Sensitivity of the network

Left plot shows the combined antenna-pattern functions (a measure of the sensitivity of the network) of a network consisting of two LIGO detectors in Hanford, one LIGO in Livingston and the Virgo detector in Italy (HHLV). Right plot shows the same for a network consisting of one LIGO detector in Hanford, LIGO Livingston, Virgo and LIGO-India (HLVI). Redder regions correspond to regions of higher sensitivity and bluer regions to regions of poorer sensitivity. Locations of the detectors (LIGO-Hanford, LIGO-Livingston, Virgo, LIGO-India, KAGRA) are marked by filled black circles. (Pic. P. Ajith)

Angular resolution of the network

Interferometric GW detectors are nearly omnidirectional instruments. The sky localization of the source is achieved by combining data from geographically separated detectors (`aperture synthesis'), and the sky-localization accuracy is proportional to the baseline width of the network. In the plot above, the projected globe shows the available networks and their baselines. HHLV network has one 3-site network. HILV has four 3 site networks with a baseline that is 1.5 times longer than the longest HHLV baseline.The light-travel time (in milliseconds) of each baseline is also shown (Pic. B. S. Sathyaprakash).

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The localization accuracy for binary neutron stars located at 160 Mpc (oriented face-on with respect to the line of sight) using GW observations. The ellipses contain the 90% localization regions for binaries located at various points in the sky (in geographic coordinate system). Left plot corresponds to the HHLV network and the tight plot to the HLVI network. Red crosses correspond to sky locations where the network would not confidently detect the binary. (Pic: S. Fairhurst)

Duty cycle of the network

Duty cycle of a detector is the fraction of time it takes quality "science data". Since the two Hanford detectors were to share the same vacuum tube, the operation of these two detectors are not completely independent — when one detector is "out of lock", it can affect the data quality of the other. If we assume that each detector has a duty cycle of 80%, and that the duty cycles of the two Hanford detectors are not independent, then the duty cycles of networks consisting of 1, 2, 3 and 4 sites is given in the figure. A minimum of 3-site is required to triangulate a source on the sky. The HHLV network consists at most of a 3-site network and its duty cycle for this configuration is 51%. HLVI, on the other hand, is a 4-site network and so consists of one 4-site configuration with a duty cycle of 41% and four 3-site configurations with a duty cycle of 41%. Thus, the duty cycle for HLVI with three or more sites is 82% as opposed to 51% of HHLV.

Further reading

• Sec. VII of the Proposal for LIGO-India.