Bulletin of the American Physical Society
APS April Meeting 2022
Volume 67, Number 6
Saturday–Tuesday, April 9–12, 2022; New York
Session Y15: Astrophysics with Gravitational Waves: Propagation EffectsRecordings Available
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Sponsoring Units: DGRAV DAP Chair: Leo Stein, University of Mississippi Room: Marquis C |
Tuesday, April 12, 2022 1:30PM - 1:42PM |
Y15.00001: Propagation of Gravitational Waves in an Inhomogeneous Universe Jared Fier In this talk, we shall present our recent studies on gravitational waves (GWs) produced by remote compact astrophysical sources. To describe such GWs properly, we introduce three scales, the typical wavelength of GWs, the scale of the cosmological perturbations, and the size of the observable universe. For GWs to be detected by the current and foreseeable detectors, we show that such GWs can be well approximated as high-frequency GWs. To simplify the field equations, we show that the spatial, traceless, and Lorentz gauge conditions can be imposed simultaneously, even when the background is not vacuum, as long as the high-frequency GW approximation is valid. Applying the general formulas, along with the geometrical optics approximation, we calculate the gravitational integrated Sachs-Wolfe effects due to the presence of the cosmological scalar and tensor perturbations, whereby the dependences of the amplitude, phase and luminosity distance of the GWs on these two kinds of perturbations are read out explicitly. |
Tuesday, April 12, 2022 1:42PM - 1:54PM |
Y15.00002: A Catalog of Dark Matter Spike Distributions: Effect on Gravitational Waves and their Detectability Nicholas Speeney Dark matter (DM) spikes are regions of over-density in the distribution of galactic DM halos, created by the growth of a central black hole inside of a DM cloud. The presence of such over-densities may affect the gravitational waveform from a binary system by changing the gravitational potential, and through dynamical friction effects. These effects can be used to probe the DM properties gravitationally, independently from proposed electromagnetic signals from DM self-annihilation. We numerically calculate DM spike distributions for Hernquist and NFW profiles following the prescription in Sadeghian et al. [Phys.Rev.D 88, 063522 (2013)] for a Schwarzschild background. We then use these DM profiles to calculate the gravitational waveform in the stationary phase approximation. We compare the DM induced dephasing to post-Newtonian corrections, and discuss implications for the detectability of signals from DM spikes. A main result of this work is a catalog of DM spikes and their GW effects, which will aid in parameter estimation of DM properties from GW measurements. |
Tuesday, April 12, 2022 1:54PM - 2:06PM |
Y15.00003: Latest results from the search for lensing signatures in gravitational-wave observations Ka Yue Alvin Li Gravitational lensing, while being extensively studied with electromagnetic waves, can also be observed with gravitational waves. In this presentation, we highlight the latest search results for gravitational lensing signatures in binary black hole merger events detected by Advanced LIGO and Advanced Virgo. We focus on estimating and interpreting lensing rates, searching for multiple gravitational-wave signals caused by strong lensing from galaxies or galaxy clusters, and wave-optics effects induced by point-mass microlenses. |
Tuesday, April 12, 2022 2:06PM - 2:18PM |
Y15.00004: Astrophysically-Informed Search of Strongly-Lensed Gravitational-Wave Transients from GWTC-1 Rico Ka Lok Lo When a compact binary merger that LIGO and Virgo see is sufficiently aligned with a gravitational potential such as that from a galaxy or a galaxy cluster at a cosmological distance, strong lensing takes place and multiple copies of the same transient will be registered by GW detectors at separate times and amplitudes as different detections. The relative amplitudes and time delays of these lensed GW signals would depend on the properties of the lens responsible, and would also affect the likelihood ratio that a set of GW signals being lensed "images" coming from the same source, or simply all coming from different sources. In this talk, I will present a search of strongly-lensed GW transients from GWTC-1 that incorporates information about the potential galaxy lenses using a simulated lens catalog, as well as information about the intrinsic properties and the redshift distribution of the GW sources. Selection effects are accounted for in this analysis, which tend to lower the likelihood ratio for the strong lensing hypothesis. |
Tuesday, April 12, 2022 2:18PM - 2:30PM |
Y15.00005: Identifying multiple images in strongly lensed gravitational wave sources Saif Ali, Evangelos Stoikos, Lindsay King, Michael Kesden Strong gravitational lensing of gravitational waves (GW) occurs when the waves from a compact binary system travel near a massive object close to the line of sight. The geometrical-optics approximation is valid only when the GW frequency is much larger than the time delay between the multiple images; otherwise, wave optics must be employed. We begin with two axially symmetric lenses, the point mass and singular isothermal sphere, which can produce at most two images. When the geometrical-optics approximation is valid, we introduce two model-independent lens parameters: the flux ratio and the time delay between the GW signals. We assess the validity of geometrical optics as a function of these lens parameters. We then calculate the mismatch between the lensed and unlensed GWs. From this, we estimate the minimum signal-to-noise ratio required to distinguish lensed GW signals from unlensed signals. We then consider a singular isothermal ellipsoid lens that can produce up to four images and compute the mismatches between lensed GW signals with two or four images. This allows us to establish whether two-image templates will suffice for the identification of GW events by more complicated lenses. |
