Abstract: Over the last decade or so, dozens of x-ray or optical transient events have been detected towards the centers of (typically) low mass galaxies. They have been interpreted as the disruption of solar mass stars by the central supermassive black holes believed to lie in the centers of most galaxies. Such events were predicted in the 1970s. Early work indicated that the emission would be thermal, with temperatures of order 50 eV, arising from an accretion disk with an inner edge at or near the innermost stable circular orbit of the black hole, and would last a month or so. Loeb and Ulmer suggested that this disk radiation would be reprocessed into the near infrared, and that the event would last a decade or so. Early detections by ROSAT had spectral energy distributions (SEDs) dominated by soft x-rays. More recent events have been detected by optical transient surveys, and have SEDs dominated by UV emission. I will revisit the reprocessing arguments, and show that they actually predict that the emission should be in the UV rather than the near IR, and they further explain the observed constant effective temperature maintained while the luminosity first increases by a factor of ten, then decreases by a factor of 100.
Yichen Shi Black Hole Initiative
Abstract: We consider a rapidly spinning black hole surrounded by an equatorial, geometrically thin, slowly accreting disk that is stationary and axisymmetric. We analytically compute the broadening of electromagnetic line emissions from the innermost part of the disk, which resides in the near-horizon region. The result is independent of the details of the disk model and therefore universal. This is an example of critical behavior in astronomy that is potentially observable by current or future telescopes.
Abstract: Within the next few years we can anticipate that the LIGO/Virgo detectors will have observed many tens or even hundreds of binary compact object merger events. One avenue to extract more information from this catalog is to stack the signals from a subset of events that are expected to share a common feature, enhancing the effective signal-to-noise ratio that the feature can be measured with. Thanks to the uniqueness properties of black holes in Einstein gravity, binary black hole mergers are ideal targets for stacking, allowing for stringent tests of dynamical, strong-field gravity, or detecting deviations from the predictions of general relativity.
I will describe an initial study exploring the utility of stacking to detect higher-order quasi-normal ringdown modes post-merger.
Though binary neutron star systems do not share such a uniqueness property, there may nevertheless be aspects of merger signals that could be enhanced using stacking. I will discuss one such example that would seek to detect a post-merger signal from the subclass of events where a hypermassive remnant forms.
Aleksi Vuorinen Helsinki Institute of Physics
Abstract: Outside the interiors of black holes, neutron stars contain the densest forms of matter in our present-day Universe. This makes them a unique laboratory for strong interaction physics, as novel phases of QCD matter may be present in their extremely dense cores, or produced at the high temperatures reached in stellar mergers. In my talk, I will concentrate on the quantitative constraints that various types of neutron star observations, including the gravitational wave signatures of their mergers, have recently set for the properties of dense nuclear and quark matter. In particular, I will demonstrate that the Equation of State of cold and dense QCD matter is significantly constrained by the known existence of two-solar-mass stars and by the recent LIGO constraint on the tidal deformabilities of the two stars involved in the gravitational wave observation GW170817.
Abstract: I will discuss some recent work on the problem of motion in general relativity, done in collaboration with Bob Geroch (Chicago). I will introduce a sense in which a collection of bodies -- including the solutions to some hyperbolic system -- could be said to "track" a curve, and then discuss some of the consequences of this idea. See arXiv:1707.04222 [gr-qc] for details.