arXiv subjects

Kanchan Soni

Publications and source records attributed to Kanchan Soni.

13 recordsLinked to original sources

arXiv2026

Prospects of electromagnetic follow-up of neutron star-black hole mergers in the LIGO-India era

Neutron star-black hole (NSBH) mergers are promising multimessenger sources, but only a subset is expected to produce detectable electromagnetic (EM) counterparts, depending on the binary mass ratio, black hole (BH) spin, and neutron star (NS) equation of state (EoS). We investigate the prospects for detecting kilonova counterparts to NSBH mergers in the LIGO-India era using the Vera C. Rubin Observatory (Rubin). We simulate GW230529-motivated NSBH populations with two effective tidal deformability ranges, estimate gravitational-wave detections for the LHV (LIGO-Livingston, LIGO-Hanford, Virgo) and LHVA (LHV with LIGO-India at Aundha) detector networks, and model the detectability of the associated kilonova emission with Rubin. We find that adding LIGO-India approximately doubles the number of EM follow-ups, primarily by increasing the duty cycle and reducing the median sky-localization area, and notably increases the number of such detections at large distances. For fixed-exposure strategies, the EM detection rate saturates at exposure times of a few hundred seconds, reflecting the trade-off between depth and sky coverage. We therefore introduce an event-specific exposure-time optimization strategy based on the expected counterpart brightness and sky-localization area. With this strategy, the expected NSBH GW+EM detection rate per year is 4.19(-3.58,+9.66) for the optimistic and 0.79(-0.68,+1.83) for the conservative population with the LHVA network, compared to 1.46(-1.25,+3.37) and 0.27(-0.23,+0.62), respectively, for LHV. The required Rubin follow-up time remains within the expected Target-of-Opportunity allocation. These results show that in the next decade, a few such NSBH follow-ups may take place, with a fraction of them in the redshift range of about 0.1-0.2, which will not only be useful for measuring the Hubble constant but can also help us in probing the Hubble parameter.

gr-qc↗

arXiv2026

Search for Sub-Solar Mass Binaries in the First Part of LIGO's Fourth Observing Run

We report the first results of a sub-solar mass compact binary search using the data from the first part of the fourth observing run (O4a) of the Advanced LIGO detectors. Sub-solar mass neutron stars and black holes are not expected to form via standard stellar evolution, and their observation would signify a new class of astrophysical objects or the discovery of a component of dark matter. Our search covers binaries with primary masses 0.1 to 2 $M_\odot$ and secondary masses 0.1 to 1 $M_\odot$. We explicitly incorporate tidal effects up to $7\times10^5$ for extremely low mass neutron stars. Due to the recent development of efficient ratio filter de-chirping frameworks, this search consisting of 25 million templates is now computationally feasible. No statistically significant candidates are identified. We place a $90\%$ confidence upper limit on the merger rate $\mathcal{R}_{90}$ for sub-solar mass black holes to be $< 2.5\times10^4\,\textrm{Gpc}^{-3} \textrm{yr}^{-1}$ for a chirp mass of 0.2 $M_\odot$. We place the first constraints for binary neutron stars with tidal deformabilities up to $\sim 7\times10^5$ and improve the merger rate estimate by a factor $\sim 3$ in comparison to previous O3 tidal searches for tidal deformabilities $< 10^4$. The advanced sensitivity of the O4a run enables an improvement in the sub-solar mass black hole merger rate limits by more than $2 \times$ over the previous three observing runs (O1-O3) combined. We constrain the effective local dark matter fraction to be $\tilde{f}_\textrm{PBH}<0.5\%$ for 0.4 $M_{\textrm{PBH}}$, approximately 1.8 times lower than the previous O1-O3 constraints. Given our model assumptions, our local dark matter fraction constraints are 2-10 times lower than the OGLE microlensing survey for $M_{\textrm{PBH}}\ge0.25$.

