arXiv subjects

Peter Raffai

Publications and source records attributed to Peter Raffai.

At least 19 recordsLinked to original sources

arXiv2026

Statistical evidence for massive black hole recoils in active galactic nuclei

We search for a population-level signature of gravitational-wave recoiling supermassive black holes: a positive correlation between dust obscuration and the magnitude of the line-of-sight velocity offset of broad emission lines relative to the host. Using the SDSS DR16 quasar catalogue, we estimate the velocity offset, $\Delta v$, as the difference between the broad H$\beta$ redshift and a noise-weighted redshift from narrow lines ([O III] 5007, [O II] 3728, and Ca II 3934). We adopt the redshift-relative colour excess $\Delta(g-i)$ as a proxy for dust column density. Analysing $\sim10^{5}$ quasars that meet basic spectral quality requirements, we find a modest but highly significant positive correlation between $|\Delta v|$ and $\Delta(g-i)$ (Spearman $r\simeq0.12$ and Pearson $r\simeq0.13$, with $p\ll10^{-10}$ in both cases). The fraction of highly obscured quasars increases with $|\Delta v|$, indicating that the correlation is driven by a dust-reddened subpopulation. The result is robust to the choice of minimum $|\Delta v|$ threshold and to the line redshift estimator (peak vs. centroid). As expected, the correlation is largely absent when velocity offsets are computed between narrow emission lines. We find systematic differences between redshifted and blueshifted subsamples, which may point to residual velocity biases or additional physical effects (e.g. winds, inflows, orientation-dependent obscuration, or asymmetric broad-line regions). Recoiling massive black holes provide a natural explanation for the observed correlation, but alternative scenarios should be explored. If confirmed, this would enable population-level constraints on massive black hole merger rates, recoil dynamics, and active galactic nuclei disc properties.

astro-ph.GA↗

arXiv2025

A Case for an Inhomogeneous Einstein-de Sitter Universe

We present a local-to-global cosmological framework in which cosmic acceleration emerges from structure formation in an inhomogeneous Einstein-de Sitter (iEdS) universe, without dark energy. The model exhibits a quasilinear coasting evolution toward an effective Milne state driven by growing inhomogeneities. We test the iEdS model with ${H_0=72.5\ \mathrm{km\ s^{-1}\ Mpc^{-1}}}$ and ${\Omega_{\mathrm{m},0}=0.272}$ using CMB, BAO, and SN Ia data. The iEdS model fits the data comparably to $\Lambda$CDM and resolves the $H_0$ tension, while yielding a cosmic age ${t_0\simeq 13.67\ \mathrm{Gyr}}$ consistent with globular-cluster estimates.

astro-ph.CO↗

arXiv2024

Cosmic chronometers, Pantheon+ supernovae, and quasars favor coasting cosmologies over the flat $\Lambda$CDM model

We test and compare coasting cosmological models with curvature parameters ${k=\left\{ -1,0,+1 \right\}}$ in ${H_0^2 c^{-2}}$ units and the flat $\Lambda$CDM model by fitting them to cosmic chronometers (CC), the Pantheon+ sample of type Ia supernovae (SNe), and standardized quasars (QSOs). We used the \texttt{emcee} code for fitting CC data, a custom Markov Chain Monte Carlo implementation for SNe and QSOs, and Anderson-Darling tests for normality on normalized residuals for model comparison. Best-fit parameters are presented, constrained by data within redshift ranges $z\leq 2$ for CCs, $z\leq 2.3$ for SNe, and $z\leq 7.54$ for QSOs. Coasting models, particularly the flat coasting model, are generally favored over the flat $\Lambda$CDM model. The overfitting of the flat $\Lambda$CDM model to Pantheon+ SNe and the large intrinsic scatter in QSO data suggest a need to refine error estimates in these datasets. We also highlight the seemingly fine-tuned nature of either the CC data or $\Omega_{\mathrm{m},0}$ in the flat $\Lambda$CDM model to an ${H_1=H_0}$ coincidence when fitting ${H(z)=H_1z+H_0}$, a natural feature of coasting models.

