Question about S5RX-stages roles
Anonymous
14 Oct 2007 18:09:53 UTC
Topic 13605
(moderation:
Quote:
First, S5R2 also was a hierarchical search by design, that is the first stage of the run is done by the volunteers' PCs that crunch workunits and send back results. Then, in a next step, the data would be post-processed and the most most promising candidate regions would be inspected more closely, hence "hierarchical search", I guess.
Not quite.
Einstein@home has been using a method called "F-statistic" (or "Fstat" for short) for searching signals in the detector data since its launch in 2005 (first analysis runs used data from S3). The most severe limitation for the sensitivity of the search is the amount of data we can send back from a client to the server.
The post-processing of the "Fstat" results can be seen as a large data-reduction process where we filter the most promising "candidates" for a gravitational wave signal out of the huge amount of Fstat results. So for the "HierarchicalSearch" we moved one post-processing step into the application that runs on participants machines, so that the same amount of data sent back would be already filtered and thus bear higher sensitive results (actually we increased the sensitivity of the search by a factor of 6 by doing that, Reinhard explained a bit more of that in his S5R3 posting and his poster linked to there). This added a second step of analysis ("Hough") to the Einstein@home Application, that post-processes the results of the first step. Actually this "first step" consists of two independent analyses of the data from two different detectors, so the "second step" can be seen as somewhat "above" the two "first steps" - hence the "Hierarchical" search.
There is another aspect of "Hierarchy", though: The grid we use to cover the (four-dimensional) parameter space is optimized in a way that we should not miss a signal, but it's too coarse to actually tell the physical parameters of a GW source. Once we have promising "candidates", we need to run another search on that area of the parameter space to find the actual parameters of the signal. This process of refinements may be necessary to be repeated.
(for the very curious: the "Hough" analysis grid also differs from the "Fstat" analysis grid, which could also be seen as a hierarchy)
With the computing power available to us now (e.g. with ATLAS) I doubt that these refinements would be done on Einstein@home, unless we actually have thousands of candidates to follow.
BM