X-ray Spectroscopy of the Accreting Millisecond Pulsar XTE J0929-314 in Outburst

(Abridged) We present the spectrum of the accretion-powered millisecond pulsar XTE J0929-314 during its 2002 outburst, measured using the LETGS onboard Chandra. The Chandra spectrum is well fit by a power-law + blackbody model with Gamma=1.55+/-0.03, kT_bb=0.65+/-0.03 keV, and R_bb,km/d_10kpc=7.6+/-0.8. No emission or absorption features are found. The neutral absorption edges are consistent with the estimated interstellar absorption along the line of sight to the source. We found no orbital modulation of the 2--10 keV X-ray flux, to a 3 sigma upper limit of 1.1%, which implies an upper limit on the binary inclination angle of i<85o for a Roche-lobe--filling companion. We also present the broadband spectrum measured over the course of the outburst by RXTE. The RXTE spectrum of XTE J0929-314 is also well fit with a power-law + blackbody model, with average values of Gamma=1.76+/-0.03, kT_bb=0.66+/-0.06 keV, and R_bb,km/d_10kpc=5.9+/-1.3. No significant evolution in the spectral shape was found. The blackbody temperature and normalization varied, but were anti-correlated such that the blackbody flux remained constant, with the power-law normalization strongly correlated to the flux of the source. We find that the difference in power-law photon indices measured from Chandra and RXTE spectra can be explained by a change in the power-law photon index at low energies, E