X‐Ray Spectroscopy of the Accreting Millisecond Pulsar XTE J0929−314 in Outburst

We present the high-resolution spectrum of the accretion-powered millisecondpulsar XTE J0929-314 during its 2002 outburst, measured using the Low EnergyTransmission Grating Spectrometer onboard the Chandra X-ray Observatory. The1.5-25.3 A (0.5-8.3 keV) Chandra spectrum is well fit by a power-law +blackbody model with photon index Gamma=1.55+/-0.03, blackbody temperaturekT_bb=0.65+/-0.03 keV, and blackbody normalization R_bb,km/d_10kpc=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 <0.007 A at 1.5 A and <0.12 A at24 A. The neutral absorption edge depths are consistent with the estimatedinterstellar absorption along the line of sight to the source. We found noorbital modulation of the 2-10 keV X-ray flux, to a 3 sigma upper limit of1.1%, which implies an upper limit on the binary inclination angle of i<85degfor a Roche-lobe-filling companion. We also present the broadband spectrummeasured over the course of the outburst by the Rossi X-ray Timing Explorer.The XTE spectrum of XTE J0929-314 is also well fit with a power-law + blackbodymodel, with average values of Gamma=1.76+/-0.03, kT_bb=0.66+/-0.06 keV, andR_bb,km/d_10kpc=5.9+/-1.3 in the 2-50 keV energy range. The blackbody fluxremained constant over the course of the outburst, while the power-law flux wasstrongly correlated to the (decreasing) flux of the source. We find that thedifference in power-law photon indices measured from Chandra and XTE spectracan be explained by a change in the power-law photon index at low energies.Comment: 7 pages, 2 figures; accepted for publication in the Astrophysical Journal (Vol. 587, April 20 issue