astro-ph.CO↗

arXiv2023

Constraints on coasting cosmological models from gravitational-wave standard sirens

We present the first test of coasting cosmological models with gravitational-wave standard sirens observed in the first three observing runs of the LIGO-Virgo-KAGRA detector network. We apply the statistical galaxy catalog method adapted to coasting cosmologies and infer constraints on the $H_0$ Hubble constant for the three fixed values of the curvature parameter $k=\left\{ -1,0,+1 \right\}$ in $H_0^2 c^{-2}$ units. The maximum posteriors and $68.3\%$ highest density intervals we obtained from a combined analysis of $46$ dark siren detections and a single bright siren detection are $H_0=\left\{68.1^{+8.5}_{-5.6},67.5^{+8.3}_{-5.2},67.1^{+6.6}_{-5.8} \right\}~\mathrm{km\ s^{-1}\ Mpc^{-1}}$, respectively. All our constraints on $H_0$ are consistent within one sigma with the $H_0$ measured with the differential age method, which provides a constraint on $H_0$ in coasting cosmologies independently from $k$. Our results constrain all cosmological models with $a(t)\propto t$ linear expansion in the luminosity distance and redshift range of the $47$ LIGO-Virgo detections, i.e. $d_\mathrm{L}\lesssim 5~\mathrm{Gpc}$ and $z\lesssim 0.8$, which practically include all (both strictly linear and quasi-linear) models in the coasting model family. As we have found, the coasting models and the $Λ$CDM model fit equally well to the applied set of gravitational-wave detections.

astro-ph.CO↗

arXiv2022

The Hitchhiker's guide to the galaxy catalog approach for gravitational wave cosmology

We outline the ``dark siren'' galaxy catalog method for cosmological inference using gravitational wave (GW) standard sirens, clarifying some common misconceptions in the implementation of this method. When a confident transient electromagnetic counterpart to a GW event is unavailable, the identification of a unique host galaxy is in general challenging. Instead, as originally proposed by Schutz (1986), one can consult a galaxy catalog and implement a dark siren statistical approach incorporating all potential host galaxies within the localization volume. Trott & Hunterer 2021 recently claimed that this approach results in a biased estimate of the Hubble constant, $H_0$, when implemented on mock data, even if optimistic assumptions are made. We demonstrate explicitly that, as previously shown by multiple independent groups, the dark siren statistical method leads to an unbiased posterior when the method is applied to the data correctly. We highlight common sources of error possible to make in the generation of mock data and implementation of the statistical framework, including the mismodeling of selection effects and inconsistent implementations of the Bayesian framework, which can lead to a spurious bias.

gr-qc↗

arXiv2022

Prospects for reconstructing the gravitational-wave signals from core-collapse supernovae with Advanced LIGO-Virgo and the BayesWave algorithm

Our current understanding of the core-collapse supernova explosion mechanism is incomplete, with multiple viable models for how the initial shock wave might be energized enough to lead to a successful explosion. Detection of a gravitational-wave signal emitted in the initial few seconds after stellar core-collapse would provide unique and crucial insight into this process. With the Advanced LIGO and Advanced Virgo detectors expected to approach their design sensitivities soon, we could potentially detect this signal from a supernova within our galaxy. In anticipation of such a scenario, we study how well the BayesWave algorithm can recover the gravitational-wave signal from core-collapse supernova models in simulated advanced detector noise, and optimize its ability to accurately reconstruct the signal waveforms. We find that BayesWave can confidently reconstruct the signal from a range of supernova explosion models in Advanced LIGO-Virgo for network signal-to-noise ratios $\gtrsim 30$, reaching maximum reconstruction accuracies of $\sim 90\%$ at SNR $\sim 100$. For low SNR signals that are not confidently recovered, our optimization efforts result in gains in reconstruction accuracy of up to $20-40\%$, with typical gains of $\sim 10\%$.

