Accurate Mass Ratio and Heating Effects in the Dual-Line Millisecond Binary Pulsar in NGC 6397

We present an analysis of the first high-resolution spectra measured from an accretion-driven millisecond X-ray pulsar in outburst. We observed XTE J1751-305 with XMM-Newton on 2002 April 7 for approximately 35 ks. Using a simple absorbed blackbody plus power-law model, we measure an unabsorbed flux of (6.6 ± 0.1) × 10^-10 ergs cm^-2 s^-1 (0.5-10.0 keV). A hard power-law component ((gamma) = 1.44 ± 0.01) contributes 83% of the unabsorbed flux in the 0.5-10.0 keV band, but a blackbody component (kT = 1.05 ± 0.01 keV) is required. We find no clear evidence for narrow or broad emission or absorption lines in the time-averaged spectra, and the sensitivity of this observation has allowed us to set constraining upper limits on the strength of important features. The lack of line features is at odds with spectra measured from some other X-ray binaries that share some similarities with XTE J1751-305. We discuss the implications of these findings on the accretion flow geometry in XTE J1751-305.