanof several years and at different brightness/hardness levels. These data prefera spin of a*~1, even when including a mild Comptonization component; however,they also show evolution of the disk atmosphere color-correction factors. Ifthe rapid spin models with standard atmosphere color-correction factors ofh_d=1.7 are to be believed, then the RXTE observations predict that 4U1957+11can range from a 3 M_sun black hole at 10 kpc with a*~0.83 to a 16 M_sun blackhole at 22 kpc with a* ~ 1, with the latter being statistically preferred.

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