Using two Chandra observations we have derived estimates of the dust distribution and distance to the eclipsing high mass X-ray binary (HMXB) Cen X-3 using the energy-resolved dust-scattered X-ray halo. By comparing the observed X-ray halos in 200 eV
bands from 2-5 keV to the halo profiles predicted by the Weingartner & Draine interstellar grain model, we find that the vast majority (about 70%) of the dust along the line of sight to the system is located within about 300 pc of the Sun, although the halo measurements are insensitive to dust very close to the source. One of the Chandra observations occurred during an egress from eclipse as the pulsar emerged from behind the mass-donating primary. By comparing model halo light curves during this transition to the halo measurements, a source distance of 5.7 +/- 1.5 kpc (68% confidence level) is estimated, although we find this result depends on the distribution of dust on very small scales. Nevertheless, this value is marginally inconsistent with the commonly accepted distance to Cen X-3 of 8 kpc. We also find that the energy scaling of the scattering optical depth predicted by the Weingartner & Draine interstellar grain model does not accurately represent the results determined by X-ray halo studies of Cen X-3. Relative to the model, there appears to be less scattering at low energies or more scattering at high energies in Cen X-3.
The low-mass X-ray binary GS 1826-238 is presently unique for its consistently regular bursting behavior. In previous Rossi X-Ray Timing Explorer (RXTE) measurements between 1997 November and 2002 July, this source exhibited (nearly) limit-cycle burs
ts with recurrence times that decreased proportionately as the persistent flux increased. Here we report additional measurements of the burst recurrence time by RXTE, Chandra, and XMM-Newton, as well as observations of optical bursts. On a few occasions we measured burst recurrence times which deviated significantly from the earlier flux-recurrence time relationship, and most of these bursts occurred earlier than would be predicted based on the X-ray flux level. The epochs with early bursts were also accompanied by unusual broadband timing signatures, with the entire power spectrum shifting to higher frequencies. Concurrent XMM-Newton observations during one of these occasions, in 2003 April, indicate that an additional soft component may be present in the spectrum containing enough flux (30% of the total) to account for the burst recurrence time discrepancy. A self-consistent interpretation for the increase in soft flux and accompanying timing changes during 2003 April is that accretion disk extends down to smaller radial distances from the source than during the other observing epochs. The RXTE observations since 2003 April show that the spectral and timing properties have nearly returned to the previously established level.