astro-ph.HE↗

arXiv2026

Beyond FINDCHIRP: Breaking the memory wall and optimal FFTs for Gravitational-Wave Matched-Filter Searches with Ratio-Filter Dechirping

A primary bottleneck in modern FFT-based matched-filter searches for gravitational waves from compact binary coalescences is not raw processor throughput, but available memory bandwidth. Standard frequency-domain implementations, such as the FINDCHIRP algorithm, rely on streaming long template waveforms and data from main memory, which leads to significant processor stalling when template durations exceed cache capacities. In this work, we introduce \textit{Ratio-Filter Dechirping} as a solution, an algorithmic restructuring of the matched filter that transforms the operation from a memory-bound Fast Fourier Transform (FFT) into a cache-efficient, compute-bound Finite Impulse Response (FIR) convolution. By utilizing a reference template to remove common orbital phase evolution, we produce slowly changing frequency-domain ratios that can be accurately implemented as short FIR filters. This method delivers a measured speedup of $8\times$ for the core filtering loop used in offline searches and should enable $>10\times$ for low-latency analysis. We find that this approach generalizes to a variety of searches that include physical features such as finite size effects, eccentricity, and precession. By dramatically reducing the computational cost of matched filtering, this approach enables the expansion of searches into dense or high-dimensional parameter spaces, such as those for eccentric or subsolar-mass signals, that are already limited by available computing budgets. Furthermore, this framework provides a natural path for hardware acceleration on GPU architectures.

astro-ph.IM↗

arXiv2025

Eccentricity signatures in LIGO-Virgo-KAGRA's BNS and NSBH binaries

Measurement of eccentricity in low-mass binary systems through gravitational waves is crucial to distinguish between various formation channels. Detecting eccentricity in these systems is challenging due to a lack of accurate eccentric waveform models and the high computational cost of Bayesian inferences. We access the eccentricities of six previously observed low-mass gravitational wave events using publicly available data from the first four observing runs of the LIGO and Virgo collaboration. We analyze the events using the new eccentric waveform model, SEOBNRv5EHM, and compare our results with the existing model, TEOBResumS-Dali. We also present the first eccentricity constraints for GW190814. To improve accuracy, we include higher-order modes in both models and optimize inference using efficient marginalization and parallelization techniques. We find that GW200105 exhibits non-negligible eccentricity, with a measured eccentricity of $e=0.135^{+0.019}_{-0.088}$ at 20 Hz (90% credible level) for TEOBResumS-Dali and $e=0.125^{+0.029}_{-0.082}$ for SEOBNRv5EHM, given a uniform eccentricity prior from 0 to 0.2. This provides moderate support for the eccentric hypothesis, with a Bayes factor of $\sim6-7$ in favor of the eccentric model. With a uniform log prior on eccentricity, the Bayes factor is reduced to 2.35. The remaining five sources are consistent with low eccentricity, with 90% upper limits from $e \leq 0.011$ to $e \leq 0.066$. We find no support for non-negligible eccentricity in GW190814. Finally, we discuss the challenges of performing Bayesian inference in eccentric, multi-modal parameter spaces, including issues related to sampling efficiency and waveform systematics.

gr-qc↗

arXiv2025

Mining the Alerts: A Preliminary Catalog of Compact Binaries from the Fourth Observing Run

