These two links will help you to find your answer:
I am now ready to be rated.
Donna Kakonge, BJ, MA, TESOL, LLB, ABD
Rather than have you try to find your answer in these documents, the explanation for the taxation of non-residents on income earned from U.S. sources is explained here:
If this answer is what you are looking for, please let me know and rate me accordingly. Regards, Judy
Is there further information required? If not, the only way I am paid is for you to rate me. Thanks. Judy