We present three spectra of Cygnus X-1 in the low/hard state from 3-1000 keV produced with RXTE and OSSE .
We demonstrate that a pure thermal Comptonization model , with Compton reflection fits the data well , with coronal temperatures of about 90 keV , optical depths of about 1.3 , and reflection fractions of about 0.25 \times 2 \pi .
We find that no additional components are necessary to fit the data , although the presence of an additional non-thermal component does result in a marginal improvement to the goodness of the fit .
Two of the observations are fit better by an almost purely non-thermal Comptonization model with \Gamma _ { inj } of \sim 3.2 for the electrons and the other parameters essentially the same as for the thermal model , while the third observation is also consistent with this model .
Observations that could break this degeneracy are discussed .
We also demonstrate that the low reflection parameter ( i.e . R / 2 \pi < 1 ) is due to a lack of strong curvature in the 30-50 keV range of the spectrum and not due to ionization effects .
The spectrum changes very little over the three weeks of the observations , in accordance with a three week correlation timescale found in the All Sky Monitor data .
We show that for a purely thermal corona , the radius of the corona must be between 3 \times 10 ^ { 6 } and 1.5 \times 10 ^ { 10 } cm , while the radius of a hybrid corona or a purely non-thermal corona is constrained to be between 10 ^ { 8 } and 10 ^ { 9 } cm .