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The impact of superfluidity and superconductivity on the magneto-thermal evolution and X-ray observations of neutron stars.

Periodic Reporting for period 1 - SF-magnetic-stars (The impact of superfluidity and superconductivity on the magneto-thermal evolution and X-ray observations of neutron stars.)

Reporting period: 2015-06-01 to 2017-05-31

Neutron stars are astrophysical objects where matter is in very extreme physical conditions, impossible to reproduce in terrestrial laboratories. More than 2000 neutron stars have been so far observed in different electromagnetic bands with quite different characteristics. In the last decades, X-ray astronomy provided a wealth of information about their thermal history and surface magnetic field. In order to interpret these data, we need to understand many aspects of the neutron star physics and, in particular, the influence of the magnetic field on the emission properties. The main purpose of this research project was to study the evolution of the magnetic field in the core of neutron stars by including the relevant mechanisms and microphysical conditions of a realistic system. The magnetic field evolution in the core is still not well understood, in particular after neutrons and protons, undergo a transition to a superfluid and superconducting state, respectively. Such a transition is predicted by nuclear theory and is expected when the star temperature drops below a giga-Kelvin. One of the most controversial mechanisms driving the evolution of magnetic fields in the core is ambipolar diffusion, suggested to be very efficient in magnetars, which are neutron stars with a magnetic field larger than ten giga-Tesla.

Strongly magnetized neutron stars also show a very rich X-ray activity which include outbursts and flares. In the tail of two giant flares (SGR 1806-20 and SGR 1900+14), which are rare and very powerful events, the power spectra revealed a series of quasi-periodic oscillations (QPOs). The detection of QPOs was very important, because has opened the possibility to use Asteroseismology to study the physics of neutron stars, i.e. to infer the properties of the system by studying its seismic wave propagation. This very powerful and well-established technique has been successfully used to study the interior of the Sun and other variable stars. With this research project we have studied the seismic vibrations of magnetars by introducing important physical ingredients in the stellar model. To exploit the potentiality of Astereoseismology, we need in fact sophisticated theoretical models and accurate templates to compare with observations.

The objectives of this research project were to provide a more detailed description of the neutron star dynamics in order to have better theoretical models for the analysis of X-ray observations. The project mainly focused on the study of the magnetic field evolution in the core of neutron stars, and on the seismology of magnetars. The combined information available from thermal history and magnetar QPOs can in fact be potentially used to determine the physical properties of highly magnetized neutron stars as well as to constrain the equation of state of dense matter.
The work carried out during the fellowship included a training period, the project development, presentation of the results in international conferences, publications in international peer-review scientific journals and in a personal web-site. Furthermore I report also other relevant work, such as the organization of conferences or referee's activity for international scientific journals. The training was continuous during all the research project, in close collaboration with the research group of Alicante. I had regular weekly meetings to discuss the relevant literature, to get practice with the numerical codes developed by the host group, and address the various theoretical and numerical issues which occurred during the project. The results from the project have been published in 3 papers in Monthly Notices of the Royal Astronomical Society (MNRAS) and made available on the Astrophysics server. Two more papers are in preparation. The results have been presented in several international conferences.

* Publications

- A. Passamonti, J. Pons (2016), MNRAS 463, 1173, “Quasi-periodic oscillations in superfluid, relativistic magnetars with nuclear pasta phases”. arXiv:1606.02132

- A. Passamonti, T. Akgün, J. Pons, J.A. Miralles (2017), MNRAS 465, 3416, “The relevance of ambipolar diffusion for neutron star evolution”. arXiv:1608.00001

- A. Passamonti, T. Akgün, J. Pons, J.A. Miralles (2017), Accepted for publication in MNRAS, “On the magnetic field evolution timescale in superconducting neutron star cores”. arXiv:1704.02016

* Conferences

1) 13-15-April 2016, University of Alicante, Spain. NewCompStar Working Group Meeting on “Origin, evolution and observable effects on the magnetic field in neutron stars”.
2) 2-6 May 2016, “Conference of the Italian Astronomical Society” SaIT, Roma, Italy
3) 4-8 July 206, Athens, Grece. Conference: European Week of Astronomy and Space Science (EWASS).
4) 13-14 September 2016, Southampton, UK: NewCompStar Working Group Meeting on “Neutron Star Oscillations and Instabilities”.
5) 26-29 October, 2016, Physikzentrum Bad Honnef, Germany. Conference on: “Neutron Stars: A Cosmic Laboratory for MAtter under Extreme Conditions”.
6) 14-16 December 2016, University of Valencia, Spain. CoCuNuT Meeting 2016.
7) 27-31 March 2017, Annual NewCompStar Conference, University of Warsaw, Poland.

* Other relevant work

- During the fellowship I refereed four papers for the journals: “Monthly Notices of the Royal Astronomical Society” and “Physical Review D”.
- April 2016. Event: “Infoday Mobility opportunities and MSCA-IF”, Valencia, Spain. Organized by Ruvid (Red de Universidades Valenciana para el fomento de la Investigacion, el Desarollo y la Innovacion). I participated in this event with a seminar to report my experience as Marie-Curie fellow and provide some tips to organize a Marie-Curie application.
The results of this project made important steps beyond the state of art of this research field. Before the development of this project, the theory of magneto-thermal evolution, despite its success, was still somewhat limited. In particular, one of the relevant mechanisms in the core, the ambipolar diffusion had been only studied with timescale estimates or simplified 1D simulations. The results obtained in this project provide the first numerical solutions for the ambipolar diffusion velocity field for 2D axi-symmetric magnetic fields, with different temperatures and microphysical conditions, which also include superfluid and superconducting constituents. The results of the present project also clarify a discrepancy arisen in the literature on the correct formulation of the induction equation in a superconducting fluid and the expected evolution timescale. During the research project we have also studied the seismic properties of magnetars and proto-neutron stars with more realistic models. The results may help the analysis of quasi-periodic oscillations observed in magnetars, or the oscillations modes expected in the gravitational wave signal of a core collapse. Making progresses in this research area is very important to connect the theoretical studies with the current and future observations in high-energy astrophysics and understand the physics of these objects.