Recent Swift Spectroscopy of Cygnus X-3
ATel #13342; Y. Zhang, B. Tetarenko, J. M. Miller (Univ. of Michigan)
on 10 Dec 2019; 15:10 UT
Credential Certification: Jon Miller (jonmm@umich.edu)
Subjects: X-ray, Binary, Black Hole, Transient
Cygnus X-3 is a high mass X-ray binary with a compact object and a Wolf-Rayet companion. It has often been monitored using the Neil Gehrels Swift Observatory, and a new observation was obtained on 2019-11-15. We reduced the XRT data using the standard HEASOFT tools. The filtered "windowed timing" mode data have a count rate of 23.66 c/s. After binning to require a minimum of 15 counts per bin, we fit the data over the 1-10 keV band using XSPEC. Fitting with a model consisting of tbabs*(gauss+gauss+gauss+diskbb), with the Gaussian lines fixed at neutral, He-like, and H-like values (6.40 keV, 6.70 keV, and 6.97 keV), a chi-squared of 1.28 was found. The blackbody was measured to be extremely hot (kT = 3.7 keV), and the emitting area to be extremely small; this indicates that the component was trying to mimic a power-law or Comptonization component. There is also a small offset in each of the lines relative to the model; this likely indicates an instrumental shift. Finally, there are negative residuals in the 2-5 keV band, potentially indicating partial covering absorption.
We next modeled the spectrum with tbabs(zmshift*(gaussian + gaussian + gaussian) + tbpcf*powerlaw). Here, the "zmshift" component simply accounts for a shift in the energy scale of the instrument; with this change, the iron lines are fit quite well. This model has a reduced chi-squared of 1.26, and it is more physically plausible and self-consistent than the previous model. The observed flux is measured to be 2.8 E-9 erg/cm^2/s; after removing the internal and line-of-sight obscuration, an unabsorbed flux of 8.4 E-9 erg/cm^2/s is implied, translating to a luminosity of 5.6 E+37 erg/s (for a distance of 7.4 kpc, see McCollough et al. 2016).
This model may be a plausible means of characterizing many of the archival Swift monitoring observations of Cygnus X-3, and examining the flux of the Fe emission lines versus several parameters of interest. We will look to systematically apply it in a future publication.
McCollough, M., et al., 2016, ApJ, 830, L36
