Detector configuration of DECIGO/BBO and identification of cosmological neutron-star binaries

Abstract

The primary target for the planned space-borne gravitational wave interferometers DECIGO/BBO is a primordial gravitational wave background (PGWB). However there exist astrophysical foregrounds and among them, gravitational waves from neutron star (NS) binaries are the solid and strong component that must be identified and subtracted. In this paper, we discuss the geometry of detector configurations preferable for identifying the NS/NS binary signals. As a first step, we analytically estimate the minimum signal-to-noise ratios (SNRs) of the binaries for several static detector configurations that are characterized by adjustable geometrical parameters, and determine the optimal values for these parameters. Next we perform numerical simulations to take into account the effect of detector motions, and find reasonable agreements with the analytical results. We show that, with the standard network formed by 4 units of triangle detectors, the proposed BBO sensitivity would be sufficient in receiving gravitational waves from all the NS/NS binaries at z ≤ 5 with SNRs higher than 25. We also discuss the minimum sensitivity of DECIGO required for the foreground identification.

Cosmology with space-based gravitational-wave detectors: Dark energy and primordial gravitational waves

“…Strictly speaking, the NS-binary foreground should be initially included and each binary signal has to be iteratively subtracted one by one for the consistency of the calculation. However, as the authors of [29] have performed, we can simplify the procedure by neglecting the contribution from the NSbinary foreground in the calculation of SNR. This simplification would be valid because large GW signals can be easily subtracted from the data to the noise level that is limited by the instrumental noise and the WD foreground.…”

“…In a recent paper [29], Yagi and Seto have performed Monte-Carlo simulations generating synthetic GW data for binaries at random sky positions and with orientations of angular-momentum axes over redshifts up to z ¼ 5. They estimated how much residuals are left after the subtraction process.…”

“…In Ref. [29], the fractional residual R NS is provided as a function of th = " e ð5Þ, where " e ð5Þ is the SNR for an edgeon NS binary at the redshift z ¼ 5. The R NS is shown in Fig.…”

“…The NS residual fraction R NS as a function of th = " e ð5Þ is given by Yagi and Seto in [29]. For the convenience of the analysis in this paper, we fit their result and provide the fitting formula [82].…”

“…For its search, one needs to accurately identify the waveforms of NS binaries and subtract them up to the level sufficiently below the amplitude of the primordial GW background [28,29]. Thus, an accurate determination of the waveforms of NS binaries is essential both for the utilization of the NS binaries as the standard siren and the detection of the primordial GW background.…”