Probing the surface magnetic field structure in RX J1856.5-3754

The evolution of magnetic field in isolated neutron stars is one of the most important ingredients in the attempt to build a unified description of these objects. A prediction of field evolution models is the existence of an equilibrium configuration, in which the Hall cascade vanishes. Recent calculations have explored the field structure in this stage, called the Hall attractor. We use X-ray data of near-by, cooling neutron stars to probe this prediction, as these sources are surmised to be close to or at Hall attractor phase. We show that the source RX J1856.5-3754 might be closer to the attractor than other sources of its class. Our modelling indicates that the properties of surface thermal emission, assuming that the star is in the Hall attractor, are in contradiction with the spectral data of RX J1856.5-3754.Comment: 9 pages, accepted to MNRA

A Comment on "A note on polarized light from Magnetars: QED effects and axion-like particles" by L.M. Capparelli, L. Maiani and A.D. Polosa

The recent detection of a large polarization degree in the optical emission of an isolated neutron star led to the suggestion that this has been the first evidence of vacuum polarization in a strong magnetic field, an effect predicted by quantum electrodynamics but never observed before. This claim was challanged in a paper by Capparelli, Maiani & Polosa (2017), according to whom a much higher polarization degree would be necessary to positively identify vacuum polarization. Here we show that their conclusions are biased by several inadequate assumptions and have no impact on the original claim.Comment: 10 pages, 2 figure

Evidence of vacuum birefringence from the polarisation of the optical emission from an Isolated Neutron Star

Isolated Neutron Stars are some of the most exciting stellar objects known to astronomers: they have the most extreme magnetic fields, with values up to $10^{15}$ G, and, with the exception of stellar-mass black holes, they are the most dense stars, with densities of $\approx 10^{14}$ g cm$^{-3}$. As such, they are perfect laboratories to test theories of electromagnetism and nuclear physics under conditions of magnetic field and density unattainable on Earth. In particular, the interaction of radiation with strong magnetic fields is the cause of the {\em vacuum birefringence}, an effect predicted by quantum electrodynamics in 1936 but that lacked an observational evidence until now. Here, we show how the study of the polarisation of the optical radiation from the surface of an isolated neutron star yielded such an observational evidence, opening exciting perspectives for similar studies at other wavelengths.Comment: 5 pages, 1 figure, Contributed to the 13th Patras Workshop on Axions, WIMPs and WISPs, Thessaloniki, May 15 to 19, 201

Extracellular vesicles: Small bricks for tissue repair/regeneration

Extracellular vesicles (EVs) are nano-sized membrane vesicles involved in intercellular communication. EVs have pleiotropic actions in physiological and pathological conditions. The ability of EVs to transports proteins, drugs and nucleic acid, to target specific cells and to increase the stability of therapeutic cargo, make EVs interesting as new devices for the treatment of human disease. In a recently published issue of European journal of pharmaceutical sciences, Silva and colleagues reviewed the ability of EVs to modulate tissue repair and regeneration, focusing on their roles and therapeutic potential as immunomodulatory messengers. In this perspective, we discussed the open questions regarding the dual role of EVs in immune system, as well as the technical limitation of the procedure for EVs isolation and administration in clinical practices. EV-based therapies require further studies to consider EVs as promising candidate for a novel cell-free therapy in the context of regeneration medicine

Observational Properties of Accreting Neutron Stars with an Optically Thick Envelope

Pulsating Ultra Luminous X-ray sources (PULXs) are thought to be X-ray bright, accreting, magnetized neutron stars. Their measured soft (<20 keV) X-ray luminosity can exceed the Eddington luminosity for a neutron star (NS) by a few orders of magnitude. Although several scenarios have been proposed to explain the different components observed in the X-ray spectrum and the characteristics of the X-ray pulse profile of these system, detailed quantitative calculations are still missing. In particular, the observed soft X-ray pulse profiles are almost sinusoidal and show an increase in the pulsed fraction (from 8 % up to even 30 %) with increasing energy. In this work, we discuss how emission originating from an optically thick envelope, expected to be formed during super-Eddington accretion, can match the observed spectra and pulse profiles

