arXiv subjects

Mohit Raj Sah

Publications and source records attributed to Mohit Raj Sah.

12 recordsLinked to original sources

arXiv2026

A New Framework to Detect Multi-Messenger Signals from Bright Sporadic Stochastic Gravitational Wave Background

The temporal dependence of the astrophysical stochastic gravitational-wave (GW) background (SGWB) in the hecto-hertz band brings a unique avenue to identify multi-messenger signals to these sources by using coincident detection in both GW and multi-band EM signals. We developed a new analysis pipeline, \textit{Multi-messenger Cross-Correlation} (MC$^2$) that can search for EM counterparts to the SGWB signal originating from both modeled and unmodeled sources by harnessing the nearly full-sky gamma-ray sky map. We provide an observation strategy that can be followed by current and future missions to discover EM counterparts to the weak GW signal hidden in the SGWB. We demonstrate the ability of this technique to drastically reduce the false alarm rates when involving EM multi-band analysis. This formalism aims towards advancing the multi-messenger observation frontier and improving our understanding of the population of bright SGWB sources present in the high-redshift universe and can also be applied to other messengers such as neutrinos in the future.

astro-ph.HE↗

arXiv2026

The First Upper Bound on the Non-Stationary Gravitational Wave Background and its Implication on the High Redshift Binary Black Hole Merger Rate

The high redshift merger rate and mass distribution of black hole binaries provide a direct probe to distinguish astrophysical black holes (ABHs) and primordial black holes (PBHs), which can be studied using the Stochastic Gravitational-Wave Background (SGWB). The conventional analyses solely based on the power spectrum are limited in constraining the properties of the underlying source population under the assumption of a non-sporadic Gaussian distribution. However, recent studies have shown that SGWB is expected to be sporadic and non-Gaussian in nature, which gives rise to non-zero \textit{spectral correlation} that depends on the high redshift merger rate and mass distribution of the compact objects. In this work, we present the first spectral covariance analysis of the SGWB using data from the LIGO--Virgo--KAGRA collaboration during the third and the first part of the fourth observing runs. Our analysis indicates that the current spectral correlation is consistent with non-stationary noise, yielding no detection and providing only upper bounds over the frequency range of 20 Hz to 100 Hz. This upper bound on the spectral correlation translates into a mass-distribution-dependent upper bound on the merger rate of PBHs. This provides a stringent upper bound on the PBH merger rate at high redshift and hence puts constraints on the PBH formation scenarios. In the future, detection of this signal will provide a new avenue to probe the high-redshift black hole population using gravitational waves.

astro-ph.HE↗

arXiv2026

Blinded Mock Data Challenge: Is the Spectral Siren Technique Robust for Measuring the Hubble Constant?

The measurement of the Hubble constant from gravitational wave (GW) sources is one of the independent avenues to shed light on the Hubble tension, which is associated with about an $8\%$ mismatch in the value of the Hubble constant inferred from low-redshift and high-redshift cosmological probes. Such a key measurement is expected from GW sources as it is a direct measurement of the Hubble constant using the luminosity distance without the need for any luminosity distance calibration. However, such a measurement relies strongly on the reliability of the independent inference of the source redshift of the GW source. As a result, it becomes pertinent to gauge the accuracy and precision of techniques in understanding their reliability in inferring redshifts of GW sources. In this work, we show the requirement of the spectral siren technique in knowing the mass distribution of BBHs across cosmic redshifts in order to make a reliable inference of the Hubble constant. We show by a blinded mock data challenge analysis the criticality in capturing the underlying metallicity dependence of the BBH mass distribution and its interplay with time-delay distribution for a robust inference of the Hubble constant using the spectral siren technique. In order to have a reliable measurement of the Hubble constant at the level required to resolve the Hubble tension in the future, the mass distribution of the BBHs needs to be independently inferred at all relevant redshifts with an accuracy less than the statistical uncertainty. Otherwise, a mismatch of the true model and the underlying assumption made in the analysis can lead to a best-fit model for the wrong value of both BBH population parameters as well as the Hubble constant.

