Polarized X-rays from magnetars: IXPE observations of AXP 4U 0142+61

In this work, we analyze the IXPE observations of the polarized X-rays coming from the AXP 4U 0142+61. Firstly, we briefly introduce neutron stars, then we take a deeper look into the magnetar model: the twisting of the magnetosphere and its consequences, the detected emission and its possible explaining models, the polarization of magnetar radiation and the role it plays in understanding to a deeper level the various mechanisms that are at work. The effects that arise due to the presence of the huge magnetic field of these sources, such as vacuum birefringence and mode switching, are taken into account. Lastly, we analyze the data received from the AXP 4U 0142+61 and we try to fit them with the various models that we computed. The latter are derived dividing the star in a 10×10 grid and assigning to each patch a temperature. We take different values for the amount of twist of the star Δϕ and for the velocity of particles flowing in the corona β, assuming a unidirectional flow of electrons which will undergo resonant Compton scattering in the magnetosphere, implemented using a Monte Carlo technique. Then, we also account for the different angles that the magnetic field of the star makes with its rotation axis, ξ, and the different viewing angles of our source, χ. Furthermore, we consider two emission models for the star: a condensed surface with either free or fixed ions. In the end, we obtain the best fit value of all the aforementioned parameters and the value of the Polarization Degree (PD) and Polarization Angle (PA) of the aforementioned source.