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Astron. Astrophys. 325, 19-26 (1997)

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5. Discussion and conclusions

In our sample of data, the gamma ray and VLBI radio flux were not measured simultaneously. This non-simultaneity may introduce a further random spread in the gamma ray - radio correlation and in the x -distribution. It is known that some of the gamma ray blazars are highly time variable. The gamma ary fluxes used here are the averaged over a period of time of two weeks, albeit the sampling of the high and low state may not be even. The VLBI radio fluxes are the instant in the sense that the observation time for a source is about one hour but the time variability of large amplitude (a few ten percent) in radio is at least days. Therefore, simultaneous observations of VLBI with gamma ray telescopes are needed to yield "average" radio fluxes for the beaming statistics.

Furthermore, such simultaneous observations are also useful in studying the correlation between radio and gamma ray emissions. Some blazars are found to have intraday variability throughout the entire electromagnetic radiation spectrum (see review by Wagner & Witzel 1995). It will be interesting and important to look at the time profiles of radio and gamma ray emission simultaneously for a specific blazar. If a good correlation is found, there must be an underlying physical link between the two emission mechanisms. If there is a time lag, there should be also a physical link which might be more indirect. Otherwise, the apparent flux correlation of moderate significance seen in Sect. 3 has to entirely result from the co-axial relativistic Doppler beaming.

In conclusion, we propose a statistical test for the gamma ray emission mechanisms in blazars in this work. It is based on the difference in the relativistic Doppler effects of gamma ray emission models, specifically inverse Compton scattering on internal photons and on external photons. The radio flux from blazar is believed to be Doppler beamed too and thus utilized as a reference to differentiate the beaming effect. The K-corrected flux ratio of gamma ray to radio x is taken to be the random variable and its distribution is derived as a power law with the index being model-dependent. A test with the currently available data gives a result of only [FORMULA] significance and a firm discrimination between the two models cannot be made. The next generation of high energy gamma ray telescopes, such as GLAST, are needed for producing a larger sample of data to give a more significant test result.

A justification of the method has been made and it is proved to be feasible for the purpose of testing the gamma ray emission mechanisms. A correlation study of gamma ray and radio data indicates that high resolution VLBI data are more suitable than low resolution data. Our Monte-Carlo simulations show that the observed correlation between gamma ray and radio flux could be purely due to the co-axial beaming effect though a physical link is a plus. The random spreads in the parameters of the Doppler factor are examined and their effect on the x -distribution is verified to be minor. The assumption of a fixed ratio of gamma ray to radio intrinsic emission is validated and the random spread associated with the ratio is proved to be small. Simultaneous observation of VLBI with gamma ray telescopes are needed to provide a better quality sample of data for the x -distribution and for exploring a physical link between the gamma ray and radio emission in blazars.

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© European Southern Observatory (ESO) 1997

Online publication: May 5, 1998