Discovery of a New Soft X-Ray Excess in the Spectrum of EXO 0748−676

Abstract

In this Letter, we present the Advanced Satellite for Cosmology and Astrophysics data from EXO 0748-676 that indicate that a previously unnoted soft X-ray excess exists in the spectrum of this source. We have modeled the intensity-selected spectra with typical candidate low-mass X-ray binary spectral models. As expected, the best performing model was the generalized thermal (GT) model and an additional component that is thought to arise from the obscuration of the X-ray radiator by dipping material within the system. The GT model, however, has an unacceptable value of χ (=3.16). We find that a previously undetected soft component exists (0.4-1.5 keV) in all intensity-selected data, which is the cause of the large value of χ in our GT model fits. We modeled the new soft excess by adding an additional component to the GT model. A variety of soft component models were examined, and the best fitting was a steep power law with one or two local components (either edges or Gaussian lines), with the number of local components needed being dependent on the intensity-selected spectrum being fit. An edge feature and power-law model best fitted the quiescent soft spectral component, while Gaussian-line and power-law models best fitted the soft spectral component in all dipping spectra. We evaluate these results in terms of the expected physical geometry of this system.