Time correlation between the radio and gamma-ray activity in blazars and the production site of the gamma-ray emission

Abstract

In order to determine the location of the gamma-ray emission site in blazars,\nwe investigate the time-domain relationship between their radio and gamma-ray\nemission. Light-curves for the brightest detected blazars from the first 3\nyears of the mission of the Fermi Gamma-ray Space Telescope are\ncross-correlated with 4 years of 15GHz observations from the OVRO 40-m\nmonitoring program. The large sample and long light-curve duration enable us to\ncarry out a statistically robust analysis of the significance of the\ncross-correlations, which is investigated using Monte Carlo simulations\nincluding the uneven sampling and noise properties of the light-curves.\nModeling the light-curves as red noise processes with power-law power spectral\ndensities, we find that only one of 41 sources with high quality data in both\nbands shows correlations with significance larger than 3-sigma (AO 0235+164),\nwith only two more larger than even 2.25-sigma (PKS 1502+106 and B2 2308+34).\nAdditionally, we find correlated variability in Mrk 421 when including a strong\nflare that occurred in July-September 2012. These results demonstrate very\nclearly the difficulty of measuring statistically robust multiwavelength\ncorrelations and the care needed when comparing light-curves even when many\nyears of data are used. This should be a caution. In all four sources the radio\nvariations lag the gamma-ray variations, suggesting that the gamma-ray emission\noriginates upstream of the radio emission. Continuous simultaneous monitoring\nover a longer time period is required to obtain high significance levels in\ncross-correlations between gamma-ray and radio variability in most blazars.\n