We report on time-resolved spectroscopy of the 63 brightest bursts of SGR J1550-5418 , detected with Fermi /Gamma-ray Burst Monitor during its 2008-2009 intense bursting episode .
We performed spectral analysis down to 4 ms time-scales , to characterize the spectral evolution of the bursts .
Using a Comptonized model , we find that the peak energy , E _ { peak } , anti-correlates with flux , while the low-energy photon index remains constant at \sim - 0.8 up to a flux limit F \approx 10 ^ { -5 } erg s ^ { -1 } cm ^ { -2 } .
Above this flux value the E _ { peak } - flux correlation changes sign , and the index positively correlates with flux reaching \sim 1 at the highest fluxes .
Using a two black-body model , we find that the areas and fluxes of the two emitting regions correlate positively .
Further , we study here for the first time , the evolution of the temperatures and areas as a function of flux .
We find that the area - kT relation follows lines of constant luminosity at the lowest fluxes , R ^ { 2 } \propto kT ^ { -4 } , with a break at higher fluxes ( F > 10 ^ { -5.5 } erg s ^ { -1 } cm ^ { -2 } ) .
The area of the high - kT component increases with flux while its temperature decreases , which we interpret as due to an adiabatic cooling process .
The area of the low - kT component , on the other hand , appears to saturate at the highest fluxes , towards R _ { max } \approx 30 km .
Assuming that crust quakes are responsible for SGR bursts and considering R _ { max } as the maximum radius of the emitting photon-pair plasma fireball , we relate this saturation radius to a minimum excitation radius of the magnetosphere , and put a lower limit on the internal magnetic field of SGR J1550-5418 , B _ { int } \gtrsim 4.5 \times 10 ^ { 15 } G .