GRG18 Day 5
The first plenary this morning was by Maria A. Papa on data analysis for ground based gravitational wave detection, with a few brief comments on LISA plans. She focussed on the most sensitive current results for different observation types, namely inspiralling compact objects, bursts, contnuous quasi-periodic phenomena and the stochastic background. The most recent results for the S3 and S4 science runs with LIGO for inspiralling objects are given in gr-qc/0704.3368. No plausible events were found. What is the expected detection rate? This is estimated using a unit of cumulative luminosity $L$ which is 10^10 times the value for the sun. For binary systems of (two times) 0.5, 1.4 or 10 solar masses respectively, the maximum distance goes like 5.7, 16.1 or 77 Mpc using the S4 analysis. This corresponds to a rate of about 1 per 1000 years, or a bit better, for black hole binaries at 100 Mpc. This should be greatly improved with the advanced detector setups previously discussed. Moreover, for S5 data (which has been taken for 20 months now) one expects roughly 1 per 10 to 800 years for a 2x10 solar mass binary.
A detection efficiency analysis was also discussed and included in the analysis. Then Papa went onto summarise the intriguing cases of searches which were triggered by electromagnetic observations. In particular, in February this year, there was a gamma ray burst GRB070201 at only 1.1 degree from the centre of M31, which is only 800 kpc from the Milky Way. Unfortunately, it appears that Hanford LIGO was the only GW facility operating at the time, but 2 detectors were in science mode. Theory suggests that GRBs are associated to either neutron star mergers or neutron star / black hole mergers. There will be another session this afternoon discussing this event with the latest data set. Another triggered search was in 2004, for the hyperflare object SGR 1806-20, which is thought to be due to magnestar quakes. The reference here is astro-ph/0703419. It is expected that a two times improvement in detector sensitivity will begin to reach interesting energy regimes for these objects.
Other sources discussed were long-lived signals from stochastic background, and continuous signals from pulsars. For one particular pulsar (Crab) the upper limit set by the first year of S5 data is lower than the limit from the spin down rate of the source. However, not all energy is emitted in gravitational waves, as discussed for example in Palomba et al (AA 354 (2000)). The S5 data can already constrain moment of inertia and ellipticity for such objects. In summary, GW observations are beginning, but just beginning, to contribute interesting astrophysical information. Papa concluded with a nice quote attributed in spirit to Kip Thorne, which says that if gravitational waves are finally observed "no cherished belief would be challenged".