X‐Ray and Radio Observations of the Cygnus OB2 Association

Abstract

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 nonthermal behavior. We obtained two ROSAT PSPC observations, a ROSAT HRI observation, and three VLA observations of the association during a 2 yr 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 frame. The fourth, Cyg OB2 No. 8A, increased in intensity by ~34%. No short-term (hourly) variability was detected. The observed X-ray characteristics (e.g., luminosity, temperature) are found to be consistent with the X-ray properties of other OB stars. The exception is Cyg OB2 No. 12, whose X-ray characteristics are found to be inconsistent with its spectral classification. Detailed spectral analyses of the PSPC data are presented for two absorption models: (1) ISM (cold absorber) and (2) Wind + ISM (warm absorber). The spectral fits suggest that the X-ray sources are located within the stellar wind, and estimates of the X-ray locations are presented. Adopting the radio-derived mass-loss rates, these X-ray locations are found to be consistent with the shock scenario proposed for OB stars. As expected, the radio emission has continued to be highly variable. Nonthermal characteristics are observed in Cyg OB2 No. 8A and No. 12. One of the most unusual nonthermal radio sources, Cyg OB2 No. 9, was found to be thermal in one of our observations. An observation of Cyg OB2 No. 5 also displayed a thermal radio spectrum. A comparison of the observed and intrinsic X-ray fluxes with the observed radio fluxes suggests that these quantities are anticorrelated; the strongest X-ray source is the weakest radio source. This is contrary to normal expectations for a wind-generated model of X-ray and radio emission. We investigate the long-term temporal behavior of both the X-ray and radio emission by comparing our newly acquired data with the previous X-ray (IPC) and radio data over the past 15 yr. Except for three events observed in Cyg OB2 No. 5, which displayed significant increases in its X-ray emission, the X-ray emission has remained relatively constant with a variability level less than 20% over this time span, whereas the radio emission has stayed highly variable with various levels of nonthermal behavior. We present a model to investigate the case in which the X-ray and radio emission are controlled 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 constant. If the stellar wind is as variable as suggested by the radio data, we believe it is highly implausible that we just happened to miss all 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 radio spectrum predicts a mass-loss rate almost 2 orders of magnitude less than that expected for a thermal radio spectrum. This lower mass-loss rate is consistent with an X-ray source located at the base of the stellar wind, contrary to the basic shock scenario. Since these stars show evidence of changing from thermal to nonthermal radio characteristics, it is difficult to understand how such a large change in mass loss did not produce a significant change in the observed X-rays.