Neutrons stars are one of the final stages of the stellar evolution of a massive star, in which a compact object made mostly of neutrons is left after a supernova event. With masses ~ 1.5 M☉ and radii of the order of ~ 10 km, neutron stars have an average density of ~ 1014 – 1015 gr cm-3, which is comparable to the density of an atomic nucleus (~2 . 1014 gr cm-3). Due to their high densities, they have strong surface gravitational fields and quantum effects dominate the properties of matter in their interiors.
Neutron stars are not just composed of neutrons but also a fraction of charged particles, such as electrons and protons, which can sustain long-lived electric current in their interiors and give rise to the strongest magnetic fields in the universe. To have an idea of the magnitude of these fields, it is interesting to compare the magnetic fields found in neutron stars with those found in more familiar contexts:
- Magnetic field of the earth: B~ 0.6 G.
- Typical refrigerator magnet: B~ 50 G.
- Medical magnetic resonance imaging: B ~ 3 . 104 G.
- Strongest pulsed magnetic field produced in a laboratory: B ~ 3 . 107 G.
- Neutron stars: up to ~ 1015 G.
Because of these high densities and strong magnetic fields, neutron stars are natural laboratories in which the properties of matter can be studied under extreme conditions that cannot be reproduced in terrestrial laboratories.
The existence of neutron stars in the Universe were first conjectured by Baade & Zwicky (1934). Since then, there have been several highlights in the study of these objects, such as the first observation of a neutron star as a radio pulsar by Hewish et al., (1968); the discovery of the binary pulsar PSR B1913+16 by Taylor & Russell (1974), which had a decaying orbit as predicted by general relativity; the association of soft gamma repeaters with starquakes in magnetars (neutron stars with super strong magnetic fields) by Thompson & Duncan (1992); and more recently, the debate whether ultra long gamma ray bursts may correspond to a supernova powered by a fast rotating magnetar.
Despite the substantial progress made in the study of neutron stars, both theoretically and observationally, there are still several fundamental questions to be solved, such as: What is the equation of the state in the neutron star interior? What is the core composition? Is the matter in the core in a superfluid state? What is the origin of the neutron star magnetic fields and how do they evolve?
The motivation for this post is to share with the reader a recent study about X-ray Dim Isolated neutron stars (XDINSs). They are relatively nearby (distance < 500 pc), middle aged (104-105 yr), strongly magnetized neutron stars (B ~ 1013 G), from which is still possible to observe the released residual-heat stored in the core after the supernova event. Their thermal emission is mainly observed in the X-ray band and, in some cases, it can also be observed in the optical band. One of the most notable characteristics of XDINSs is that their spectra are purely thermal, non-affected by the emission from a surrounding supernova remnant nor by magnetospheric activity. Therefore, by observing these XDINSs we are able to directly see the neutron star surface.
Figure 1: Left panel: X-ray image of the XDINS RX J1856.5-3754 (Credit: NASA/SAO/CXC/J.Drake et al.). Right panel: RX J1856.5-3754 moving across the sky with speed of 108 km s–1.
In general, the radiation from a neutron star is expected to be reprocessed by an atmosphere, which, due to the strong gravitational field, should be very thin, of the order of ~ 1 cm. However, current atmosphere models are incompatible with the spectra of XDINSs. It has also been proposed that the strong magnetic fields present in XDINSs may drive a phase transition, turning the gaseous atmosphere into a condensed, metallic surface. However, the poorly known physics of matter in the strong magnetic fields have hampered efforts to obtain reliable conclusions regarding the surface of XDINSs.
In spite of the unknown state of the matter in the surface of XDINSs, it is expected that a strong magnetic field should introduce an anisotropy in the medium in which electromagnetic waves are propagating. This translates into a net degree of polarization in the radiation emitted from the surface of XDINSs. At the same time, it is natural to think that the polarization properties should be different whether the emission comes from an atmosphere or condensed surface.
Figure 2: Phase-averaged polarization fraction in the X-ray band (considering different viewing geometries) for the thermal emission from a condensed surface (left panel) and gaseous atmosphere (right panel). In the plots, the horizontal axis correspond to the angle between the spin and magnetic axes of the star, ξ, and the vertical axis correspond to the angle between the star spin axis and the line of sight, χ.
In a recent paper, we studied the properties of the polarized thermal emission from XDINSs. We modelled the polarized optical and X-ray emission for the case of the nearest and brightest XDINS, RX J1856.5-3754. Since the the radiation pattern depends on the viewing geometry of the star, the polarization properties were studied considering different viewing angles for the star spin, magnetic axis, and line of sight. We found that for a range of viewing angles, the polarization fraction of the thermal radiation from a gaseous atmosphere is significantly higher than that from a condensed surface. Therefore, by performing polarimetry observations it is possible to discriminate whether this star has an atmosphere or condensed surface. This result is interesting as it may helps us to constrain the properties of matter under very strong magnetic fields and can be used as an scientific case for future missions of X-ray polarimetry.
The results of this paper will be presented in the first XIPE conference meeting in Valencia (May, 2016), and later in Cospar conference in Istanbul (August, 2016).
 You can find more information about neutron star in a previous post (Silvia Zane)
 Gonzalez Caniulef, D., Zane, S., Taverna, R., Turolla, R., & Wu, K. 2016, http://arxiv.org/abs/1604.01552
(Post by Denis Gonzalez )