Chandra and RXTE Spectra of the Burster GS 1826-238

Using simultaneous observations from the Chandra X-Ray Observatory and the Rossi X-Ray Timing Explorer (RXTE), we investigated the low-mass X-ray binary (LMXB) and "clocked burster" GS 1826-238 with the goal of studying its spectral and timing properties. The uninterrupted Chandra observation captured six bursts (RXTE saw three of the six), yielding a recurrence time of 3.54 ± 0.03 hr. Using the proportional counter array on board RXTE, we made a probable detection of 611 Hz burst oscillations in the decaying phases of the bursts with an average rms signal amplitude of 4.8%. The integrated persistent emission spectrum can be described as the dual Comptonization of ~0.3 keV soft photons by a plasma with kTe ~ 20 keV and τ ~ 2.6 (interpreted as emission from the accretion disk corona), plus the Comptonization of hotter ~0.8 keV seed photons by a ~6.8 keV plasma (interpreted as emission from or near the boundary layer). We discovered evidence for a neutral Fe Kα emission line, and we found interstellar Fe L II and Fe L III absorption features. The burst spectrum can be fit by fixing the disk Comptonization parameters to the persistent emission best-fit values and adding a blackbody. The temperature of the boundary layer seed photons was tied to the blackbody temperature. The blackbody/seed photon temperature at the peak of the burst is ~1.8 keV and returns to ~0.8 keV over 200 s. The blackbody radius is consistent with Rbb ≈ 10.3-11.7 km, assuming a distance of 6 kpc, though this value cannot be interpreted as the physical size of the neutron star due to partial covering of the stellar surface by the accretion disk. By accounting for the fraction of the surface that is obscured by the disk as a function of binary inclination, we determined the source distance must actually be near 5 kpc in order for the stellar radius to lie within the commonly assumed range of 10-12 km. The order of magnitude increase in flux at burst peak is seen to cause Compton cooling of the electron plasma surrounding the disk, as the plasma temperature decreases to ~3 keV at burst onset, and then slowly returns to the persistent emission value after about 150 s.