Centrum Astronomii
Uniwersytet Mikołaja Kopernika

Probing Jet Structures using Optical Variability in Blazars

dr Gopal Bhatta (OA UJ)

Active Galactic Nuclei (AGN) are the most luminous and long-lived objects in the universe. They are powered by the accretion of matter onto supermassive black holes. The sources being visible from cosmological distances and therefore key to the exploration of the universe at large, the understanding of AGNs lies at the forefront of extragalactic research as well as modern cosmology. Blazars, a subset of radio-loud AGNs with their relativistic jets pointed towards us, are known to exhibit multi-frequency flux variability in diverse timescales ranging from a few hours to a few years. The variability is believed to originate from the regions close to the central engine; more likely due to modulations in the accretion disk which propagate along the jets. Although several competing theoretical models are proposed, the details of the conditions leading to the launch of the relativistic jets in AGNs remain elusive. In such context, variability study proves to be an excellent tool to probe the region near the central region and the jet structures in AGNs, and thereby offering insight into the innermost regions. Here I present the results of our studies of the jet structures of the two widely studied BL Lac objects, 0716+714 and OJ 287, using long- and short-term multi-band optical observations: I) To characterize intraday variability in blazars, a novel approach of the multi-site observation campaigns through a large collaborative effort was adopted. We organized two photo-polarimetric campaigns to intensely monitor the blazar 0716+714 through whole earth blazar telescope (WEBT) consortium. The campaigns resulted in unprecedentedly dense observations allowing us to perform robust analyses. We found many interesting results including high amplitude variability, bluer-when-brighter trend, well correlated multi-band emission, characterization of variability by red-noise processes, highly polarized (~60%) microflares, looping behavior in Stokes' parameters Q-U plane and variable correlation between flux and polarization degree depending on the orientation of polarization angle in reference to the jet-base. In addition, our modeling of the light curves based on the the shocks propagating along the turbulent jets scenario constrained the probable largest and smallest size of the compact emission sites in the relativistic outflows. II) In blazar OJ 287, we discovered possible quasi-periodic signal around the periods of ∼ 400 and ∼ 800 days, with a significance (over the underlying colored noise) of ≥ 99%. However, in the analysis of light curve from the year 2015, we did not find any clear indication of any quasi-periodic oscillations (QPO) in shorter timescales.

20.03.2017 - 11:15
Sala seminaryjna KRA