Research Summary

According to Einstein’s theory of general relativity, gravitational waves (GWs) are wavelike distortions in the fabric of space-time created by moving masses and propagating at the speed of light. Although there had been several experimental attempts throughout the 20th century to detect GWs, the first direct detection was only achieved in September 2015 by the LIGO-Virgo Collaboration (LVC) using the two LIGO detectors (see Abbott et al. 2016). As a member of the LVC since 2007 and as the co-founder of the first LIGO group in Hungary, I had the privilege of being involved in this great scientific achievement. The expected common detection of GW signals in the near future will not only allow testing Einstein’s general relativity under various conditions in the strong field regime, but it will also revolutionize astronomical observations by providing a new carrier of information that can be utilized in exploring the Universe.

In the Hungarian LIGO group at Eötvös Loránd University, I am supervising multiple research projects that support GW observations, from improving scientific understanding of astrophysical sources of GWs, to developing algorithms that can identify signals in the noisy outputs of GW detectors, and to optimizing operations of GW detectors. One of the greatest potentials for scientific discoveries in the near future is in the field of multi-messenger astronomy, in which a network of GW detectors provide triggers for electromagnetic (EM) follow-up observations of and host identifications for localized GW source candidates. Our LIGO group supports the EM follow-up observations both by providing a value-added all-sky catalog of galaxies optimized for identifying host galaxy candidates (see our project website), and by incorporating GW source models in the complex optimization of target selections and observing strategies.

Also, my personal research has partially been focusing on developing novel data processing methods to detect long-duration GW emission of compact objects, and to propose tests of models of such GW sources and their host environments. In one such ongoing project, I have lead the development of a statistical method that can allow searching for supermassive black holes (BHs) kicked out from galactic nuclei during their formation. In a paper we published in January 2016 (Raffai, Haiman, Frei 2016), we have shown that observing a possible correlation between the line-of-sight velocity and obscuration of quasars (supermassive BHs accreting material) by their dusty surroundings can be used to statistically confirm the existence of BHs recoiling due to anisotropic GW emission. We are now in the process of searching for this correlation in data sets of large-scale quasar catalogs. If both the correlation and our explanation for it survives all tests, not only will it provide the first statistical evidence for GW recoils of supermassive BHs, but it would allow testing the dynamics and distribution of mass in galactic nuclei.

Interests

Gravitational waves
Compact objects
Multi-messenger astronomy
Active galactic nuclei
Cosmology

Students Advised in Group Research

János Takátsy

M.Sc. Student

Personal website

Bence Bécsy

M.Sc. Student

Personal website

Andor Budai

PhD Student

Gergely Dálya

PhD Student

Personal website

I find it a great honor to be able to work with a group from the most talented students in Hungary, all having strong commitments to high-quality research in astrophysics. Students in our group are given mentoring at many levels according to their needs, and strong support in advancing their research, scientific interest, and academic career.

Research Projects

A Statistical Search for Recoiling Supermassive Black Holes in Active Galactic Nuclei
Looking for signs of supermassive black holes recoiling due to anisotropic gravitational-wave emission

I have lead the development of a statistical method that can allow searching for supermassive black holes (BHs) kicked out from galactic nuclei during their formation from a pair of merging BHs. According to theoretical predictions, such recoiling BHs should exist, with many of them engaging in an oscillating motion in the centers of their host galaxies after receiving a kick from anisotropic gravitational-wave (GW) emission. In a paper we published in November 2015 (Raffai, Haiman, Frei 2015), we have shown that observing a possible correlation between the line-of-sight velocity and obscuration of quasars (supermassive BHs accreting material) by their dusty surroundings can be used to statistically confirm the existence of BHs recoiling due to GW emission. We are now in the process of searching for this correlation in data sets of large-scale quasar catalogs. If both the correlation and our explanation for it survives all tests, not only will it provide the first statistical evidence for GW recoils of supermassive BHs in galactic nuclei, but it would allow testing the dynamics of the central BHs and the distribution of mass in galactic nuclei.
GLADE: a full-sky catalog of galaxies for the era of advanced gravitational-wave detectors
Constructing a full-sky catalog of galaxies for electromagnetic follow-up observations of gravitational-wave candidates

This project is lead by Gergely Dálya, and is being carried out with the participation of Gábor Galgóczi, László Dobos, Rafael de Souza, and Zsolt Frei, from Eötvös Loránd University. The work is coordinated with the the Burst, EM Follow-up, and GRB Working Groups of the LIGO-Virgo Collaboration (LVC).

