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Title:
Discovery of a New Soft X-Ray Excess in the Spectrum of EXO 0748-676
Authors:
Thomas, Brian; Corbet, Robin; Smale, Alan P.; Asai, Kazumi; Dotani, Tadayasu
Publication:
Astrophysical Journal Letters v.480, p.L21 (ApJL Homepage)
Publication Date:
05/1997
Origin:
APJ
ApJ Keywords:
STARS: INDIVIDUAL ALPHANUMERIC: EXO 0748-676, X-RAYS: STARS
DOI:
10.1086/310611
Bibliographic Code:
1997ApJ...480L..21T

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 chi 2_ν (=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 chi 2_ν 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.
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