arXiv subjects

Vaibhav Tiwari

Publications and source records attributed to Vaibhav Tiwari.

At least 19 recordsLinked to original sources

arXiv2026

The Chirp-Mass Ladder: A New Rung Emerges

The population of binary black holes (BBHs) observed through gravitational waves (GWs) now includes 256 events with the release of GWTC-5.0, enabling more detailed studies. The inferred chirp-mass distribution shows prominent peaks at approximately $7.5M_{\odot}$, $14M_{\odot}$, and $27M_{\odot}$, with subsequent peaks spaced by approximately a factor of two. A parsimonious explanation for this structured distribution is a hierarchical merger scenario, in which the first peak arises from mergers of black holes of stellar origin, and higher-mass peaks arise from repeated mergers. Notably, with the addition of new observations, an intermediate peak near $19M_{\odot}$ emerges. This feature was anticipated in earlier work as a consequence of intergenerational mergers involving second- and third-generation (G) black holes, highlighting the predictive expectations of the hierarchical-merger interpretation. Furthermore, two groups of $1G+2G$ mergers recently reported in separate studies can be understood as distinct rungs---$1G+2G$ and $3G+4G$---within this hierarchical chirp-mass ladder, a unification that describes both spin transitions with a single mechanism. Although we observe expected correlations between mass ratios and spins in multiple events across the mass range, the lack of clear signatures across all rungs invites investigation into the role of hierarchical mergers in shaping the BBH population.

astro-ph.HE↗

arXiv2026

Population of Binary Black Holes Inferred from One Hundred and Fifty Gravitational Wave Signals

The LIGO-Virgo-KAGRA collaborations have reported gravitational wave signals from more than 150 binary black holes in the fourth catalog (GWTC-4). Here, we investigate the population properties of these binary black holes using the mixture-model framework Vamana. We present one-dimensional distributions of masses and spins, explore their correlations, and examine their evolution with redshift. These features may reflect astrophysical processes associated with binary black hole formation channels, although most remain poorly constrained. A notable feature is a peak near $10M_\odot$ in the primary mass and $8M_\odot$ in the chirp mass. Additionally, the primary and secondary masses correlate uniquely, producing pronounced peaks in the chirp mass around $14M_\odot$ and $27M_\odot$. The three peaks are roughly separated by a factor of two. A simple explanation for such well-placed peaks is a hierarchical merger scenario, in which the first peak arises from mergers of black holes of stellar origin, and higher-mass peaks arise from repeated mergers of black holes from lower-mass peaks. Although most binaries do not exhibit the high spins and characteristic mass ratios expected from hierarchical mergers, those that do are associated with the peaks observed in the chirp mass distribution.

astro-ph.HE↗

arXiv2026

Geo-Data-Driven HD Map Generation Workflow with Integrated Reference-Free Constraint-Based Verification

High-definition (HD) maps are core artifacts for automated driving systems, but their generation commonly relies on sensor-intensive mobile mapping campaigns, while quality assessment often depends on high-precision reference data. These dependencies make HD map engineering costly and difficult to apply in settings where specialised measurement data or independently measured reference maps are unavailable. This paper presents an engineering-oriented geo-data-driven workflow for HD map generation with integrated representation-level verification. The workflow uses openly available geo-engineering datasets as the primary input source and transforms them into lane-level HD map representations of existing road environments through explicit intermediate representations and processing stages. To assess the generated representations without external reference maps, the workflow integrates executable constraint-based verification into the engineering process. Selected constraints are derived from specifications relevant to automated driving and road-design guidelines. They are evaluated directly on the generated lanelet-based representation to detect geometric, topological, and elevation-related inconsistencies. The workflow is evaluated using real-world shapefile-based road-network data from four cities in Lower Saxony, Germany, and controlled defect-injection scenarios. The real-world evaluation shows that the generated map representations satisfy the selected constraints in the evaluated scenarios, while the defect-injection study demonstrates complete detection of the considered defect types without observed false positives. The results indicate that geo-data-driven HD map generation with integrated executable verification can provide a modular and inspectable complement to sensor-intensive mapping workflows under reduced sensing and reference-data availability.

cs.RO↗

arXiv2026

Connected Dependability Cage: Run-Time Function and Anomaly Monitoring for the Development and Operation of Safe Automated Vehicles

