Session M13: Gravitation and Astrophysics

Prospects for observing dynamically formed stellar mass black hole binaries with gravitational waves

Stellar mass black hole binaries are expected to be produced through dynamical interactions within globular clusters. Dynamically formed black hole binaries are likely to have higher chirp masses than binaries formed in the field. We show that these systems are observable with space-based gravitational wave detectors at extragalactic distance as far as the the Virgo cluster.   

Binary Neutron Star Mergers: Prospects for Multimessenger Observations

We evolve a binary system of two, equal-mass neutron stars in a quasi-circular orbit through and past merger. We consider different nuclear equations of state, which vary from soft to quite stiff, and allow for magnetization of the system and neutrino cooling via a leakage scheme. Here, I focus on potential observables, other than gravitational waves, produced mainly by the hot, strongly magnetized matter resulting from the merger and study their dependence on both the equation of state and the initial magnetic field strength.   

Integrated GW-EM Follow-up Analysis

Advanced Gravitational-Wave (GW) detectors such as Advanced LIGO and Advanced Virgo are expected to become operational for observation runs in 2015, with an expected ultimate improvement in sensitivity over previous configurations by a factor of 10 by 2019. There are many potential electromagnetic (EM) counterparts to GWs including short and long gamma-ray bursts (GRBs) and kilonovae. While SGRBs and LGRBs predominantly emit in the X-ray, and the recently-observed kilonova primarily in the infrared, all three sources are expected to have detectable traces in the optical band, albeit requiring very sensitive optical telescopes. In order to aid in the optimization of GW trigger follow-up procedures, we perform an end-to-end analysis feasibility study using synthesized Advanced detector data simulating a GW detection with a theoretical EM counterpart injected into archival optical images. We use images from Robotic Optical Transient Search Experiment (ROTSE) and Palomar Transient Factory (PTF), and inject candidate events following observed lightcurves of SGRBs, LGRBs, and kilonovae. The use of Zernike PSF decomposition on candidate objects offers a fast way to identify point sources, speeding up the automated identification of transient sources in the images. We present our method of transient recovery and the latest results of our feasibility study of a joint GW-EM observation.   

Probing the Milky Way electron density using multi-messenger astronomy

Multi-messenger observations of ultra-compact binaries in both gravitational waves and electromagnetic radiation supply highly complementary information, providing new ways of characterizing the internal dynamics of these systems, as well as new probes of the galaxy itself. Electron density models, used in pulsar distance measurements via the electron dispersion measure, are currently not well constrained. Simultaneous radio and gravitational wave observations of pulsars in binaries provide a method of measuring the average electron density along the line of sight to the pulsar, thus giving a new method for constraining current electron density models. We present this method and assess its viability with simulations of the compact binary component of the Milky Way using the public domain binary evolution code, BSE.   

Neutron Stars are Follicly Challenged

Black holes satisfy certain no-hair relations through which all multipole moments of the spacetime can be specified in terms of just a few quantities, like their mass and spin angular momentum. I will describe how neutron stars and quark stars also satisfy similar no-hair relations that are approximately independent of their equation of state. I will show how these results hold for both slowly- and rapidly-rotating stars in full General Relativity, provided the stars are uniformly rotating and uncharged. I will then explain why such relations may be relevant to observations of the pulse profile of hot spots on rotating neutron stars with NICER, as well as how they could be used to test General Relativity with binary pulsar and gravitational wave observations.   

Why neutron stars have three hairs

Neutron stars have recently been found to enjoy a certain `baldness' in their multipolar structure which is independent of the equation of state (EoS) of dense nuclear matter. This is reminiscent of the black hole no-hair relations, and in stark contrast to regular stars. Why is this? Is it because realistic EoSs are sufficiently similar, or because GR effects are especially important, or because the nuclear matter is `cold'? We explore the physics behind these and more hypotheses, and give a convincing explanation for the true origin of the three-hair relations.   

I-Love-Q Anisotropically

Recent work shows that rotating incompressible stars with anisotropic matter in the weak-field limit become prolate, which is rather counter-intuitive. We construct slowly-rotating, incompressible and anisotropic stellar solutions in full General Relativity valid to quadratic order in spin and show that the stellar shape shifts from prolate to oblate as one increases the relativistic effect. Anisotropic stars are also interesting because they can be more compact than isotropic stars, and can even be as compact as black holes. We present how stellar multipole moments approach the black hole limit as one increases the compactness, suggesting that they reach the black hole limit continuously.   

Hydrodynamic circularization in stellar tidal disruption events

In the tidal disruption of a star by a black hole, the process by which debris forms a disk and generates flares and/or jets is not well-understood. We investigate this process using a new numerical tool to simulate the circularization of debris from the disrupted star. A post-Newtonian hydrodynamic simulation (FNC) of the star itself as it is torn apart is combined with a fully general relativistic hydrodynamics simulation (Harm3d) of the subsequent motion of the stellar debris as it orbits the black hole. The characteristic length scale at which the tidal streams merge to form an accretion flow is much larger than classical expectation. Furthermore, the time for accumulation of mass into the flow is significantly longer than the characteristic orbital period of most tightly-bound tidal streams. We discuss how these results can be applied to recently observed tidal disruption candidates.   

Gravitomagnetic Acceleration of Black Hole Accretion Disk Matter to Polar Jets

It is shown that the motion of the neutral masses in an accretion disk orbiting a black hole creates a magnetic-like (gravitomagnetic) field that vertically accelerates neutral particles near the accretion disk away from the disk and then inward toward the axis of the accretion disk. Moreover, as the accelerated material nears the axis, a frame-dragging effect twists the trajectories around the axis thus contributing to the formation of a narrow polar jet emanating from the poles.