Joint LSC/Source Modeling Meeting, 20-24 March 2002

Patrick R Brady, University of Wisconsin-Milwaukee
patrick@gravity.phys.uwm.edu

In parallel with the March meeting of the LIGO Scientific Collaboration (LSC), there was a workshop to discuss formation of source working groups for interferometric gravitational-wave detectors. The workshop was motivated by the growing need for a tighter coupling of the gravitational-wave source-analysis community to the experimental gravitational-wave projects. The meeting took place at the LIGO Observatory in Livingston, Louisiana over three days interlaced between the LSC meeting. Many of the presentations are available from the LSC web site [1].

On the evening of the first day, the source analysts gave a series of short (15 minute) presentations about the state of source modeling as a field. Over the course of a long evening from 4pm until 10pm, attendees were shown the breadth of ongoing work. The presentations covered inspiral of compact binaries, binary black hole mergers, binary neutron star mergers, neutron-star black-hole mergers, stellar collapse and neutron star vibrations. Despite the short time available for each presentation and the long evening, presenters attempted to give a flavor of their work, where it stands, and where it is going.

The second day of the meeting was given over to the formal sessions of the LSC in the morning, and to presentations about how source analysts might interface with data analysts in the afternoon.

The third day of the meeting began with a series of presentations/panel discussions by data analysts. These presentations were intended to initiate the discourse on the need for information about sources. Considerable emphasis was placed on data analysis techniques other than matched-filtering, which the source analysis community is already familiar with. The discussion focused on the use of time-frequency methods in the detection of gravitational waves, emphasizing how information from the source analysts can be used to develop data analysis algorithms even in the absence of knowing the full waveform. Two examples of urgent needs for the ground-based detectors are: (1) Information on the phase evolution of waves from the late (``IBBH") stage of compact binary inspiral (where post-Newtonian expansions fail). (2) Information from numerical relativity simulations of black-hole merger waveforms which can be used in applying the excess-power algorithm. During the session on LISA data analysis, the needs were shown to be somewhat different at this time. For example, the first urgent need is to acquire sufficient science information to firm up LISA's noise curve by December 2003.

By the end of the session, it was clear that source analysis should be guided by the data analysis needs, and the development of data analysis algorithms will be more effective if source analysts participate to some degree. As the community moves from the goal of first detection of gravitational waves toward gravitational-wave astronomy, interpretation of the data will inevitably lead to closer coupling between source analysts and data analysts; when that happens, those with experience will benefit most from the mutual give and take.

Separate from this meeting, NASA and NSF have been discussing a possible new initiative to fund source modeling efforts in the gravitational wave community. Members of a joint NASA/NSF panel, chaired by Saul Teukolsky, were present at the meeting and took the opportunity to obtain feedback for their report. An open discussion, in which Teukolsky explained the panel's mandate¹ took place on the afternoon of the second day. The discussion was animated and helped to identify the needs of the community. The report from the Teukolsky panel is now available [2].

The third day finished with a discussion of how to coordinate the efforts of the two communities more coherently. Several different methods were advocated: closer connection with the experiments [for example, through the Astrophysical Source Identification and Signatures (ASIS) group of the LSC], groups organized by source and led by their members. Once again, the discussion was animated and brought out problems with each of the possible ways to organize. In many cases, the purpose of such organization was called into question. In the end, however, it was clear that those involved in target-source data analysis had a strong desire to bring the source analysis community closer to the data analysis activities. Moreover, there is a community which wants a closer connection to the gravitational-wave experiments without direct obligation to provide deliverables to the LSC or another collaboration. The outcome of the discussions was the formation of several source groups to facilitate communication among its members with two goals:

1. The data analysts should educate the source analysts about gravitational-wave data analysis, and most especially about what kinds of source-analysis information will be useful, and in what ways, in data analysis.
2. The source analysts should educate the data analysts about source analysis and simulations, and most especially about what kinds of information it will be possible to supply for the data analysis and on what timescales.