The advancement of automated vehicles introduces complex safety challenges, particularly in dynamic and unpredictable environments where AI-enabled perception systems must operate reliably. Ensuring compliance with safety standards such as ISO 26262 and ISO/PAS 21448 (SOTIF) is essential for addressing system malfunctions and mitigating unsafe behavior in unknown scenarios. However, as automation levels increase, vehicles must go beyond conventional functional safety by incorporating fail-operational capabilities that enable continued safe operation during system or component failures and the handling of unfamiliar or degraded operational conditions. To address these safety concerns, we propose the Connected Dependability Cage, an architectural framework designed to enable hierarchical fail-operational behavior in AI-enabled perception systems. This framework integrates two complementary monitoring mechanisms: a Function Monitor that oversees multiple heterogeneous AI-based perception pipelines and detects inconsistencies through a voting mechanism, and an Anomaly Monitor that evaluates the reliability of AI perception by detecting unknown or novel objects in scenes that may be excluded from the training dataset. In the presence of critical discrepancies, the system supports graceful degradation, ultimately enabling a transition to a minimal-risk maneuver strategy. Furthermore, whenever either monitor raises a safety flag, an automated data recording process is initiated to facilitate iterative system development and continuous improvement. Both monitors have been implemented and validated through extensive vehicle testing, demonstrating their practical effectiveness in real-world applications.

cs.RO↗

arXiv2025

Examining the Gap in the Chirp Mass Distribution of Binary Black Holes

The mass distribution of binary black holes inferred from gravitational wave measurements is expected to shed light on their formation scenarios. An emerging structure in the mass distribution indicates the presence of multiple peaks around chirp masses of $8M_\odot$, $14M_\odot$, and $27M_\odot$. In particular, there is a lack of observations between chirp masses of 10 and 12 $M_\odot$. In this article, we report that observations significantly favour the model supporting suppression of the rate in a narrow chirp mass range compared to the model that doesn't include suppression at a confidence greater than 99.5\%. Using another test, which measures the deviation between the inferred chirp mass distributions from the two models, we conservatively estimate a 95\% confidence in the presence of a feature. A lack of confidence has been reported in the presence of a gap around a comparable location in the component mass distribution. The differing conclusions are due to a unique correlation between the primary~(heavier of the two masses) and the secondary~(lighter of the two masses) masses of binary black holes. This correlation results in increased clustering of measured chirp masses around specific values.

astro-ph.HE↗

arXiv2025

Varaha: A promising sampler for obtaining gravitational wave posteriors

Nested sampling is often used in Bayesian statistics problems in astronomy. It operates with a set of live points, iteratively replacing the point with the lowest likelihood with a new point of higher likelihood. Each iteration reduces the enclosed volume by a known factor. The estimated sampling density and the likelihood values of both new and old live points quantify the enclosed probability mass. Although robust, nested sampling often discards a majority of the sampled points ($\sim 99.9\%$) at which likelihood was calculated. Here, we present an efficient method to explicitly calculate the sampling density for small dimensional problems~(ten or less), thereby removing the need to discard samples. The points' sampling density and likelihood values constitute the posterior distribution. We build on the existing version of the sampler Varaha and present an alternate version that is significantly more efficient for expensive likelihoods. These samplers specifically focus on obtaining compact binary parameters from their gravitational wave signals. They provide a viable alternative to nested sampling when the full fifteen-dimensional space is sampled separately for observer-dependent parameters and parameters intrinsic to the binary.

astro-ph.HE↗

arXiv2023

What's in a binary black hole's mass parameter?

The black hole masses measured from gravitational wave observations appear to cluster around specific mass values. Consequently, the primary~(and chirp) mass distribution of binary black holes inferred using these measurements shows four emerging peaks. These peaks are approximately located at a primary~(chirp) mass value of 10$M_\odot$~(8$M_\odot$), 20$M_\odot$~(14$M_\odot$), 35$M_\odot$~(28$M_\odot$) and 63$M_\odot$~(49$M_\odot$). Although the presence of the first and third peaks has been attributed to binary black hole formation in star clusters or due to the evolution of stellar binaries in isolation, the second peak has received relatively less attention because it lacks significance in the primary mass distribution. In this article, we report that confidence in the second peak depends on the mass parameter we choose to model the population on. Unlike primary mass, this peak is significant when modelled on the chirp mass. We discuss the disparity as a consequence of mass asymmetry in the observations that cluster at the second peak. Finally, we report this asymmetry as part of a potential trend in the mass ratio distribution manifested as a function of the chirp mass, but not as a function of primary mass, when we include the observation GW190814 in our modelling. The chirp mass is not a parameter of astrophysical relevance. Features present in the chirp mass, but not in the primary mass, are relatively difficult to explain and expected to garner significant interest.

