arXiv subjects

Amanda M. Farah

Publications and source records attributed to Amanda M. Farah.

11 recordsLinked to original sources

arXiv2026

What You Don't Know Won't Hurt You: Self-Consistent Hierarchical Inference with Unknown Follow-up Selection Strategies

Many astronomical surveys prompt follow-up observations, but the decision process through which candidates are selected for follow-up can be difficult to model. This poses a challenge when inferring properties of the intrinsic population of astrophysical sources, rather than those of the set of objects detected by the survey and often-incomplete follow-up observations. We alleviate this problem by demonstrating that explicitly modeling of the follow-up selection process is not required for self-consistent inference of the intrinsic population. Using the framework of hierarchical Bayesian inference, we show that the intrinsic population can be accurately inferred even when the decision to follow up candidates strongly correlates with latent parameters of interest. We provide several worked examples, showing that the precision of posterior constraints can depend on the follow-up process and that one may have to model a population of contaminants if the initial selection is imperfect. Our result could dramatically simplify population inference that incorporates uncoordinated follow-up from multiple observers triggered by the deluge of candidates from surveys like LSST, Gaia, and next-generation gravitational-wave interferometers.

astro-ph.IM↗

arXiv2026

The steep redshift evolution of the hierarchical binary black hole merger rate may cause the $z$-$χ_{\rm eff}$ correlation

There is growing evidence from gravitational-wave observations that some merging black holes are created from previous mergers. Using the prediction that these hierarchically merged black holes have dimensionless spin magnitudes of $χ\approx 0.69$, we identify a subpopulation in the gravitational-wave data consistent with a hierarchical-merger origin in dense star clusters. This subpopulation's primary mass distribution peaks at $17.0^{+18.3}_{-4.4},\mathrm{M}_{\odot}$, which is approximately twice as large as its secondary mass distribution's mode ($10.5^{+29.7}_{-4.7},\mathrm{M}_{\odot}$), and its spin tilt distribution is consistent with isotropy. Our inferred secondary mass distributions imply that isolated binary evolution may still be needed to explain the entirety of the $9\,\mathrm{M}_{\odot}$ peak. Surprisingly, we find that the rate of hierarchical mergers may evolve more steeply with redshift than the rest of the population ($98.0\%$ credibility): the fraction of all binary black holes that are hierarchically formed at $z=0.1$ is $0.03^{+0.05}_{-0.02}$, compared to $0.09^{+0.11}_{-0.07}$ at $z=1$. This provides an explanation for the previously discovered broadening of the effective spin distribution with redshift. Our results have implications for star cluster formation histories, as they suggest the potential existence of a high-redshift population of massive, compact clusters.

astro-ph.HE↗

arXiv2026

The maximum mass ratio of hierarchical binary black hole mergers may cause the $q$-$χ_{\rm eff}$ correlation

Regardless of their initial spins, the merger of two roughly equal mass black holes (BHs) produces a remnant BH of dimensionless spin $0.69$. Such remnants can merge with other BHs in dense stellar environments and produce hierarchical mergers. Analyzing the latest catalog binary black hole (BBH) mergers from the LIGO-Virgo-KAGRA detectors, we identify a subpopulation with primary spins consistent with such hierarchical mergers. Consistent with astrophysical expectations for mergers of second-generation BHs with first-generation BHs, we find that this subpopulation has mass ratios below $0.59^{+0.18}_{-0.23}$. We also infer that $19$-$88\%$ of the BBH population below this mass ratio is consistent with belonging to the hierarchically merged population. Our results offer a natural explanation for the narrowing of the effective inspiral spin distribution with mass ratio observed in other studies.

astro-ph.HE↗

arXiv2026

Gravitational lensing rarely produces high-mass outliers to the compact binary population

All gravitational-wave signals are inevitably gravitationally lensed by intervening matter as they propagate through the Universe. When a gravitational-wave signal is magnified, it \emph{appears} to have originated from a closer, more massive system. Thus, high-mass outliers to the gravitational-wave source population are often proposed as natural candidates for strongly lensed events. However, when using a data-driven method for identifying population outliers, we find that high-mass outliers are not necessarily strongly lensed, nor will the majority of strongly-lensed signals appear as high-mass outliers. This is both because statistical fluctuations produce a larger effect on observed binary parameters than does lensing magnification, and because lensing-induced outliers must originate from intrinsically high-mass sources, which are rare. Thus, the appearance of a single lensing-induced outlier implies the existence of many other lensed events within the catalog. We additionally show that it is possible to constrain the strong lensing optical depth, which is a fundamental quantity of our Universe, with the detection or absence of high-mass outliers. However, constraints using the latest gravitational-wave catalog are weak$\unicode{x2014}$we obtain an upper limit on the optical depth of sources at redshift $1$ magnified by a factor of $5$ or more of $τ(μ\geq5,z=1)\leq 0.035 \unicode{x2014}$and future observing runs will not make an outlier-based method competitive with other probes of the optical depth. However, the full inferred population of compact binaries may be more informative of the distribution of lenses in the Universe, opening a unique opportunity to access the high-redshift Universe and constrain cosmic structures.

