Thermal emission from magnetised neutron stars

X-ray dim isolated neutron stars and magnetars are neutron stars with strong magnetic fields, B 1013 1015 G. Under these magnetic fields, the properties of the matter and the vacuum are expected to change dramatically. As such, these sources are (unique) natural laboratories for studying fundamental physics in the strong magnetic field regime. In this thesis, I study the polarised thermal emission from both X-ray dim isolated neutron stars and magnetars. First, I developed a ray tracing code that allows me to compute the polarisation properties of X-ray dim isolated neutron stars by considering different surface emission models and by accounting for QED vacuum birefringence effects. I show that combined optical and X-ray polarimetric observations can allow us to infer the state of the matter at the surface of X-ray dim isolated neutron stars. Then, these models are confronted with the optical VLT polarimetric observation of RX J1856.5-3754, the nearest and brightest X-ray dim isolated neutron star. I found that the observed polarisation fraction, 16 5%, can be explained if vacuum birefringence is present, the first ever evidence for this QED effect. In order to confirm this measurement by future missions of Xray polarimetry, I model the polarisation properties of magnetars, active sources with outburst activity in which the X-ray flux decay is explained in the context of an untwisting magnetosphere. I show that if both vacuum birefringence and an untwisting magnetosphere are present, then during the X-ray flux decay of the source, the polarisation angle can change up to 23 (for an untwisting of DF = 0:5 rad). Finally, I model the atmosphere of magnetars heated by a particle bombardment. I compute the thermal emission in the case of a grey atmosphere and discuss the potential spectrum of the source and polarisation signal

Polarized thermal emission from X-ray Dim Isolated Neutron Stars: the case of RX J1856.5-3754

The observed polarization properties of thermal radiation from isolated, cooling neutron stars depend on both the emission processes at the surface and the effects of the magnetized vacuum which surrounds the star. Here we investigate the polarized thermal emission from X-ray Dim Isolated Neutron Stars, taking RX J1856.5-3754 as a representative case. The physical conditions of the star outermost layers in these sources is still debated, and so we consider emission from a magnetized atmosphere and a condensed surface, accounting for the effects of vacuum polarization as the radiation propagates in the star magnetosphere. We have found that, for a significant range of viewing geometries, measurement of the phase-averaged polarization fraction and phase-averaged polarization angle at both optical and X-ray wavelengths allow us to determine whether this neutron star has an atmosphere or a condensed surface. Our results may therefore be relevant in view of future developments of soft X-ray polarimeters.Comment: 12 pages, 12 figures, accepted for publication in MNRA

Atmosphere of strongly magnetized neutron stars heated by particle bombardment

The magnetosphere of strongly magnetized neutron stars, such as magnetars, can sustain large electric currents. The charged particles return to the surface with large Lorentz factors, producing a particle bombardment. We investigate the transport of radiation in the atmosphere of strongly magnetized neutron stars, in the presence of particle bombardment heating. We solve the radiative transfer equations for a gray atmosphere in the Eddington approximation, accounting for the polarization induced by a strong magnetic field. The solutions show the formation of a hot external layer and a low (uniform) temperature atmospheric interior. This suggests that the emergent spectrum may be described by a single blackbody with the likely formation of a optical/infrared excess (below ~ 1 eV). We also found that the polarization is strongly dependent on both the luminosity and penetration length of the particle bombardment. Therefore, the thermal emission from active sources, such as transient magnetars, in which the luminosity decreases by orders of magnitude, may show a substantial variation in the polarization pattern during the outburst decline. Our results may be relevant in view of future X-ray polarimetric missions such as IXPE and eXTP.Comment: 16 pages, 9 figures, accepted for publication in MNRA

The X-ray Polarization Probe mission concept

The X-ray Polarization Probe (XPP) is a second generation X-ray polarimeter following up on the Imaging X-ray Polarimetry Explorer (IXPE). The XPP will offer true broadband polarimetery over the wide 0.2-60 keV bandpass in addition to imaging polarimetry from 2-8 keV. The extended energy bandpass and improvements in sensitivity will enable the simultaneous measurement of the polarization of several emission components. These measurements will give qualitatively new information about how compact objects work, and will probe fundamental physics, i.e. strong-field quantum electrodynamics and strong gravity.Comment: submitted to Astrophysics Decadal Survey as a State of the Profession white pape

Neutron star radius measurement from the ultraviolet and soft X-ray thermal emission of PSR J0437-4715

International audienceWe analysed the thermal emission from the entire surface of the millisecond pulsar PSR J0437−4715 observed in the ultraviolet and soft X-ray bands. For this, we calculated non-magnetized, partially ionized atmosphere models of hydrogen, helium, and iron compositions and included plasma frequency effects that may affect the emergent spectrum. This is particularly true for the coldest atmospheres composed of iron (up to a few per cent changes in the soft X-ray flux). Employing a Markov chain Monte Carlo method, we found that the spectral fits favour a hydrogen atmosphere, disfavour a helium composition, and rule out iron atmosphere and blackbody models. By using a Gaussian prior on the dust extinction, based on the latest 3D map of Galactic dust, and accounting for the presence of hot polar caps found in the previous work, we found that the hydrogen atmosphere model results in a well-constrained neutron star radius |${R_{\rm NS}}= 13.6^{+0.9}_{-0.8}{\, {\rm km}}$| and bulk surface temperature |${T_{\rm eff}^{\infty }}=\left(2.3\pm 0.1\right){\times 10^{5}}{\, {\rm K}}$|⁠. This relatively large radius favours a stiff equation of state and disfavours a strange quark composition inside neutron stars

Physics and astrophysics of strong magnetic field systems with eXTP

In this paper we present the science potential of the enhanced X-ray Timing and Polarimetry (eXTP) mission for studies of strongly magnetized objects. We will focus on the physics and astrophysics of strongly magnetized objects, namely magnetars, accreting X-ray pulsars, and rotation powered pulsars. We also discuss the science potential of eXTP for QED studies. Developed by an international Consortium led by the Institute of High Energy Physics of the Chinese Academy of Sciences, the eXTP mission is expected to be launched in the mid 2020s