Fourth international LISA symposium

Peter Bender, JILA, University of Colorado

The 4th International LISA (Laser Interferometer Space Antenna) Symposium was held at Penn State on July 20-24, 2002. It followed earlier Symposia at the Rutherford Appleton Laboratory near Oxford (1996), at Caltech (1998), and at the Albert Einstein Institute in Golm, Germany (2000).

The present status of LISA is that it is being planned as a joint mission of ESA and NASA. It officially is a Cornerstone Mission in the ESA program, but with the time schedule to be determined in consultation with NASA. From the NASA side, LISA is in the 5 Year Strategic Plan of the Office of Space Science, which means that the OSS hopes to obtain approval for the mission within 5 years. For planning purposes, a possible launch date of about 2011 is being considered.

The Symposium had 9 half-day sessions. The first gave a broad view of gravitational wave astronomy in the LISA epoch, and started with a talk on Science with LISA by Bernard Schutz. Next, Karsten Danzmann described LISA Technology and the Future. The last two talks were by David Shoemaker on Ground-Based GW Interferometers in the LISA Epoch, and by Massimo Cerdonio on Acoustic GW Detectors for the 2012 Timeframe.

Three of the remaining sessions covered LISA Sources and Source Science. One talk by Gijs Nelemans discussed galactic binaries, including their abundance and what can be learned from their signals. The main topic, however, was massive black holes, and their important role in astrophysics.

One important issue is how black holes grow to the roughly $10^5 M_{\rm Sun}$ or larger range, or whether they form with high masses by sudden collapse of large gas clouds or supermassive stars. Another issue is whether information about galaxy formation can be obtained from coalescences of massive black holes already present in pregalactic or galactic structures that merge. Existing and new information relevant to these topics was given in a number of talks by astrophysicists, that were among the highlights of the Symposium. As an example, Tom Abel discussed calculations of the probable mass distribution of the first stars to form, with the mass range up to about $300 M_{\rm Sun}$ appearing to have been favored. Thus many roughly $100 M_{\rm Sun}$ black holes may have formed early from rapid stellar evolution, and some could have gotten into galactic nuclei where massive black holes were being formed.

Another topic discussed in some detail was coalescences of stellar mass black holes and compact stars with massive black holes. Such highly unequal mass coalescences occur when compact objects in the density cusp around a massive black hole get scattered in close enough to start losing substantial energy by gravitational radiation. Sterl Phinney and Steinn Sigurdsson described both recent and earlier work in this area. In addition, the strong tests of general relativity that can be obtained from observing such events or more equal mass coalescences were discussed by Cliff Will.

Some of the other theoretical and data analysis topics included modeling binary black hole coalescence, radiation reaction in strong fields, formation of neutron stars and stellar mass black holes, templates for highly unequal mass coalescences, analysis of galactic binary signals, and the higher harmonic GW content of black hole binary inspiral signals. Tom Prince finished up the sessions on sources by reviewing the LISA science requirements and science challenges.

The highlights of the experimental and technology development sessions were the detailed discussions of the gravitational reference sensors, which also can be described as free mass units. Each of these devices provides a freely floating test mass inside a housing with capacitive electrodes on its inside to detect any relative motion. What gives LISA its high sensitivity, even at frequencies below $10^{-4} Hz$, is keeping the spurious accelerations of the test masses extremely low. The three LISA spacecraft will be in solar orbit roughly 20 deg. behind the Earth, and form a nearly equilateral triangle 5 million km on a side. Micronewton thrusters are used to keep the spacecraft nearly motionless with respect to the test masses, despite solar radiation pressure and other non-gravitational forces on them. Laser distance measurements are made continuously between the test masses in the different spacecraft, and extremely small GW changes in the distances at periods of roughly a day to a second can be detected.

Almost all aspects of the mission can be tested on the ground. However, it does not appear possible to ground-test the gravitational reference sensors for spurious accelerations to the desired level. Thus ESA is planning a technology validation mission called SMART-2 for a 2006 launch, with demonstrating the gravitational reference sensor performance and the micronewton thrusters as the main objective of the mission. A European LISA Technology Package will be carried to accomplish this, as well as a NASA-provided package called the Disturbance Reduction System. The decision was made recently to provide the US package under the NASA New Millennium Technology Validation Program, since the principles demonstrated will be valuable for a number of other US missions besides LISA.

Two of the major talks were by Stefano Vitale and by William Folkner on the ESA and NASA test packages respectively. They laid the groundwork for additional talks on the detailed design of the units, and on the analysis of the noise sources for them. These included discussions of interferometric measurements of the tiny residual motions of the test masses, control of charging of the test masses due to cosmic rays, mechanisms for caging the test masses during launch, flight experience with accelerometers having many similarities to the gravitational reference sensors, and overall modeling of the expected performance.

Other LISA experimental and technology development talks included two by John Armstrong and Jean-Yver Vinet on the ``time-delay interferometry" approach that will be used in LISA. This method has been developed for cancelling out the effects of phase noise in the lasers and some other undesired effects. Plans for making the required phase measurements on the laser signals also were described, and European work on development of the micronewton thrusters was reviewed. In addition, reports were given on the modeling of the motions of the coupled spacecraft and test mass systems, and on modeling of the whole LISA mission.

One other session was included to review progress on the design and construction of ground-based GW detectors. This included interferometric detectors such as LIGO, VIRGO, GEO-600, and TAMA, and also acoustic detectors. TAMA already is operating fairly frequently, and the first science run for LIGO is scheduled for starting in August, with parallel operations planned by GEO-600 and TAMA. Progress also is continuing with the acoustic detectors, including the development of the first spherical detectors, MiniGRAIL and two others, in The Netherlands, Brazil, and Italy.

The Symposium was sponsored by the National Science Foundation Center for Gravitational Wave Physics at Penn State. Co-sponsors included NASA's Jet Propulsion Laboratory, and travel assistance for some European attendees was provided by the European Space Agency and the new Albert-Einstein-Institute for Experimental Gravitation. The very efficient and helpful Chair of the Scientific Organizing Committee was Sam Finn from Penn State.

Jorge Pullin 2002-09-23