We present a preliminary catalog of compact binary merger candidates from the ongoing fourth observing run (O4) of Advanced LIGO, Virgo, and KAGRA, based on an analysis of public alerts distributed through GraceDB as of May 2025. We developed and applied methods to estimate the source-frame chirp mass for each candidate by utilizing information from public data products, including source classification probabilities, sky localizations, and observatory status. Combining our O4 analysis with previous catalogs, we provide updated estimates for the local merger rate density. For sources with chirp mass characteristic of binary neutron stars ($[1, 1.5]\,M_\odot$), we find a rate of $56^{+99}_{-40}$ $\textrm{Gpc}^{-3}\,\textrm{yr}^{-1}$. For systems in the expected neutron star--black hole chirp mass range ($[1.5, 3.5]\,M_\odot$), the rate is $36^{+32}_{-20}$ $\textrm{Gpc}^{-3}\,\textrm{yr}^{-1}$, and for heavier binary black holes ($[3.5, 100]\,M_\odot$), we estimate a rate of $19^{+4}_{-2}$ $\textrm{Gpc}^{-3}\,\textrm{yr}^{-1}$. This work provides an early glimpse into the compact binary population being observed in O4; we identify a number of high-value candidates up to signal-to-noise $\sim 80$, which we expect to enable precision measurements in the future.

astro-ph.HE↗

arXiv2025

Potential science with GW250114 -- the loudest binary black hole merger detected to date

On January 14, 2025 the LIGO interferometers detected a gravitational wave from the merger of two black holes, GW250114. Using publicly available information, we estimate that the signal-to-noise ratio (SNR) of GW250114 was $\sim 80$. This would make it three to four times louder than any other gravitational wave detected to date. GW250114 therefore offers a unique opportunity to make precise measurements of its source parameters and to test general relativity. In anticipation of its public data release, we analyze a set of simulated signals that have parameters similar to what we estimate for GW250114 and explore what new insights may be gained from this significant event. We investigate how well the component spins may be constrained, whether any eccentricity may be measured, what quasi-normal modes (QNMs) may be detected in the post-merger signal, how well the black hole area theorem may be constrained, and what constraints may be expected on sub-dominant inspiral-merger-ringdown modes. We find that it should be possible to measure a non-zero eccentricity at $20\,$Hz ($e_{20}$) if GW250114 has $e_{20} \gtrsim 0.05$. We also find that at least one overtone of the dominant QNM should be detectable in the ringdown of GW250114, with a Bayes factor of $O(10^3)$ after marginalizing over all timing uncertainties.

gr-qc↗

arXiv2025

A $\chi^2$ statistic for the identification of strongly lensed gravitational waves from compact binary coalescences

Gravitational waves (GWs) emanated by stellar mass compact binary coalescences (CBCs), and lensed by galaxy- or cluster-scale lenses, will produce two or more copies of the GW signal. These will have identical phase evolution but differing amplitudes. Such lensing signatures are expected to be detected by the end of the LIGO-Virgo-Kagra's (LVK's) fifth observing run (O5). In this work, we propose a novel $\chi_{\mathrm{lens}}^2$ statistic to segregate pairs of detected GW events as either lensed or unlensed, using templates typically used in GW searches. The statistic is an application of the generalized $\chi^2$ discriminator described in \citet{dhurandhar2017}, tailored to probe the similarity (or lack thereof) between the phase evolutions of two CBC signals. We assess the performance of $\chi_{\mathrm{lens}}^2$ on a realistic astrophysical dataset of lensed and unlensed CBCs detectable in O4, assuming a single LIGO-like detector at design sensitivity. We find that we can correctly identify lensed events with efficiencies comparable to existing Bayesian and machine learning methods. Evaluating $\chi_{\mathrm{lens}}^2$ is orders of magnitude faster than Bayesian methods. Moreover, the statistics of $\chi_{\mathrm{lens}}^2$, in stationary Gaussian noise, are fully understood, in contrast to machine learning methods. $\chi_{\mathrm{lens}}^2$ can, therefore, be used to rapidly and accurately weed out the vast majority of unlensed candidate pairs and identify lensed pairs.

