Source confusion from neutron star binaries in ground-based gravitational wave detectors is minimal
Aaron D. Johnson, Katerina Chatziioannou, Will M. Farr
Abstract
Upgrades beyond the current second generation of ground-based gravitational wave detectors will allow them to observe tens of thousands neutron star and black hole binaries. Given the typical minute-to-hour duration of neutron star signals in the detector frequency band, a number of them will overlap in the time-frequency plane, resulting in a nonzero cross-correlation. We examine ``source confusion'' arising from overlapping signals whose time-frequency tracks cross. Adopting the median observed merger rate of $100\text{ }\text{ }{\mathrm{Gpc}}^{\ensuremath{-}3}\text{ }{\mathrm{yr}}^{\ensuremath{-}1}$, each neutron star binary signal overlaps with an average of 42(4)[0.5] other signals when observed from 2(5)[10] Hz. The vast majority of overlaps occur at low frequencies where the inspiral evolution is slow: 91% of time-frequency overlaps occur in band below 5 Hz. The combined effect of overlapping signals does not satisfy the central limit theorem and source confusion cannot be treated as stationary, Gaussian noise: on average 0.91(0.17)[0.05] signals are present in a single adaptive time-frequency bin centered at 2(5)[10] Hz. We quantify source confusion under a realistic neutron star binary population and find that parameter uncertainty typically increases by less than 1% unless there are overlapping signals whose detector-frame chirp mass difference is $\ensuremath{\lesssim}0.01{M}_{\ensuremath{\bigodot}}$ and the overlap frequency is $\ensuremath{\gtrsim}40\text{ }\text{ }\mathrm{Hz}$. Out of $1\ifmmode\times\else\texttimes\fi{}{10}^{6}$ simulated signals, 0.14% fall within this region of detector-frame chirp mass differences, but their overlap frequencies are typically lower than 40 Hz. Source confusion for ground-based detectors, where events overlap instantaneously, is significantly milder than the equivalent Laser Interferometer Space Antenna problem, where many classes of events overlap for the lifetime of the mission.