Constraints on the neutron star magnetic field of the two X-ray transients SAX J1808.4-3658 and Aql X-1

We present a comprehensive X-ray study of four years of pointed RXTE observations of GRS 1915+105 in the X-state. We interpret the behavior of the hard power law tail spectrum as coming from inverse Compton scattering of soft disk photons on a thermally dominated hybrid corona above the accretion disk. GRS 1915+105 shows a strong, variable reflection amplitude. As in other BHC and in Seyfert galaxies, a correlation between the power law slope and the reflection was found. Also, the radio fluxes at 2.25 GHz and 15 GHz correlate with the power law slope, thus revealing a connection between the outflowing matter and the comptonizing region in the X-states.