MG14 - Talk detail

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Abstract

We propose a new parametric framework to describe in generic metric theories of gravity the spacetime of axisymmetric black holes. In contrast to similar approaches proposed so far, we do not use a Taylor expansion in powers of $M/r$, where $M$ and $r$ are the mass of the black hole and a generic radial coordinate, respectively. Rather, we use a continued-fraction expansion in terms of a compactified radial coordinate. This choice leads to superior convergence properties and allows us to approximate a number of known metric theories with a much smaller set of coefficients. The measure of these coefficients via observations of near-horizon processes can be used to effectively constrain and compare arbitrary metric theories of gravity.

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Session

Accepted

Yes

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Time

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Abstract

Determining the equation of state of matter at nuclear density and hence the structure of neutron stars has been a riddle for decades. We show how the imminent detection of gravitational waves from merging neutron star binaries can be used to solve this riddle. Using a large number of accurate numerical-relativity simulations of binaries with nuclear equations of state, we find that the postmerger emission is characterized by two distinct and robust spectral features. While the high-frequency peak has already been associated with the oscillations of the hypermassive neutron star produced by the merger and depends on the equation of state, a new correlation emerges between the low-frequency peak, related to the merger process, and the total compactness of the stars in the binary. More importantly, such a correlation is essentially universal, thus providing a powerful tool to set tight constraints on the equation of state.

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Session

Accepted

Yes

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Time

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Title

Abstract

A large international effort is under way to assess the presence of a shadow in the radio emission from the compact source at the center of our galaxy, Sag-A*. If detected, this shadow would provide the first evidence of the existence of black holes and that Sag-A* is a supermassive black hole. In addition, the shape of the shadow could be used to learn about extreme gravity near the event horizon and to determine which theory of gravity better describes the observations. The mathematical description of the shadow has so far been done making use of a number of simplifying assumptions that are unlikely to be offered by the real observational data. We here provide a general formalism to describe the shadow as an arbitrary polar curve expressed in terms of a Legendre expansion. Our formalism does not presume any knowledge of the properties of the shadow and offers a number of routes to characterize the properties of the curve and its distortions that can be implemented across different analyzing teams.

Pdf file

Session

Accepted

Yes

Order

Time

Talk

Title

Abstract

The merger of a binary of neutron stars provides natural explanations for many of the features of short gamma-ray bursts (SGRBs), such as the generation of a hot torus orbiting a rapidly rotating black hole, which can then build a magnetic jet and provide the energy reservoir to launch a relativistic outflow. However, this scenario has problems explaining the recently discovered long-term and sustained X-ray emission associated with the afterglows of a subclass of SGRBs. We propose a new model that explains how an X-ray afterglow can be sustained by the product of the merger and how the X-ray emission is produced before the corresponding emission in the gamma- band, though it is observed to follow it. Overall, our paradigm combines in a novel manner a number of well- established features of the emission in SGRBs and results from simulations. Because it involves the propagation of an ultra-relativistic outflow and its interaction with a confining medium, the paradigm also highlights a unifying phenomenology between short and long GRBs.

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