astro-ph.CO↗

arXiv2026

The First Upper Bound on the Nano-Hertz Gravitational Waves and Galaxy Cross-Correlation signal using 15-year NANOGrav Data and DESI Galaxy Survey

The recent detection of a common-spectrum stochastic signal by multiple pulsar timing array (PTA) collaborations has provided tentative evidence for a nanohertz (nHz) stochastic gravitational-wave background (SGWB). This signal can be widely interpreted as originating from a cosmic population of inspiraling supermassive black hole binaries (SMBHBs). Current PTA analyses primarily constrain the SGWB power spectrum and its auto-angular power spectrum. However, the supermassive black holes will produce an underlying correlation with the large-scale structure of the Universe, which can help in understanding the formation and evolution of the binaries. In this work, we develop a new analysis pipeline PyGxGW-PTA for studying the cross-correlation of nHz GW signal with galaxy surveys ($C^{\rm g\, GW}_\ell$) and obtain the first constraint on the SGWB and galaxy distribution cross-correlation using the NANOGrav 15-year dataset in combination with the DESI galaxy catalog. We find no statistically significant correlation between the SGWB and the large-scale distribution of DESI galaxies and using an optimal estimator we put an upper bound on $C^{\rm g\, GW}_{\ell=8} < 0.0083$ at $95\%$ C.I. This yields the first observational upper limit on the spatial correlation between the nHz SGWB and the large-scale structure of the Universe, establishing the observational groundwork for future multi-tracer analyses that will combine PTA data with next-generation galaxy surveys to unveil the SMBHB-galaxy correlation.

astro-ph.CO↗

arXiv2025

The Route to Unveil the Cosmic Genealogy of Supermassive Black Hole Binaries Using Nano-Hertz Gravitational Waves and Galaxy Surveys

The nano-hertz (nHz) stochastic gravitational wave background (SGWB), generated by unresolved supermassive black hole binaries (SMBHBs), provides a unique probe of their population and its cosmic evolution. In this work, we explore the potential of uncovering the SMBHB population and its redshift dependence by combining the SGWB signal and its anisotropies with galaxy distribution through cross-correlation analyses. Using a Fisher analysis technique, we show that the SGWB power spectrum alone can not provide any information on the evolutionary history of SMBHBs, whereas the inclusion of the angular power spectrum of the SGWB and its cross-correlation with the galaxy distribution substantially improves constraints on the redshift evolution parameters. Assuming pulsar timing array (PTA) configurations achievable in the Square Kilometre Array (SKA) era, we find that the combined use of isotropic and anisotropic SGWB signals, together with galaxy surveys, can provide valuable measurements of the redshift evolution of the SMBH-galaxy connection and the frequency distribution of SMBHBs. These results highlight the potential of joint GW-galaxy studies to address the long-standing open question of SMBH growth and evolution across cosmic time.

astro-ph.CO↗

arXiv2025

An Accurate Modeling of Nano-hertz Gravitational Wave Signal from Eccentric Supermassive Binary Black Holes: An Essential Step Toward a Robust Discovery

The stochastic gravitational wave background (SGWB) in the nanohertz (nHz) regime, detectable by pulsar timing arrays (PTAs), provides a promising probe of the cosmic population of supermassive black hole binaries (SMBHBs). These binaries are expected to retain significant eccentricity throughout their evolution. We present a new technique to model the nHz SGWB by incorporating eccentricity into a multi-scale adaptive simulation framework. Using the time-domain eccentric waveform model ESIGMAHM, we generate realistic GW signals from astrophysical populations of SMBHBs. Unlike circular binaries, eccentric systems emit across multiple frequencies, introducing spectral correlations between frequency bins. These correlations provide a novel observational signature of the eccentricity distribution of the SMBHB population. In this work, we adopt simplified power-law models for the eccentricity distribution. While this does not capture the full complexity of galactic environments, it effectively highlights the key features of GW emission from eccentric binaries and their imprint on the SGWB. Our approach advances nHz GW signal modeling by incorporating eccentricity at small scales, enabling more realistic predictions and offering a new avenue for probing SMBHB astrophysics with future PTA observations.

