arXiv subjects

Ignacio Magaña Hernandez

Publications and source records attributed to Ignacio Magaña Hernandez.

At least 19 recordsLinked to original sources

arXiv2026

Normalizing Flow Emulator for Gravitational Wave Detection Probability in the LIGO/Virgo/KAGRA O4 Observing Run

Efficient and accurate estimation of the detection probability of a gravitational wave event given its source parameters remains a challenging problem because it conventionally requires expensive Monte Carlo injection campaigns. We present a machine learning-based approach using Normalizing Flows (NFs) to estimate the detection probability of gravitational wave events for a LIGO/Virgo/KAGRA O4-like detector network configuration. We utilize the publicly available injection dataset as training and test data. We test our method and implementation by evaluating the raw NF performance as a generative model based on its ability to recover the correct parameter distributions of detected events. In addition, we compare the detection efficiencies predicted by the NF emulator with those from Monte Carlo importance sampling and find good agreement. Furthermore, estimating the detection efficiency for a wide variety of population models, including those with sharp features, can be more precise than estimates obtained via Monte Carlo importance sampling. We show that the NF emulator can be used in population analysis of gravitational wave sources, similar to previous neural network approaches. The training code and trained models from this work are publicly available.

gr-qc↗

arXiv2026

One Year and One Night to 1% in $\mathbf{H_0}$: Efficient Spectroscopic Strategy for Dark Siren Cosmology

Gravitational-wave events from compact binary coalescences (CBCs) can be used as standard sirens: they encode the luminosity distance to their sources, which yields a measurement of the Hubble constant ($H_0$) if the source's redshift is known. In the absence of an electromagnetic counterpart, $H_0$ can still be inferred statistically from the cataloged galaxies within the event's sky localization volume via the dark siren, or galaxy catalog method. In this work we examine how the depth of a volume-limited galaxy catalog affects this inference and identify the most efficient spectroscopic strategy for an unbiased measurement of $H_0$ at a median precision of $0.98\%$. The strategy is to carry out spectroscopic surveys to a depth of $r \sim 19$ for the ten best-localized events observed in one year by the LIGO Hanford, LIGO Livingston, and LIGO-India network at A$^\#$ sensitivity. Such a campaign requires a minimum of one night of electromagnetic follow-up observations, or five nights under the most conservative assumptions. For the Hanford, Livingston, and Virgo network at A$+$ sensitivity, the same ten-event strategy yields a median precision of $2.63\%$.

astro-ph.CO↗

arXiv2026

Electromagnetic Follow-up of the Sub-Solar Mass Gravitational Wave Candidate S251112cm: Kilonova Constraints and a Coincident IIb Supernova

On November 12th, 2025 the LIGO--Virgo--KAGRA (LVK) collaboration reported gravitational waves (GWs) from a compact object merger candidate (S251112cm) with at least one sub-solar mass component. Using the Dark Energy Camera (DECam), the Fraunhofer Telescope at Wendelstein Observatory (FTW), and the Zwicky Transient Facility (ZTF), we surveyed $56\%$ of the GW localization region beginning $2.4$~hours after the GW alert. We find no kilonova (KN) counterpart, and use radiative-transfer models to rule out $42\%$ (ZTF), $68\%$ (DECam), and $92\%$ (FTW) of the KN models as possible emission from this GW candidate. Within the recently proposed disk-fragmentation (``superkilonova'') model for generating sub-solar mass neutron star mergers from stellar core-collapse, the delay between the supernova explosion time and the GW merger time is estimated to be less than a few days. Searching this time window prior to the GW event, we identify and spectroscopically classify a IIb supernova (SN~2025adtq), with a spatial association odds ratio of $\log_{10}\mathcal{I} \approx 4.8$, a chance coincidence probability of ${\sim}2$--$9\%$, and an estimated explosion time ${\sim}2$ days prior to S251112cm. SN~2025adtq is the second Type~IIb supernova found in spatial and temporal coincidence with a sub-solar mass GW candidate, following the previously reported S250818k/SN~2025ulz association; jointly, we measure an odds ratio that favors the association hypothesis over the null, however, when conditioned on finding a coincident supernova by chance, the odds ratio disfavors association. Together, these results provide suggestive but inconclusive evidence for the superkilonova formation channel.