astro-ph.HE↗

arXiv2023

VARAHA: A Fast Non-Markovian sampler for estimating Gravitational-Wave posteriors

This article introduces VARAHA, an open-source, fast, non-Markovian sampler for estimating gravitational-wave posteriors. VARAHA differs from existing Nested sampling algorithms by gradually discarding regions of low likelihood, rather than gradually sampling regions of high likelihood. This alternative mindset enables VARAHA to freely draw samples from anywhere within the high-likelihood region of the parameter space, allowing for analyses to complete in significantly fewer cycles. This means that VARAHA can significantly reduce both the wall and CPU time of all analyses. VARAHA offers many benefits, particularly for gravitational-wave astronomy where Bayesian inference can take many days, if not weeks, to complete. For instance, VARAHA can be used to estimate accurate sky locations, astrophysical probabilities and source classifications within minutes, which is particularly useful for multi-messenger follow-up of binary neutron star observations; VARAHA localises GW170817 $\sim 30$ times faster than LALInference. Although only aligned-spin, dominant multipole waveform models can be used for gravitational-wave analyses, it is trivial to extend this algorithm to include additional physics without hindering performance. We envision VARAHA being used for gravitational-wave studies, particularly estimating parameters using expensive waveform models, analysing subthreshold gravitational-wave candidates, generating simulated data for population studies, and rapid posterior estimation for binary neutron star mergers.

astro-ph.HE↗

arXiv2022

Binary black hole mergers from merged stars in the Galactic field

The majority of massive stars are found in close binaries which: (i) are prone to merge and (ii) are accompanied by another distant tertiary star (triples). Here, we study the evolution of the stellar post-merger binaries composed of the merger product and the tertiary companion. We find that post-merger binaries originating from compact stellar triples with outer semi-major axes $a_{\rm out,init}\lesssim10^1\,-\,10^2\,\rm AU$ provide a new way to form binary black hole mergers in the galactic field. By means of a population synthesis, we estimate their contribution to the total black hole merger rate to be $\mathcal{R}(z=0)=0.3\,-\,25.2\,\rm Gpc^{-3}\,yr^{-1}$. Merging binary black holes that form from stellar post-merger binaries have exceptionally low mass ratios. We identify a critical mass ratio $q\simeq0.5$ below which they dominate the total black hole merger rate in the field. We show that after including their additional contribution, the mass ratio distribution of binary black hole mergers in the galactic field scenario is in better agreement with that inferred from gravitational wave detections.

astro-ph.GA↗

arXiv2022

Understanding how fast black holes spin by analysing data from the second gravitational-wave catalogue

The Advanced LIGO and Virgo detectors have now observed approximately 50 black-hole-binary mergers, from which we can begin to infer how rapidly astrophysical black holes spin. The LIGO-Virgo Collaboration (LVC) analysis of detections up to the end of the first half of the third observing run (O3a) appeared to uncover a distribution of spin magnitudes that peaks at $\sim$0.2. This is surprising: is there a black-hole formation mechanism that prefers a particular, non-zero spin magnitude, or could this be the cumulative effect of multiple formation processes? We perform an independent analysis of the most recent gravitational-wave catalogue, and find that (a) the support for the LVC spin-magnitude is tenuous; in particular, adding or removing just one signal from the catalogue can remove the statistical preference for this distribution, and (b) we find potential evidence for two spin sub-populations in the observed black holes; one with extremely low spins and one with larger spin magnitudes. We make the connection that these spin sub-populations could be correlated with the mass of the binary, with more massive binaries preferring larger spin magnitudes, and argue that this may provide evidence for hierarchical mergers in the second gravitational-wave catalogue.

gr-qc↗

arXiv2022

Exploring Features in the Binary Black Hole Population

Vamana is a mixture model framework that infers the astrophysical distribution of chirp mass, mass ratio, and spin component aligned with the orbital angular momentum for the binary black hole population. We extend the mixing components in this framework to also model the redshift evolution of merger rate and report all the major one and two-dimensional features in the Binary Black Hole population using the 69 gravitational wave signals detected with a false alarm rate $<1\mathrm{yr}^{-1}$ in the third Gravitational-Wave Transient Catalog (GWTC)-3. Endorsing our previous report and a corroborating recent report from LIGO Scientific, Virgo, and KAGRA Collaborations, we observe the chirp mass distribution has multiple peaks and a lack of mergers with chirp masses $10 \textrm{--} 12M_\odot$. In addition, we observe aligned spins show mass dependence with heavier binaries exhibiting larger spins, mass ratio shows a dependence on the chirp mass but not on the aligned spin, and the redshift evolution of the merger rate for the peaks in the mass distribution is disparate. These features possibly reflect the astrophysics associated with the binary black hole formation channels. However, additional observations are needed to improve our limited confidence in them.

