Soft X-ray emission from the Low Mass X-ray Binary Cyg X-2
Proceedings of VII Microquasar Workshop: Microquasars and Beyond — PoS(MQW7)
Suzaku observed results of Cyg X-2 are reported. Cyg X-2 is a Low-Mass X-ray Binary (LMXB) with a weakly-magnetized neutron star (NS) and its luminosity is always close to the Eddington luminosity. Suzaku observed the source in the normal branch (NB) and flaring branch (FB). Seven spectra were extrated according to different positions in the color-color diagram. The continuum of these spectra is moderately well described with the Eastern model, a conbination of soft disk blackbody and hard
... body emission. However, all the spectra have broad excess around 1 keV. This excess cannot be explained with the contribution of dust scattering and is thought to be related with the source itself, since comparison of spectra come from including and excluding the central core of the X-ray image does not show any difference in their spectral shape. The broad excess can be represented by either a broad Gaussian with the center energy of ∼ 0.7 keV or a blackbody model with the temperature of ∼ 0.2 keV. When the broad Gaussian model is applied to all the 7 spectra, the center energy and width are obtained as almost constant at 0.7 keV and 340 eV, respectively, and the equivalent width is 170 -300 eV which increases continuously from NB to FB. The equivalent width obtained by Suzaku is much larger than the total of narrow lines detected by previous grating observations. In additon, if the line is assumed to be broad, the estimated line emitting region becomes too close to the NS to prevent ions from the fullyphotoionized plasma. These results indicate that the 1 keV excess cannot be explained by the line components physically. Therefore, we suppose that this excess is represented by the 0.2 keV blackbody component. Such a temperature is lower than neither that of the NS surface (∼ 2 keV) nor the innermost temperature of the accretion disk (∼ 1 keV), the 0.2 keV emission is thought to be from the outer part of the disk. This interpretation is consistent with the picture of a slim disk, where the relative flux from the outer disk (the lower temperature emission) to that from the inner part increases compared with the standard disk.