arXiv subjects

Otto A. Hannuksela

Publications and source records attributed to Otto A. Hannuksela.

51 records · Page 3Linked to original sources

arXiv2022

GOLUM: A fast and precise methodology to search for, and analyze, strongly lensed gravitational-wave events

Like light, gravitational waves can be gravitationally lensed by massive objects along their travel path. Strong lensing produces several images from the same binary coalescence and is forecasted to have a promising rate in ground-based gravitational detectors. To search for this effect in the data, one would, in principle, have to analyze all the possible combinations of the individual detected events, whose number will be ever-increasing. To keep up with the rising computational cost, we propose a fast and precise methodology to analyze strongly-lensed gravitational wave events. The method works by effectively using the posterior of the first image as prior for the second image. Thanks to its increased speed tractability, this method enables the joint analysis of more than two images. In addition, it opens the door to new strong lensing studies where a large number of injections is required.

gr-qc↗

arXiv2021

On the identification of individual gravitational wave image types of a lensed system using higher-order modes

Similarly to light, gravitational waves can be gravitationally lensed as they propagate near massive astrophysical objects such as galaxies, stars, or black holes. In recent years, forecasts have suggested a reasonable chance of strong gravitational-wave lensing detections with the LIGO-Virgo-KAGRA detector network at design sensitivity. As a consequence, methods to analyse lensed detections have seen rapid development. However, the impact of higher-order modes on the lensing analyses is still under investigation. In this work, we show that the presence of higher-order modes enables the identification of the individual images types for the observed gravitational wave events when two lensed images are detected, which would lead to unambiguous confirmation of lensing. In addition, we show that higher-order mode content can be analyzed more accurately with strongly lensed gravitational wave events.

gr-qc↗

arXiv2021

Beyond the detector horizon: Forecasting gravitational-wave strong lensing

When gravitational waves pass near massive astrophysical objects, they can be gravitationally lensed. The lensing can split them into multiple wave-fronts, magnify them, or imprint beating patterns on the waves. Here we focus on the multiple images produced by strong lensing. In particular, we investigate strong lensing forecasts, the rate of lensing, and the role of lensing statistics in strong lensing searches. Overall, we find a reasonable rate of lensed detections for double, triple, and quadruple images at the LIGO--Virgo--KAGRA design sensitivity. We also report the rates for A+ and LIGO Voyager and briefly comment on potential improvements due to the inclusion of sub-threshold triggers. We find that most galaxy-lensed events originate from redshifts $z \sim 1-4$ and report the expected distribution of lensing parameters for the observed events. Besides forecasts, we investigate the role of lensing forecasts in strong lensing searches, which explore repeated event pairs. One problem associated with the searches is the rising number of event pairs, which leads to a rapidly increasing false alarm probability. We show how knowledge of the expected galaxy lensing time delays in our searches allow us to tackle this problem. Once the time delays are included, the false alarm probability increases linearly (similar to non-lensed searches) instead of quadratically with time, significantly improving the search. For galaxy cluster lenses, the improvement is less significant. The main uncertainty associated with these forecasts are the merger-rate density estimates at high redshift, which may be better resolved in the future.

astro-ph.HE↗

arXiv2021

A fast and precise methodology to search for and analyse strongly lensed gravitational-wave events

Gravitational waves, like light, can be gravitationally lensed by massive astrophysical objects such as galaxies and galaxy clusters. Strong gravitational-wave lensing, forecasted at a reasonable rate in ground-based gravitational-wave detectors such as Advanced LIGO, Advanced Virgo, and KAGRA, produces multiple images separated in time by minutes to months. These images appear as repeated events in the detectors: gravitational-wave pairs, triplets, or quadruplets with identical frequency evolution originating from the same sky location. To search for these images, we need to, in principle, analyze all viable combinations of individual events present in the gravitational-wave catalogs. An increasingly pressing problem is that the number of candidate pairs that we need to analyse grows rapidly with the increasing number of single-event detections. At design sensitivity, one may have as many as $\mathcal O(10^5)$ event pairs to consider. To meet the ever-increasing computational requirements, we develop a fast and precise Bayesian methodology to analyse strongly lensed event pairs, enabling future searches. The methodology works by replacing the prior used in the analysis of one strongly lensed gravitational-wave image by the posterior of another image; the computation is then further sped up by a pre-computed lookup table. We demonstrate how the methodology can be applied to any number of lensed images, enabling fast studies of strongly lensed quadruplets.

