Observation of the Cygnus OB2 Association with Suzaku and Long-Term Light Curves

Abstract

We obtained a Suzaku 41-ks observation of the four brightest early type stars in the Cygnus OB2 association (Cyg OB2 Nos. 5, 8a, 9, and 12). The spectra of two of the stars, Nos. 9 and 12, could be fitted by a two-temperature thin-thermal collisional-ionization equilibrium model, while those of Nos. 5 and 8a required a three-temperature model. Star No. 8a showed a time variation both above 3 keV and below 1.5 keV. The time scale of the variation of the soft X-rays is $\sim$ 8 $\times$ 10 $^{4}$ s, while that above 3 keV is $\sim$ 2 $\times$ 10 $^{4}$ s. We found that the variation above 3 keV can be explained by temperature changes in the hottest plasma of the three-temperature model. This result restricts the size and density of the hottest component of the three-temperature model. We also produced long-term light curves of the four stars for over 30 yr. We confirmed the orbital modulation in the intensity of No. 8a, and discovered a possible orbital modulation of 2.35 yr in the light curve of No. 9.