Speaker: Manisha Shrestha
A new polarimetric approach to GRB jets and kilonovae emission in the era of gravitational waves
Over the past decade, there have been large strides forward in multi-messenger studies of transient phenomena resulting from the merger of compact objects. The first detection of the electromagnetic counterpart of a gravitational wave source as a short gamma-ray burst (GRB) with kilonova emission has shown us that at least a subset of neutron star mergers produces short GRBs and kilonova emission. GRBs are extremely energetic events of cosmological origin. They are broadly divided into two types based on the gamma-ray duration; long GRBs (>2seconds), and short GRBs (<2 seconds). Long GRBs are thought to be originated from explosions of very massive stars and short GRBs are thought to be produced by the merger of compact binaries. Kilonova emission is produced by the radioactive decay of the unstable heavy elements produced in these merger events. Since these events happen at cosmological distances, they can not be resolved using traditional astronomical techniques. However, polarimetric observations of GRBs and kilonovae emission can allow us to start the exploration of the structure and magnetic field configurations of GRB relativistic jets and as well as density distribution of kilonova emission. Liverpool Telescope (LT) has utilized rapidly rotating polaroids to create a series of polarimeters that have successfully detected early-time optical polarimetry of various GRBs. I will present photometric and polarimetric results of various GRBs observed by the RINGO3 polarimeter onboard the LT. I will discuss how polarimetric detection for a long GRB 191016A along with photometric data constraint the energy injection mechanism for the central engine. In addition, I will present how polarization depends on various properties of GRBs such as photometric decay index, isotropic energy of GRBs, redshift etc. Finally, I will present simulation techniques utilized to predict the light curve and spectropolarimetry of kilonovae emission and preliminary results of these simulations.
Speaker: Maria Werhahn
Simulating galaxy formation with cosmic rays: the multi-frequency view
We aim at understanding the underlying physics of the non-thermal emission processes arising from cosmic rays (CRs) in star-forming galaxies, that lead to the emergence of the observed FIR-radio correlation, as well as the FIR-gamma-ray relation. Assuming calorimetry, i.e. that CRs lose most of their energy due to emission processes, these relations emerge naturally. However, this explanation implies steeper radio spectra in comparison to observations and additionally, in the high-density environments of starburst galaxies, this assumption might not be fulfilled. In order to dissect these processes, we perform high-resolution magneto-hydrodynamic simulations of galaxies using the moving-mesh code AREPO with self-consistent CR physics. We calculate steady-state spectra of CRs including all relevant cooling and escape losses. From these, we obtain multi-frequency spectra due to all non-thermal emission processes of our simulated galaxies (dwarfs to Milk-Way sized). These range from the radio, dominated by synchrotron emission from primary electrons, up to the TeV energy regime, that comprises leptonic inverse Compton and bremsstrahlung emission as well as gamma-ray emission from neutral pion decay. Within our model, we match the observed relations and gamma-ray spectra of nearby galaxies. Furthermore, we uncover the underlying processes that enable the FIR-radio correlation to be maintained even in starburst galaxies and find that thermal free-free-emission naturally explains the observed radio spectra in star-forming galaxies like M82 and NGC 253, thus solving the riddle of flat radio spectra and a tight FIR-radio correlation.
