Physics & Astronomy ETDs

Publication Date

Summer 7-15-2019


The recent discovery of gravitational waves (GWs) by the LIGO collaboration has opened a new observing window on the universe, but it is limited to the GWs in the frequency range of 10-1000 Hz. The main motivation of this thesis is to consider the possibility of detecting low frequency (nHz) GWs. In the pursuit of these waves, we need to understand their source of origin and build a detector with the required sensitivity. Low-frequency waves are expected as a result of coalescing binary supermassive black holes (SMBBHs). We hope to detect these waves in the near future using pulsar timing arrays (PTAs). Thus, this thesis can be divided into two parts: searching for SMBBHs and using pulsars for improving the sensitivity of PTAs.

SMBBHs are expected to form as a result of galaxy mergers. However, despite numerous attempts, so far we have been able to and only a handful of these systems. This raises various questions about the merger rate, the timescale of a merging process, and the sensitivity and resolving power of the available instruments. 0402+379 is the most compact confirmed SMBBH, with two compact cores at a separation of 7.3 pc. We have studied this source at 5, 8, 15 and 22 GHz using the Very Long Baseline Array (VLBA) over a timeline of 12 years. With some assumptions about the orbit, we measure the orbital period P ~ 28000 years and SMBBH mass M ~ 15 x 10^9 Solar Mass. A strong frequency-dependent core shift is evident, which we use to infer magnetic fields near the jet base. Subsequently, we search for more compact SMBBHs so that we can study their orbital motion within our lifetime. We select a sample of 18 sources from the VLBA imaging polarimetry survey based on their morphology and host galaxy properties. We also include NGC7674, which was recently claimed to be an SMBBH candidate. We observe these sources at multiple frequencies using the VLBA.

PTAs consist of a large array of pulsars which are being used to detect the GWs. However, when the signal from a pulsar traverses the interstellar medium (ISM), its signal gets affected, contributing to PTA noise. In order to improve the sensitivity of PTAs, it is necessary to understand the properties of the ISM. The main effects of the ISM are dispersion, scattering, and scintillation. All these effects are strongly dependent on frequency such that they have a large impact at lower frequencies. Hence, in order to study these effects, we study a sample of eight pulsars at frequencies below 100 MHz using the Long Wavelength Array. This provides us with insights into the distribution of inhomogeneities in the ISM which we hope will help in improving the sensitivity of PTAs.

Degree Name


Level of Degree


Department Name

Physics & Astronomy

First Committee Member (Chair)

Greg Taylor

Second Committee Member

Kevin Stovall

Third Committee Member

Rouzbeh Allahverdi

Fourth Committee Member

Edl Schamiloglu




Gravitational Waves, Supermassive Binary Black Holes, Pulsars, Scattering, ISM

Document Type