astro-ph.HE↗

arXiv2021

Statistical search for angular non-stationarities of long gamma-ray burst jets using Swift data

In Budai et al. (2020) we argued that angular non-stationarities of gamma-ray burst (GRB) jets can result in a statistical connection between the angle values deduced from jet break times and the variabilities of prompt light curves. The connection should be an anti-correlation if luminosity densities of jets follow a power-law or a uniform profile, and a correlation if they have a Gaussian profile. In this follow-up paper, we search for the connection by measuring Spearman's rank correlation coefficient in a sample of 19 long GRBs observed by the Swift satellite. Using 16 of the GRBs with well-defined angle measurements, we find $ρ= -0.20_{-0.09}^{+0.09}$ and $p = 0.46_{-0.19}^{+0.23}$. Adding three more GRBs to the sample, each with a pair of equally possible angle values, can strengthen the anti-correlation to $ρ=-0.31_{-0.08}^{+0.07}$ and $p=0.19_{-0.09}^{+0.14}$. We show that these results are incompatible with non-stationary jets having Gaussian profiles, and that $\gtrsim\!100$ GRBs with observed afterglows would be needed to confirm the potential existence of the angle-variability anti-correlation with $3σ$ significance. If the connection is real, GRB jet angles would be constrainable from prompt gamma light curves, without the need of afterglow observations.

astro-ph.HE↗

arXiv2021

Bayesian Reconstruction of Gravitational-wave Signals from Binary Black Holes with Nonzero Eccentricities

We present a comprehensive study on how well gravitational-wave signals of binary black holes with nonzero eccentricities can be recovered with state of the art model-independent waveform reconstruction and parameter estimation techniques. For this we use BayesWave, a Bayesian algorithm used by the LIGO-Virgo Collaboration for unmodeled reconstructions of signal waveforms and parameters. We used two different waveform models to produce simulated signals of binary black holes with eccentric orbits and embed them in samples of simulated noise of design-sensitivity Advanced LIGO detectors. We studied the network overlaps and point estimates of central moments of signal waveforms recovered by BayesWave as a function of $e$, the eccentricity of the binary at 8 Hz orbital frequency. BayesWave recovers signals of near-circular ($e\lesssim0.2$) and highly eccentric ($e\gtrsim0.7$) binaries with network overlaps similar to that of circular ($e=0$) ones, however it produces lower network overlaps for binaries with $e\in[0.2,0.7]$. Estimation errors on central frequencies and bandwidths (measured relative to bandwidths) are nearly independent from $e$, while estimation errors on central times and durations (measured relative to durations) increase and decrease with $e$ above $e\gtrsim0.5$, respectively. We also tested how BayesWave performs when reconstructions are carried out using generalized wavelets with linear frequency evolution (chirplets) instead of sine-Gaussian wavelets. We have found that network overlaps improve by $\sim 10-20$ percent when chirplets are used, and the improvement is the highest at low ($e<0.5$) eccentricities. There is however no significant change in the estimation errors of central moments when the chirplet base is used.

astro-ph.HE↗

arXiv2020

Interpreting gravitational-wave burst detections: constraining source properties without astrophysical models

We show that for detections of gravitational-wave transients, constraints can be given on physical parameters of the source without using any specific astrophysical models. Relying only on fundamental principles of general relativity, we can set upper limits on the size, mass, and distance of the source solely from characteristics of the observed waveform. If the distance of the source is known from independent (e.g. electromagnetic) observations, we can also set lower limits on the mass and size. As a demonstration, we tested these constraints on binary black hole signals observed by the LIGO and Virgo detectors during their first and second observing runs, as well as on simulated binary black hole and core-collapse supernova signals reconstructed from simulated detector data. We have found that our constraints are valid for all analyzed source types, but their efficiency (namely, how far they are from the true parameter values) strongly depends on the source type, ranging from being in the same order of magnitude to a several orders of magnitude difference. In cases when a gravitational-wave signal is reconstructed without waveform templates and no astrophysical model on the source is available, these constraints provide the only quantitative characterization of the source that can guide the astrophysical modeling process.