astro-ph.HE↗

arXiv2021

VAMANA: Modeling Binary Black Hole Population with Minimal Assumptions

The population analysis of compact binaries involves the reconstruction of some of the gravitational wave (GW) signal parameters, such as, the mass and the spin distribution, that gave rise to the observed data. This article introduces VAMANA, which reconstructs the binary black hole population using a mixture model and facilitates excellent density measurement as informed by the data. VAMANA uses a mixture of weighted Gaussians to reconstruct the chirp mass distribution. We expect Gaussian mixtures to provide flexibility in modeling complex distributions and enable us in capturing details in the astrophysical chirp mass distribution. Each of the Gaussian in the mixture is combined with another Gaussian and a power-law to simultaneously model the spin component aligned with the orbital angular momentum and the mass ratio distribution, thus also allowing us to capture their variation with the chirp mass. Additionally, we can also introduce broadband smoothing by restricting the Gaussian mixture to lie within a threshold distance of a predefined reference chirp mass distribution. Using simulated data we show the robustness of our method in reconstructing complex populations for a large number of observations. We also apply our method to the publicly available catalog of GW observations made during LIGO's and Virgo's first and second observation runs and present the reconstructed mass, spin distribution, and the estimated merger rate of binary black holes.

astro-ph.HE↗

arXiv2021

The Emergence of Structure in the Binary Black Hole Mass Distribution

We use the gravitational wave signals from binary black hole merger events observed by LIGO and Virgo to reconstruct the underlying mass and spin distributions of the population of merging black holes. We reconstruct the population using the mixture model framework VAMANA (Tiwari 2020) using observations in GWTC-2 occurring during the first two observing runs and the first half of the third run (O1, O2, and O3a). Our analysis identifies a structure in the chirp mass distribution of the observed population. Specifically, we identify peaks in the chirp mass distribution at 8, 14, 26, and 45 M and a complementary structure in the component mass distribution with an excess of black holes at masses of 9, 16, 30 and 57 M_. Intriguingly, for both the distributions, the location of subsequent peaks are separated by a factor of around two and there is a lack of mergers with chirp masses of 10-12 M. The appearance of multiple peaks is a feature of a hierarchical merger scenario when, due to a gap in the black-hole mass spectrum, a pile-up occurs at the first peak followed by mergers of lower mass black-holes to hierarchically produce higher mass black-holes. However, cross-generation merger peaks and observations with high spins are also predicted to occur in such a scenario that we are not currently observing. The results presented are limited in measurement accuracy due to small numbers of observations but if corroborated by future gravitational wave observations these features have far-reaching implications.

astro-ph.HE↗

arXiv2020

Applying Cosmological Principle to Better Probe the Redshift Evolution of Binary Black Hole Merger Rate

Gravitational waves inform about the probable distances at which an observed signal originated. This information when combined over multiple observations is used in the modeling of the redshift evolution of the merger rate. This is an important aspect of binary black hole population analysis which is expected to have close ties with the star formation history of the universe or dynamical evolution of star clusters. At the least, it can probe the time delay between the star formation and merger of remnants. However, due to the degeneracy between the inclination angle of the binary and the luminosity distance, the measured luminosity distance has large uncertainty that limits our ability to precisely measure the redshift distribution of mergers. In this letter, we show that by imposing the cosmological principle it is possible to suppress this uncertainty and better discriminate between different distributions modeling the redshift evolution of binary black hole merger rate. Additionally, we show that by making a comparison with an analysis that does not make such imposition we can probe the cosmological principle.

