Exploring and Investigating the Physics of Time-Domain Multi-Messenger Astronomy across the Electromagnetic Spectrum & Beyond (FINESST 2019 A Hajela)

The joint detection of gravitational waves (GWs) and light from the neutron star merger GW170817 signaled the birth of a new field: multi-messenger astronomy with gravitational waves. GW170817 confirmed the long-sought connection between short gamma-ray bursts and binary neutron star (BNS) mergers. GW170817 was detected across the electromagnetic spectrum, from the gamma-rays to the radio band. As of ~600 days since the birth of this new field of investigation, the most pressing still open questions that motivates this proposal are the following: (i) Are all BNS capable of launching ultra-relativistic jets like GW170817? (ii) What is the nature of the merger remnant? The interpretation of the gradual rise in the light curve of the afterglow for ~160 days post-merger and a broadband afterglow spectrum which is really well modeled by a simple-power law divided the community between two outflow models which explained all the observed features. Based on future observations and an intense debate, the community reached a consensus that the non-thermal emission in GW170817 is produced in an off-axis ultra-relativistic collimated outflow. Three observational factors that played a key role in distinguishing between the two models were (a) evolution of the light curve post-peak (b) evolution of synchrotron cooling frequency and (c) superluminal motion of the centroid of GW170817 in the radio image. Even though the nature of outflow is resolved for GW170817, we do not know if all BNS mergers have an ultra-relativistic collimated outflow. To optimize the process of understanding the underlying physics of these events, I will develop a software which creates a large number of light curves using ‘BoxFit’ as well as semi-analytical methods. I aim to process the light curves to match the observations in real-time. This will help constrain the jet parameters: E_k (kinetic energy), n (density) and epsilon_e and epsilon_B (fraction of internal energy partitioned between electrons and magnetic field respectively) along with the jet-opening angle and the angle the jet makes with the line of sight of the observer. When simulating afterglow models of short gamma-ray bursts, we have a N~10 dimensional parameter-space which results in a high degeneracy between model parameters. Some of the parameters are easy to constrain, but others like the density, n and energy, E_k are not. If we could constrain, say n, we can greatly improve the constraints on E_k. Accordingly, I plan to develop a new technique to independently constrain the ambient density of the GW event using the diffused X-ray emission from the BNS host galaxy. I will further study the temporal variability in the X-rays that is indicative of the product of the neutron star merger, i.e if it is a black hole (BH) or a neutron star (NS). X-ray flares are powered by the reactivation of the central engine. In the case of BH, the central engine could be an accretion disk powering the flare, whereas in the case of NS, it could be the internal magnetic dissipation in the outflow. By sampling the X-ray observations at shorter timescales, we can trace the temporal decay of the flare and distinguish between the remnants. Finally, I will develop an observational strategy following the future GW events to optimally trigger the observations with different NASA facilities like Swift, Chandra and NuStar, which would increase the efficacy of the above efforts to understand the science behind these new kind of events. As a result, we will be to able to systematically constrain the model parameters, including