### Conveners

#### Compact Stars as Laboratories for Testing Strong Gravity: Block 1

- Aurora Maria Perez Martinez (Instituto de Cibernetica Matematica y Fisica ICIMAF)
- Cesar Augusto Zen Vasconcellos (ICRANet/UFRGS)

### Description

There are reasons to believe the 21st century will be the best ever for astrophysics: the James Webb Space Telescope will extend nearly twenty times the present observational limit of visible light; neutrino massiveness opens a new window for exploration on dark energy and dark matter physics and is expected to provide insights into the fate of the Universe; the Higgs boson may allow for an understanding of the weakness of gravity; gravitational waves produced at the birth of the Universe and by compact stellar objects (supermassive black holes, black hole/neutron star mergers, gamma-ray bursts, white dwarf inspirals) have unveiled a new area of astronomy. Framed by this background, compact stars represent an unique astrophysical laboratories for probing the fabric of space-time and the building blocks of matter and their interactions at physical regimes not attainable in terrestrial laboratories. The strong gravitational fields of compact stars - black holes, pulsars, neutron, and exotic stars - provide this way an unique test ground for strong gravity and modified theories of gravity and can offer restrictions for extended theories of general relativity. The aim of this session is to bring together researchers from cosmology, particle physics, nuclear theory and astrophysics, working on these topics from different but complementary viewpoints.

We propose two models for constant density relativistic perfect-fluid spheres supported by thin shell configurations. These models are obtained from the Schwarzschild constant density star solution: the first via the collapse of the external layers of the fluid into a thin shell by performing a matching with the exterior Schwarzschild solution at a matching radius smaller than the star radius;...

Thermal evolution of neutron stars is studied in the $f(R)=R+\alpha R^{2}$ theory of gravity.

We first describe the equations of stellar structure and evolution for a spherically symmetric spacetime plus a perfect fluid at rest.

We then present numerical results for the structure of neutron stars using four dense matter equations of state and a series of gravity theories for

$\alpha$...

We present our studies on the neutrino pairs annihilation into electron-positron pairs ($\nu{\bar \nu}\to e^-e^+$) near the surface of a neutron star in the framework of extended theories of gravity. The latter modifies the maximum energy deposition rate near to the photonsphere and it might be several orders of magnitude greater than that computed in the framework of General Relativity. These...

Neutron stars in scalar-tensor theories may undergo spontaneous scalarization, which is important for probing the theories with binary pulsar and gravitational wave observations. Since the effect is nonlinear, most studies of spontaneous scalarization were carried out numerically. In the first part of my talk, I explain how one can compute the effect of scalarization analytically based on a...

Gravitational-wave(GW) sources can serve as standard sirens to probe cosmology by measuring their luminosity distance and redshift. Such standard sirens are also useful to probe theories beyond General Relativity with a modified GW propagation. Previous studies on the latter assume multi-messenger observations so that the luminosity distance can be measured with GWs while the redshift is...

Neutron stars are ideal astrophysical sources to probe general relativity due to their large compactnesses and strong gravitational fields. For example, binary pulsar and gravitational wave observations have placed stringent bounds on certain scalar-tensor theories in which a massless scalar field is coupled to the metric through matter. A remarkable phenomenon of neutron stars in such...

We present the first application of a hierarchical Markov Chain Monte Carlo (MCMC) follow-up on continuous gravitational-wave candidates from real-data searches. The follow-up uses an MCMC sampler to draw parameter-space points following the F-statistic. As outliers are narrowed down, coherence time increases, imposing more restrictive phase-evolution templates. We introduce a novel Bayes...

In this work we investigate neutron stars (NS) in f (R, T ) gravity

for the case R + 2λT , R is the Ricci scalar, and T the trace of the

energy-momentum tensor. The hydrostatic equilibrium equations are

solved considering realistic equations of state (EOS). The NS masses

and radii obtained are subject to a joint constrain from massive pulsars

and the event GW170817. The parameter λ needs...

A. Perez Martinez^{1T}, M. A. Perez-Garcia^{1(∗)}, E. Rodriguez Querts^{2} and A. Romero Jorge^{2}

Vacuum in presence of magnetic field exhibits birrefrigence. We have obtained this effect from linear correction of dispersion relation of photon travelling perpendicular to the magnetic field valid even for magnetic fields close to...

In this contribution we identify two scenarios for the evolutionary branch cut universe. In the first scenario, the universe evolves continuously from the negative complex cosmological time sector, prior to a primordial singularity, to the positive one, circumventing continuously a branch cut, and no primordial singularity occurs in the imaginary sector, only branch points. In the second...