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 Å (0.5-8.3 keV) Chandra spectrum is well fitted by a power-law+blackbody model with photon index Γ = 1.55 ± 0.03, blackbody temperature kT_bb = 0.65 ± 0.03 keV, and blackbody normalization R_bb,km/d_{10 kpc} = 7.6 ± 0.8. No emission or absorption features are found in the high-resolution spectrum, with a 3 σ equivalent width upper limit of less than 0.007 Å at 1.5 Å and less than 0.12 Å at 24 Å. 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 σ upper limit of 1.1%, which implies an upper limit on the binary inclination angle of i ≲ 85° 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 Γ = 1.76 ± 0.03, kT_bb = 0.66 ± 0.06 keV, and R_bb,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.