Analysis of spatial mode sensitivity of gravitational wave interferometer and targeted search for gravitational radiation from the Crab pulsar

Abstract

Over the last several years the Laser Interferometer Gravitational Wave Observatory (LIGO) has been making steady progress in improving the sensitivities of its three interferometers, two in Hanford, Washington, and one in Livingston, Louisiana. These interferometers have reached their target design sensitivities and have since been collecting data in their fifth science run for well over a year. On the way to increasing the sensitivities of the interferometers, difficulties with increasing the input laser power, due to unexpectedly high optical absorption, required the installation of a thermal compensation system. We describe a frequency resolving wavefront sensor, called the phase camera, which was used on the interferometer to examine the heating effects and corrections of the thermal compensation system. The phase camera was also used to help understand an output mode cleaner which was temporarily installed on the Hanford 4 km interferometer. Data from the operational detectors was used to carry out two continuous gravitational wave searches directed at isolated neutron stars. The first, targeted RX J1856.5-3754, now known to be outside the LIGO detection band, was used as a test of a new multi interferometer search code, and compared it to a well tested single interferometer search code and data analysis pipeline. The second search is a targeted search directed at the Crab pulsar, over a physically motivated parameter space, to complement existing narrow time domain searches. The parameter space was chosen based on computational constraints, expected final sensitivity, and possible frequency differences due to free precession and a simple two component model.

(cont.) An upper limit on the strain of gravitational radiation from the Crab pulsar of 1.6 x 10-24 was found with 95% confidence over a frequency band of 6 x 10-3 Hz centered on twice the Crab pulsar's electromagnetic pulse frequency of 29.78 Hz. At the edges of the parameter space, this search is approximately 105 times more sensitive than the time domain searches. This is a preliminary result, presently under review by the LIGO Scientific Collaboration.