Talk detail

Participant	Macedo, Caio Filipe Bezerra
Institution	Universidade Federal do Para - Rua Augusto Correa 1 - Belem - Para - Brazil

Session	BH4	Accepted	Yes	Order	17	Time	17:40	10'
Talk	Oral abstract	Title	Slowly Rotating Anisotropic Neutron Stars in General Relativity and Scalar-Tensor Theory
Coauthors	Silva, Hector O.; Berti, Emanuele; Crispino, Lu\'is C. B. Crispino
Abstract	Some models (such as the Skyrme model, a low-energy effective field theory for QCD) suggest that the high-density matter prevailing in neutron star interiors may be significantly anisotropic. Anisotropy is known to affect the bulk properties of nonrotating neutron stars in General Relativity. In this paper we study the effects of anisotropy on slowly rotating stars in General Relativity. We also consider one of the most popular extensions of Einstein's theory, namely scalar-tensor theories allowing for spontaneous scalarization (a phase transition similar to spontaneous magnetization in ferromagnetic materials). Anisotropy affects the moment of inertia of neutron stars (a quantity that could potentially be measured in binary pulsar systems) in both theories. We find that the effects of scalarization increase (decrease) when the tangential pressure is bigger (smaller) than the radial pressure, and we present a simple criterion to determine the onset of scalarization by linearizing the scalar-field equation. Our calculations suggest that binary pulsar observations may constrain the degree of anisotropy or even, more optimistically, provide evidence for anisotropy in neutron star cores.
Pdf file

Session	BH6	Accepted	Yes	Order	7	Time	17:15	15'
Talk	Oral abstract	Title	Absorption of planar massless scalar waves by Bardeen regular black holes
Coauthors	Crispino, Lu\'is C. B.
Abstract	Accretion of fields by black holes is a subject of great interest in physics. It is known that accretion plays a fundamental role in active galactic nuclei and in the evolution of black holes. Accretion of fundamental fields is often related to the study of absorption cross section. Basically all black holes for which absorption of fields has been studied so far present singularities. However, even within general relativity, it is possible to construct regular black holes: objects with event horizons but without singularities. Many physically motivated regular black hole solutions have been proposed in the past years, demanding the understanding of their absorption properties. We study the absorption of planar massless scalar waves by Bardeen regular black holes. We compare the absorption cross section of Bardeen and Reissner–Nordstr\"om black holes, showing that the former always have a bigger absorption cross section for fixed values of the field frequency and of the normalized black hole charge. We also show that it is possible for a Bardeen black hole to have the same high-frequency absorption cross section of a Reissner–Nordstr\"om black hole. Our results suggest that, in mid-to-high-frequency regimes, regular black holes can have compatible properties with black holes with singularities, as far as absorption is concerned.
Pdf file