PULXs as Accreting Magnetars: Observational Manifestations

Pulsating Ultra Luminous X-ray sources (PULXs) are thought to be X-ray bright, accreting, magnetized neutron stars, and could be the first and only evidence for the existence of magnetars in binary systems. Their apparent soft (< 20 keV) X-ray luminosity can exceed the Eddington luminosity for a neutron star (NS) by a few orders of magnitude. Although several scenarios have been proposed to explain the different components observed in the X-ray spectra and the characteristics of the X-ray lightcurve of these system, detailed quantitative calculations are still missing. In particular, the observed soft X-ray lightcurves are almost sinuosidal and show an increase in the pulsed fraction (from 8% up to even 30%) with increasing energy. Here, we present how emission originating from an optically thick envelope, expected to be formed during super-Eddington accretion, can result in pulsed fractions similar to observations

Polarized emission from strongly magnetized sources

Anomalous X-ray pulsars (AXPs) and Soft gamma repeaters (SGRs) form together a single class of astrophysical sources, commonly associated to magnetars. New-generation X-ray polarimeters will play a key role in assessing the nature of these sources by directly probing the star magnetic field. In the highly magnetized environment radiation is expected to be strongly polarized and such a measure will be easily within reach of IXPE and eXTP. Polarization measurements will eventually confirm the presence of ultra-strong magnetic fields, probing the magnetar scenario. In this work we will discuss theoretical expectations for the polarization signature of AXPs and SGRs and present numerical simulations for the detector response of the polarimeters currently under construction. We will also show how these sources can be used to test vacuum birefringence, a QED effect predicted by Heisemberg and Euler in the Thirties and not experimentally verified as yet

Modelling Magnetar Behaviour with 3D Magnetothermal Simulations

The observational properties of isolated NSs are shaped by their magnetic field and surface temperature. They evolve in a strongly coupled fashion, and modelling them is key in understanding the emission properties of NSs. Much effort was put in tackling this problem in the past but only recently a suitable 3D numerical framework was developed. We present a set of 3D simulations addressing both the long-Term evolution (≈ 104-106 yrs) and short-lived outbursts (â 1 yr). Not only a 3D approach allows one to test complex field geometries, but it is absolutely key to model magnetar outbursts, which observations associate to the appearance of small, inherently asymmetric hot regions. Even though the mechanism that triggers these phenomena is not completely understood, following the evolution of a localised heat injection in the crust serves as a model to study the unfolding of the event

A strong X-ray polarization signal from the magnetar 1RXS J170849.0-400910

Magnetars are the most strongly magnetized neutron stars, and one of the most promising targets for X-ray polarimetric measurements. We present here the first Imaging X-ray Polarimetry Explorer observation of the magnetar 1RXS J170849.0-400910, jointly analyzed with a new Swift observation and archival NICER data. The total (energy- and phase-integrated) emission in the 2–8 keV energy range is linerarly polarized, at a ∼35% level. The phase-averaged polarization signal shows a marked increase with energy, ranging from ∼20% at 2–3 keV up to ∼80% at 6–8 keV, while the polarization angle remains constant. This indicates that radiation is mostly polarized in a single direction. The spectrum is well reproduced by a combination of either two thermal (blackbody) components or a blackbody and a power law. Both the polarization degree and angle also show a variation with the spin phase, and the former is almost anticorrelated with the source counts in the 2–8 and 2–4 keV bands. We discuss the possible implications and interpretations, based on a joint analysis of the spectral, polarization, and pulsation properties of the source. A scenario in which the surface temperature is not homogeneous, with a hotter cap covered by a gaseous atmosphere and a warmer region in a condensed state, provides a satisfactory description of both the phase- and energy-dependent spectro-polarimetric data. The (comparatively) small size of the two emitting regions, required to explain the observed pulsations, does not allow to reach a robust conclusion about the presence of vacuum birefringence effects