Wednesday Colloquium


"Gravitational waves from rapidly rotating neutron stars"

Brynmor Haskell (NCAC, Warsaw)

The recent detection of gravitational waves by Advanced LIGO is the beginning of the era of gravitational wave astronomy. Neutron stars are one of the main targets for current and future gravitational wave detectors, and detecting signals from such sources would allow us to constrain fundamental physics in regimes inaccessible with terrestrial experiments. In this talk I will discuss `continuous’ signals from rapidly rotating neutron stars, and discuss some of the main emission mechanisms that have been proposed, i.e. deformations, or ‘mountains’, sustained by the crust or magnetic field, and unstable modes. In particular I will focus on the r-mode instability in Low Mass X-ray Binaries, and show how combining gravitational wave and X-ray observations can constrain the interior dynamics of the star.


"Creating and Building the Hubble Space Telescope"

Charles Robert O’Dell (Vanderbilt University, Nashville, TN)

Unlike Athena, who sprang full grown from the forehead of Zeus, the Hubble Space Telescope had a long and difficult gestation. It was one of the original goals in building Earth orbiting satellites, but finally came under serious consideration only in 1971. The next two decades first saw battles to gain support from astronomers and financial support from the US and European governments. The next phase saw the challenges of designing and building something that had never been done before—a long duration observatory in space. I’ll then close out with an explanation of the problems with the primary mirror, how these were corrected, and a brief report on the observatory’s condition.


"New vision of the local Group and the specific case of Andromeda II"

Sylvain Fouquet (NCAC, Warsaw)

Since 2000, our vision of the Local Group has evolved a lot. Thanks to new instruments, measures and large surveys, the number of dwarfs galaxies have been multiplied by nearly 3 and new constrains has been found to understand the formation and evolution of the dwarf galaxies. However, a lot of questions that I will present remain open. In this general field of research, I will focus on one specific M31 dwarf galaxy: Andromeda II. I present an evolutionary model for the origin of Andromeda II. The model is an extension of the scenario proposed by Łokas et al. 2014 involving a major merger between two gas-rich disky dwarf galaxies. I also take into account the interaction between the remnant of the merger and M31. This could explain the lack of gas in And II and the sudden stop of the star formation 5 Gyr ago."


"Black Holes and Neutron Stars in our Galaxy as Laboratories for Strong Gravity"

Tomaso Belloni (Osservatorio Astronomico di Brera)

Binary systems containing a black hole or a neutron star offer the best possibility to test prediction of General Relativity in the strong field regime. The plasma stripped from the non-degenerate companion star reaches the space time in the immediate vicinity of the compact object and releases strong X-ray emission. The spectral and variability properties of this emission contain the signatures of predicted effects such as the presence of an innermost stable orbit and black hole spin. I discuss the current observational status with particular emphasis onto sub-second time variability, which constitutes the most direct measurement of the properties of the plasma accreting onto a collapsed star. I present recent results that led to a precise measurement of the spin of a stellar-mass black hole through the measurement of GR effects from fast time variability.


"Thermal instability (or not?) in three-dimensional, global, radiative GRMHD simulations of geometrically thin discs"

Bhupendra Mishra (NCAC, Warsaw)

I shall present results of a set of three-dimensional, general relativistic radiation magnetohydro- dynamics simulations of thin accretion discs to test their thermal stability. We consider two cases, one that is initially radiation-pressure dominated and expected to be thermally unstable and another that is initially gas-pressure dominated and expected to remain stable. Indeed, we find that cooling dominates over heating in the radiation-pressure-dominated case, causing the disc to collapse vertically on the local cooling timescale. On the other hand, the gas-pressure- dominated case, which was run for twice as long as the radiation-pressure-dominated case, remains stable, with heating and cooling roughly in balance. Because the radiation-pressure- dominated disc collapses to the point that we are no longer able to resolve it, we had to terminate the simulation. Thus, we do not know for sure whether it might find a much thinner, stable solution or if it will make a transition to unstable expansion and exhibit limit-cycle be- havior. However, the fact that the cooling rate seems to be dropping faster than heating as the disc collapses suggests that the disc may be headed toward a stable, albeit thinner, solution. It is not clear, though, if the disc will remain radiation-pressure-dominated in that new state.