astro-ph.HE↗

arXiv2025

Lucky Strikes: On the Origins of GW190814 Through Isolated Binary Evolution

The asymmetric nature of GW190814, particularly its mass ratio ($q \approx 1/10$), has made its astrophysical origin elusive. We explore isolated binary evolution as a potential explanation for GW190814's formation. Using the binary population synthesis code COSMIC, and the backpop sampling technique to map the observed parameters of GW190814 to the initial conditions of Zero Age Main Sequence binary stars while simultaneously inferring the astrophysical prescriptions for common envelope evolution, stable mass transfer and natal kick kinematics that are needed for its formation and eventual merger. We find that the initial conditions for the binary stellar population that forms GW190814 do not stand out significantly from massive star populations observed in the Local Group. Our backpop simulations recover a dominant formation pathway where the first Roche overflow phase includes a common envelope evolution and the second Roche overflow phase remains stable. Our findings suggest that natal kicks imparted during compact object formation play the strongest role in forming GW190814-like systems. Specifically, our models require a low magnitude first natal kick (independent of direction) that prevents the binary from unbinding and a large second natal kick with its direction in the plane of the orbit and toward the binary's center of mass. The second natal kick strength and direction crucially increases the orbital eccentricity, leading to shorter delay times, and thus enabling mergers within a Hubble time. We estimate the chance probability for GW190814-like events that experience such a lucky kick and find that it occurs in $\sim20\%$ of systems if natal kicks are randomly oriented. We discuss the astrophysical implications for the formation of asymmetric GW190814-like systems under the context of binary stellar evolution.

astro-ph.HE↗

arXiv2025

AT2025ulz and S250818k: Investigating early time observations of a subsolar mass gravitational-wave binary neutron star merger candidate

Over the past LIGO--Virgo--KAGRA (LVK) observing runs, it has become increasingly clear that identifying the next electromagnetic counterparts to gravitational-wave (GW) neutron star mergers will likely be more challenging compared to the case of GW170817. The rarity of these GW events, and their electromagnetic counterparts, motivates rapid searches of any candidate binary neutron star (BNS) merger detected by the LVK. We present our extensive photometric and spectroscopic campaign of the candidate counterpart AT2025ulz to the low-significance GW event S250818k, which had a ${\sim} 29\%$ probability of being a BNS merger. We demonstrate that during the first five days, the luminosity and color evolution of AT2025ulz are consistent with both kilonova and shock cooling models, although a Bayesian model comparison shows preference for the shock cooling model, underscoring the ambiguity inherent to early data obtained over only a few days. Continued monitoring beyond this window reveals a rise and color evolution incompatible with kilonova models and instead consistent with a supernova. This event emphasizes the difficulty in identifying the electromagnetic counterparts to BNS mergers and the significant allotment of observing time necessary to robustly differentiate kilonovae from impostors.

astro-ph.HE↗

arXiv2025

Spectral siren cosmology from gravitational-wave observations in GWTC-4.0

Gravitational wave standard sirens offer a promising avenue for cosmological inference, particularly in measuring the expansion history of the universe. Traditionally, bright sirens require an electromagnetic counterpart to determine the redshift of the emission source while dark sirens rely on the presence of complete galaxy catalogs over large sky regions. Spectral sirens, using GW data alone, can circumvent these limitations by leveraging features in the mass distribution of compact binaries. With the recent release of the Gravitational-Wave Transient Catalog 4 (GWTC-4.0), the number of significant binary black hole (BBH) merger candidates has increased to 153, enabling more robust population studies and cosmological constraints. This work builds upon previous spectral siren analyses by analyzing the latest BBH observations with parametric and non-parametric models. In particular, we consider a parametric approach using the Powerlaw + Peak and Broken Powerlaw + 2 Peaks models as well as a more flexible non-parametric model based on Gaussian processes. We find broad consistency in the inferred Hubble constant $H_0$ constraints across models. Our most constraining result is from the Gaussian Process model, which, combined with the GW170817 bright siren measurement, results in $H_0 = 69^{+7}_{-6} \ \mathrm{km\,s^{-1}\,Mpc^{-1}}$, a 10% precision measurement. For the Powerlaw + Peak and Broken Powerlaw + 2 Peaks we find fractional uncertainties of 17% and 13% respectively.