astro-ph.CO↗

arXiv2025

A White Paper on The Multi-Messenger Science Landscape in India

The multi-messenger science using different observational windows to the Universe such as Gravitational Waves (GWs), Electromagnetic Waves (EMs), Cosmic Rays (CRs), and Neutrinos offer an opportunity to study from the scale of a neutron star to cosmological scales over a large cosmic time. At the smallest scales, we can explore the structure of the neutron star and the different energetics involved in the transition of a pre-merger neutron star to a post-merger neutron star. This will open up a window to study the properties of matter in extreme conditions and a guaranteed discovery space. On the other hand, at the largest cosmological scales, multi-messenger observations allow us to study the long-standing problems in physical cosmology related to the Hubble constant, dark matter, and dark energy by mapping the expansion history of the Universe using GW sources. Moreover, the multi-messenger studies of astrophysical systems such as white dwarfs, neutron stars, and black holes of different masses, all the way up to a high redshift Universe, will bring insightful understanding into the physical processes associated with them that are inaccessible otherwise. This white paper discusses the key cases in the domain of multi-messenger astronomy and the role of observatories in India which can explore uncharted territories and open discovery spaces in different branches of physics ranging from nuclear physics to astrophysics.

astro-ph.HE↗

arXiv2025

Blinded Mock Data Challenge for Gravitational-Wave Cosmology-I: Assessing the Robustness of Methods Using Binary Black Holes Mass Spectrum

Gravitational Wave (GW) sources are standard sirens that provide an independent way to map the cosmic expansion history by combining with an independent redshift measurement either from an electromagnetic counterpart for a bright siren or using different statistical techniques for dark sirens. In this analysis, we perform the first Blinded Mock Data Challenge (\texttt{Blinded-MDC}) to test the robustness in inferring the value of Hubble constant $H_0$ for a dark siren technique which depends on astrophysical mass distribution of Binary Black Holes (BBHs). We have considered different analysis setups for the \texttt{Blinded-MDC} to test both statistical and systematic uncertainties and demonstrate the capabilities in inferring $H_0$ with detector sensitivity as per the fourth observation run of LIGO-Virgo-KAGRA. We find that when the astrophysical population of BBHs matches with the underlying assumption of the model, a cosmological pipeline can recover the injected parameters using the observed mass distribution. However, when the mock mass distribution of the astrophysical population depends slightly on redshift and one is ignorant about it in analyzing the data, it can cause a systematic discrepancy in the inferred value of $H_0$ by about $1.5σ$, above the statistical fluctuations due to noise and a limited number of events. In the future, elaborate studies will be required to mitigate systematic uncertainties due to unknown astrophysical complexities. This MDC framework sets the road map for inspecting the precision and accuracy of standard siren cosmology and provides the first insight into the robustness of the population-dependent cosmology inference in a blinded analysis setup.

astro-ph.CO↗

arXiv2025

Multi-Tracer Correlated Stacking: A Novel Way to Discover Anisotropy in nano-Hz Stochastic Gravitational Wave Background

The isotropic stochastic gravitational wave background (SGWB) generated by a population of supermassive black hole binaries (SMBHBs) provides a unique window into their cosmic evolution. In addition to the isotropic power spectrum, the anisotropic component of the signal carries additional information about the supermassive black holes (SMBHs) and host galaxy connection. The measurement of this signal is usually carried out by angular power spectra, which is only a sufficient measure for a Gaussian and statistically isotropic distribution of SMBHBs, where the statistical properties of a field remain unchanged across the sky. In contrast, the contribution from SMBHBs in nano-hertz SGWB will be hosted by fewer massive galaxies, making the nano-hertz background anisotropic and non-Gaussian. As a result, the performance of angular power spectra in extracting the underlying physics is limited. In this work, we propose a novel technique called the \texttt{Multi-Tracer Correlated Stacking}, which enables the detection of anisotropies in the SGWB by stacking the signal from regions of the sky with tracers of BHs such as active galactic nucleus (AGNs), quasars, bright galaxies, etc., that can be mapped up to high redshift. We demonstrate this technique on a simulated supermassive BHBs distribution using an AGN catalog, which maps the underlying matter distribution approximately up to redshift $z=5$. This stacking technique uniquely distinguishes between isotropic and anisotropic distributions of SGWB source, surpassing the capabilities of angular power spectrum-based methods in detecting anisotropic signals. This highlights the effectiveness of this technique in detecting anisotropic SGWB signals and in the future, this technique can play a crucial role in its discovery.

