OVRO 40 Meter Blazar Monitoring Program: Location of the Gamma-Ray Emission Region in Blazars by the Study of Correlated Variability at Radio and Gamma-Rays

Walter Max-Moerbeck, J L Richards (Purdue U)

V Pavlidou (U of Crete)

T J Pearson, A C S Readhead (Caltech)

Fermi LAT Collaboration (NRAO)

OVRO 40 Meter Blazar Monitoring Program: Location of the Gamma-Ray Emission Region in Blazars by the Study of Correlated Variability at Radio and Gamma-Rays

Blazars are powerful, variable emitters from radio to gamma-ray wavelengths. Even though the general picture of synchrotron emission at low energies and inverse Compton at the high energy end is well established, many important aspects of these remarkable objects are still not well understood. For example, even the location of the gamma-ray emission region is still not clearly established, with some theories locating it close to the black hole/accretion disk while others place it at parsec scales in the radio jet. Since mid-2007 we have carried out a large scale monitoring program at 15 GHz using the OVRO 40 m telescope. We are currently observing about 1700 blazars twice per week. The sample includes all the Fermi-LAT detected blazars north of declination -20°. Here, we study the existence of correlated variability between these two bands for 86 sources bright enough to be detected weekly by Fermi-LAT. The existence of correlated variability can be interpretedvas an indication of a related spatial locations for the radio and gamma-ray emission, making the evaluation of its statistical significance a key goal of our program. A study of the statistical significance of these cross-correlations is presented along with a discussion of the Monte Carlo simulations used to evaluate them. These time lag measurements along with constraints from very long baseline interferometry observations allow us to estimate the distance from the black hole at which gamma-rays are produced. The development of time series techniques as the ones we present there will be crucial for future multi-wavelength monitoring programs combining LSST and other facilities.

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