gr-qc↗

arXiv2019

A statistical method to detect non-stationarities of gamma-ray burst jets

We propose a method to detect possible non-stationarities of gamma-ray burst jets. Assuming that the dominant source of variability in the prompt gamma light curve is the non-stationarity of the jet, we show that there should be a connection between the variability measure and the characteristic angle of the jet derived from the jet break time of the afterglow. We carried out Monte Carlo simulations of long gamma-ray burst observations assuming three radial luminosity density profiles for jets and randomizing all burst parameters, and created samples of gamma light curves by simulating jets undergoing Brownian motions with linear restoring forces. We were able to demonstrate that the connection between the variability and the characteristic angle is an anti-correlation in case of uniform and power-law jet profiles, and a correlation in case of a Gaussian profile. We have found that as low as $50$ $(144)$ gamma-ray burst observations with jet angle measurements can be sufficient for a $3σ (5σ) $ detection of the connection. The number of observations required for the detection depends on the underlying jet beam profile, ranging from 50 (144) to 237 (659) for the four specific profile models we tested.

astro-ph.HE↗

arXiv2019

Eccentricity distributions of eccentric binary black holes in galactic nuclei

Galactic nuclei are expected to be one of the main sites for formations of eccentric binary black holes (EBBHs), with an estimated detection rate of $\mathcal{O}(1-100$ yr$^{-1})$ with Advanced LIGO (aLIGO) detectors operating at design sensitivity. The two main formation channels of these binaries are gravitational capture and the secular Kozai-Lidov mechanism, with expectedly commensurable formation rates. We used Monte Carlo simulations to construct the eccentricity distributions of EBBHs formed through these channels in galactic nuclei, at the time their gravitational-wave signals enter the aLIGO band at $10$ Hz. We have found that the proportion of binary black holes entering the aLIGO band with eccentricities larger than $0.1$ is $\sim 10$ percent for the secular Kozai-Lidov mechanism, and $\sim 75$ percent for gravitational capture. We show that if future EBBH detection rates with aLIGO will be dominated by EBBHs formed in galactic nuclei, then the proportions of EBBHs formed through the two main channels can be constrained to a $Δ\mathcal{F}= 0.2$ wide one-sigma confidence interval with a few tens of observations, even if parameter estimation errors are taken into account at realistic levels.

astro-ph.HE↗

arXiv2017

Parameter estimation for gravitational-wave bursts with the BayesWave pipeline

We provide a comprehensive multi-aspect study on the performance of a pipeline used by the LIGO-Virgo Collaboration for estimating parameters of gravitational-wave bursts. We add simulated signals with four different morphologies (sine-Gaussians, Gaussians, white-noise bursts, and binary black hole signals) to simulated noise samples representing noise of the two Advanced LIGO detectors during their first observing run. We recover them with the BayesWave (BW) pipeline to study its accuracy in sky localization, waveform reconstruction, and estimation of model-independent waveform parameters. BW localizes sources with a level of accuracy comparable for all four morphologies, with the median separation of actual and estimated sky locations ranging from 25.1$^{\circ}$ to 30.3$^{\circ}$. This is a reasonable accuracy in the two-detector case, and is comparable to accuracies of other localization methods studied previously. As BW reconstructs generic transient signals with sine-Gaussian wavelets, it is unsurprising that BW performs the best in reconstructing sine-Gaussian and Gaussian waveforms. BW's accuracy in waveform reconstruction increases steeply with network signal-to-noise ratio (SNR$_{\rm net}$), reaching a $85\%$ and $95\%$ match between the reconstructed and actual waveform below SNR$_{\rm net} \approx 20$ and SNR$_{\rm net} \approx 50$, respectively, for all morphologies. BW's accuracy in estimating central moments of waveforms is only limited by statistical errors in the frequency domain, and is affected by systematic errors too in the time domain as BW cannot reconstruct low-amplitude parts of signals overwhelmed by noise. The figures of merit we introduce can be used in future characterizations of parameter estimation pipelines.