astro-ph.CO↗

arXiv2025

Astrophysics informed Gaussian processes for gravitational-wave populations: Evidence for the onset of the pair-instability supernova mass gap

We analyze binary black hole (BBH) mergers from the latest Gravitational Wave Transient Catalog (GWTC-3) using a flexible, non-parametric framework to infer the underlying black hole mass distribution. Our model employs Gaussian Processes (GPs) with astrophysically motivated priors to represent the mass distribution, assuming both component masses are independently drawn from a common mass function. This approach enables us to capture complex features in the population without relying on rigid parametric forms. Motivated by predictions from binary stellar evolution, we focus on the presence of a mass gap in the range $40-120~ M_\odot$, attributed to the pair-instability supernova (PISN) and pulsational PISN (PPISN) processes. Using our GP-based model, we find for the first time, strong evidence for the onset of this mass gap, locating its lower edge at approximately $45-60 ~M_\odot$. We observe a suppression in the BBH merger rate when comparing the latest constraints against our GP model, by a factor of $10-60$ at $\sim60~M_\odot$, corresponding to the minimum of the mass gap. Additionally, we find evidence of a subpopulation of mergers populating the mass gap around $70 ~M_\odot$, which we argue is due to hierarchical mergers, as well as of a feature in the $40-50~ M_\odot$ range, albeit with low significance, which we attribute to the predicted PPISN build up. We discuss the astrophysical implications of this result in light of GW231123, a recently reported BBH merger with a total mass potentially above the PISN gap, suggesting the need for revised models of massive stellar evolution or alternative formation channels.

astro-ph.HE↗

arXiv2025

Kilonova constraints for the LIGO/Virgo/KAGRA neutron star merger candidate S250206dm: GW-MMADS observations

Gravitational wave (GW) neutron star mergers with an associated electromagnetic counterpart constitute powerful probes of binary evolution, the production sites of heavy elements, general relativity, and the expansion of the universe. Only a handful of candidate GW binary mergers during the fourth LIGO/Virgo/KAGRA observing run (O4) so far are believed to include a neutron star. We present optical-near infrared follow-up observations of the candidate neutron-star black hole GW merger S250206dm. This is the first high-significance mass gap neutron star-black hole candidate observed by multiple GW detectors (thus having a significantly smaller sky localization than one-detector events), offering the first opportunity to effectively follow up a GW event of this kind. Our GW MultiMessenger Astronomy DECam Survey (GW-MMADS) campaign consisted of a wide-field search using the Dark Energy Camera (DECam) and T80-South (T80S), as well as galaxy-targeted observations using the Southern Astrophysical Research (SOAR) imager and the Wendelstein 2.1m 3-channel camera. No viable kilonova counterpart was found in our observations. We use our observation depths to place competitive constraints on kilonova models similar to or brighter than the GW170817 kilonova AT 2017gfo within our observed fields, ruling out 100\% of such models with SOAR galaxy-targeted observations and $\sim43$\% (48\%) with DECam (DECam and T80S).

astro-ph.HE↗

arXiv2025

A new bump in the night: evidence of a new feature in the binary black hole mass distribution at $70~M_{\odot}$ from gravitational-wave observations

