Session U11: Gravitational Wave Astronomy II and Compact Objects

LIGO data and the stochastic gravitational wave background

From the analysis of scientific data taken in late 2002, LIGO was able to improve the constraints on $\Omega_{\rm GW}$ by a factor of order $10^4$ over the best, previously available, interferometer result. With data taken in early 2005, and still not at the end of commissioning, LIGO could improve on its own first result by another factor of order $10^6$. Now, with commissioning complete, and more than half way through accumulating a year of useable data, LIGO is approaching a position of being able to challenge some of the more exotic theories of the early universe. With the completion of the current data run, and the implementation of proposed upgrades to the interferometers - even enhanced LIGO - further constraints on a cosmically generated stochastic background of gravitational waves will become a reality. Practical prospects for this are presented, along with anticipated consequences for the understanding of our cosmological origins.   

Constraining off-Kerr deviations using intermediate mass ratio inspirals in Advanced LIGO

The inspiral of a stellar mass compact object into an intermediate mass black hole is a promising source for the detection of gravitational waves in Advanced LIGO. The gravitational waves from such intermediate mass ratio inspirals (IMRIs) will act as a probe of the spacetime structure of the massive central body. If the IMRI central body is a Kerr black hole, then the multipolar structure of it's spacetime is determined only by its mass and spin. We discuss the prospects with which Advanced LIGO can measure deviations of the central body's spacetime from that of Kerr. We find the central body's quadrupole moment and tidal coupling can be measured with modest but interesting accuracy.   

Externally triggered searches of gravitational-wave bursts with Tikhonov regularization technique

Searches for gravitational waves with arbitrary waveforms benefit from combining data from several detectors in a coherent way. An algorithm for such coherent detection, based on the Tikhonov regularization scheme, is presented here. In the absence of prior knowledge of the signal, the regularization functional is uniform with regard to the location of the source in the sky and the frequency of the anticipated signal. Such searches for gravitational-wave bursts can be greatly facilitated by coincidences with electro-magnetic observations, such as those provided by the X-ray and gamma-ray detectors. These externally-triggered network searches for bursts will be conducted using the data from several interferometric gravitational wave detectors, including LIGO, GEO and Virgo. The results from trial runs of this algorithm on simulated data are described.   

Searching for Gravitational-Wave Bursts Associated with Gamma-Ray Bursts during the LIGO S5 Run

We present the status of a search for short-duration gravitational-wave bursts (GWBs) associated with gamma-ray bursts (GRBs) detected by gamma-ray satellite experiments during LIGO's ongoing S5 science run. There is now a sample of more than 100 GRB triggers, most of which were observed by the Swift satellite, that is available for this GRB-GWB search. The search involves calculating the crosscorrelation between two LIGO interferometer data streams surrounding the GRB trigger time. We search for associated gravitational radiation from single GRBs, and also apply statistical tests to search for a gravitational-wave signature associated with the collective sample.   

Simulation of a Galactic Population of Gravitational Wave Emitting Pulsars

We have simulated using a Monte Carlo process few plausible galactic populations of Gravitational Wave emitting neutron stars. The simulation is based on a range of astrophysical models to determine statistical distributions of neutron stars' parameters that are relevant to the search of Gravitational Waves through detectors as LIGO or VIRGO (and the advanced versions of such detectors). In particular we have focused on neutron stars' spatial distribution in the Galaxy, proper motion, frequency and spin-down distributions and likely ellipticities. Some preliminary results of our simulations with interesting implications for the Gravitational Wave detection are discussed.   

Core-collapse supernova simulation with effective general relativity

We have performed multi-dimensional numerical radiation hydrodynamic simulations of core-collapse supernovae. Our numerical code can handle realistic nuclear reactions as well as spectral neutrino transport coupled with fluid motions. In order to take into account a strong general relativistic (GR) gravity that is important in this context, we employ an effective GR gravity formalism recently proposed by Marek et al.(Astron.\& Astrophys., 445, 273 (2006)). The formalism is exact in a static spherical star and has shown a good agreement with GR collapse simulation. We will present our preliminary results in this talk.   

Astrophysical constraints on BH-NS and NS-NS mergers and short GRBs

Gravitational-wave detectors are expected to observe binary mergers in the near future, improving on our understanding of compact binary formation and evolution. In this talk we describe the range of merger rates expected from state-of-the-art population synthesis models for the Milky Way; we summarize existing observational constraints in the Milky Way; and we describe how constraints improve our understanding of binary evolution, using existing (electromagnetic) and expected future (gravitational-wave) observations. However, because long delays can occur between binary birth and merger and because most star formation occurred long ago, binaries born long ago in old elliptical galaxies can also contribute significantly to the present-day merger rate. Using recent results on the cosmological census and star formation history, we summarize the presently plausible range of LIGO detection rates. Though these additional uncertainties complicate astrophysical interpretations of LIGO detections, we suggest that additional observations of short GRBs and of merging binary parameter distributions can reduce the associated ambiguity and allow gravitational-wave observations to significantly constrain binary evolution.   

Black hole formation by rotational instabilities

We investigate the role of rotational instabilities in the context of black hole formation in relativistic stars. In addition to the standard scenario - an axially symmetric dynamical instability forming a horizon at the star's center - the recently found low-$T/|W|$ instabilities are shown to lead to fragmentation and off-center horizon formation in differentially rotating stars. This process might be an alternative pathway to produce SMBHs from supermassive stars with inefficient angular momentum transport.   

Light Curves of Millisecond-Period Accreting X-ray Pulsars

We present the first raytracing computations for light emitted from the surface of a rapidly rotating neutron star in order to construct light curves for X-ray pulsars and bursters. These calculations are for realistic models of rapidly rotating neutron stars which take into account both the correct exterior metric and the oblate shape of the star. Our motivation is to test the validity of the commonly used Schwarzschild + Doppler approximation (S+D). We find that the most important difference between the exact calculation and the S+D approximation arising from rotation comes from the oblate shape of the rotating star. We find that approximating a rotating neutron star as a sphere introduces serious errors in fitted values of the star's radius and mass if the rotation rate is very large. We will explain the simple geometric origin of the effect and present simple approximations that allow the oblate shape to be properly included in raytracing programs which use the Schwarzschild metric.