### Sala de Grados (1st floor)

#### Facultad de Ciencias Fisicas UCM

Plaza de las Ciencias 1, Madrid, Spain
Description

A workshop to discuss the latest developments in Neutron Star Theory, of particular relevance for WG1, 2 and 3 of the PHAROS COST action.

It is organized by members of the Departamento de Fisica Teorica and the Institute of Particle and Cosmos Physics at the Universidad Complutense de Madrid, where it convenes (map) on June 10th.

It is simultaneous to the Vth Iberian-Russian Congress in Madrid and joint registration is possible.

Registration
Registration form for the workshop "Neutron Star Theory- Madrid", 10-11 Oct. 2019
Participants
• Alexander Potekhin
• Andrey Chugunov
• Antonio Bernardo
• Arus Harutyunyan
• Daniela Doneva
• Eva Lope Oter
• Francisco Navarro-Lerida
• Jose Luis Blázquez-Salcedo
• Jose Manuel Alarcon
• José Antonio Oller
• Laura Tolos
• Luis Fraile
• M. Angeles Perez-Garcia
• Manuel González Romero
• Maxim Dvornikov
• Mikhail Gusakov
• Morgane Fortin
• Nikolay Shchechilin
• Ricard Aguilera Miret
• Vasily Dommes
• Víctor González
• Wolfgang Kastaun
• Xavier Viñas
• Thursday, 10 October
• 08:35 09:00
Registration and welcome 25m Sala de Grados (1st floor) CHAIRMAN FOR THE SESSION: FELIPE LLANES ()

### Sala de Grados (1st floor) CHAIRMAN FOR THE SESSION: FELIPE LLANES

• 09:05 09:40
Numerical simulations of neutron star mergers 35m Sala de Grados (1st floor) ()

### Sala de Grados (1st floor)

The recent multi-messenger observation campaign of event GW170817, attributed to merging neutron stars, opens exciting new possibilities to test fundamental laws of nature ranging from general relativity to nuclear physics. Deriving a single consistent model explaining the observed gravitational waves and electromagnetic counterparts is, however, a very complex undertaking. A key element in this is given by numerical simulations of relativistic hydrodynamics and magnetohydrodynamics. I will give an overview on the state of modeling the merger- and early post-merger phase, with emphasis on the challenges of predicting observable counterparts such as short gamma ray bursts and kilonovae. I will also present simulation results visualizing the complexity of the early post-merger phase which is crucial to the subsequent evolution.

Speaker: Dr. Kastaun Wolfgang (Max Planck Institute for Gravitational Physics Hanover)
• 09:45 10:20
Quasinormal modes of neutron stars with scalar hair 35m Sala de Grados (1st floor)

### Sala de Grados (1st floor)

#### Facultad de Ciencias Fisicas UCM

Plaza de las Ciencias 1, Madrid, Spain

Using the quasinormal mode formalism, we study the ringdown phase of gravitational waves emitted from neutron stars. We consider several alternative models of gravity that include additional scalar degrees of freedom in the action. As a result, in these theories the compact objects are typically surrounded by a non-trivial scalar field. An analysis of the quasinormal modes of these objects reveals differences with respect to General Relativity.

Speaker: Dr. Jose Luis Blázquez-Salcedo
• 10:25 10:45
Gravitational wave echoes from strange stars 20m Sala de Grados (1st floor)

### Sala de Grados (1st floor)

#### Facultad de Ciencias Fisicas UCM

Plaza de las Ciencias 1, Madrid, Spain

We discuss the emission process of GW echoes from ultracompact strange stars. Strange stars are extremely compact, and exotic, objects consisting of deconfined quark matter. If in the merging of two neutron stars a strange star is formed, the GW emission process should result in a GW echo at a frequency of the order of tens of kHz. This would allow to (indirectly) probe the equation of state of quark matter at extremely high densities.