Following the workshop in March, five source groups have formed. (In fact, another group centered on sources of stochastic gravitational waves is also forming.) At least two facilitators have been identified within each group (one data analyst and one source analyst) in the hope that they can help the groups get started. The facilitators' roles may become less important as the groups begin to self-organize. Information about the groups can be found at http://www.lsc-group.phys.uwm.edu/gwawg along with links about membership and to mailing lists for each group. Anybody interested in participating should join the mailing list which will be the venue for preliminary discussion/organization. The five existing groups are listed here with a descriptive paragraph adapted from the web site:

Inspiral of Comparable Mass Binaries

This group will focus on the late stages of inspiral, up to the final plunge and merger, for BH/BH, NS/BH, and NS/NS binaries. Source analysis is currently being carried out via post-Newtonian methods, resummation methods, and numerical relativity methods. Data analysis is currently via matched filters using post-Newtonian (or kludged) waveforms, and by the fast chirp transform. Source Analysis Facilitators: Bala Iyer and Thomas Baumgarte
Data Analysis Facilitator: Jolien Creighton

Binary Black Hole Mergers

This group will focus on plunge from the innermost stable circular orbit and the final merger of binary black holes. Source analysis is currently by numerical relativity techniques. Data analysis is currently via various time-frequency techniques. Source Analysis Facilitators: Bernd Bruegman and Luis Lehner
Data Analysis Facilitator: Patrick Brady

NS/NS and NS/BH Binary Mergers

This group will focus on plunge induced by combined general relativity and tidal couplings, bar formation and evolution, oscillations of neutron stars. It will also focus on tidal disruption of the neutron star by the black hole in NS/BH binaries. Source analysis is currently by numerical simulations (Newtonian, post-Newtonian, and fully relativistic). Data analysis is currently via various time-frequency techniques. Source Analysis Facilitators: Fred Rasio and Masaru Shibata
Data Analysis Facilitator: Ben Owen

Stellar Collapse

This group will focus on core collapse at end of stellar evolution to form a neutron star, a centrifugally hung-up protoneutron star, and/or a black hole; accretion-induced collapse of a white dwarf; collapse of a supermassive star for LISA; dynamical or secular instabilities of neutron stars. Source analysis is currently by numerical simulations (Newtonian, post-Newtonian, and fully relativistic), and by study of stability and dynamics of equilibria models. Data analysis is currently via time-frequency techniques. Source Analysis Facilitator: Tony Mezzacapa
Data Analysis Facilitator: Warren Anderson

Inspiral of Compact Objects into Supermassive Black Holes

This group is already rather far along in its organization and work, and is attached to LISA Working Group 1. It focuses on white dwarfs, neutron stars, and stellar mass black holes captured into small-periastron orbits around a supermassive black hole in a galactic nucleus; evolution of the orbits under radiation reaction and via perturbations of other orbiting objects and via interaction with accretion disks; transition to plunge and capture. Source analysis is via black-hole perturbation theory and N-body simulations for capture computations as input for estimating capture rates. Data analysis is expected to be by hierarchical techniques that involve mixed coherent (matched-filter) and incoherent methods. Data analysis work includes separating the strongest sources from the background of weaker inspiral waves and WD/WD binary waves. Source Analysis Facilitator: Scott Hughes
Data Analysis Facilitator: Curt Cutler

Most of these groups are embryonic and all interested parties are encouraged to join in and help define their activities.

References:

[1] LIGO Scientific Collaboration web site:
web/lsc/lsc.html">http://www.ligo.caltech.edu/LIGO${}_{}$web/lsc/lsc.html.

[2] Preliminary Report of the Task Group for an NSF/NASA Computational Initiative on Gravitational Wave Science is available from http://gravity.phys.psu.edu/~tggweb/ or http://astrogravs.gsfc.nasa.gov.