Research Projects

In the 1970s, jets emanating from the central region of galaxies were proposed as the mechanism by which luminous radio lobes of active galaxies are powered (Blandford & Rees 1974, MNRAS, 169, 395). Since then there has been considerable progress in our understanding of the jets (for recent reviews, see Blandford et al. 2019, ARA&A, 57, 467; Hovatta & Lindfors 2019, New Astron. Rev., 87, 101541). It has been known that the jets contain magnetized plasma, originate near the central supermassive black hole, and propagate at relativistic speeds. In blazars, jet emission is beamed in the observer's frame and hence its apparent brightness is much higher than the other parts of the active galactic nuclei. Hence, blazars are ideal for studying jets. Blazar jets tend to show broadband emission which implies that it contains very high energy (~TeV) particles while fast variability indicates that the emitting particles are accelerated to very high energy at as short as ~hours timescale. Hence, jets are unique laboratories to probe those exotic phyical processes. In addition, jets interact with its surroundings within and outside their host galaxy. It can push the galactic gas away or heat it, causing a drop in star formation and consequently affect galaxy evolution (McNamara & Nulsen 2007, ARA&A, 45, 117). Therefore, knowing the physical and geometric parameters and emission processes of jets is a crucial aspect of modern astronomical research. Our current knowledge of relativistic jets in blazars have been accumulated by investigation of their spectral energy distribution and multi-wavelength (MW) flux variability as well as radio and mm-wave interferometric imaging, which can provide pc-scale resolution. Strongly polarized emission is a characteristic property of blazar jets. Therefore, analysis and interpretation of polarization in blazar emission is an effective tool of probing the jets. However, until recently, polarization observations were limited to optical and lower-frequency bands. A new era of blazar studies has begun with the launch of Imaging X-ray Polarimetry Explorer (IXPE), which is being used to study the X-ray polarization in tens of blazars since 2022. It has provided crucial information about the highest energy particles, which generate the X-rays. In this proposal we plan to develop and utilize a numerical model to calculate the polarization fraction (P) and EVPA from an existing self-developed theoretical multi-zone model of time-variable nonthermal emission from blazar jets. The latter has been previously used by our group to interpret observed flux and spectral variability (Kundu et al. 2022, MNRAS, 510, 3688; Barat et al. 2022, MNRAS, 515, 1655; Das & Chatterjee 2025, MNRAS, 539, 3582; Kundu et al. 2025, MNRAS, 540, 1379}. Comparison of the results generated from the model proposed to be developed here with the rapidly growing database of P and EVPA, and their variability at X-ray and other wave bands will be utilized to reveal certain details about the magnetic field structure, emission processes, and acceleration mechanisms in the jets of blazars, which could not be probed with the existing imaging, spectroscopy and variability studies. In addition to addressing several long-standing questions in blazar physics, IXPE and supporting MW observations have given rise to new questions: Direction of EVPA, in some cases, is not as expected in the shock acceleration scenario; variability of flux and P are not always correlated; observed connection between large swings of EVPA and GeV outbursts has not been explained. It has been suggested that the higher value of P in the X-ray band compared to that at lower energies may result from the curved nature of the energy distribution of the emitting particles without requiring energy stratification of the jet but that requires further confirmation from numerical studies. We intend to address the above issues using the work proposed here.