Speaker: Massimo Mannarelli (INFN-LNGS)
• 10:50 11:20
Coffee Break 30m Gabinete CHAIRMAN FOR THE NEXT SESSION: JOSE MANUEL ALARCON ()

### Gabinete CHAIRMAN FOR THE NEXT SESSION: JOSE MANUEL ALARCON

• 11:20 11:40
Mixed configurations of tensor-multi-scalar solitons and neutron stars 20m Sala de Grados (1st floor)

### Sala de Grados (1st floor)

#### Facultad de Ciencias Fisicas UCM

Plaza de las Ciencias 1, Madrid, Spain

We consider special classes of massive tensor-multi-scalar theories of gravity whose target space metric admits Killing field(s) with a periodic flow. For such tensor-multi-scalar theories we show that there exist mixed configurations of tenor-multi-scalar solitons and neutron stars. The influence of the curvature of the target space on the structure of the mixed configurations is studied for two explicit scalar-tensor theories. The stability of the obtained compact objects is examined and it tuns out that the stability region spans to larger central energy densities and central values of the scalar field compared to the pure neutron stars or the pure tensor-multi-scalar solitons. Some astrophysical implications of the mixed soliton-star configurations are also discussed.

• 11:45 12:20
Thermodynamically consistent equation of state for an accreted crust 35m Sala de Grados (1st floor)

### Sala de Grados (1st floor)

#### Facultad de Ciencias Fisicas UCM

Plaza de las Ciencias 1, Madrid, Spain

Neutron stars (NSs) in low-mass X-ray binaries have an accreted crust, whose equation of state and composition differs from that in isolated NSs. To determine it, one usually makes a number of simplifying assumptions regarding both thermodynamics and kinetics of crust matter. We critically revise some of these assumptions and propose new thermodynamically consistent derivation of the crust equation of state. As a by-product of this work, we also present a simple formula showing how much heat is released in the non-equilibrium crust per accreted baryon.

Speaker: Mikhail Gusakov (Ioffe Institute)
• 12:25 12:45
EoS in neutron stars - recent results on cooling and tidal deformabilities 20m Sala de Grados (1st floor)

### Sala de Grados (1st floor)

#### Facultad de Ciencias Fisicas UCM

Plaza de las Ciencias 1, Madrid, Spain

We report on our recent hadronic equation of state for the interior of neutron stars obtained from a new parameterization of the FSU2 relativistic mean-field functional that satisfies these latest astrophysical constraints and, at the same time, reproduces the properties of nuclear matter and finite nuclei while fulfilling the restrictions on high-density matter deduced from heavy-ion collisions. We also present our latest cooling simulations for isolated neutron stars as well as tidal deformabilities of hybrid stars in twin-star configurations.

Speaker: Laura Tolos
• 12:50 13:25
Bulk viscosity of baryonic matter with trapped neutrinos and its application to neutron star mergers 35m Sala de Grados (1st floor)

### Sala de Grados (1st floor)

#### Facultad de Ciencias Fisicas UCM

Plaza de las Ciencias 1, Madrid, Spain

We study bulk viscosity arising from weak current Urca processes in dense baryonic matter at and beyond nuclear saturation density. We consider the temperature regime where neutrinos are trapped and therefore have non-zero chemical potential. We model the nuclear matter in a relativistic density functional approach, taking in to account the trapped neutrino component. We find that the resonant maximum of the bulk viscosity would occur at or below the neutrino trapping temperature, so in the neutrino trapped regime the bulk viscosity decreases with temperature as $T^{-2}$ , this decrease being interrupted by a drop to zero at a special temperature where the proton fraction becomes density-independent and the material scale-invariant. The bulk viscosity is larger for matter with lower lepton fraction, i.e., larger isospin asymmetry. We find that bulk viscosity in the neutrino-trapped regime is smaller by several orders than in the neutrino-transparent regime. We also estimate the damping timescales of the oscillations in neutron star mergers and find that bulk viscosity in neutrino-trapped matter is probably not strong enough to affect the evolution of these events.

Speaker: Dr. Arus Harutyunyan (Byurakan Astrophysical Observatory, Yerevan State University)
• 13:30 15:00
Lunch break 1h 30m Gabinete (ground floor) CHAIRWOMAN FOR THE NEXT SESSION: M. ANGELES PEREZ GARCIA ()

### Gabinete (ground floor) CHAIRWOMAN FOR THE NEXT SESSION: M. ANGELES PEREZ GARCIA

• 15:05 15:25
nEoS: Neutron Star Equation of State from hadron physics alone 20m Sala de Grados (1st floor)

### Sala de Grados (1st floor)

#### Facultad de Ciencias Fisicas UCM

Plaza de las Ciencias 1, Madrid, Spain

We contribute a publicly available set of tables and code to provide Equations of State (EoS) for matter at neutron star densities. Our EoS's are constrained only by input from hadron physics and fundamental principles, without feedback from neutron star observations, and so without relying on General Relativity. They can therefore be used to test General Relativity itself, as well as modified gravity theories, with neutron star observables, without logical circularity. We have adapted state of the art results from NN chiral potentials for the low-density limit, pQCD results for the asymptotically high-density EoS, and use monotonicity and causality as the only restrictions for intermediate densities, for the EoS sets to remain as model-independent as is feasible today.

