LIGO Document G1600001-v1

Model-Based Cross-Correlation Search for Gravitational Waves from Scorpius X-1

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Seminar given at ICTS Bangalore, 2016 January 11

The low-mass x-ray binary (LMXB) Scorpius X-1 (Sco X-1, presumed to be a binary consisting of a neutron star which is accreting matter from a low-mass companion) is one of the most promising potential sources of gravitational waves (GWs) which may be observed by the generation of GW detectors---such as Advanced LIGO, Advanced Virgo and KAGRA---which will begin operation this year with the first Advanced LIGO observing run, and Advanced Virgo and KAGRA observations expected to follow in the coming years. Nonaxisymmetric deformations in the neutron star can give rise to gravitational radiation, most of which is emitted at twice the rotation frequency of the neutron star. Such deformations can be maintained by the accretion of matter onto the neutron star. It has been conjectured (e.g., by Bildsten in 1998) that the neutron star's rotation may be in an approximate equilibrium state, where the spin-up torque due to accretion is balanced by the spin-down due to GWs. Scorpius X-1's high x-ray flux implies a high accretion rate, which makes it the most promising potential source of observable GWs among known LMXBs.

Since Sco X-1 is not seen as a pulsar, its rotation frequency is unknown. There is also residual uncertainty in the orbital parameters which determine the Doppler modulation of the signal, monochromatic in the neutron star's rest frame, which reaches the solar-system barycenter. This parameter uncertainty limits the effectiveness of the optimal coherent search for periodic GWs, as the parameter space resolution needed for a fully coherent search renders the projected computing costs unfeasible. I describe a tunable semi-coherent method based on cross-correlation of data over a shorter coherence time, which allows a tradeoff between sensitivity and computing time. I present sensitivity estimates for hypothetical searches of advanced LIGO and advanced Virgo data, as well results of analysis of "mock data" containing simulated signals injected into simulated Gaussian noise.

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