Studies of electromagnetic counterparts to gravitational-wave sources and the nonlinear gravitational-wave memory effect

Abstract

In this dissertation, theoretical/computational results are presented from the investigations of three different topics within the general research areas of gravitational wave astrophysics and electromagnetic counterparts. First, general relativistic force-free electromagnetic theory and its application to black hole magnetospheres are discussed. In this connection, simulations of a binary black hole merger are examined using the open-source software, GiRaFFE, which is used to model black holes’ magnetospheres and to study supermassive black-hole binary mergers in an external magnetic field. In the simulations, a helical magnetic field structure around each black hole is observed. Electromagnetic energy flux is observed during the inspiral and the merger phases. The emission becomes stronger as the binary move from inspiral to the merger phase. The electromagnetic emission during binary black hole merger is accompanied by gravitational waves (GW). A better understanding of electromagnetic emissions along with results from gravitational waves provides us with a better understanding of the physics of the source. In this connection, this work investigates the effect of the inclination of binary on electromagnetic emission. The second topic investigated in this work is an interesting prediction of general relativity called the non-linear memory of GW. The effect of black-hole spins on non-linear memory from supermassive black-hole binary mergers is examined and its impact on the timing residuals observed by pulsar timing arrays is analyzed. The non-linear memory contribution to the gravitational waveform is obtained for inspiraling spinning black holes. These results are generalizations of the work by Favata [1] to the spinning case. The memory contribution during the merger is obtained by using the formula by Favata [1] and is applied to publicly available Simulating eXtreme Spacetimes (SXS) [2] data. Further, our results are used to compute the post-fit timing residual growth during a two-week interval of time for supermassive black holes in the mass range from 108 to 1010 M⊙ . This work would be useful for the search of Burst with Memory (BWM) events with the future Pulsar Timing Array (PTA) which would have higher sensitivity. In literature, researchers have modeled the BWM events as step functions but in this dissertation, it have been investigated how the expected results could change if more accurate models for nonlinear memory waveforms are used. The third topic investigated in this work looks at the accuracy of the gravitational wave- iii forms as given by various models using the recently proposed super- momentum balance law