We analyze the confident binary black hole (BBH) detections from the third Gravitational-Wave Transient Catalog (GWTC-3) with an alternative mass population model in order to capture features in the mass distribution beyond the Powerlaw + Peak model. We find that the peak of a second power law characterizes the $\sim 30-35~ M_\odot$ bump, such that the data marginally prefers a mixture of two power laws for the mass distribution of binary components over a Powerlaw + Peak model with a Bayes Factor $\log_{10}\mathcal{B}$ of 0.24. This result may imply that the $\sim 30-35~ M_\odot$ feature represents the onset of a second population of BBH mergers (e.g. from a dynamical formation channel) rather than a specific mass feature over a broader distribution. When an additional Gaussian bump is allowed within our power law mixture model, we find a new feature in the BH mass spectrum at $\sim65-70~M_\odot$ ($\log_{10}\mathcal{B}$ = 0.29 compared to Powerlaw + Peak). This new feature may be consistent with hierarchical mergers, and constitute $\sim3\%$ of the BBH population. This model also recovers a maximum mass of $58^{+32}_{-14}~M_\odot$ for the second power law, consistent with the onset of a pair-instability supernova mass gap.

astro-ph.HE↗

arXiv2024

On the association of GW190425 with its potential electromagnetic counterpart FRB 20190425A

Recent work by Moroianu et al. (2022) has suggested that the binary neutron star (BNS) merger GW190425 might have a potential fast radio burst (FRB) counterpart association, FRB 20190425A, at the 2.8$σ$ level of confidence with a likely host galaxy association, namely UGC10667. The authors argue that the observations are consistent with a long-lived hypermassive neutron star (HMNS) that formed promptly after the BNS merger and was stable for approximately 2.5 hours before promptly collapsing into a black hole. Recently, Bhardwaj et al. (2023) conclusively associated FRB 20190425A with UGC10667, potentially providing a direct host galaxy candidate for GW190425. In this work, we examine the multi-messenger association based on the space-time localization overlaps between GW190425 and the FRB host galaxy UGC10667 and find that the odds for a coincident association are $\mathcal{O}(5)$. We validate this estimate by using a Gaussian Process (GP) density estimator. Assuming that the association is indeed real, we then perform Bayesian parameter estimation on GW190425 assuming that the BNS event took place in UGC10667. We find that the viewing angle of GW190425 excludes an on-axis system at $p(θ_v>30^o)\approx99.99$\%, highly favouring an off-axis system similar to GRB 170817A. We also find a slightly higher source frame total mass for the binary, namely, $m_{\rm{total}} = 3.42^{+0.34}_{-0.11} M_{\odot}$, leading to an increase on the probability of prompt collapse into a black hole and therefore disfavors the long-lived HMNS formation scenario. Given our findings, we conclude that the association between GW190425 and FRB 20190425A is disfavoured by current state-of-the-art gravitational-wave analyses.

astro-ph.HE↗

arXiv2024

Beyond Gaps and Bumps: Spectral Siren Cosmology with Non-Parametric Population Models

Gravitational wave standard sirens typically require electromagnetic (EM) data to obtain redshift information to constrain cosmology. Difficult to find EM counterparts for bright sirens and galaxy survey systematics for dark sirens make cosmological constraints with spectral sirens, a gravitational wave data-only approach, extremely appealing. In this work, we use the GWTC-3 BBH detections as spectral sirens to constrain the BBH population and the underlying cosmological expansion with a flexible model for the black hole mass spectrum. We use a binned Gaussian process to model the BBH mass distribution in the source frame without any astrophysical assumptions on the shape and or inclusion (or lack of) features that drive the cosmological constraints as the redshifted detector frame masses become consistent with the underlying astrophysical mass distribution features. For GWTC-3 we find a measurement on the Hubble constant of $H_0=73.0^{+13.3}_{-7.7} \ {\rm{km \ s^{-1} \ Mpc^{-1}}}$ at $68\%$ C.L. when combined with that obtained from the bright standard siren analysis with GW170817 and its associated host galaxy NGC 4993. We find an improved estimate for the Hubble constant of around a factor of 1.4 times better than the GW170817 measurement alone. We validate our nonparametric spectral siren approach with simulations and benchmark its scalability and constraining performance when compared with parametric methods.