gr-qc↗

arXiv2024

Assessing the potential of LIGO-India in resolving the Hubble tension

Conclusive determination of the Hubble constant ($H_0$) is a major challenge in current astronomy due to the observed discrepancies between early and late universe measurements. Joint detections of gravitational waves (GW) and electromagnetic (EM) signals from binary neutron star (BNS) mergers promise an independent probe for $H_0$. However, this requires tens of such detections, requiring decades of observation with the present detectors. LIGO-India can significantly accelerate this process. LIGO-India's addition to the detector network could boost event detections by $\sim 70\%$ and double the number of properly localized ("triple-coincidence") detections. Through end-to-end simulations, we show that LIGO-India could increase the EM follow-up rate of kilonovae by $\sim 2-7$ times, reducing the $H_0$ estimation error by a factor of $\approx 1.65-2.82$ for Vera Rubin LSST, thus significantly reducing the observation time required to attain the necessary precision. Moreover, LIGO-India can improve sky localization precision by many folds ($\sim 5$), allowing much deeper EM follow-ups, potentially reducing the time needed to resolve the "Hubble tension" from decades to a few years.

astro-ph.CO↗

arXiv2024

Hierarchical searches for subsolar-mass binaries and the third-generation gravitational wave detector era

The detection of gravitational waves (GWs) from coalescing compact binaries has become routine with ground-based detectors like LIGO and Virgo. However, beyond standard sources such as binary black holes and neutron stars and neutron star black holes, no exotic sources revealing new physics have been discovered. Detecting ultra-compact objects, such as subsolar mass (SSM) compact objects, offers a promising opportunity to explore diverse astrophysical populations. However, searching for these objects using standard matched-filtering techniques is computationally intensive due to the dense parameter space involved. This increasing computational demand not only challenges current search methodologies but also poses significant obstacles for third-generation (3G) ground-based GW detectors. In the 3G era, signals may last tens of minutes, and detection rates could reach one per minute, requiring efficient search strategies to manage the computational load of long-duration signals. In this paper, we demonstrate a hierarchical search strategy designed to address the challenges of searching for long-duration signals, such as those from SSM compact binaries, and the anticipated issues with 3G detectors. We show that by adopting optimization techniques in a two-stage hierarchical approach, we can efficiently search for the SSM compact object in the current LIGO detectors. Our preliminary results show that conducting matched filtering at a lower frequency of 35 Hz improves the signal-to-noise ratio by 6% and enhances the detection volume by 10-20%, compared to the standard two-detector PyCBC search. This improvement is achieved while reducing computational costs by a factor of 2.5.

gr-qc↗

arXiv2024

Detectability of eccentric binary black holes with PyCBC and cWB pipelines during the third observing run of LIGO-Virgo-KAGRA

Detecting binary black hole (BBH) mergers with quantifiable orbital eccentricity would confirm the existence of a dynamical formation channel for these binaries. The current state-of-the-art gravitational wave searches of LIGO-Virgo-KAGRA strain data focus more on quasicircular mergers due to increased dimensionality and lack of efficient eccentric waveform models. In this work, we compare the sensitivities of two search pipelines, the matched filter-based \texttt{PyCBC} and the unmodelled coherent Wave Burst (\texttt{cWB}) algorithms towards the spinning eccentric BBH mergers, using a multipolar nonprecessing-spin eccentric signal model, \texttt{SEOBNRv4EHM}. Our findings show that neglecting eccentricity leads to missed opportunities for detecting eccentric BBH mergers, with \texttt{PyCBC} exhibiting a $10-20\, \%$ sensitivity loss for eccentricities exceeding $0.2$ defined at $10$ Hz. In contrast, \texttt{cWB} is resilient, with a $10\, \%$ sensitivity increase for heavier ($\mathcal{M} \ge 30 \, \text{M}_{\odot}$) eccentric BBH mergers, but is significantly less sensitive than \texttt{PyCBC} for lighter BBH mergers. Our fitting factor study confirmed that neglecting eccentricity biases the estimation of chirp mass, mass ratio, and effective spin parameter, skewing our understanding of astrophysical BBH populations, fundamental physics, and precision cosmology. Our results demonstrate that the current search pipelines are not sufficiently sensitive to eccentric BBH mergers, necessitating the development of a dedicated matched-filter search for these binaries. Whereas, burst searches should be optimized to detect lower chirp mass BBH mergers as eccentricity does not affect their search sensitivity significantly.