Our group has created a value-added full-sky catalog of galaxies, named as Galaxy List for the Advanced Detector Era, or GLADE. The purpose of this project is (i) to identify host galaxy candidates for gravitational-wave (GW) sources detected and localized by advanced GW detectors, (ii) to support target selections for electromagnetic (EM) follow-up observations of GW candidates, and (iii) to identify host galaxy candidates for poorly localized EM transients, such as gamma-ray bursts observed by the InterPlanetary Network. The catalog is already being used by the LVC and by external collaborators in all three areas.

GLADE has nearly 2 million galaxies (about 40 times more than the catalog used by the LVC in the initial detector era), and a high level of completeness (100% within 73 Mpc and 53% within 300 Mpc) within the Advanced LIGO range for binary neutron stars, which are the main targets for joint GW-EM observations. GLADE was originally constructed by combining and matching sources from four previously existing galaxy catalogs: GWGC, 2MPZ, 2MASS XSC, and HyperLEDA. As according to theoretical models, B-band magnitudes of galaxies can be used as a proxy for binary neutron star formation, we considered it as a crucial requirement to have B-band magnitude data for all galaxies listed in GLADE. Providing distance estimates for all galaxies is also necessary in determining the likelihood of a galaxy being the host of an event, as well as in calculating the completeness of GLADE as a function of distance. Therefore we used a quantile regression forest machine learning technique to calculate B-band magnitudes and distances for a total of ~550,000 galaxies where these parameters were previously unavailable.

We are still working on extending GLADE by matching it with additional galaxy catalogs as they become available. There are also ongoing attempts to improve the precision of the machine learning technique used in estimating missing galaxy parameters. Finally, we are working on developing a technique for estimating stellar masses of GLADE galaxies, since theoretical models suggest that they can be used as an alternative proxy for binary neutron star formation and for GW events in general. These upgrades will make GLADE an even more powerful tool for the LVC and for the broader astrophysical community as well.

For more information on the GLADE project, please visit our project website.
Parameter estimation of gravitational-wave signals
Studying the performance of methods estimating parameters of gravitational-wave signals, and identifying astrophysical models that can be tested with the achievable precision

This project is lead by Bence Bécsy under my advisership, and is coordinated with the Burst and Parameter Estimation Working Groups of the LIGO-Virgo Collaboration.

With the Advanced LIGO detectors in operation, we expect regular detections of gravitational-wave (GW) signals in the near future. GWs are not part of the electromagnetic spectrum, thus these observations will provide a novel tool for astrophysicists to study the physics and properties of GW sources.

The only way to obtain information on a GW source that we can utilize in astrophysical research is giving estimations on values of its astrophysical parameters (e.g. source mass, spin, orbital parameters, etc.). This means that it is crucial to develop precise and efficient methods that can extract the values of these source parameters from the detected waveforms.

In this project, we focus on characterizing the performance of one of the most advanced parameter estimation algorithms, named as BayesWave Burst (BWB). BWB uses Gaussian-modulated sinusoids to reconstruct the complete waveform of a GW signal. This by itself is a challenging task given that GW signals are embedded in high levels of noise. BWB then uses Bayesian statistical methods to estimate all model-independent parameters of the source from the reconstructed waveform. The reconstructed waveform can also be used to estimate model-dependent source parameters. Thus, by testing and tuning BWB, we can identify the broadest range of astrophysical models that can be tested through GW detections, and we can also motivate further improvements of BWB to broaden the range of testable models.

In case of coincident detection of a GW signal with multiple detectors, sky coordinates of the GW source are among the model-independent parameters that are estimated by BWB. If the sky localization is accurate enough, electromagnetic telescopes can be pointed towards the source location in order to observe a possible afterglow of the event leading to GW emission. It is therefore important to test the sky localization accuracy of BWB. In order to do this, we use multiple figures of merit in a broad range of tests to characterize the performance of BWB. Additionally, we work on a study on the waveform reconstruction capabilities of BWB. Both this and the source localization study involve injecting a large number of simulated GW signals into mock aLIGO noise samples, and carrying out computationally intensive data processing with aLIGO computer clusters.
Testing host models with gravitational-wave detections of eccentric binary black holes
Studying the possibility of testing models of globular clusters and galactic nuclei with future gravitational-wave detections of eccentric binary black holes

This project is lead by János Takátsy under my advisership.