astro-ph.CO↗

arXiv2024

Searching for binary black hole sub-populations in gravitational wave data using binned Gaussian processes

Astrophysically motivated population models for binary black hole observables are often insufficient to capture the imprints of multiple formation channels. This is mainly due to the strongly parametrized nature of such investigations. Using a non-parametric model for the joint population-level distributions of binary black hole component masses and effective inspiral spins, we find hints of multiple subpopulations in the third gravitational wave transient catalog. The higher (more positive) spin subpopulation is found to have a mass-spectrum without any feature at the $30-40M_{\odot}$, which is consistent with the predictions of isolated stellar binary evolution, simulations for which place the pile up due to pulsational pair-instability supernovae near $50M_{\odot}$ or higher. The other sub-population with effective spins closer to zero shows a feature at $30-40M_{\odot}$ and is consistent with binary black holes formed dynamically in globular clusters, which are expected to peak around $30M_{\odot}$. We also compute merger rates for these two subpopulations and find that they are consistent with the theoretical predictions of the corresponding formation channels. We validate our results by checking their robustness against variations of several model configurations and by analyzing large simulated catalogs with the same model.

astro-ph.HE↗

arXiv2023

Non-parametric inference of the population of compact binaries from gravitational wave observations using binned Gaussian processes

The observation of gravitational waves from multiple compact binary coalescences by the LIGO-Virgo-KAGRA detector networks has enabled us to infer the underlying distribution of compact binaries across a wide range of masses, spins, and redshifts. In light of the new features found in the mass spectrum of binary black holes and the uncertainty regarding binary formation models, non-parametric population inference has become increasingly popular. In this work, we develop a data-driven clustering framework that can identify features in the component mass distribution of compact binaries simultaneously with those in the corresponding redshift distribution, from gravitational wave data in the presence of significant measurement uncertainties, while making very few assumptions on the functional form of these distributions. Our generalized model is capable of inferring correlations among various population properties such as the redshift evolution of the shape of the mass distribution itself, in contrast to most existing non-parametric inference schemes. We test our model on simulated data and demonstrate the accuracy with which it can re-construct the underlying distributions of component masses and redshifts. We also re-analyze public LIGO-Virgo-KAGRA data from events in GWTC-3 using our model and compare our results with those from some alternative parametric and non-parametric population inference approaches. Finally, we investigate the potential presence of correlations between mass and redshift in the population of binary black holes in GWTC-3 (those observed by the LIGO-Virgo-KAGRA detector network in their first 3 observing runs), without making any assumptions about the specific nature of these correlations.

gr-qc↗

arXiv2023

Challenges for Fast Radio Bursts as Multi-Messenger Sources from Binary Neutron Star Mergers

Fast radio bursts (FRBs) are a newly discovered class of radio transients that emerge from cosmological sources and last for $\sim$ a few milliseconds. However, their origin remains a highly debated topic in astronomy. Among the plethora of cataclysmic events proposed as potential progenitors, binary neutron star (BNS) mergers have risen as compelling candidates for at least some subset of apparently non-repeating FRBs. However, this connection should not be drawn solely on the basis of chance coincidence probability. In this study, we delineate necessary criteria that must be met when considering an association between FRBs and BNS mergers, focusing on the post-merger ejecta environment. To underscore the significance of these criteria, we scrutinize the proposed association between GW190425 and FRB20190425A. Our investigation meticulously accounts for the challenging condition that the FRB signal must traverse the dense merger ejecta without significant attenuation to remain detectable at 400 MHz. Furthermore, we find that if the FRB is indeed linked to the gravitational wave event, the GW data strongly support a highly off-axis configuration, with a probability of the BNS merger viewing angle $p(θ_v$ $>$ 30$^{\circ}$) to be $\approx$ 99.99%. Our findings therefore strongly exclude an on-axis system, which we find, on the other hand, to be required in order for this FRB to be detectable. Hence, we conclude that GW190425 is not related to FRB20190425A. We also discuss implications of our results for future detections of coincident multi-messenger observations of FRBs from BNS remnants and GW events and argue that BNS merger remnants cannot account for the formation of > 1% of FRB sources. This observation suggests that short gamma-ray bursts should not be used to explain global attributes of the FRB host population.

