Swift XRT Observations of Cygnus X-3

ATel #1655; S. Moore, J. M. Miller (University of Michigan)
on 11 Aug 2008; 16:13 UT
Credential Certification: Jon Miller (jonmm@umich.edu)

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We report on SWIFT/XRT observations of the well-known X-ray binary and jet source Cygnus X-3. We analyzed observations that were made on 2004 December 28 (3 observations), 2005 June 10, 2006 May 2, May 10, May 13, May 17, July 26, July 27, 2007 December 2, 2008 April 18, and April 23-30.

Using XSPEC v11 and fitting the spectra with a power-law in the 2-10 keV band, some spectra showed evidence for three Fe emission lines at 6.4, 6.7, and 6.97 keV, consistent with low-ionization Fe, and He-like and H-like Fe. We therefore attempted to analyze all 20 spectra with a model consisting of a power-law plus 3 Gaussian lines. In general, these lines each have a width consistent with sigma=0.1 keV, and this value was fixed in all fits.

We obtained power-law indices ranging from 0.8 +/- 0.1 to 2.8 +/- 0.2 (90 percent confidence errors). The total flux values ranged from 0.78 +/- 0.3 E-9 cgs to 20.2 +/- 0.3 E-9 cgs.

We examined the possibility of variations in the continuum and line properties with orbital phase in Cygnus X-3. The total flux is not correlated with the phase of Cygnus X-3. The flux of each Fe line (6.4, 6.7, and 6.97 keV) also appears to be uncorrelated with orbital phase.

Finally, we examined the possibility that the iron line flux might vary with the 7-10 keV ionizing flux, as might be expected in a simple photoionization scenario. Despite variations in continuum flux by a factor of three, there are no obvious correlations between line flux and continuum flux. The lack of any strong correlations suggests that geometrical changes accompany changes in the power-law index and flux.

Figures supporting this ATEL can be found here.