A globular cluster (GC) is a tightly bound spherical collection of hundreds of thousands of old stars. The proper modelling of GCs can be difficult due to the high number of constituents, each interacting with one another. Accordingly, there are many different analytic and numeric GC models competing against each other. Thus, an efficient observational method is required to test these models, and to find the most realistic one. Electromagnetic observations are limited in this regard, because they cannot provide us information about the deeper structure of GCs. However, as we point out in this new study, detections of gravitational-waves (GWs) from eccentric binary black holes (EBBHs) could serve as a tool for testing and constraining these GC models.

EBBHs are expected to form in dense stellar systems, such as GCs. Properties of GCs affect the formation of EBBHs within them, and consequently, we may gain information about properties of GCs by detecting EBBHs and reconstructing their parameters. Our goal is to determine the minimal number of EBBH detections with GW detectors that allows testing implications of different GC models on the observable distribution of EBBH parameters, such as orbital eccentricity and pericenter distance at the time EBBH signals enter the sensitive band of Advanced LIGO (aLIGO). Our study is the first one that aims testing possible models of dense stellar environments using GW signals of EBBHs.

EBBHs are proposed to be sources of GWs detectable by aLIGO. These binaries are expected to be detectable from great distances due to the strength and spectral richness of their GW signal. When aLIGO reaches its full sensitivity, which is proposed to happen in 2019, the expected rate of EBBH observations will be 5-20/year. This detection rate can already provide enough data to carry out an actual test of GC models on a reasonable timescale.

In 2015, János Takátsy was awarded with the 3rd prize of the Conference of Scientific Students’ Associations (the most famous and recognized competition in Hungary for undergraduate researchers) for his leading contributions to this project.
Targeted search for long-duration gravitational-wave transients
Developing a search pipeline targeting long-duration gravitational-wave signals, and applying it in searches for gravitational-wave signals from long gamma-ray bursts

I have participated in the development of a search pipeline (named as “Stochastic Transient Analysis Multi-detector Pipeline” or STAMP) dedicated to focus on gravitational-wave transients in the 1-1000 second time scale. STAMP was the first data processing pipeline developed by LIGO-Virgo Collaboration (LVC) members that covered this duration range of signals. STAMP uses cross-correlated spectra of multiple detectors to construct frequency-time (ft-)maps of cross-power.

STAMP applies a pattern-recognition algorithm on the ft-maps that incorporates an image processing method developed by members of the EGRG group. The first test of STAMP was carried out by searching for long-duration gravitational-wave transients in coincidence with long gamma-ray bursts (GRB) detected by the Swift telescope during the S5 LIGO run. In order to determine the time window in which STAMP searches are to be carried out around a GRB trigger, I participated in a study aiming to explore the temporal characteristics of emission processes in GRB engines.

STAMP is still the long-duration search pipeline applied by the LVC in processing data of second-generation gravitational-wave detectors.
Optimal networks of future gravitational-wave detectors
Searching for the best geographical locations and orientations of future gravitational-wave detectors

This work is being carried out with the participation of researchers at Eötvös Loránd University, at University of Glasgow, and at Columbia University in the City of New York.

We study ways of characterizing and optimizing the efficiency of detection and signal reconstruction of networks of multiple gravitational-wave (GW) detectors. We do this work in strong collaboration with the LIGO Columbia and Glasgow groups.

As part of this project, we have applied different N-detector figures-of-merit (FoMs) suggested by several authors, including FoMs characterizing the transient source localization accuracy of networks of detectors, in a complex optimization process aiming to maximize the scientific output of GW detector networks operating in the near or far future. The project has lead to publishing the paper Raffai+ 2013. In this paper (among other published results) we have suggested an optimal location and orientation for the proposed LIGO-India detector in terms of N-detector FoMs, considering the future five-detector network of LIGO-India, aLIGO Hanford, aLIGO Livingston, AdvVirgo, and KAGRA.

Further studies in this project resulted with another paper published in CQG in 2015, with a lead author from the University of Glasgow group (Hu, Y.). This paper was among the few ones selected for being highlighted in the CQG+ website. In this work, we used a Markov Chain Monte Carlo method to optimize the locations of future generations of GW detectors in order to maximize their scientific output. By using public databases, we have developed a new perspective on the world map, presenting the allowable regions for building new GW detectors based on expected construction limitations and local noise levels.

Within the framework of this project, we are currently working on time-dependent characterizations of different advanced detector networks. If we assume that we know the location of a chosen, observed source or host in the sky (e.g. known pulsars, or the Galactic Center region), we can quantify how sensitive a detector network is towards that direction at different times of a day. Comparing these source-specific FoMs for different networks of detectors, we can calculate which sub-network of detectors is the most sensitive among all at different times of a day and towards a specific source or host.