2005ApJ...619..492W

We observed the accretion-driven millisecond X-ray pulsars XTE J0929-314 and XTE J1751-305 in their quiescent states using Chandra. From XTE J0929-314 we detected 22 source photons (in the energy range 0.3-8 keV) in ∼24.4 ks, resulting in a background-corrected time-averaged count rate of 9±2x10^–4 counts/s. The small number of photons detected did not allow for a detailed spectral analysis of the quiescent spectrum, but we can demonstrate that the spectrum is harder than simple thermal emission, which is what is usually presumed to arise from a cooling neutron star that has been heated during the outbursts. Assuming a power-law model for the time-averaged (averaged over the whole observation) X-ray spectrum, we obtain a power-law index of 1.8^+0.6_–0.5and an unabsorbed X-ray flux of 6⁺⁴_–2x10^–15 ergs/s/cm2 (for the energy range 0.5-10 keV), resulting in a 0.5-10 keV X-ray luminosity of 7⁺⁵_–2x10³¹ (d/10 kpc)² ergs/s, with d the distance toward the source in kiloparsecs. No thermal component could be detected; such a component contributed at most 30% to the 0.5-10 keV flux. Variability in the count rate of XTE J0929-314 was observed at the 95% confidence level. We did not conclusively detect XTE J1751-305 in our ∼43 ks observation, with 0.5-10 keV flux upper limits between 0.2 and 2.7x10^–14 ergs/s/cm2 depending on assumed spectral shape, resulting in 0.5-10 keV luminosity upper limits of (0.2-2)x10³² (d/8 kpc)² ergs/s. We compare our results with those obtained for other neutron star X-ray transients in their quiescent state, and in particular with the quiescent properties of SAX J1808.4-3658. Using simple accretion disk physics in combination with our measured quiescent luminosity of XTE J0929-314 and the luminosity upper limits of XTE J1751-305 and the known spin frequency of the neutron stars, we can constrain the magnetic field of the neutron stars in XTE J0929-314 and XTE J1751-305 to be less than 3x10⁹d/10kpc and 3-7x10⁸d/8kpc G (depending on assumed spectral shape of the quiescent spectrum), respectively.