X-ray spectroscopy of the accreting millisecond pulsar XTE J0929-314 in outburst

We present the high-resolution spectrum of the accretion-powered millisecond pulsar XTE J0929-314 during its 2002 outburst, measured using the Low Energy Transmission Grating Spectrometer on board the Chandra X-Ray Observatory. The 1.5-25.3 Angstrom (0.5-8.3 keV) Chandra spectrum is well fitted by a power-law+ blackbody model with photon index Gamma = 1.55 +/- 0.03, blackbody temperature kT(bb) = 0.65 +/- 0.03 keV, and blackbody normalization R-bb,R-km/d(10) (kpc) = 7.6 +/- 0.8. No emission or absorption features are found in the high-resolution spectrum, with a 3 sigma equivalent width upper limit of less than 0.007 Angstrom at 1.5 Angstrom and less than 0.12 Angstrom at 24 Angstrom. The neutral absorption edge depths 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 iless than or similar to85degrees for a Roche lobe filling companion. We also present the broadband spectrum measured over the course of the outburst by the Rossi X-Ray Timing Explorer (RXTE). The RXTE spectrum of XTE J0929-314 is also well fitted 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,R-km/d(10 kpc) = 5.9 +/- 1.3 in the 2-50 keV energy range. The blackbody flux remained constant over the course of the outburst, while the power-law flux was strongly correlated to the (decreasing) 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.