Gravitational wave astronomy opened dramatically in September 2015 with the LIGO discovery of a distant and massive binary black hole coalescence. The more recent discovery of a binary neutron star merger, followed by a gamma ray burst and a kilonova
, reinforces the excitement of this new era, in which we may soon see other sources of gravitational waves, including continuous, nearly monochromatic signals. Potential continuous wave (CW) sources include rapidly spinning galactic neutron stars and more exotic possibilities, such as emission from axion Bose Einstein clouds surrounding black holes. Recent searches in Advanced LIGO data are presented, and prospects for more sensitive future searches discussed.
Scorpius X-1 (Sco X-1) and X-ray transient (XTE) J1751-305 are Low-Mass X-ray Binaries (LMXBs) that may emit continuous gravitational waves detectable in the band of ground-based interferometric observatories. Neutron stars in LMXBs could reach a tor
que-balance steady-state equilibrium in which angular momentum addition from infalling matter from the binary companion is balanced by angular momentum loss, conceivably due to gravitational-wave emission. Torque-balance predicts a scale for detectable gravitational-wave strain based on observed X-ray flux. This paper describes a search for Sco X-1 and XTE J1751-305 in LIGO Science Run 6 data using the TwoSpect algorithm, based on searching for orbital modulations in the frequency domain. While no detections are claimed, upper limits on continuous gravitational-wave emission from Sco X-1 are obtained, spanning gravitational-wave frequencies from 40 to 2040 Hz and projected semi-major axes from 0.90 to 1.98 light-seconds. These upper limits are injection validated, equal any previous set in initial LIGO data, and extend over a broader parameter range. At optimal strain sensitivity, achieved at 165 Hz, the 95% confidence level random-polarization upper limit on dimensionless strain $h_0$ is approximately $1.8 times 10^{-24}$. Closest approach to the torque-balance limit, within a factor of 27, is also at 165 Hz. These are the first upper limits known to date on $r$-mode emission from this XTE source. Upper limits are set in particular narrow frequency bands of interest for J1751-305. The TwoSpect method will be used in upcoming searches of Advanced LIGO and Virgo data.
LIGO, the Laser Interferometer Gravitational-wave Observatory, has been designed and constructed to measure gravitational wave strain via differential arm length. The LIGO 4-km Michelson arms with Fabry-Perot cavities have auxiliary length control se
rvos for suppressing Michelson motion of the beam-splitter and arm cavity input mirrors, which degrades interferometer sensitivity. We demonstrate how a post-facto pipeline (AMPS) improves a data sample from LIGO Science Run 6 with feedforward subtraction. Dividing data into 1024-second windows, we numerically fit filter functions representing the frequency-domain transfer functions from Michelson length channels into the gravitational-wave strain data channel for each window, then subtract the filtered Michelson channel noise (witness) from the strain channel (target). In this paper we describe the algorithm, assess achievable improvements in sensitivity to astrophysical sources, and consider relevance to future interferometry.
The LIGO Scientific Collaboration and Virgo Collaboration have carried out joint searches in LIGO and Virgo data for periodic continuous gravitational waves. These analyses range from targeted searches for gravitational-wave signals from known pulsar
s, for which precise ephemerides from radio or X-ray observations are used in matched filters, to all-sky searches for unknown neutron stars, including stars in binary systems. Between these extremes lie directed searches for known stars of unknown spin frequency or for new unknown sources at specific locations, such as near the galactic center or in globular clusters. Recent and ongoing searches of each type will be summarized, along with prospects for future searches using data from the Advanced LIGO and Virgo detectors.
Gravitational wave science should transform in this decade from a study of what has not been seen to a full-fledged field of astronomy in which detected signals reveal the nature of cataclysmic events and exotic objects. The LIGO Scientific Collabora
tion and Virgo Collaboration have recently completed joint data runs of unprecedented sensitivities to gravitational waves. So far, no gravitational radiation has been seen (although data mining continues). It seems likely that the first detection will come from 2nd-generation LIGO and Virgo interferometers now being installed. These new detectors are expected to detect ~40 coalescences of neutron star binary systems per year at full sensitivity. At the same time, research and development continues on 3rd-generation underground interferometers and on space-based interferometers. In parallel there is a vigorous effort in the radio pulsar community to detect ~several-nHz gravitational waves via the timing residuals from an array of pulsars at different locations in the sky. As the dawn of gravitational wave astronomy nears, this review, intended primarily for interested particle and nuclear physicists, describes what we have learned to date and the prospects for direct discovery of gravitational waves.
