This parameter pulls double duty, used to enter the radiation temperature in units of 10K in the case of a blackbody or bremsstrahlung input model. It also is used to input the power-law index (in energy), , in the case of a power-law model. Note that is defined as in so generally will be less than zero (this is the opposite of the convention used by xspec). XSTAR always works with specific luminosity in units erg s erg, and never uses or , for example.