X-RAY AND RADIO OBSERVATIONS OF THE CYGNUS OB2 ASSOCIATION

Several OB stars in the Cygnus OB2 association are among the strongest stellar x-ray and radio sources in the Galaxy. The radio emission is particularly unusual, displaying a high level of variability and nonth ermal behavior. We obtained two ROSAT PSPC observations, a ROSAT HRI o bservation, and three VLA observations of the association during a 2 y r time span. Our study will focus on four stars, Cyg OB2 No. 5, No. 8A , No. 9, and No. 12. Three of the four (Cyg No. 5, No. 9, and No. 12) were relatively constant in their x-ray emission over the 2 yr time fr ame. The fourth, Cyg OB2 No. 8A, increased in intensity by similar to 34%. No short-term (hourly) variability was detected. The observed x-r ay characteristics (e.g., luminosity, temperature) are found to be con sistent with the x-ray properties of other OB stars. The exception is Cyg OB2 No. 12, whose x-ray characteristics are found to be inconsiste nt with its spectral classification. Detailed spectral analyses of the PSPC data are presented for two absorption models: (1) ISM (cold abso rber) and (2) Wind + ISM (warm absorber). The spectral fits suggest th at the x-ray sources are located within the stellar wind, and estimate s of the x-ray locations are presented. Adopting the radio-derived mas s-loss rates, these x-ray locations are found to be consistent with th e shock scenario proposed for OB stars. As expected, the radio emissio n has continued to be highly variable. Nonthermal characteristics are observed in Cyg OB2 No. 8A and No. 12. One of the most unusual nonther mal radio sources, Cyg OB2 No. 9, was found to be thermal in one of ou r observations. An observation of Cyg OB2 No. 5 also displayed a therm al radio spectrum. A comparison of the observed and intrinsic x-ray fl uxes with the observed radio fluxes suggests that these quantities are anticorrelated; the strongest x-ray source is the weakest radio sourc e. This is contrary to normal expectations for a wind-generated model of x-ray and radio emission. We investigate the long-term temporal beh avior of both the x-ray and radio emission by comparing our newly acqu ired data with the previous x-ray (IPC) and radio data over the past 1 5 yr. Except for three events observed in Cyg OB2 No. 5, which display ed significant increases in its x-ray emission, the x-ray emission has remained relatively constant with a variability level less than 20% o ver this time span, whereas the radio emission has stayed highly varia ble with various levels of nonthermal behavior. We present a model to investigate the case in which the x-ray and radio emission are control led by stellar wind properties and find that the predicted variability should be comparable in both emission processes. This is not observed . It is very intriguing that every time we observe the radio emission, it is different, whereas the x-ray emission always appears to be cons tant. If the stellar wind is as variable as suggested by the radio dat a, we believe it is highly implausible that we just happened to miss a ll periods of x-ray variability. We also investigate the implications of the observed nonthermal radio spectrum of Cyg OB2 No. 8A. Using the synchrotron emission model of White, we find that this nonthermal rad io spectrum predicts a mass-loss rate almost 2 orders of magnitude les s than that expected for a thermal radio spectrum. This lower mass-los s rate is consistent with an x-ray source located at the base of the s tellar wind, contrary to the basic shock scenario. Since these stars s how evidence of changing from thermal to nonthermal radio characterist ics, it is difficult to understand how such a large change in mass los s did not produce a significant change in the observed x-rays.