14. Searching in coincidence with gamma-ray bursts
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Orbiting satellite detectors observe extremely energetic, non-repeating bursts of gamma-ray radiation from cosmological sources, about once a day. These gamma-ray bursts (GRBs) can be subdivided into two phenomenological, distinct categories: short-duration (< 2s) bursts, which have generally harder spectra, and long-duration (> 2s) bursts, with generally softer spectra. While collapsars leading to the birth of a compact object (a black hole or a neutron star) are known to be the central engine of long GRBs, there is currently no sound explanation for the origin of short GRBs. However, also in this case, astrophysical evidence points to compact objects: the merger of compact binary systems comprising at least a neutron star is believed to trigger short GRBs. Both progenitor categories (collapsars and neutron star binaries) are expected to be sources of transient gravitational waves (GWs). The detection of a gravitational wave signal in coincidence with a GRB would provide tremendous insight in the astrophysical origin of these events and in the mechanisms that power them. Additionally, a collection of gravitational wave merger signals associated to short GRB events with a measured redshift would enable scientists to perform a systematics-free measurement of the Hubble parameter at low redshift. In turn, this would provide constraints on cosmological models.
Within the hypothesis on the origin of GRBs discussed above, the emitted gravitational wave signal would be observable within seconds of the onset of the gamma-ray detection. Therefore, scientists use gravitational wave detectors to perform searches triggered by GRB events. These aim at detecting gravitational radiation at a time and sky location that are consistent with a given GRB. Two distinct data analysis methods are employed.
- Optimal searches with graviational waveform templates that target compact binary mergers in coincidence with short GRBs. Once the advanced detectors will be fully operational, these will be sensitive to short GRBs out to about 400 Mpc in the case of neutron star-neutron star mergers and to about 1 Gpc in the case of neutron star-black hole mergers.
- Unmodelled, burst-type search techniques that target both short and long GRBs. In the latter case, these approaches area advantageous because astrophysical uncertainties and chaotic dynamics do not allow us to currently model the gravitational wave emission of long GRB progenitors.
Cardiff University’s Gravitational Physics Group is at the forefront in developping data analysis software and techniques for both types of searches, and it leads the worldwide endeavour to detect gravitational waves in coincidence with GRB events.