Investigation of multiwavelength correlations for the VHE gamma-ray blazar markarian 421 from 1995 to 2009

Abstract

The blazar Markarian 421 is an excellent laboratory for studying the physical processes within the jets of active galactic nuclei. In this thesis, a multiwavelength database has been compiled using data from the Whipple Observatory, the Milagro Observatory, the Fermi Gamma-ray Space Telescope, the RXTE satellite, the Very Long Baseline Array, the Metsahovi Radio Observatory and the University of Michigan Radio Astronomy Observatory. The Whipple data comprise 878.4 hours of high-quality y-ray observations taken with the 10m atmospheric Cherenkov telescope over the 14-year period 1995 - 2009; these observations constitute an unprecedented database for examining the long-term very-high-energy (VHE) variability of this source. The level of variability at VHE y-ray energies was examined on timescales from days to years. Over the 14-year period the annual mean rate is highly variable with values ranging from 0.18 ± 0.02 Crab units to 2.25 ± 0.31 Crab units; a test for constant rate gives a %2 value of 2669 for 13 degrees of freedom. Within each season the rate is highly variable from month to month with mean monthly rate from less than 0.1 Crab to more than 4 Crab; on the basis of the y? test, the most variable season was 2003-2004 while the least variable was 1996-1997. The mean nightly rate exceeds 2 Crab for -6% of the nights on which the source was observed. On a run-by-run basis (28 minutes), the rate is over 1 Crab for 30% of the runs and over 3 Crab for 5% of the runs. For the other energy bands, the level of variability is found to be lower. The various datasets were examined for correlations, both quasi-simultaneous and with time delays. In particular, an investigation was carried out to determine whether changes in the radio morphology of Markarian 421 can be associated with the y-ray emission. Over the 14-year timescale, a clear correlation was found between the X-ray flux and the TeV flux for zero delay; such correlations have been reported before but only on shorter timescales. No significant evidence was found for correlation on any timescale either between the TeV emission and the flux in any other energy band or between the TeV emission and the radio morphology. Searches for periodicity in the X-ray and y-ray signals showed no evidence for periodic behaviour on any timescale between 2 months and 7 years.