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ATEL # 1656; K. J. Perry, J. M. Miller (University of Michigan)
on 11 Aug 2008; 18:11 UT
Password Certification: Jon Miller (firstname.lastname@example.org)
Subjects: X-ray, Binaries, Neutron Stars
We report on a series of observations of the neutron star X-ray binary Circinus X-1, taken by Swift between 2 Jan 2004 until 1 August 2007. The observations were fit with single-component power-law and blackbody models. In both sets of fits, the column density of (partly interstellar) low-energy absorption was allowed to vary because Circinus X-1 is known to have a variable wind. The distance was assumed to be 8 kpc (Lei et al. 2007).
For the powerlaw model, the absorption was found to range from 2 E+22 to 4 E+22, the power-law index was found to range from 1.1 to 1.9, and the luminosity was found to range from 2 E+35 cgs to 1.0 E+37 cgs. Fractional errors on individual parameter values are generally about 30-50% owing to the count-limited nature of the spectra.
Under the blackbody model, the absorption was found to range from 0 to 1.8 E+22, the blackbody temperature was found vary between 1.2 to 2.8 keV, and the luminosity was found to range from 1.3 E+35 to 6.6 E+36 cgs. Here again, fractional errors were generally at the 30-50% level.
The power-law index showed no systematic variation with luminosity. Similarly, the blackbody temperature showed no systematic variation with luminosity. This may be due to the rather limited sensitivity of the spectra, or arise because the variations in column density are spurious and mask spectral variability. If the luminosity dependence of the spectral parameters is real, it may suggest that the flux observed from Circinus X-1 is largely scattered emission rather than direct emission, at least in low flux states.
Figures supporting this ATEL can be found here.