• Cygnus region
• Fermi-LAT
• gamma-ray astrophysics
• PSR J2021+4026
• pulsars

In this thesis I present a full analysis of Fermi-LAT data from PSR J2021+4026, the first variable gamma-ray pulsar. This 256 ms young radio-quiet pulsar is one of the brightest objects in the gamma-ray sky and lies within the Cygnus region, a star-forming region of the Galactic plane populated by several bright sources.
I perform an updated analysis of the source with the latest Fermi-LAT data release, covering almost 12 years from August 2008 to May 2020 and fully exploiting the LAT performance. My goal is to study the variability of spectral and timing characteristics in detail.
PSR J2021+4026 shows quasi-periodic state changes at intervals of about 3 years. In October 2011 the measured photon flux dropped by about 13% in a timescale of a few days and, at the same time, the spin-down rate increased by about 5%. After a slow recovery in December 2014, a similar state change occurred in February 2018. The source also shows slight variations in the pulse profile and in the spectral energy distribution, parameterized as a power law with an exponential cutoff.
The observed variations in spectrum, flux and spin-down rate suggest a change in the strength of the magnetic field and in the structure of the magnetosphere at the poles. I discuss previously proposed hypotheses of phenomena that could induce this behavior, pointing out the difficulties encountered in building a realistic model for such a unique source in the gamma-ray sky.