X-ray Polarization

The polarization of light provides two observables, polarization degree and angle, that are very sensitive to the anisotropies of the emission region; therefore, polarization, combined with spectral and timing analysis, can be a powerful tool to study the geometry of astronomical sources. Regular astrophysical observations of X-ray polarization will soon become a reality, as several observatories with an X-ray polarimeter on board are now being developed. The NASA SMEX mission IXPE (Weisskopf et al. 2016), in the 1–10 keV energy range, and the Indian POLIX (5–30 keV), are both scheduled to launch in 2021, as well as the rocket-based REDSox (0.2–0.8 keV) (Gaenther et al. 2017), while the balloon-borne X-Calibur (Beilicke et al. 2014) and PoGO+ (Chauvin et al. 2018) are already flying. Additionally, the Chinese–European eXTP (Zhang et al. 2016) (1– 10 keV), is scheduled for launch in 2025, while the narrow band (250 eV) LAMP (She et al. 2015) and the broad band (0.2–60 keV) XPP (Krawczynski et al. 2019; Jahoda et al. 2019) are still at the concept stage. Many of these upcoming polarimeters employ the gas pixel detector technology, or GPD, which was recently put to test by the CubeSat PolarLight (Feng et al. 2019), a small polarimeter without optics, that was able to measure the polarization of the Crab nebula (Feng et al. 2020). I am a member of the science teams of IXPE and eXTP. Here you can find some of our white papers:

QED and black holes

Quantum electrodynamics or QED is the relativistic quantum field theory of electrodynamics. It is usually thought to apply only to the realm of the very small. However, its effects can be important on macroscopic scales in extreme environments, like the ones attained inside and around astrophysical compact objects, such as neutron stars and black holes. In my work, I have shown that the QED effect of vacuum birefringence can modify the polarization of the X-rays emitted from black-hole accretion disks. It is therefore important to include this effect in our calculations if we want to understand future polarimetry observations, also because it can tell us about the magnetic field threading the disk.

Read more:
Vacuum birefringence and the x-ray polarization from black-hole accretion disks
Probing Black Hole Magnetic Fields with QED

Accreting X-ray Pulsars

Accreting X-ray pulsars are highly magnetized neutron stars that live in a binary and accrete material from a companion star. X-ray pulsars have been known since the 70s, but the geometry of accretion and the physics behind the X-ray spectral formation are still debated. Polarimetry will be an incredible tool to finally understand these neutron stars, as they will provide two new observables, polarization degree and angle, that can help constrain the different models. In my work, I developed new realistic models that describe the polarized emission from the luminous X-ray pulsar Hercules X-1. In contrast with previous works, my models predict the polarization parameters independently of spectral formation, and consider the structure and dynamics of the accretion column, as well as the additional effects on propagation due to general relativity and quantum electrodynamics.

Read more:
Polarisation of Accreting X-ray Pulsars. I. A New Model
Polarisation of Accreting X-ray Pulsars. II. Hercules X-1

Magnetars

Magnetars are isolated neutron stars that are powered by their magnetic field. They were discovered because of their strong bursting activity; however, many magnetars are seen in a quiescent state as persistent X-ray sources, and their spectra are interpreted as a mixture of thermal and magnetospheric emission. In my work, I derived the polarization in the soft X-rays (0.5–10 keV) of persistent magnetars in quiescence, as for example 4U 0142+61, in the context of different physical models.

Read more:
Polarimetry of Magnetars and Isolated Neutron Stars