Improving the detection sensitivity to primordial stochastic gravitational waves with reduced astrophysical foregrounds

One of the primary targets of third-generation (3G) ground-based gravitational wave (GW) detectors is detecting the stochastic GW background (SGWB) from early universe processes. The astrophysical foreground from compact binary mergers will be a major contamination to the background, which must be reduced to high precision to enable the detection of primordial background. In this work, we revisit the limit of foreground reduction computed in previous studies, point out potential problems in previous foreground cleaning methods and propose a novel cleaning method subtracting the approximate signal strain and removing the average residual power. With this method, the binary black hole foreground is reduced with fractional residual energy density below $10^{-4}$ for frequency $f\in (10, 10^2)$ Hz, below $10^{-3}$ for frequency $f\in (10^2, 10^3)$ Hz and below the detector sensitivity limit for all relevant frequencies in our simulations. Similar precision is achieved to clean the foreground from binary neutron stars (BNSs) that are above the detection threshold, so that the residual foreground is dominated by sub-threshold BNSs, which will be the next critical problem to solve for detecting the primordial SGWB in the 3G era.Comment: 12 pages, 4 fig

Science Potential for Stellar-mass Black Holes as Neighbors of Sgr A*

It has been suggested that there is possibly a class of stellar-mass black holes (BHs) residing near (distance $\le 10^3 M$) the galactic center massive black hole, Sgr A*. Possible formation scenarios include the mass segregation of massive stellar-mass black holes and/or the disk migration if there was an active accretion flow near Sgr A* within $\mathcal{O}(10)$ Myr. In this work, we explore the application of this type of objects as sources of space-borne gravitational wave detectors, such as Laser Interferometer Space Antenna (LISA). We find it is possible to probe the spin of Sgr A* based on the precession of the orbital planes of these stellar-mass black holes moving around Sgr A*. We also show that the dynamical friction produced by accumulated cold dark matter near Sgr A* generally produces small measurable phase shift in the gravitational waveform. In the case that there is an axion cloud near Sgr A*, the dynamical friction induced modification to gravitational waveform is measurable only if the mass of the axion field is in a narrow range of the mass spectrum. Gravitational interaction between the axion cloud and the stellar-mass black holes may introduce additional precession around the spin of Sgr A*. This additional precession rate is generally weaker than the spin-induced Lense-Thirring precession rate, but nevertheless may contaminate the spin measurement in a certain parameter regime. At last, we point out that the multi-body gravitational interaction between these stellar-mass black holes generally causes negligible phase shift during the LISA lifetime.Comment: 12 pages, 11 figures v2: new Fisher parameter added; threshold SNR changes slightly; v3: matches with the published version

Repeating Fast Radio Bursts from Neutron Star Binaries: Multi-band and Multi-messenger Opportunities

Recent observations indicate that magnetars may commonly reside in merging compact binaries and at least part of fast radio bursts (FRBs) are sourced by magnetar activities. It is natural to speculate that a class of merging neutron star binaries may have FRB emitters. In this work, we study the observational aspects of these binaries - particularly those with FRB repeaters, which are promising multi-band and multi-messenger observation targets of radio telescopes and ground based gravitational wave detectors as the former telescopes can probe the systems at a much earlier stage in the inspiral than the latter. We show that observations of FRB repeaters in compact binaries have a significant advantage in pinning down the binary spin dynamics, constraining neutron star equation of state, probing FRB production mechanisms, and testing beyond standard physics. As a proof of principle, we investigate several mock observations of FRB pulses originating from pre-merger neutron star binaries, and we find that using the information of FRB arriving times alone, the intrinsic parameters of this system (including the stellar masses, spins, and quadrupole moments) can be measured with high precision, and the angular dependence of the FRB emission pattern can also be well reconstructed. The measurement of stellar masses (with an error of $\mathcal{O}(10^{-6}-10^{-5})$) and quadrupole moments (with an error of $\mathcal{O}(1\%-10\%)$) may be an unprecedented discriminator of nuclear equations of state in neutron stars. In addition, we find the multi-band and multi-messenger observations of this binary will be sensitive to alternative theories of gravity and beyond standard models, e.g., dynamical Chern-Simons gravity and axion field that is coupled to matter.Comment: 20 pages, 12 fig