• 15:30 15:50
An universal upper limit for shear modulus of neutron star crust 20m Sala de Grados (1st floor)

### Sala de Grados (1st floor)

#### Facultad de Ciencias Fisicas UCM

Plaza de las Ciencias 1, Madrid, Spain

In this talk I discuss elastic properties of neutron star crust within model of static Coulomb crystal (ions are treated as point charges; electron screening and ion motion are neglected). I start from discussion of the elasticity tensor of the perfect crystal and demonstrate that it has additional symmetry, which do not depend on the actual crystalline structure and composition. As a particular result of this symmetry, the effective (Voigt averaged) shear modulus of the polycrystalline matter shown to be equal to $-2/15$ of the Coulomb (Madelung) energy density of non-deformed state. This result is applicable even in case of disordered multicomponent structure of the crust. Usage of the linear mixing rule combined with the ion sphere model allows to suggest universal upper limit for the effective shear modulus of polycrystalline or disordered neutron star crust, which can be written as $\sum_i 0.12 n_i Z_i^{5/3} e^2/a_e$. Here summation is taken over ion species, $n_i$ and $Z_i$ are number density and charge of ions of type $i$, respectively. Finally $a_e=(4 pi n_e/3)^{-1/3}$ is electron sphere radius. Quasineutrality condition $n_e=\sum Z_i n_i$ is assumed.

Speaker: Dr. Andrey Chugunov (Ioffe Institute)
• 15:55 16:15
The crust of accreting neutron stars within simplified reaction network 20m Sala de Grados (1st floor)

### Sala de Grados (1st floor)

#### Facultad de Ciencias Fisicas UCM

Plaza de las Ciencias 1, Madrid, Spain

We construct a simplified reaction network model for calculating the multicomponent composition of accreted neutron star crust and apply it to probe the sensitivity of the composition to the choice of the theoretical atomic mass model. Following Haensel & Zdunik [A&A 229 (1990) 117], our approach considers the evolution of matter with increasing pressure and employs Gibbs energy formalism to estimate energy release in nuclear reactions. Following Steiner [Phys. Rev. C 85 (2012) 055804], we handle nuclear reactions kinetics in a multicomponent matter within the simplified stepwise method. Based on characteristic reactions timescales we explicitly specify ‘preference rules’ for different types of nuclear reactions, which are applied if several reactions are energetically allowed. For simplicity, we set the initial composition to be pure 56Fe. We demonstrate that our network reproduces basic features of detailed reaction network by Lau et al. [ApJ 859 (2018) 62], if the same mass model is applied (AME2016[Chinese Physics C36 (2016) 1287]+FRDM95[Atomic Data and Nuclear Data Tables 59 (1995) 185]+DZ31[Phys. Rev. C 52 (1995) R23]). However, usage of HFB21[A&A 549 (2013) A106]+DZ31 mass table for experimentally unknown nuclei leads to a substantially different composition of the inner crust, in particular, the impurity parameter tends to increase with density. We conclude that integrated heat per nucleon, heating profile, and composition of inner crust depends on the selection of the atomic mass model. Moreover, these properties essentially depend on the approach applied for merging of different mass tables.

Speaker: Nikolay Shchechilin (Ioffe Institute)
• 16:20 16:50
Coffee Break CHAIRMAN FOR THE NEXT SESSION: FRANCISCO NAVARRO 30m Gabinete ()

### Gabinete

• 16:50 17:25
Heat transport in neutron stars 35m Sala de Grados (1st floor)

### Sala de Grados (1st floor)

#### Facultad de Ciencias Fisicas UCM

Plaza de las Ciencias 1, Madrid, Spain

I will review the heat transport mechanisms that are most important for
thermal structure of neutron stars and thermal radiation from
neutron-star surfaces. The effects of the heat transport on thermal
evolution and thermal states of different classes of neutron stars will
be highlighted. Special attention will be given to the effects of strong
magnetic fields, to recent developments in the theory and to comparison
with observations.