gr-qc↗

arXiv2023

Obtaining Statistical Significance of Gravitational Wave Signals in Hierarchical Search

Gravitational Wave (GW) astronomy has experienced remarkable growth in recent years, driven by advancements in ground-based detectors. While detecting compact binary coalescences (CBCs) has become routine, searching for more complex ones, such as mergers involving eccentric and precessing binaries and sub-solar mass binaries, has presented persistent challenges. These challenges arise from using the standard matched filtering algorithm, whose computational cost increases with the dimensionality and size of the template bank. This urges the pressing need for faster search pipelines to efficiently identify GW signals, leading to the emergence of the hierarchical search strategy. This method looks for potential candidate events using a sparse template bank in the first stage, followed by dense templates around potential events in the second stage. Although the hierarchical search speeds up the standard PyCBC analysis by more than a factor of 20, as demonstrated in a previous work~\cite{kanchan_hierarchical}, assigning statistical significance to detected signals was done in a heuristic way. In this article, we present a robust approach for background estimation in a two-stage hierarchical search. Our method models background triggers from time-shifted triggers in a two-detector network, extrapolating to higher statistic values. Through an extensive injection campaign for a population of simulated signals on real data, we test the effectiveness of our background estimation approach. The results show our method achieves a sensitive volume-time product comparable to the standard two-detector PyCBC search. This equivalence holds for an inverse false alarm rate of 10 years and chirp mass $1.4-10~\text{M}_\odot$, substantially reducing computational cost with a remarkable speed-up of nearly 13 times compared to PyCBC analysis.

astro-ph.IM↗

arXiv2022

Hierarchical search for compact binary coalescences in the Advanced LIGO's first two observing runs

Detection of many compact binary coalescences (CBCs) is one of the primary goals of the present and future ground-based gravitational-wave (GW) detectors. While increasing the detectors' sensitivities will be crucial in achieving this, efficient data analysis strategies can play a vital role. With given computational power in hand, efficient data analysis techniques can expand the size and dimensionality of the parameter space to search for a variety of GW sources. Matched filtering based analyses that depend on modeled signals to produce adequate signal-to-noise ratios for signal detection may miss them if the parameter space is too restrained. Specifically, the CBC search is currently limited to non-precessing binaries only, where the spins of the components are either aligned or anti-aligned to the orbital angular momentum. A hierarchical search for CBCs is thus well motivated. The first stage of this search is performed by matched filtering coarsely sampled data with a coarse template bank to look for candidate events. These candidates are then followed up for a finer search around the vicinity of an event's parameter space. Performing such a search leads to enormous savings in computational cost. Here we report the first successful implementation of the hierarchical search as a PyCBC-based production pipeline to perform a complete analysis of LIGO observing runs. With this, we analyze Advanced LIGO's first and second observing run data. We recover all the events detected by the PyCBC (flat) search in the first GW catalog, GWTC-1, published by the LIGO-Virgo collaboration, with nearly the same significance using a scaled background. In the analysis, we get an impressive factor of 20 reduction in computation compared to the flat search. With a standard injection study, we show that the sensitivity of the hierarchical search remains comparable to the flat search within the error bars.

gr-qc↗

arXiv2016

Everyday Radio Telescope

We have developed an affordable, portable college level radio telescope for amateur radio astronomy which can be used to provide hands-on experience with the fundamentals of a radio telescope and an insight into the realm of radio astronomy. With our set-up one can measure brightness temperature and flux of the Sun at 11.2 GHz and calculate the beam width of the antenna. The set-up uses commercially available satellite television receiving system and parabolic dish antenna. We report the detection of point sources like Saturn and extended sources like the galactic arm of the Milky way. We have also developed python pipeline, which are available for free download, for data acquisition and visualization.

physics.ed-ph↗