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Paper: The Spectrum and Accurate Location of GRS 1758-258
Volume: 262, The High Energy Universe at Sharp Focus: Chandra Science
Page: 241
Authors: Heindl, W. A.; Smith, D. M.
Abstract: GRS 1758-258 and its sister source 1E 1740.7-2942 were the first objects dubbed "micro-quasars". Their hard X-ray spectra are typical of Galactic black hole candidates, and they are apparently associated with time variable cores of double-lobed radio sources, reminiscent of extra-galactic radio sources. This morphology, seen on a parsec scale within the Milky Way, earned them their nickname. We observed GRS 1758-258 four times with Chandra. Two HRC-I observations were made in 2000 September-October spanning an intermediate-to-hard spectral transition (identified with the RXTE). Another HRC-I and an ACIS/HETG observation were made in 2001 March following a hard-to-softs transition to a very low flux state. We report results from these observations, including the accurate source position, a search for sub-arcsecond X-ray jets, and the X-ray spectrum following the low-state transition. Preliminary work shows that the position is consistent at the sub-arcsecond level with the proposed radio counterpart, and that despite the presence of arcminute scale radio lobes no X-ray jets are observed. The spectrum is consistent with a multi-color disk-blackbody (with inner temperature about 0.5 keV) plus a power-law. No obvious emission lines (which might indicate relativistic jets a la SS433) are detected. In the framework of a model we developed to explain RXTE observations, we interpret the spectrum as the direct disk flux, observed after the near-complete loss of a Comptonizing corona in the low-state transition.
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