Speaker: Dr. Alexander Potekhin (Ioffe Institute)
• 17:30 17:50
Generation of strong magnetic fields in neutron stars driven by the permanent mean spin source 20m Sala de Grados (1st floor)

### Sala de Grados (1st floor)

#### Facultad de Ciencias Fisicas UCM

Plaza de las Ciencias 1, Madrid, Spain

We discuss the contribution of the chiral separation effect, which is proportional to the mean spin of plasma, to the system of anomalous MHD equations. It is found that the chiral imbalance in inhomogeneous matter gets supported by the chiral separation effect versus its washing out by spin flipping collisions in plasma. Thus, the magnetic field instability, driven by the chiral magnetic effect, becomes possible. Then we consider the application of this phenomenon to the generation of strong magnetic fields in magnetars. For this purpose, we discuss the saturation regime of the alpha parameter in the Parker thin layer dynamo. We find that a seed magnetic field can be amplified by several orders of magnitude in the time comparable with magnetar ages.
References
1. M. Dvornikov and V. B. Semikoz, Permanent mean spin source of the chiral magnetic effect in neutron stars, JCAP 06 (2019) 053, arXiv:1904.05768.
2. M. Dvornikov and V. B. Semikoz, Magnetic helicity evolution in a neutron star accounting for the Adler-Bell-Jackiw anomaly, JCAP 08 (2018) 021, arXiv:1805.04910.

Speaker: Maxim Dvornikov (Institute of Terrestrial Magnetism, Ionosphere and Radiowave Propagation)
• 17:55 18:10
A second look to the Polyakov Loop Nambu-Jona-Lasinio model at finite baryonic density 15m Sala de Grados (1st floor)

### Sala de Grados (1st floor)

#### Facultad de Ciencias Fisicas UCM

Plaza de las Ciencias 1, Madrid, Spain

We revisit the Polyakov Loop coupled Nambu-Jona-Lasinio model that maintains the Polyakov loop dynamics in the limit of zero temperature. This is of interest for astrophysical applications in the interior of neutron stars. For this purpose we re-examine the form of the potential for the deconfinement order parameter at finite baryonic densities. Since the modification of this potential at any temperature is formally equivalent to assigning a baryonic charge to gluons, we develop a more general formulation of the present model that cures this spurious effect and is normalized to match the asymptotic behaviour of the QCD equation of state given by $\mathcal{O}(\alpha_s^2)$ and partial $\mathcal{O}(\alpha_s^3\ln^2\alpha_s)$ perturbative results.

Speaker: Dr. O. Ivanitskyi / M. Angeles Pérez García
• Friday, 11 October
• 09:15 09:50
Resumming the fermionic in-medium ladder diagrams to all orders (Russian-Iberian congress) by Jose Antonio Oller 35m Aula M2 (ground floor) ()

### Aula M2 (ground floor)

• 09:55 10:30
Study of the core-crust transition in neutron stars with finite-range interactions: the dynamical method (Russian-Iberian Congress) by Xavier Viñas 35m Aula M2 (ground floor) ()

### Aula M2 (ground floor)

• 10:35 10:55
Gravitation wave driven CFS instability of neutron stars and current observational constraints (Russian-Iberian Congress) by Andrey Chugunov 20m Aula M2 (ground floor) ()

### Aula M2 (ground floor)

• 11:00 11:30
Coffee Break 30m Gabinete ()

### Gabinete

• 11:30 12:05
Constraints in Neutron Stars Properties 35m Aula M2 (ground floor) ()

### Aula M2 (ground floor)

I will review the current constraints on the neutron star properties from multi-messenger astrophysical observations. I will then discuss if and how they complement the constraints from nuclear experiments and calculations.

Speaker: Morgane Fortin (CAMK PAN)
• 12:10 12:45
Neutron stars in modified theories of gravity - models, astrophysical implications and gravitational wave emission 35m Aula M2 (ground floor) ()

### Aula M2 (ground floor)

The rapid advance of the neutron star observations offers unprecedented opportunities to test different modification of general relativity. Our talk focuses on a variety of solutions describing neutron stars in alternatives theories of gravity, as well as their astrophysical implications. A special emphasis is made on the gravitational wave emission after neutron star mergers and the possible observable phenomena.

Speaker: Daniela Doneva (University of Tuebingen)
• 12:50 13:25
Measuring H_0 in PTA with gravitational waves (Russian-Iberian congress) 35m Aula M2 (ground floor) ()

### Aula M2 (ground floor)

Speaker: Domenec Espriu
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