Investigating the connection between optical and very high energy gamma-ray emission of BL Lac objects

Abstract

In this thesis, I have performed systematic comparison of the occurrence of optical and very high-energy (VHE) gamma-ray high states in BL Lac objects (BL Lacs). BL Lacs are the most numerous extragalactic VHE gamma-ray sources. They emit electromagnetic radiation from radio to VHE gamma-ray band and this emission is extremely variable, timescales ranging from years to days to hours. Optical emission is synchrotron emission, and VHE gamma-rays are inverse Compton emission. The VHE emission mechanism is either synchrotron self-Compton process (SSC) or external Compton process (EC) but both predict a connection between optical and gamma-ray flares. Also, several mechanisms may affect the time-lags between optical and gamma-ray flares but it is impossible to predict them accurately. Optical to VHE gamma-ray connection should also be affected by the synchrotron peak frequency of the source since higher peak frequency in the X-ray band leads to the variability in the optical domain to be in some cases very small and the synchrotron flares have the best visibility in the X-ray range as is seen for blazars Mrk 421 and Mrk 501, for example. A correlation between the two bands would hint a common origin for both VHE and optical emission. To find the correlation I first try to characterize the optical variability timescales to understand in which time windows I should compare the optical and VHE gamma-ray fluxes. To determine the optical flares from the light curves, I used a statistical method called Bayesian blocks along with hill-climbing algorithm. After this I checked if these flares have a common duration. The flare durations were distributed rather uniformly, but the distribution peaked around 14 days, which we used as a minimum window when comparing the fluxes. In VHE gamma-ray band the data is very sparse for most of the BL Lac objects and can consist only of one to few observations. We defined uniform criteria on how to define high and low states in VHE gamma-ray band and applied it to all objects, also to those few that had several years of data. I analyzed how well the high states coincide within the two bands. I calculated the average optical fluxes for each light curve and within each VHE observation window. The result was that on average the optical flux within high VHE flux state windows is higher than the average optical flux. This result was investigated further, and by simulating light curves we could find out the chance probability of this occurring in a repetition of simulated data. The result was that the chance probability is very low for the high state sample. The findings of this thesis are in line with previous studies with much smaller samples. Even if the sample of this thesis is significantly larger, further observations in VHE gamma-ray band are needed to remove the observational biases from our sample. Furthermore, real correlation studies between optical and VHE gamma-ray band would be needed, as more VHE gamma-ray light curves will become available.