astro-ph.HE↗

arXiv2025

Kilonova emission from GW230529 and mass gap neutron star-black hole mergers

The detection of the gravitational wave event GW230529, presumably a neutron star-black hole (NSBH) merger, by the LIGO-Virgo-KAGRA (LVK) Collaboration marks an exciting discovery for multimessenger astronomy. The black hole (BH) has a high probability of falling within the "mass gap" (mg) between the neutron star (NS) and the BH mass distributions. Because of the relatively low primary mass, this system has a higher likelihood of producing an electromagnetic counterpart than previously detected NSBH mergers. We analyze the potential kilonova (KN) emission from GW230529 and find that, if the source was an NSBH merger, there is a $\sim $2-$28\%$ probability (depending on the assumed equation of state) that it produced a KN peaking at $\sim 1$ day post-merger with $g \lesssim 23.5$ and $i < 23$. Hence, it could have been detected by ground-based telescopes. If instead the event was a binary neutron star (BNS) merger, the probability of KN production drops to $\sim $0-$10\%$. Motivated by these results, we simulate a broader population of mgNSBH mergers expected during the fifth LIGO/Virgo/KAGRA observing run (O5) and find a $2$-$3\%$ chance of KN production per event. Such KNe would typically be fainter than GW230529, with $g \lesssim 26$ and $i \lesssim 25$. Based on these findings, DECam-like instruments may be able to detect up to $\sim 70\%$ of future mgNSBH KNe, corresponding to $1-2$ multimessenger mgNSBH per year in O5.

astro-ph.HE↗

arXiv2024

What is the nature of GW230529? An exploration of the gravitational lensing hypothesis

On the 29th of May 2023, the LIGO-Virgo-KAGRA Collaboration observed a compact binary coalescence event consistent with a neutron star-black hole merger, though the heavier object of mass 2.5-4.5 $M_\odot$ would fall into the purported lower mass gap. An alternative explanation for apparent observations of events in this mass range has been suggested as strongly gravitationally lensed binary neutron stars. In this scenario, magnification would lead to the source appearing closer and heavier than it really is. Here, we investigate the chances and possible consequences for the GW230529 event to be gravitationally lensed. We find this would require high magnifications and we obtain low rates for observing such an event, with a relative fraction of lensed versus unlensed observed events of $2 \times 10^{-3}$ at most. When comparing the lensed and unlensed hypotheses accounting for the latest rates and population model, we find a 1/58 chance of lensing, disfavoring this option. Moreover, when the magnification is assumed to be strong enough to bring the mass of the heavier binary component below the standard limits on neutron star masses, we find high probability for the lighter object to have a sub-solar mass, making the binary even more exotic than a mass-gap neutron star-black hole system. Even when the secondary is not sub-solar, its tidal deformability would likely be measurable, which is not the case for GW230529. Finally, we do not find evidence for extra lensing signatures such as the arrival of additional lensed images, type-II image dephasing, or microlensing. Therefore, we conclude it is unlikely for GW230529 to be a strongly gravitationally lensed binary neutron star signal.

gr-qc↗

arXiv2024

Things that might go bump in the night: Assessing structure in the binary black hole mass spectrum

Several features in the mass spectrum of merging binary black holes (BBHs) have been identified using data from the Third Gravitational Wave Transient Catalog (GWTC-3). These features are of particular interest as they may encode the uncertain mechanism of BBH formation. We assess if the features are statistically significant or the result of Poisson noise due to the finite number of observed events. We simulate catalogs of BBHs whose underlying distribution does not have the features of interest, apply the analysis previously performed on GWTC-3, and determine how often such features are spuriously found. We find that one of the features found in GWTC-3, the peak at $\sim35\,M_{\odot}$, cannot be explained by Poisson noise alone: peaks as significant occur in $1.7\%$ of catalogs generated from a featureless population. This peak is therefore likely to be of astrophysical origin. The data is suggestive of an additional significant peak at $\sim10\,M_{\odot}$, though the exact location of this feature is not resolvable with current observations. Additional structure beyond a power law, such as the purported dip at $\sim14\,M_{\odot}$, can be explained by Poisson noise. We also provide a publicly-available package, \texttt{GWMockCat}, that creates simulated catalogs of BBH events with correlated measurement uncertainty and selection effects according to user-specified underlying distributions and detector sensitivities.

astro-ph.HE↗

arXiv2024

Two of a Kind: Comparing big and small black holes in binaries with gravitational waves

