Second Gravitational Wave Phenomenology Workshop

Neil Cornish, Montana State University
It is likely that Sam Finn had to twist some arms to get a good turnout of astrophysicists at the First Gravitational Wave Phenomenology Workshop in November 2001, but two years later, with the scent of data in the air, he would have had a hard time keeping them out of the second workshop, which took place at Penn State on November 6-8, 2003.

The goal of the workshop was to foster dialog between observers, astrophysical source modelers, and data analysts, on the question of how gravitational wave observations could be used to ``inform our understanding of the cosmos'' (to quote the workshop website, The workshop was designed to be discussion heavy, with short talks providing the starting point for longer discussions.

The meeting began with an update on the three types of gravitational wave detectors currently in operation, and the implications that these observations have on astrophysical models. Stan Whitcomb described the various ground based Laser Interferometers; Giovanni Prodi talked about Acoustic (Bar) Detectors; and Andrea Lommen discussed Pulsar Timing Arrays. A common theme was that even null detections can be used to place bounds on astrophysical processes, and while the current upper limits are not that strong in most cases, we are already doing gravitational wave astronomy. Andrea gave one example where Pulsar Timing had been able to rule out the suggestion that the Radio Galaxy 3C66B harbors a supermassive black hole binary of $5.4 \times 10^{10} \, M_{\odot}$. Another common theme in the detector session was optimism for the future. Stan boldly predicted that LIGO and VIRGO would reach their design sensitivities in 2004, Giovanni saw a bright future for dual resonant bars, and Andrea foresaw Pulsar Timing Arrays being able to detect the gravitational wave background produced by supermassive black hole binaries. The impact of the observations on our understanding of Neutron Star Physics, Stellar Populations and Burst Sources was then addressed by Ben Owen, Vicky Kalogera and Andrew MacFadyen. Ben outlined how targeted searches for periodic signals from know Pulsars could be used to constrain the material properties of a Neutron Star's crust. Vicky emphasized that large uncertainties in the theoretical models mean that any observational input - upper bounds or a few direct detections - would strongly constrain certain stellar population models. As an illustration of this point, Vicky showed how the recent discovery of a third binary pulsar in our galaxy has lead to a six-fold increase in the predicted Neutron Star Inspiral event rate for LIGO. In the same vein, Andrew explained how uncertainties in the modeling of Gamma Ray bursts would be reduced by co-ordinated gravitational and electromagnetic observations, even if no gravitational wave counterparts were found.

The second day of the workshop was devoted to gravitational wave source astrophysics and source modeling. Brad Hansen reviewed the evidence for Intermediate Mass Black Holes (IMBH) and brought up the interesting possibility that an IMBH might be responsible for dragging the observed population of young, massive stars into the lair of the supermassive black hole at the galactic center. Christian Cardall and Adam Burrows described supernova modeling and what the gravitational wave signatures might look like. They had good news for gravitational wave astronomers - supernova simulations fizzle unless there is asymmetry. Adam showed some possible waveforms with very interesting ringing structure. Milos Milosavljevic took on the ``last parsec problem'' and concluded that it probably wasn't much of a problem - a range of physical mechanisms could succeed in getting black hole binaries close enough to evolve under radiation reaction. Leor Barak brought us up to date on the impressive progress that has been made in solving the self-force problem, and how the new calculation schemes were being used to model extreme mass ratio inspiral. Deirdre Shoemaker and Masaru Shibata led the discussion on numerical relativity, and its application in the areas of black hole and neutron star binary evolution. Both conceded that significant problems remained to be solved, especially when black holes enter the picture, but they also showed how partial results were better than no results. Shibata drew an analogy between numerical relativity and the operation of the laser interferometers. The system is up and running, and while the performance is below the design goals, they are collecting data and learning as they go.

The final day had us looking to the future, and the promise of advanced detectors. Sam Finn discussed second generation ground based laser interferometers, and emphasized how astrophysical considerations will play a role in making design choices. Eugenio Coccia described how future acoustic detectors, with their improved sensitivities and wider bandpasses, can provide coverage of the very high frequency portion of the spectrum. The final speaker, Neil Cornish, got a little carried away with the ``advanced detector'' theme, and after a brief description of the Laser Interferometer Space Antenna (LISA) and its potential impact, he went on to describe two post-LISA missions. These follow-on missions use multiple detectors for improved angular resolution, and in the case of the Big Bang Observatory, co-aligned interferometers for detecting the Cosmic Gravitational Wave Background.

A proper account of the workshop would report on the discussions that followed the talks, as these discussions involved all the workshop participants, and took us well beyond the formal presentations. But lacking extensive notes, I'll have to skip the best part and simply encourage people to attend the third meeting in the series.

Jorge Pullin 2004-03-12