astro-ph.HE↗

arXiv2018

Constraining black-hole spins with gravitational wave observations

The observation of gravitational-wave signals from merging black-hole binaries enables direct measurement of the properties of the black holes. An individual observation allows measurement of the black-hole masses, but only limited information about either the magnitude or orientation of the black hole spins is available, primarily due to the degeneracy between measurements of spin and binary mass ratio. Using the first six black-hole merger observations, we are able to constrain the distribution of black-hole spins. We perform model selection between a set of models with different spin population models combined with a power-law mass distribution to make inferences about the spin distribution. We assume a fixed power-law mass distribution on the black holes, which is supported by the data and provides a realistic distribution of binary mass-ratio. This allows us to accurately account for selection effects due to variations in the signal amplitude with spin magnitude, and provides an improved inference on the spin distribution. We conclude that the first six LIGO and Virgo observations (Abbott et al. 2016a, 2017a,b,c) disfavour highly spinning black holes against low spins by an odds-ratio of 15:1; thus providing strong constraints on spin magnitudes from gravitational-wave observations. Furthermore, we are able to rule out a population of binaries with completely aligned spins, even when the spins of the individual black holes are low, at an odds ratio of 22,000:1, significantly strengthening earlier evidence against aligned spins (Farr et al. 2017). These results provide important information that will aid in our understanding on the formation processes of black-holes.

gr-qc↗

arXiv2018

Estimation of the Sensitive Volume for Gravitational-wave Source Populations Using Weighted Monte Carlo Integration

The population analysis and estimation of merger rates of compact binaries is one of the important topics in gravitational wave (GW) astronomy. The primary ingredient in these analyses is the population-averaged sensitive volume. Typically, sensitive volume, of a given search to a given simulated source population, is estimated by drawing signals from the population model and adding them to the detector data as injections. Subsequently injections, which are simulated gravitational waveforms, are searched for by the search pipelines and their signal-to-noise ratio (SNR) is determined. Sensitive volume is estimated, by using Monte-Carlo (MC) integration, from the total number of injections added to the data, the number of injections that cross a chosen threshold on SNR and the astrophysical volume in which the injections are placed. So far, only fixed population models have been used in the estimation of the merger rates. However, as the scope of population analysis broaden in terms of the methodologies and source properties considered, due to an increase in the number of observed GW signals, the procedure will need to be repeated multiple times at a large computational cost. In this letter we address the problem by performing a weighted MC integration. We show how a single set of generic injections can be weighted to estimate the sensitive volume for multiple population models; thereby greatly reducing the computational cost. The weights in this MC integral are the ratios of the output probabilities, determined by the population model and standard cosmology, and the injection probability, determined by the distribution function of the generic injections. Unlike analytical/semi-analytical methods, which usually estimate sensitive volume using single detector sensitivity, the method is accurate within statistical errors, comes at no added cost and requires minimal computational resources.

astro-ph.HE↗

arXiv2017

Localization of binary mergers with gravitational-wave detectors of second and third generation

The observation of gravitational wave signals from binary black hole mergers has established the field of gravitational wave astronomy. It is expected that future networks of gravitational wave detectors will possess great potential in probing various aspects of astronomy. An important consideration for successive improvement of current detectors or establishment on new sites is knowledge of the minimum number of detectors required to perform precision astronomy. We attempt to answer this question by assessing ability of future detector networks in detecting and localizing binary neutron stars mergers in the sky. This is an important aspect as a good localization ability is crucial for many of the scientific goals of gravitational wave astronomy, such as electromagnetic follow-up, measuring the properties of compact binaries throughout cosmic history, and cosmology. We find that although two detectors at improved sensitivity are sufficient to get a substantial increase in the number of observed signals, at least three detectors of comparable sensitivity are required to localize majority of the signals, typically to within around 10 deg$^{2}$ --- adequate for follow-up with most wide field of view optical telescopes.

gr-qc↗

arXiv2015

A Proposed Search for the Detection of Gravitational Waves from Eccentric Binary Black Holes

Most of compact binary systems are expected to circularize before the frequency of emitted gravitational waves (GWs) enters the sensitivity band of the ground based interferometric detectors. However, several mechanisms have been proposed for the formation of binary systems, which retain eccentricity throughout their lifetimes. Since no matched-filtering algorithm has been developed to extract continuous GW signals from compact binaries on orbits with low to moderate values of eccentricity, and available algorithms to detect binaries on quasi-circular orbits are sub-optimal to recover these events, in this paper we propose a search method for detection of gravitational waves produced from the coalescences of eccentric binary black holes (eBBH). We study the search sensitivity and the false alarm rates on a segment of data from the second joint science run of LIGO and Virgo detectors, and discuss the implications of the eccentric binary search for the advanced GW detectors.

gr-qc↗