Saugata Barat

We know that several galaxies possess a very bright core which can outshine the entire galaxy. Such systems are called Active Galactic Nuclei (AGNs) and are formed as a result of matter accretion onto the central supermassive black hole. A fraction of such AGNs are known to possess superluminal jet like structures which have been directly observed using radio interferometry techniques. When such relativistic jets are pointed towards our line of sight, most of the emission from these sources is dominated by the jet emission because of Doppler boosting effects. Blazars exhibit a broad band spectra from radio all the way upto TeV energies. Blazars are characterized by a double humped Spectral Energy Distribution(SED) with the lower energy hump coming from synchrotron radiation of electrons/positrons in the presence of strong magnetic fields. The higher energy bump is associated with inverse Compton scattering of lower energy seed photons which could be produced internally within the jet from synchrotron mechanism and the comptonization process is known as synchrotron self-Compton (SSC), or they could be incident on the jet from external sources like fast moving broad line region (BLR) clouds and dusty molecular torus surrounding the AGN. Blazars are also characterized by their multiwavelength correlated variability. Blazar light curves have shown variability on timescales from hours to months. Different variability timescales relate to different physical phenomenon. The blazar outbursts have also been shown to be correlated to the emission of radio 'knots' from the AGN core from VLBI observations.

The aim of our work is to statistically study correlated multiwavelength flares or 'flare pairs' and constrain blazar jet properties from such observations. For this purpose we use a sample of 15 bright blazars observed by FERMI telescopes in the GeV band with simulataneous optical and near IR observations from the SMARTS programme. From the well sampled GeV abd R Band light curves comprising of almost 11 years of data, we identify flares of the order of months and obtain a statistical sample of rise and decay timescales and amplitudes of the flare pairs. The distribution of the ratio of energy dissipated in the optical and GeV flares shown in Figure 2b of my poster. It is evident that the GeV flares are much more energetic as compared to the optical flares. Figure 3a and 3b show the distribution of the asymmetry parameter for rise and fall timescales of the flare pairs. The asymmetry parameter has been defined in that section of the poster.



We have develeoped a 1D jet emission model. We use the Internal Shock Model to energize the electrons. The idea is that standing shock fronts are produced within the jets due to collision of plasma blobs moving down the jet and as quiscent electron populations cross the shock front, they are energized to relativistic energies. These electrons cool by emitting synchrotron and inverse compton radiation. In our model we include both SSC and EC with external seed photns being taken into account from BLR and torus regions. the BLR is assumed to be located around 1pc and the torus is assumed to be located around 10 pc. The seed photons from the BLR are in UV and fro the torus are in IR. In Figure 4a we show the distribution of optical/GeV energy ratio for all the blazars in our sample. We select the blazar PKS 2326-502. The ratio for this blazar is around 0.01 and from its observed SED we decide on a energy scale for the shock paraameterized into gamma_max in our simulation. Figure 4b shows the ratio of Optical/GeV flares as a function of distance from the central engine. We can conclude that the GeV emitting region must be around 10pc ie far way from the BLR region and around the molecular torus. The physical explanation for this is that the GeV photons for such low synchrotron peak blazars originates from EC from BLR seed photons, hence very close to the BLR the GeV photons are strongly produced, as we go further away the energy ratios match with our observations. we can make similar predictions about the other blazars too. Locating the GeV emitting region is a long debated issue in the blazar community and in previous works it has been discussed that the emission region must lie in the parsec scale. These studies were based on hour scale flares. We have independently studied month scale flares on a large sample of blazars and come to a similar conclusion.