The independent variables are X and T? If you insist on using the Curve Fitting Toolbox, the thing to do would be to regard this as a surface in X and T and use sfit().
It is noteworthy, though, that your model is linear lnX and 1/T. So, you could really probably do a simple linear fit with mldivide(). In other words, just solve the linear equations for k, a, and b using normal linear algebraic operations.