astro-ph.HE↗

arXiv2015

A Statistical Method to Search for Recoiling Supermassive Black Holes in Active Galactic Nuclei

We propose an observational test for gravitationally recoiling supermassive black holes (BHs) in active galactic nuclei, based on a correlation between the velocities of BHs relative to their host galaxies, |Δv|, and their obscuring dust column densities, Σ_{dust} (both measured along the line of sight). We use toy models for the distribution of recoil velocities, BH trajectories, and the geometry of obscuring dust tori in galactic centres, to simulate 2.5x10^5 random observations of recoiling quasars. BHs with recoil velocities comparable to the escape velocity from the galactic centre remain bound to the nucleus, and do not fully settle back to the centre of the torus due to dynamical friction in a typical quasar lifetime. We find that |Δv| and Σ_ {dust} for these BHs are positively correlated. For obscured (Σ_{dust}>0) and for partially obscured (0<Σ_{dust}<~2.3 g/m^2) quasars with |Δv|>=45 km/s, the sample correlation coefficient between log10(|Δv|) and Σ_{dust} is r_{45} = 0.28+/-0.02 and r_{45} = 0.13+/-0.02, respectively. Allowing for random +/-100 km/s errors in |Δv| unrelated to the recoil dilutes the correlation for the partially obscured quasars to r_{45} = 0.026+/-0.004 measured between |Δv| and Σ_{dust}. A random sample of >~3,500 obscured quasars with |Δv|>=45 km/s would allow rejection of the no-correlation hypothesis with 3 sigma significance 95% of the time. Finally, we find that the fraction of obscured quasars, F_{obs}(|Δv|), decreases with |Δv| from F_{obs}(<10 km/s)>~0.8 to F_{obs}(>10^3 km/s)<~0.4. This predicted trend can be compared to the observed fraction of type II quasars, and can further test combinations of recoil, trajectory, and dust torus models.

astro-ph.GA↗

arXiv2015

Global Optimization for Future Gravitational Wave Detectors' Sites

We consider the optimal site selection of future generations of gravitational wave detectors. Previously, Raffai et al. optimized a 2-detector network with a combined figure of merit. This optimization was extended to networks with more than two detectors in a limited way by first fixing the parameters of all other component detectors. In this work we now present a more general optimization that allows the locations of all detectors to be simultaneously chosen. We follow the definition of Raffai et al. on the metric that defines the suitability of a certain detector network. Given the locations of the component detectors in the network, we compute a measure of the network's ability to distinguish the polarization, constrain the sky localization and reconstruct the parameters of a gravitational wave source. We further define the `flexibility index' for a possible site location, by counting the number of multi-detector networks with a sufficiently high Figure of Merit that include that site location. We confirm the conclusion of Raffai et al., that in terms of flexibility index as defined in this work, Australia hosts the best candidate site to build a future generation gravitational wave detector. This conclusion is valid for either a 3-detector network or a 5-detector network. For a 3-detector network site locations in Northern Europe display a comparable flexibility index to sites in Australia. However for a 5-detector network, Australia is found to be a clearly better candidate than any other location.

astro-ph.IM↗

arXiv2013

Detecting Long-Duration Narrow-Band Gravitational Wave Transients Associated with Soft Gamma Repeater Quasi-Periodic Oscillations

