Chandra High Resolution Spectroscopy of the Burst Spectrum of EXO 0748-67

We have observed EXO0748-67 for approximately 300 ksec with the High Energy Transmission Grating Spectrometer on Chandra. A total of 35 Type I X-ray bursts occurred during our observation, and from these we obtained a composite burst spectrum with high sensitivity in the Fe K band. Along with the spectrum of GS1826-238, this is the highest sensitivity short-wavelength photospheric spectrum of a neutron star ever obtained. The total summed spectrum does not appear to exhibit a clear, narrow Fe Ly alpha absorption line; a provisional upper limit on the equivalent width of an unresolved feature is approximately 5 mA (11 eV). We present the spectroscopy and briefly discuss the physical implications of our analysis in the context of attempts to measure the fundamental properties of neutron stars by photospheric spectroscopic techniques. Analysis of the broad-band characteristics of the short-wavelength spectrum suggests that the majority of the bursts shows significant photospheric radius expansion. We will present the results of a spectroscopic analysis performed on subsets of the data selected by the apparent state of the photosphere, to allow for the fact that any narrow Fe absorption spectrum strongly depends on the physical parameters of the atmosphere. This research was supported by NASA, through Chandra grant GO4-5045A.