gr-qc↗

arXiv2021

Constraints on ultralight scalar bosons within black hole spin measurements from LIGO-Virgo's GWTC-2

Clouds of ultralight bosons - such as axions - can form around a rapidly spinning black hole, if the black hole radius is comparable to the bosons' wavelength. The cloud rapidly extracts angular momentum from the black hole, and reduces it to a characteristic value that depends on the boson's mass as well as on the black hole mass and spin. Therefore, a measurement of a black hole mass and spin can be used to reveal or exclude the existence of such bosons. Using the black holes released by LIGO and Virgo in their GWTC-2, we perform a simultaneous measurement of the black hole spin distribution at formation and the mass of the scalar boson. We find that the data strongly disfavors the existence of scalar bosons in the mass range between $1.3\times10^{-13}~\mathrm{eV}$ and $2.7\times10^{-13}~\mathrm{eV}$ for a decay constant $f_a\gtrsim 10^{14}~\mathrm{GeV}$. The statistical evidence is mostly driven by the two {binary black holes} systems GW190412 and GW190517, which host rapidly spinning black holes. The region where bosons are excluded narrows down if these two systems merged shortly ($\sim 10^5$ years) after the black holes formed.

gr-qc↗

arXiv2021

Searching for ultralight bosons within spin measurements of a population of binary black hole mergers

Ultralight bosons can form clouds around rotating black holes if their Compton wavelength is comparable to the black hole size. The boson cloud spins down the black hole through a process called superradiance, lowering the black hole spin to a characteristic value. It has been suggested that spin measurements of the black holes detected by ground-based gravitational-wave detectors can be used to constrain the mass of ultralight bosons. Unfortunately, a measurement of the individual black hole spins is often uncertain, resulting in inconclusive results. Instead, we use hierarchical Bayesian inference to combine information from multiple gravitational-wave sources and obtain stronger constraints. We show that hundreds of high signal-to-noise ratio gravitational-wave detections are enough to exclude (confirm) the existence of non-interacting bosons in the mass range $\left[10^{-13},3\times 10^{-12}\right]~\rm{eV}$ $\left([10^{-13},10^{-12}]~\rm{eV}\right)$. The precise number depends on the distribution of black hole spins at formation and the mass of the boson. From the few uninformative spin measurements of binary black hole mergers detected by LIGO and Virgo in their first two observing runs, we cannot draw statistically significant conclusions.

gr-qc↗

arXiv2020

Stellar-mass microlensing of gravitational waves

When gravitational waves pass through the nuclear star clusters of galactic lenses, they may be microlensed by the stars. Such microlensing can cause potentially observable beating patterns on the waveform due to waveform superposition and magnify the signal. On the one hand, the beating patterns and magnification could lead to the first detection of a microlensed gravitational wave. On the other hand, microlensing introduces a systematic error in strong lensing use-cases, such as localization and cosmography studies. We show that diffraction effects are important when we consider GWs in the LIGO frequency band lensed by objects with masses $\lesssim 100 \, \rm M_\odot$. We also show that the galaxy hosting the microlenses changes the lensing configuration qualitatively, so we cannot treat the microlenses as isolated point mass lenses when strong lensing is involved. We find that for stellar lenses with masses $\sim 1 \, \rm M_\odot$, diffraction effects significantly suppress the microlensing magnification. Thus, our results suggest that gravitational waves lensed by typical galaxy or galaxy cluster lenses may offer a relatively clean environment to study the lens system, free of contamination by stellar lenses. We discuss potential implications for the strong lensing science case. More complicated microlensing configurations will require further study.

astro-ph.HE↗

arXiv2020

Localizing merging black holes with sub-arcsecond precision using gravitational-wave lensing