Tuesday, April 12, 2022 2:30PM - 2:42PM |
Y15.00006: Lensing or luck? False alarm probabilities for gravitational lensing of gravitational waves Mesut Caliskan, Jose Maria Ezquiaga, Otto A Hannuksela, Daniel Holz Strong gravitational lensing of gravitational wave sources is expected to be detected in the upcoming LIGO/Virgo/KAGRA observing runs. However, definitively distinguishing pairs of lensed sources from random associations is a challenging problem. We investigate the degree to which non-lensed events mimic lensed ones because of the overlap of parameters due to a combination of random coincidence and errors in parameter estimation. We construct a mock catalog of lensed and non-lensed events. We find that the probability of a false alarm based on coincidental overlaps of the chirp mass, sky location, and coalescence phase are approximately 11%, 1%, and 10% per pair, respectively. Combining the three, we arrive at a false alarm probability per pair of 10-4. As the number of events, N, in the GW catalogs increases, the number of random pairs of events increases as ∼Ν2. Meanwhile, the number of lensed events will increase linearly with N, implying that for sufficiently high N, the false alarms will always dominate over the actual lensing events. This issue can be compensated for by placing higher thresholds on the lensing candidates (e.g., selecting a higher signal-to-noise (SNR) threshold), which will lead to better parameter estimation and thus lower false alarm rates per pair, at the cost of dramatically decreasing the size of the lensing sample (by ∼SNR3). We show that with our simple overlap criteria for current detectors at design sensitivity, the false alarms will win for realistic lensing rates (≤10-3) even when selecting the highest SNR pairs. These results highlight the necessity to design alternative identification criteria for conclusive detection of strong lensing. |
Tuesday, April 12, 2022 2:42PM - 2:54PM |
Y15.00007: Detection Prospects for Retrolensing Gravitational Wave Images in Hierarchical Triple Systems Yijun Wang, Yanbei Chen Gravitational wave (GW) retrolensing is a relativistic strong lensing scenario where the GW source is located between the lensing black hole and the observer. Such GW images pass close to the lens and have highly bent trajectories. While such retrolensing images are typically much fainter than the primary images, they become significantly magnified when the GW sources traverse near or through the lensing caustics. These images may then be discernible in future space-based GW observatories such as LISA and DECIGO. In this study, we investigate the detection prospects of retrolensing GW images in nearly edge-on hierarchical triple systems where a stellar-mass binary orbits a supermassive Kerr black hole. Under this scenario, the detection possibility is enhanced since Kerr spacetime admits extended caustics, and the separation between the lens and the source can be significantly smaller than the distance between the triple system and the observer. Our study also complements existing literature on repeated lensing in triple systems. |
Tuesday, April 12, 2022 2:54PM - 3:06PM |
Y15.00008: The detection and parameter estimation challenges of Type-II lensed binary black hole signals Ajit K Mehta Strong lensing can produce three types of images denoted as Type-I, Type-II and Type-III corresponding to the minimum, saddle and maximum of the total arrival time of the lensed GWs. The Type II images, in particular, receive a non-trivial phase shift of $\pi/2$ (for positive frequencies). This phase shift can introduce (additional) distortions in the strains produced by the Type-II image of the binary black hole (BBH) signals depending on the morphology of the signals, e.g., when they have contributions from higher harmonics (HMs) and/or precession. The optical depth to the Type-II images is nearly the same as the strong lensing (from galaxies) and thus are very likely to be observed in the near future. In this work, we investigate the potential applicability of these distortions in helping to identify the Type-II signals individually and also the systematic biases that could arise in the inferences of their parameters if they are unknowingly recovered with the unlensed GW templates. We show that at high SNRs ($\gtrsim 100$), and thus mostly relevant for 3G detectors and beyond, most Type-II images would be identifiable. These identified images will, however, have significantly biased estimates of the parameters (such as, sky location, distance, inclination, etc.) and in many cases fully biased. Thus, in the future, using Type-II lensed template for such signals would be necessary to avoid the potentially wrong astrophysical implications otherwise. |
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