I recently started the study of vector bundles on $\mathbb{P}^n$, and started to read Rao's article 'A family of vector bundles on $\mathbb{P}^3$'. There, there is a notion of spectrum of a vector bundle. I searched this in some other articles, even in the excellent book "Vector Bundles on Complex Projective Space" by Okonek, Schneider and Spindler, but I cannot find the definition of spectrum.

Does anyone has any reference where I can find this?

Thank you so much.