LIGO Document P1400217-v3

Gravitational waves from Sco X-1: A comparison of search methods and prospects for detection with advanced detectors

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The low-mass X-ray binary Scorpius X-1 (Sco X-1) is potentially the most luminous source of continuous gravitational-wave radiation for interferometers such as LIGO and Virgo. Unlike for recycled pulsars, this radi- ation would be sustained by active accretion of matter from its binary companion. With the Advanced Detector Era fast approaching, work is underway to develop an array of robust tools for maximizing the science and detection potential of Sco X-1. We describe the plans and progress of a project designed to compare the nu- merous independent search algorithms currently available. A mock-data challenge is employed by which the search pipelines are tested for their relative proficiencies in parameter estimation, dependence upon parameter space volume, computational efficiency, robustness, and most importantly, search sensitivity. The mock-data challenge data contains an ensemble of Scorpius X-1 (Sco X-1) type signals to allow a useful comparison of search methodologies, and simulated noise assuming the expected Advanced LIGO/VIRGO best strain sensi- tivity (4 10−24 Hz−1/2) in the 100-500 Hz band. A factor of 2 in strain separates the quietest detected signal, at 6.8 10−26 strain, from the torque-balance limit at a spin frequency of 300 Hz, although this limit could range from 1.2 10−25 (25 Hz) to 2.2 10−26 (750 Hz) depending on the unknown frequency of Sco X-1. (For emission due to quadrupolar deformation, the gravitational wave frequency will be twice the spin frequency.) With future improvements to the search algorithms and using advanced detector data, our expectations for probing below the theoretical torque-balance strain limit are optimistic.
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