AAS 205th Meeting, 9-13 January 2005
Session 102 Neutron Stars
Poster, Wednesday, January 12, 2005, 9:20am-6:30pm, Exhibit Hall

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[102.02] Chandra and RXTE Studies of the Burster GS 1826--238

T. W. J. Thompson, R. E. Rothschild, J. A. Tomsick (Center for Astrophysics and Space Science, University of California, San Diego, La Jolla, CA 92093), H. L. Marshall (Center for Space Research, 77 Vassar Street, Massachusetts Institute of Technology, Cambridge, MA 02139)

Using simultaneous observations from the Chandra X-Ray Observatory and the Rossi X-Ray Timing Explorer, we investigate the low-mass X-ray binary (LMXB) and "clocked burster" GS 1826--238 with the goals of studying its broadband spectral properties and using the burst-induced temporal variability in the X-ray halo to determine the distance to the source. In a 68 ks Chandra exposure, we detect 6 type I X-ray bursts, and during 30 ks of observation with RXTE, we see 4 bursts. We present 0.6--200 keV energy spectra of both the persistent and burst emission. The persistent spectrum is fit with a black body plus comptonization model with hydrogen column density NH \approx 4.5 x 1020 cm-2. The column density that we obtain is substantially lower than what has been reported by previous authors. We attempt to observe the temporal variability of the source in the X-ray halo, however, the scattering optical depth of the source is too small to unambiguously detect halo flux over the point spread function (PSF). We derive an upper limit on the fractional halo intensity (Ihalo=1 - e-\tau_{sca}) of about 2.5% which corresponds to a scattering optical depth \tausca ~ 0.03. Thus, the geometrical distance determination technique could not be successfully implemented. Nonetheless, we have developed a method which can be used for variable sources with more prominent halos.

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