RXTE observations of 4U 1728-34

We analyze the transition episodes from this source observed with BeppoSAX and RXTE satellites. We find that the X-ray broadband energy spectra of 4U 1728-34 during all spectral states can be modeled by a combination of a thermal (blackbody-like) component, a Comptonized component (which we herein denote as COMPTB), and a Gaussian component.

Spectral analysis using this model provides evidence that the photon power-law index {Gamma} is almost constant ({Gamma}=1.99+/-0.02) when kT_e_ changes from 15 to 2.5keV during these spectral transitions.