Digital archive of theses discussed at the University of Pisa


Thesis etd-06272019-000502

Thesis type
Tesi di laurea magistrale
Thesis title
Study of Periodic Polarized X-ray Emission with IXPE
Course of study
relatore Prof. Baldini, Luca
  • X-ray
  • tmsp
  • pulsar
  • polarimetry
  • ixpe
  • gps
Graduation session start date
X-ray polarization of astronomical sources is an almost unexplored field of high energy astrophysics, with a single signi cant measurement performed more than 40 years ago for the Crab Nebula by the Orbiting Solar Observatory OSO-8.
The importance of this emission property is that it carries direct information on the geometry and magnetic field configuration of the source. The lack of experimental measurements is due to the difficulty in realizing conventional X-ray polarimeters (typically based on Thomson scattering and Bragg reflection) with sufficient sensitivity, along with the necessity of comparatively long observation times.
In recent years, the advent of a new generation of high-sensitivity polarimeters based on the photoelectric effect, led to the selection of the Imaging X-ray Polarimetry Explorer (IXPE) mission as the next NASA SMEX (SMall EX-plorer), with a launch scheduled for 2021. IXPE will provide a fundamental opportunity to study polarization of many kind of X-ray sources, like pulsar wind nebulae, supernova remnants, X-ray binaries, active galactic nuclei, magnetars and pulsars (both isolated and accreting). The polarimetric sensitivity will be complemented by moderate spectral, imaging and timing capabilities, providing simultaneous access to all properties carried by electromagnetic radiation.
The IXPE payload is composed of three identical X-ray telescopes, operating independently in the 2-8 keV energy range, each with a Mirror Module Assembly (grazing incidence X-ray optics) and a polarization-sensitive imaging detector (a Gas Pixel Detector, or GPD) at the focus. The GPD, constituting the main Italian responsibility for the mission, is designed as a proportional gas detector with an Application-Specific Integrated Circuit (ASIC) used as a pixelized collecting anode and a Gas Electron Multiplier (GEM) as amplification stage.
It exploits the photoelectric effect in a gas. Since for polarized X-rays photo-electrons are preferentially emitted along the direction of the electric field, the polarization can be measured statistically by imaging the photoelectron track of each event and reconstructing its direction. Having a realistic observation-simulation framework is of crucial importance for the IXPE mission in order to assess the sensitivity of the instrument, analyze a specific science case and formulate the observation plan. In our case, being X-ray polarimetry largely a new field, the collaboration could only make a limited re-use of existing tools, and had to develop many of the technology from scratch. During my thesis work I did participate actively in the development of such a simulation and analysis framework, called ixpeobssim. This simulation framework is based on the Python programming language and the SciPy stack. Starting from an arbitrary source model (including morphological, temporal, spectral and polarimetric information), it uses the instrument response functions to produce fast and realistic observation simulations. The generated events list can be directly fed into the standard X-ray visualization and analysis tools, including XSPEC, which make it a useful tool not only for simulating observations of astronomical sources, but also to develop and test end-to-end analysis chains. The work described in this thesis is focused on the implementation of a series of simulation features and analysis tools dedicated to the study of the pulsars, with particular emphasis on transitional millisecond pulsars.
Millisecond pulsars are believed to be the descendants of old, magnetized neutron stars (NSs), hosted in low mass X-ray binaries (LMXB). The transfer of angular momentum from companion to NS through accretion is considered the responsible of the spin-up of slowly rotating NSs to millisecond spinning sources. This recycling scenario switch of the NS radio pulsation, leading to an accretion phase where the X-ray pulsation and the spin frequency can be detected. In this case, the system takes on the typical features of an accretion millisecond X-ray pulsar (AMXP). The recycling scenario of MSPs has been supported by the so called missed link pulsar PSRJ1023+0038 (J1023), the first observed to alternate a radio pulsar phase to an X-ray pulsed state, powered by accretion, usually referred to us as transitional millisecond pulsar (or tMSP).
The nature of tMSP pulsation due to accretion ow onto the NS (if any) is not well understood.
In this thesis a simulation of J1023 observation with IXPE is presented, bringing its state of art coming from literature. The emission model assumed is the AMXP in low-luminosity state. The polarization degree is resulting from the optical polarization and the X-ray pulsed luminosity, that provide an average polarization degree equal to 8%. The analysis made to the generated events optimally reproduces the spectrum and the pulse profile of the input model for two months of observation. The analysis of the polarization degree of these events allows to resolve two phase intervals of the source emission, providing two comparable polarization degree values. Thus during two months of observation we will be able to measure significantly the mean X-ray polarization degree of J1023.
Everything I developed is available from the official software package of the IXPE collaboration and it can be used to study these kind of sources. This work opens up the possibility to new perspectives, such as the possibility of simulating orbital phase modulation for binary source models (that will improve the J1023 simulation itself), simulating the brightest millisecond pulsars (like SAXJ1808) and verifying if its polarization is resolvable in phase by IXPE. So in the end, regard to J1023, I say that its low brightness does not allow it to be part of observational priorities of IXPE, but given its peculiarity, it is desirable to insert the source among the last targets of the first two years of mission or in the first renewable year.