On-axis sensitivity curve for protoMIRAX. The horizontal bars are the minimum detectable fluxes at a level of 3σ at an atmospheric depth of 2.7 g cm^-2 and a zenith angle of 30°. The integration time is 8 hours. Also shown are the spectra of 4 X-ray sources that will be observed in the first balloon flight. The source spectra were taken from Sizun et al. (2004; Crab), Grebenev et al. (1995; 1E 1740.7−2942), Sunyaev et al. (1991b; GRS 1758−258) and Dieters et al. (1991; GX 1 + 4).