astro-ph.CO↗

arXiv2024

Imprints of Supermassive Black Hole Evolution on the Spectral and Spatial Anisotropy of Nano-Hertz Stochastic Gravitational-Wave Background

The formation and evolution of supermassive black holes (SMBHs) remains an open question in the field of modern cosmology. The detection of nanohertz (n-Hz) gravitational waves via pulsar timing arrays (PTAs) in the form of individual events and the stochastic gravitational wave background (SGWB) offers a promising avenue for studying SMBH evolution across cosmic time, with SGWB signal being the immediately detectable signal with the currently accessible telescope sensitivities. By connecting the galaxy properties in the large scale (Gpc scale) cosmological simulation such as \texttt{MICECAT} with the small scale ($\sim$ Mpc scale) galaxy simulations from \texttt{ROMULUS}, we show that different scenarios of galaxy-SMBH evolution with redshift leads to a frequency-dependent spatial anisotropy in the SGWB signal. The presence of slow evolution of the SMBHs in the Universe leads to a pronounced blue anisotropic spectrum of the SGWB. In contrast, if SMBHs grow faster in the Universe in lighter galaxies, the frequency-dependent spatial anisotropy exhibits a more flattened anisotropic spectrum. This additional aspect of the SGWB signal on top of the monopole SGWB signal, can give insight on how the SMBHs form in the high redshift Universe and its interplay with the galaxy formation from future measurements.

astro-ph.CO↗

arXiv2024

Discovering the Cosmic Evolution of Supermassive Black Holes using Nano-Hertz Gravitational Waves and Galaxy Surveys

The formation and evolution of supermassive black holes (SMBHs) in the Universe remains an open question in cosmology. We show for the first time that the evolution of SMBHs with redshift leads to a unique signature on the angular cross-correlation power spectrum between the multi-frequency nano-hertz (nHz) stochastic gravitational wave (SGWB) and the galaxy density in the Universe. By using galaxy catalogs from the upcoming Rubin LSST Observatory in synergy with the nHz SGWB signal accessible from the Square Kilometer Array, we can measure this signal with a signal-to-noise ratio above five, thereby opening a new observational window to the cosmic evolution of SMBHs across redshift. This discovery space that can be opened by the cross-correlation of the nHz SGWB will not be possible by any other currently known techniques.

astro-ph.CO↗

arXiv2023

Non-Stationary Astrophysical Stochastic Gravitational-Wave Background: A New Probe to the High Redshift Population of Binary Black Holes

The astrophysical Stochastic Gravitational Wave Background (SGWB) originates from the mergers of compact binary objects that are otherwise undetected as individual events, along with other sources such as supernovae, magnetars, etc. The individual GW signal is time-varying over a time scale that depends on the chirp mass of the coalescing binaries. Another timescale that plays a role is the timescale at which the sources repeat, which depends on the merger rate. The combined effect of these two leads to a breakdown of the time-translation symmetry of the observed SGWB and a correlation between different frequency modes in the signal covariance matrix of the SGWB. Using an ensemble of SGWB due to binary black hole coalescence, calculated using simulations of different black hole mass distributions and merger rates, we show how the structure of the signal covariance matrix varies. This structure in the signal covariance matrix brings additional information about the sources on top of the power spectrum. We show that there is a significant improvement in the Figure of Merit by using this additional information in comparison to only power spectrum estimation for the LIGO-Virgo-KAGRA (LVK) network of detectors with the design sensitivity noise with two years of observation. The inclusion of the off-diagonal correlation in the covariance of the SGWB in the data analysis pipelines will be beneficial in the quest for the SGWB signal in LVK frequency bands as well as in lower frequencies and in getting an insight into its origin.

gr-qc↗