The current gravitational-wave localization methods rely mainly on sources with electromagnetic counterparts. Unfortunately, a binary black hole does not emit light. Due to this, it is generally not possible to localize these objects precisely. However, strongly lensed gravitational waves, which are forecasted in this decade, could allow us to localize the binary by locating its lensed host galaxy. Identifying the correct host galaxy is challenging because there are hundreds to thousands of other lensed galaxies within the sky area spanned by the gravitational-wave observation. However, we can constrain the lensing galaxy's physical properties through both gravitational-wave and electromagnetic observations. We show that these simultaneous constraints allow one to localize quadruply lensed waves to one or at most a few galaxies with the LIGO/Virgo/Kagra network in typical scenarios. Once we identify the host, we can localize the binary to two sub-arcsec regions within the host galaxy. Moreover, we demonstrate how to use the system to measure the Hubble constant as a proof-of-principle application.

astro-ph.HE↗

arXiv2020

lensingGW: a Python package for lensing of gravitational waves

Advanced LIGO and Advanced Virgo could observe the first lensed gravitational waves in the coming years, while the future Einstein Telescope could observe hundreds of lensed events. Ground-based gravitational-wave detectors can resolve arrival time differences of the order of the inverse of the observed frequencies. As LIGO/Virgo frequency band spans from a few $\rm Hz$ to a few $ \rm kHz$, the typical time resolution of current interferometers is of the order of milliseconds. When microlenses are embedded in galaxies or galaxy clusters, lensing can become more prominent and result in observable time delays at LIGO/Virgo frequencies. Therefore, gravitational waves could offer an exciting alternative probe of microlensing. However, currently, only a few lensing configurations have been worked out in the context of gravitational-wave lensing. In this paper, we present lensingGW, a Python package designed to handle both strong and microlensing of compact binaries and the related gravitational-wave signals. This synergy paves the way for systematic parameter space investigations and the detection of arbitrary lens configurations and compact sources. We demonstrate the working mechanism of lensingGW and its use to study microlenses embedded in galaxies.

astro-ph.CO↗

arXiv2020

Lensed or not lensed: Determining lensing magnifications for binary neutron star mergers from a single detection

Advanced LIGO and Advanced Virgo could observe the first lensed gravitational wave sources in the coming years, while the future Einstein Telescope could observe hundreds of lensed events. It is, therefore, crucial to develop methodologies to distinguish between lensed from unlensed gravitational-wave observations. A lensed signal not identified as such will lead to biases during the interpretation of the source. In particular, sources will appear to have intrinsically higher masses. No robust method currently exists to distinguish between the magnification bias caused by lensing and intrinsically high-mass sources. In this work, we show how to recognize lensed and unlensed binary neutron star systems through the measurement of their tidal effects for highly magnified sources as a proof-of-principle. The proposed method could be used to identify lensed binary neutron stars, which are the chief candidate for lensing cosmography studies. We apply our method on GW190425, finding no evidence in favor of lensing, mainly due to the poor measurement of the event's tidal effects. However, we expect that future detections with better tidal measurements can yield better constraints.

astro-ph.HE↗

arXiv2020

Probing the existence of ultralight bosons with a single gravitational-wave measurement

Light bosons, proposed as a possible solution to various problems in fundamental physics and cosmology, include a broad class of candidates for beyond the Standard Model physics, such as dilatons and moduli, wave dark matter and axion-like particles. If light bosons exist in nature, they will spontaneously form "clouds" by extracting rotational energy from rotating massive black holes through superradiance, a classical wave amplification process that has been studied for decades. The superradiant growth of the cloud sets the geometry of the final black hole, and the black hole geometry determines the shape of the cloud. Hence, both the black hole geometry and the cloud encode information about the light boson. For this reason, measurements of the gravitational field of the black hole/cloud system (as encoded in gravitational waves) are over-determined. We show that a single gravitational wave measurement can be used to verify the existence of light bosons by model selection, rule out alternative explanations for the signal, and measure the boson mass. Such measurements can be done generically for bosons in the mass range $[10^{-16.5},10^{-14}]$ eV using LISA observations of extreme mass-ratio inspirals.

