Dear Editors, We thank the referee for all the comments and corrections. We have revised our manuscript. In what follows we offer our responses to the referee. With best regards, A.L. Müller ---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- Reviewer: "Section 2, first paragraph, line 5: when the authors state that the proton density at R_sp is low, do they refer to the stellar wind? I think so, but it is unclear." Authors: We have rewritten this sentence. We refer to the fact that the relativistic protons produce negligible radiation because the particle densities at R_sp are very low (matter and photon densities). We cannot include the corresponding calculations because of space limitation. ---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- Reviewer: "Next line: As shown in Fig. 1 ---> As sketched in Fig. 1" Authors: Thank you for the correction, we changed it. ---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- Reviewer: "Item 1, page 3,, 3rd line: young stars, which typical ---> whose typical" Authors: We corrected it. Thank you. ---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- Reviewer: "Item 3: 'Eq (2) is a good approximation' is a vague statement wothout further evidence that this is correct. I recommend to rephrase the sentence." Authors: We rephrased this sentence. ---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- Reviewer: "Section 3.1, first paragraph, lines 3-5: The maximum energy proportionality is for single or multiple collisions? Please, specify." Authors: We specified that it is the maximum energy at an individual shock. ---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- Reviewer: "There are a number of references of relevant works in the field that are missing, but I guess this is due to space limitation." Authors: Indeed, we needed to constrain the number of references due to space limitation.