When modeling the population of merging binary black holes, analyses have generally focused on characterizing the distribution of primary (i.e. more massive) black holes in the binary, while simplistic prescriptions are used for the distribution of secondary masses. However, the secondary mass distribution and its relationship to the primary mass distribution provide a fundamental observational constraint on the formation history of coalescing binary black holes. If both black holes experience similar stellar evolutionary processes prior to collapse, as might be expected in dynamical formation channels, the primary and secondary mass distributions would show similar features. If they follow distinct evolutionary pathways (for example, due to binary interactions that break symmetry between the initially more massive and less massive star), their mass distributions may differ. We present the first analysis of the binary black hole population that explicitly fits for the secondary mass distribution. We find that the data is consistent with a $\sim30\,M_{\odot}$ peak existing only in the distribution of \emph{secondary} rather than primary masses. This would have major implications for our understanding of the formation of these binaries. Alternatively, the data is consistent with the peak existing in both component mass distributions, a possibility not included in most other previous studies. In either case, the peak is observed at $31.4_{-2.6}^{+2.3}\,M_{\odot}$, which is shifted lower than the value obtained in previous analyses of the marginal primary mass distribution, placing this feature in further tension with expectations from a pulsational pair-instability supernova pileup.

astro-ph.HE↗

arXiv2024

No need to know: astrophysics-free gravitational-wave cosmology

Gravitational waves (GWs) from merging compact objects encode direct information about the luminosity distance to the binary. When paired with a redshift measurement, this enables standard-siren cosmology: a Hubble diagram can be constructed to directly probe the Universe's expansion. This can be done in the absence of electromagnetic measurements as features in the mass distribution of GW sources provide self-calibrating redshift measurements without the need for a definite or probabilistic host galaxy association. This ``spectral siren'' technique has thus far only been applied with simple parametric representations of the mass distribution, and theoretical predictions for features in the mass distribution are commonly presumed to be fundamental to the measurement. However, the use of an inaccurate representation leads to biases in the cosmological inference, an acute problem given the current theoretical uncertainties in the population. Here, we demonstrate that spectral sirens can accurately infer cosmological parameters without prior assumptions for the shape of the mass distribution. We apply a flexible, non-parametric model for the mass distribution of compact binaries to a simulated catalog of 1,000 GW events, consistent with expectations for the next LVK observing run. We find that, despite our model's flexibility, both the source mass model and cosmological parameters are correctly reconstructed. We predict a $5.8\%$ measurement of $H_0$, keeping all other cosmological parameters fixed, and a $6.4\%$ measurement of $H(z=0.9)$ when fitting for multiple cosmological parameters ($1σ$ uncertainties). This astrophysically-agnostic spectral siren technique will be essential to arrive at precise and unbiased cosmological constraints from GW source populations.

astro-ph.CO↗

arXiv2023

Updated observing scenarios and multi-messenger implications for the International Gravitational-wave Network's O4 and O5

An advanced LIGO and Virgo's third observing run brought another binary neutron star merger (BNS) and the first neutron-star black hole mergers. While no confirmed kilonovae were identified in conjunction with any of these events, continued improvements of analyses surrounding GW170817 allow us to project constraints on the Hubble Constant ($H_0$), the Galactic enrichment from $r$-process nucleosynthesis, and ultra-dense matter possible from forthcoming events. Here, we describe the expected constraints based on the latest expected event rates from the international gravitational-wave network (IGWN) and analyses of GW170817. We show the expected detection rate of gravitational waves and their counterparts, as well as how sensitive potential constraints are to the observed numbers of counterparts. We intend this analysis as support for the community when creating scientifically driven electromagnetic follow-up proposals. During the next observing run O4, we predict an annual detection rate of electromagnetic counterparts from BNS of $0.43^{+0.58}_{-0.26}$ ($1.97^{+2.68}_{-1.2}$) for the Zwicky Transient Facility (Rubin Observatory).

astro-ph.HE↗

arXiv2022

Bridging the Gap: Categorizing Gravitational-Wave Events at the Transition Between Neutron Stars and Black Holes

We search for features in the mass distribution of detected compact binary coalescences which signify the transition between neutron stars and black holes. We analyze all gravitational wave detections by LIGO-Virgo-KAGRA made through the end of the first half of the third observing run, and find clear evidence for two different populations of compact objects based solely on gravitational wave data. We confidently (99.3%) find a steepening relative to a single power law describing neutron stars and low-mass black holes below $2.4^{+0.5}_{-0.5}\text{ M}_{\odot}$, which is consistent with many predictions for the maximum neutron star mass. We find suggestions of the purported lower mass gap between the most massive neutron stars and the least massive black holes, but are unable to conclusively resolve it with current data. If it exists, we find the lower mass gap's edges to lie at $2.2^{+0.7}_{-0.5}\text{ M}_{\odot}$ and $6.0^{+2.4}_{-1.4}\text{ M}_{\odot}$. We re-examine events that have been deemed "exceptional" by the LIGO-Virgo-KAGRA collaborations in the context of these features. We analyze GW190814 self-consistently in the context of the full population of compact binaries, finding support for its secondary to be either a neutron star or a lower mass gap object, consistent with previous claims. Our models are the first to accommodate this event, which is an outlier with respect to the binary black hole population. We find that GW200105 and GW200115 probe the edges of, and may have components within, the lower mass gap. As future data improve global population models, the classification of these events will also improve.

astro-ph.HE↗