We have performed an in-depth concept study of a gravitational wave data analysis method which targets repeated long quasi-monochromatic transients (triggers) from cosmic sources. The algorithm concept can be applied to multi-trigger data sets in which the detector-source orientation and the statistical properties of the data stream change with time, and does not require the assumption that the data is Gaussian. Reconstructing or limiting the energetics of potential gravitational wave emissions associated with quasi-periodic oscillations (QPOs) observed in the X-ray lightcurve tails of soft gamma repeater flares might be an interesting endeavour of the future. Therefore we chose this in a simplified form to illustrate the flow, capabilities, and performance of the method. We investigate performance aspects of a multi-trigger based data analysis approach by using O(100 s) long stretches of mock data in coincidence with the times of observed QPOs, and by using the known sky location of the source. We analytically derive the PDF of the background distribution and compare to the results obtained by applying the concept to simulated Gaussian noise, as well as off-source playground data collected by the 4-km Hanford detector (H1) during LIGO's fifth science run (S5). We show that the transient glitch rejection and adaptive differential energy comparison methods we apply succeed in rejecting outliers in the S5 background data. Finally, we discuss how to extend the method to a network containing multiple detectors, and as an example, tune the method to maximize sensitivity to SGR 1806-20 flare times.

astro-ph.IM↗

arXiv2012

Multimessenger Science Reach and Analysis Method for Common Sources of Gravitational Waves and High-energy Neutrinos

We present the baseline multimessenger analysis method for the joint observations of gravitational waves (GW) and high-energy neutrinos (HEN), together with a detailed analysis of the expected science reach of the joint search. The analysis method combines data from GW and HEN detectors, and uses the blue-luminosity-weighted distribution of galaxies. We derive expected GW+HEN source rate upper limits for a wide range of source parameters covering several emission models. Using published sensitivities of externally triggered searches, we derive joint upper limit estimates both for the ongoing analysis with the initial LIGO-Virgo GW detectors with the partial IceCube detector (22 strings) HEN detector and for projected results to advanced LIGO-Virgo detectors with the completed IceCube (86 strings). We discuss the constraints these upper limits impose on some existing GW+HEN emission models.

astro-ph.HE↗

arXiv2011

Opportunity to Test non-Newtonian Gravity Using Interferometric Sensors with Dynamic Gravity Field Generators

We present an experimental opportunity for the future to measure possible violations to Newton's 1/r^2 law in the 0.1-10 meter range using Dynamic gravity Field Generators (DFG) and taking advantage of the exceptional sensitivity of modern interferometric techniques. The placement of a DFG in proximity to one of the interferometer's suspended test masses generates a change in the local gravitational field that can be measured at a high signal to noise ratio. The use of multiple DFGs in a null experiment configuration allows to test composition independent non-Newtonian gravity significantly beyond the present limits. Advanced and third-generation gravitational-wave detectors are representing the state-of-the-art in interferometric distance measurement today, therefore we illustrate the method through their sensitivity to emphasize the possible scientific reach. Nevertheless, it is expected that due to the technical details of gravitational-wave detectors, DFGs shall likely require dedicated custom configured interferometry. However, the sensitivity measure we derive is a solid baseline indicating that it is feasible to consider probing orders of magnitude into the pristine parameter well beyond the present experimental limits significantly cutting into the theoretical parameter space.

gr-qc↗

arXiv2011

Long gravitational-wave transients and associated detection strategies for a network of terrestrial interferometers

Searches for gravitational waves (GWs) traditionally focus on persistent sources (e.g., pulsars or the stochastic background) or on transients sources (e.g., compact binary inspirals or core-collapse supernovae), which last for timescales of milliseconds to seconds. We explore the possibility of long GW transients with unknown waveforms lasting from many seconds to weeks. We propose a novel analysis technique to bridge the gap between short O(s) burst analyses and persistent stochastic analyses. Our technique utilizes frequency-time maps of GW strain cross-power between two spatially separated terrestrial GW detectors. The application of our cross-power statistic to searches for GW transients is framed as a pattern recognition problem, and we discuss several pattern-recognition techniques. We demonstrate these techniques by recovering simulated GW signals in simulated detector noise. We also recover environmental noise artifacts, thereby demonstrating a novel technique for the identification of such artifacts in GW interferometers. We compare the efficiency of this framework to other techniques such as matched filtering.

astro-ph.IM↗