03/29/2023
By Ankur Roy

The Kennedy College of Science, Department of Physics, invites you to attend a master's thesis defense by Ankur Roy entitled "Modeling the emission geometry of a pulsar undergoing Type-II outburst using Polestar."

Date: Thursday, April 6, 2023
Time: 3:30 to 5 p.m.
Location: Via Zoom. Zoom link will be available to those interested, please contact Ankur_Roy@student.uml.edu at least 24 hours prior to the defense to request access.

Committee Members:

  • Advisor Silas Laycock, Professor, Physics & Applied Physics, University of Massachusetts Lowell
  • Ofer Cohen, Associate Professor, Physics & Applied Physics, University of Massachusetts Lowell
  • Nishant Agarwal, Assistant Professor, Physics & Applied Physics, University of Massachusetts Lowell

Abstract: The endeavor behind this study is to understand the emission characteristics of High mass X-ray binaries as they undergo major outbursts. The major study conducted revolved around the transient pulsar SMC X-2 situated in the Small Magellanic Cloud (SMC) galaxy. Apart from being one of the earliest extra galactic pulsars to be identified, this X-ray source also holds the distinction to have scaled one of the highest luminosity levels within the SMC. In a particular episode in 2015, the pulsar underwent a Type-II outburst and the study documented here tries to understand the time evolution of the emission modes of the pulsar as it underwent that event. The pulse profiles for the same were investigated and were found to be luminosity dependent. The transition of the shape and depth of the profiles corroborated the fact that the pulsar evolved from the super-critical emission stage to the sub-critical phase. To better understand this emission mode, the geometrical model \textit{Polestar} was used. The model also yielded for the first time an approximate geometry of the pulsar, which gave the inclination value of the spin axis and the dipole moment of the neutron star (NS). Further, it was also understood how the emission changes can be related to change in the number of peaks in pulse profiles. The optical activity of the pulsar during the same epoch also gave us hints to further look into the activity of the pulsar during the outburst. On examining the extended optical light curve for the pulsar during the epoch using latest OGLE data, a correlation was identified between the optical and X-ray emission. Additionally the updated orbital period of the system was found. The comprehensive study of the Be- X-ray binary system (BeXRB) also yielded facts about the magnetic field and dipole moment which was then compared to that of canonical NSs. Additionally this thesis also sheds light on a parallel study conducted on a system of binary BeXRB s in the SMC. While one of them was discovered and its properties were known more than two decades ago, a neighbouring pulsar previously unknown was later found to be in outburst in 2017. My study encompassed comparing the historical X-ray activity for both the sources from multiple observatories like \textit{Chandra}, \textit{XMM-Newton} and \textit{NuSTAR}. A similar detailed optical study carried on the candidates yielded more definitive information about the new pulsar. The X-ray studies also unearthed archival observations from \textit{RXTE}, which showed that the erstwhile unknown pulsar probably had shown periodic pulsations previously as well but went unnoticed due to its tendency to remain quiescent mostly. In conclusion, this thesis intends to shed additional light on the nature of BeXRBs during outbursts and their characteristics.