astro-ph.HE↗

arXiv2023

Localization of binary neutron star mergers with a single Cosmic Explorer

Next-generation ground-based gravitational-wave detectors, such as Cosmic Explorer (CE), are expected to be sensitive to gravitational-wave signals with frequencies as low as 5 Hz, allowing signals to spend a significant amount of time in the detector frequency band. As a result, the effects caused by the rotation of the Earth become increasingly important for such signals. Additionally, the length of the arms of these detectors can be comparable to the wavelength of detectable gravitational waves, which introduces frequency-dependent effects that are not significant in current-generation detectors. These effects are expected to improve the ability to localize compact binary coalescences in the sky even when using only one detector. This study aims to understand how much these effects can help in localization. We present the first comprehensive Bayesian parameter estimation framework that accounts for all these effects using \textsc{Bilby}, a commonly used Bayesian parameter estimation tool. We focus on sky localization constraints for binary neutron star events with an optimal signal-to-noise ratio of 1000 with one detector at the projected CE sensitivity. We find that these effects help localize sources using one detector with sky areas as low as 10 square degrees. Moreover, we explore and discuss how ignoring these effects in the parameter estimation can lead to biases in the inference.

astro-ph.HE↗

arXiv2023

Follow-up Analyses to the O3 LIGO-Virgo-KAGRA Lensing Searches

Along their path from source to observer, gravitational waves may be gravitationally lensed by massive objects. This results in distortions of the observed signal which can be used to extract new information about fundamental physics, astrophysics, and cosmology. Searches for these distortions amongst the observed signals from the current detector network have already been carried out, though there have as yet been no confident detections. However, predictions of the observation rate of lensing suggest detection in the future is a realistic possibility. Therefore, preparations need to be made to thoroughly investigate the candidate lensed signals. In this work, we present some of the follow-up analyses and strategies that could be applied to assess the significance of such events and ascertain what information may be extracted about the lens-source system from such candidate signals by applying them to a number of O3 candidate events, even if these signals did not yield a high significance for any of the lensing hypotheses. For strongly-lensed candidates, we verify their significance using a background of simulated unlensed events and statistics computed from lensing catalogs. We also look for potential electromagnetic counterparts. In addition, we analyse in detail a candidate for a strongly-lensed sub-threshold counterpart that is identified by a new method. For microlensing candidates, we perform model selection using a number of lens models to investigate our ability to determine the mass density profile of the lens and constrain the lens parameters. We also look for millilensing signatures in one of the lensed candidates. Applying these additional analyses does not lead to any additional evidence for lensing in the candidates that have been examined. However, it does provide important insight into potential avenues to deal with high-significance candidates in future observations.

gr-qc↗

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

Measuring the polarization content of gravitational waves with strongly lensed binary black hole mergers

Alternative theories of gravity predict up to six distinct polarization modes for gravitational waves. Strong gravitational lensing of gravitational waves allows us to probe the polarization content of these signals by effectively increasing the number of observations from the same astrophysical source. The lensing time delays due to the multiple observed lensed images combined with the rotation of the Earth allows for effective non-collocated interferometers to be defined with respect to the source location and hence probe the alternative polarization amplitudes with more observations. To measure these amplitudes, we jointly fit the image observations to a single gravitational wave signal model that takes into account the image magnifications, time delays, and polarization mode amplitudes. We show that for certain systems, we can make a measurement of the relative mode amplitudes for lensed events with two detectable images.

gr-qc↗