astro-ph.HE↗

arXiv2019

Extreme Dark Matter Tests with Extreme Mass Ratio Inspirals

Future space-based laser interferometry experiments such as LISA are expected to detect $\cal O$(100--1000) stellar-mass compact objects (e.g., black holes, neutron stars) falling into massive black holes in the centers of galaxies, the so-called extreme-mass-ratio inspirals (EMRIs). If dark matter forms a "spike" due to the growth of the massive black hole, it will induce a gravitational drag on the inspiraling object, changing its orbit and gravitational-wave signal. We show that detection of even a single dark matter spike from the EMRIs will severely constrain several popular dark matter candidates, such as ultralight bosons, keV fermions, MeV--TeV self-annihilating dark matter, and sub-solar mass primordial black holes, as these candidates would flatten the spikes through various mechanisms. Future space gravitational wave experiments could thus have a significant impact on the particle identification of dark matter.

astro-ph.CO↗

arXiv2018

Discovering intermediate-mass black hole lenses through gravitational wave lensing

Intermediate-mass black holes are the missing link that connects stellar-mass to supermassive black holes and are key to understanding galaxy evolution. Gravitational waves, like photons, can be lensed, leading to discernable effects such as diffraction or repeated signals. We investigate the detectability of intermediate-mass black hole deflectors in the LIGO-Virgo detector network. In particular, we simulate gravitational waves with variable source distributions lensed by an astrophysical population of intermediate-mass black holes, and use standard LIGO tools to infer the properties of these lenses. We find detections of intermediate-mass black holes at $98\%$ confidence level over a wide range of binary and lens parameters. Therefore, we conclude that intermediate-mass black holes could be detected through lensing of gravitational waves in the LIGO-Virgo detector network.

gr-qc↗

arXiv2018

Empirical tests of the black hole no-hair conjecture using gravitational-wave observations

We show that second-generation gravitational-wave detectors at their design sensitivity will allow us to directly probe the ringdown phase of binary black hole coalescences. This opens the possibility to test the so-called black hole no-hair conjecture in a statistically rigorous way. Using state-of-the-art numerical relativity-tuned waveform models and dedicated methods to effectively isolate the quasi-stationary perturbative regime where a ringdown description is valid, we demonstrate the capability of measuring the physical parameters of the remnant black hole, and subsequently determining parameterized deviations from the ringdown of Kerr black holes. By combining information from $\mathcal{O}(5)$ binary black hole mergers with realistic signal-to-noise ratios achievable with the current generation of detectors, the validity of the no-hair conjecture can be verified with an accuracy of $\sim 1.5\%$ at $90\%$ confidence.

gr-qc↗

arXiv2018

Classifying Lensed Gravitational Waves in the Geometrical Optics Limit with Machine Learning

Gravitational waves are theorized to be gravitationally lensed when they propagate near massive objects. Such lensing effects cause potentially detectable repeated gravitational wave patterns in ground- and space-based gravitational wave detectors. These effects are difficult to discriminate when the lens is small and the repeated patterns superpose. Traditionally, matched filtering techniques are used to identify gravitational-wave signals, but we instead aim to utilize machine learning techniques to achieve this. In this work, we implement supervised machine learning classifiers (support vector machine, random forest, multi-layer perceptron) to discriminate such lensing patterns in gravitational wave data. We train classifiers with spectrograms of both lensed and unlensed waves using both point-mass and singular isothermal sphere lens models. As the result, classifiers return F1 scores ranging from 0.852 to 0.996, with precisions from 0.917 to 0.992 and recalls ranging from 0.796 to 1.000 depending on the type of classifier and lensing model used. This supports the idea that machine learning classifiers are able to correctly determine lensed gravitational wave signals. This also suggests that in the future, machine learning classifiers may be used as a possible alternative to identify lensed gravitational wave events and to allow us to study gravitational wave sources and massive astronomical objects through further analysis.

astro-ph.IM↗