Session DD02: V: Gravitational Waves

Improved upper limits on gravitational wave emission from NS 1987A

We report on a new search for continuous gravitational waves from NS 1987A, the neutron star born in SN 1987A, using open data from Advanced LIGO and Virgo’s third observing run (O3).  The search covered frequencies from 35 to 1050 Hz, more than five times the band of the only previous gravitational wave search to constrain NS 1987A.  It used an improved code and coherently integrated from 5.10 days to 14.85 days depending on frequency. No astrophysical signals were detected. By expanding the frequency range and using O3 data, this search improved on strain upper limits from the previous search and was sensitive at the highest frequencies to ellipticities of 1.6 × 10⁻⁵ and r-mode amplitudes of 4.4 × 10⁻⁴, both an order of magnitude improvement over the previous search and both well within the range of theoretical predictions.   

Recovering dark-matter effects in gravitational waveforms using parametrized post-Einsteinian templates

As a massive black hole (BH) grows in a dark matter (DM) halo, an over-density may form near the BH, called a DM spike. A binary formed from the massive BH with the DM spike and a compact secondary produces gravitational waves that dephase from vacuum binaries without DM. The planned LISA detector will be sensitive enough to detect this dephasing and characterize the DM spike surrounding the massive BH. We show that using gravitational waveforms without the DM effect to analyze this non-vacuum merger biases the measured parameters. However, these biases can be significantly reduced, and the DM effects determined, by using a waveform that includes the parameterized post-Einsteinian model that was originally developed for capturing non-GR effects.   

Experimental Investigation of Shielding of a Dynamic Gravitational Field

With many advances in gravitational wave research (The LIGO Scientific Collaboration et al., 2015), fully controlled laboratory experiments on dynamic gravitational fields become more and more important (Hirakawa et al., 1980; Astone et al., 1998; Ross et al., 2021, Brack et al. 2022). For static fields, it has not been possible to measure any generally accepted shielding effects for gravitational fields (Majorana, 1920; Braginsky et al., 1963; Unnikrishnan et al., 2000). To the authors’ knowledge, no corresponding experiments are known for dynamic fields, with attenuation expected to be proportional to frequency squared from theory (Weber, 1966).

Usually, dynamic experiments consist of a transmitter system, i.e. a periodically moving mass distribution, and a detector system. Two such systems have been described recently (Brack et al., 2022, 2023). In the latter, the transmitter system consists of  two rotating tungsten bars. The detector consists of a high Q (10⁴), 42 Hz resonant bending beam of 1m length made of titanium. Its motion is analyzed using three calibrated laser Doppler vibrometers and multichannel lock-in amplifiers. Of paramount importance is the passive and active vibration isolation of the detector from ambient noise and crosstalk from the transmitter. The interaction is quantitatively modelled with high precision and excellent agreement between theory and experiment. The transmitters' homogeneity is characterized using neutron tomography. Here, we present a new feature of the setup, where a ~400kg metal shield (with dimensions 1.4m x 0.25m x d, where d is its thickness) is periodically lowered in between transmitter and detector. Two methods are presented to analyze the signals for the two shield positions (with/without shield): Measuring the frequency response of the detector and fitting a single degree of freedom response function or a continuous near resonance excitation with fixed frequency. Currently an upper bound for the relative change of the response signal of about 10^-3 has been established for a brass shield at a frequency of more than four orders of magnitude higher than previous work.

    

Regularizing Parameterized Black Hole Spacetimes with Kerr Symmetries (I): Metric

Parameterized Kerr spacetimes allow us to test the nature of black holes in model-independent ways. In the series of talks, we focus on the parameterized spacetime preserving Killing symmetries of a Kerr spacetime. In this first talk, we will show that an unphysical divergence may appear in the metric if arbitrary functions in the metric are expanded about infinity and truncated at a finite order. To remedy this, we propose to redefine the arbitrary functions so that the divergence disappears, at least for several known black hole solutions that can be mapped to the parameterized Kerr spacetime. We then explain the Petrov type of the refined parameterized Kerr spacetime.   

Regularizing Parameterized Black Hole Spacetimes with Kerr Symmetries (II): Observables

Parameterized Kerr spacetimes allow us to test the nature of black holes in model-independent ways. In the series of talks, we focus on the parameterized spacetime preserving Killing symmetries of a Kerr spacetime. In this second talk, we will show from the metric one can derive expressions describing physical observables of black holes. We will discuss the gravitational-wave ringdown frequencies and the shapes of black hole shadows for several example deviation parameters.   

A hybrid model for comparable-mass binary-black-hole gravitational waveforms

Gravitational waves (GW) from binary-black-hole mergers have three parts: inspiral, merger and ringdown. Post-Newtonian (PN) and black-hole-perturbation (BHP) theories model the inspiral and ringdown parts of the waveform, respectively. A hybrid approximation method was previously used during the merger by applying PN and BHP theories at the same times in different spatial regions and matching the results at a boundary where the theories were either both valid or had errors that did not affect the waveform. The prior work used leading PN dynamics and non-rotating BHP theory; this led to errors during the late inspiral and disagreement with the dominant quasinormal mode (QNM) frequency observed in numerical relativity (NR) simulations during the ringdown, respectively. We modified the hybrid method to achieve a better match with NR waveforms as follows: During the inspiral we used Effective-One-Body (EOB) dynamics rather than PN, and we evolved the waveform on an effective BHP theory using a modified Poschl-Teller potential tuned to match the dominant QNM of the remnant black hole. By optimizing the potential, the matching region, and adding an effective 1PN parameter, we could match NR waveforms for comparable mass ratios up to q = 8 with the accuracy of order 10^-3.   

Challenges in Highly-Parallel Simulations of the Einstein Equations

In this talk we discuss some of the significant challenges to efficient, large-scale numerical relativity simulations. These challenges include  developing efficient adaptive mesh refinement algorithms with many levels of refinement and finding optimum refinement criteria, and numerical issues related to the structure of the Einstein Equations themselves. The latter is particularly important when optimizing algorithms for GPU computation.