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Directed searches for broadband extended gravitational-wave emission in nearby energetic core-collapse supernovae

Core-collapse supernovae are factories of neutron stars and stellar mass black holes. Type Ib/c supernovae stand out as potentially originating in relatively compact stellar binaries and their branching ratio of about 1\% into long gamma-ray bursts. The most energetic events probably derive from central engines harboring rapidly rotating black holes, wherein accretion of fall-back matter down to the Inner Most Stable Circular Orbit (ISCO) offers a window to {\em broadband extended gravitational-wave emission} (BEGE). To search for BEGE, we introduce a butterfly filter in time-frequency space by Time Sliced Matched Filtering. To analyze long epochs of data, we propose using coarse grained searches followed by high resolution searches on events of interest. We illustrate our proposed coarse grained search on two weeks of LIGO S6 data prior to SN 2010br $(z=0.002339)$ using a bank of up to 64 thousand templates of one second duration covering a broad range in chirp frequencies and bandwidth. Correlating events with signal-to-noise ratios $>6$ from the LIGO L1 and H1 detectors each reduces to a few events of interest. Lacking any further properties reflecting a common excitation by broadband gravitational radiation, we disregarded these as spurious. This new pipeline may be used to systematically search for long duration chirps in nearby core-collapse supernovae from robotic optical transient surveys using embarrassingly parallel computing.