Digital archive of theses discussed at the University of Pisa


Thesis etd-08242021-232854

Thesis type
Tesi di laurea magistrale
Thesis title
Population synthesis to study Gravitational Waves and Electromagnetic observations of isolated Pulsars
Course of study
relatore Prof. Razzano, Massimiliano
  • pulsar
  • population synthesis
  • gravitational waves
  • electromagnetic observations
Graduation session start date
Release date
The Universe provides us a vast amount of information about explosions, collisions or other exotic processes. Thanks to the multimessenger astronomy, a new astrophysical research method, we have a more complete knowledge of astrophysical processes through different messengers. In fact, each messenger teaches us different properties and characteristics of the sources, its environment or history, allowing us to be more efficient in reconstructing the sky position of these transient sources and to trigger other messengers detectors and telescopes.

Pulsars are extraordinary astrophysical sources, since they are interesting multiwave High Energy emitters and natural laboratories for testing laws of physics under extreme conditions. Due to their extraordinary density, these neutron stars are commonly classified by modern astrophysics as compact objects and, thanks to their multiwave radiation emission in the whole electromagnetic spectrum, they can be exploited to simultaneously carry out multiwavelength and multimessenger research. Moreover, isolated rotating neutron stars can also be a source of continuous gravitational waves over a period that is long compared to our observation time provided that they have non-spherically symmetric shape. The rotational motion of the system with a frequency nu generates a quasi-monochromatic radiation
signal and emits a gravitational wave with a frequency equal to 2*nu. In addition, the gravitational waves depend on the spin frequency and on the star deformation, i.e. on its ellipticity.

This thesis presents a new work and results on population synthesis study on Galactic neutron stars, focusing on their multimessenger detection with electromagnetic and gravitational wave observations. In order to understand the parameters distribution of Galactic pulsars and their multimessenger detectability, we can perform simulations of pulsar populations to generate artificial pulsars that satisfy the criteria of detection of existing surveys, and then infer the distributions and properties like gravitational waves detectability by comparing the simulated pulsars with those detected in surveys. Nowadays, continuous gravitational waves from rotating, non-axisymmetric neutron stars have not yet been detected and we have only upper limits on their intrinsic asymmetry. Once a realistic set of initial conditions is given, we can constrain population synthesis models using electromagnetic observation. Our aim is to estimate the fraction of the Galactic simulated population of neutron stars that are detectable by actual and future ground-based gravitational waves detectors Advanced LIGO, Advanced Virgo and third generation interferometers as the Einstein Telescope. Our study is focused on the population point of view and from it estimates the number of pulsars that could be detected and effectively considered as continuous gravitational waves sources.

Population synthesis studies of Galactic neutron stars have been performed by many authors and have been widely used to study the initial parameters and their evolution. In this thesis, we implement a software to simulate a population of neutron stars in the Milky Way and to investigate their properties, calculating the evolution of their trajectories and characteristics from their birth forward in time until today. We investigate the birth and evolution of Galactic isolated radio pulsars. We begin by estimating their birth distribution in the spiral arms of the Galactic disk, their surface magnetic field and their initial period. We then evolved in time in the Paczynski Galactic gravitational potential with a Runge-Kutta 4th Order based population model, including magnetic field decay on a timescale of 2 Myr. Finally, we use the electromagnetic observations in order to gauge and constrain our simulations, filtering their radio-flux through a selected set of radio survey thresholds.

Using an exponentially decaying ellipticity model, we analyze the detectability of our set of simulated pulsar population separately in each detector. We decided to use a simplified two parameter phenomenological model of the neutron star asymmetry evolution: using the ellipticity and the timescale as parameters. Varying these two quantities, we explore a 2D space of different possible mechanism of asymmetry generation and we infer the number of observable pulsars.