arXiv subjects

Gabriela Sato-Polito

Publications and source records attributed to Gabriela Sato-Polito.

At least 19 recordsLinked to original sources

arXiv2026

Bridging the Population Synthesis of Supermassive Binary Black Holes and the Gravitational Wave Background

Pulsar Timing Arrays (PTAs) constrain population properties of supermassive binary black holes (SMBHBs) through the observation of the gravitational wave background (GWB). Unlike other approaches that interpolate population-synthesis libraries or only consider the mean of the strain spectrum, here we capture its full strain probability density directly from semi-analytic population models. We apply our new method to the semi-analytic SMBHB population model, independently reproducing the parameter estimation for this model performed by the NANOGrav Collaboration with their 15-yr data. We also show the extent to which discrete SMBHB contributions to the GWB resolve degeneracies in the population parameter space. Finally, using the source-count intensity as the intermediate product in our calculation, we map PTA observations, as a proof of principle, to the SMBHB model based on the galaxy merger prescriptions from numerical hydrodynamical simulations "Illustris". We find the effect of delay times $τ$ between kiloparsec and subparsec SMBHB separations following galaxy mergers, finding $h_{\rm c}$ spanning $(1$-$6)\times10^{-16}$ and $N_{\rm c}$ spanning $(0.3$-$7.1)\times10^{-3}$ for $τ$ up to 8 Gyr.

astro-ph.HE↗

arXiv2026

Anisotropies in the PTA gravitational wave background: what can they teach us about supermassive black hole binaries?

The gravitational wave background detected by pulsar timing arrays is sourced by a finite population of supermassive black hole binaries, and is therefore anisotropic. We ask what measuring that anisotropy can teach us about the population, using models that span a wide range of effective source counts, all normalized to the measured background amplitude. We find four things. First, the expected anisotropy is produced by the single brightest binary: a dipole at the level of the published NANOGrav 95% upper limit would require one source to supply about 60% of the power in the band. Second, no model that also reproduces the measured strain spectrum contains a source that bright. In every case the loudest binary stays below the NANOGrav continuous-wave upper limit at every frequency it covers, consistent with the joint search of the 15-year data, which finds no resolved source. Third, because the anisotropy is produced by one source, compressing the sky to an angular power spectrum discards the phase information that locates it. Such a search is never more sensitive than looking for the source directly, and is strictly worse once more than a dipole is kept. Fourth, the published upper limits on the angular power spectrum therefore reflect the analysis prior rather than the data: they coincide with the 95th percentile of the prior induced by the square-root spherical harmonic basis adopted in the analysis. What does constrain the population today is the shape of the strain spectrum. Rare, bright sources depress the median spectrum below its mean, and fitting the measured spectrum already disfavors mass functions dominated by binaries above about 10^10 solar masses.

astro-ph.CO↗

arXiv2026

Summary statistic for pulsar timing arrays

The timing residuals produced by gravitational-wave signals can be described as an incoherent (pulsar term) contribution and a coherent (Earth term) map on the sky, which PTAs measure at the locations of the timed pulsars. The observed Earth term map and the variance induced by the pulsar term contain all of the information about any GW signal available to a PTA (assuming pulsar distances are unknown). Furthermore, any type of signal produces on average the same angular correlation function, the Hellings and Downs curve, which decays steeply with multipole as $C_\ell \propto 1/[(\ell+2)(\ell+1)\ell(\ell-1)]$. This suggests that the signal is inherently low-dimensional and therefore only a small number of parameters are needed to fully characterize it. We present an expression for the PTA likelihood that makes the dependence on the Earth term map and pulsar term variance explicit, and show that only a few spherical harmonic coefficients are needed to capture most of the information about the signal. To quantify this in a realistic setting, we compute the Fisher matrix of the amplitude of a stochastic background or a deterministic point source assuming the noise properties and sky locations of the pulsars in the NANOGrav 15yr dataset. We find that $\ell_{\rm max}=2$ of the Earth term map and the monopole of the pulsar term variance retain $\sim 95\%$ of the information about the signal. For a point source, including the dipole of the pulsar term variance is important to achieve a similar fraction.

astro-ph.CO↗

arXiv2026

A Joint Optimal Search for Gravitational Waves from Resolved and Unresolved Supermassive Binary Black Holes with Pulsar Timing Arrays

We introduce, from first principles, a joint model of the gravitational wave background (GWB) and brightest supermassive black hole binary (SMBHB) sources that may be individually resolvable in Pulsar Timing Array (PTA) searches for gravitational waves. We propose the characteristic number of SMBHB sources, $N_{\rm c}$, as a detection statistic for the astrophysical origin of the GWB. We then demonstrate how the brightest SMBHBs assist in resolving $N_{\rm c}$. Applying our method to the simulated NANOGrav 15-year data, which replicates all aspects of real data's known noise, observations, and the inferred GWB power spectrum, we demonstrate direct astrophysical limits on the strain amplitude of individually resolvable SMBHBs. We find that 21 of 114 SMBHB candidates from active galactic nuclei observations are in tension with the NANOGrav's observations. In contrast, only one candidate is in tension with the NANOGrav data based on the upper limits reported in the original analysis. Constraining the Poisson-specific characteristic number of SMBHBs, $N_{\rm c}$, at ${\rm yr}^{-1}$, we outline implications for the population properties of SMBHBs. Based on our new model applied to the simulated NANOGrav data, we calculate the probability of detecting GWs from isolated SMBHB in the 15-year data to be 2\% at the ${\rm SNR}=5$ level. Our projection towards the expected NANOGrav 20-year data suggests an increase to 5\%. With this, we estimate the probability of finding an outlier with an SNR of 2 in the NANOGrav 20-year data to be $40\%$.

astro-ph.HE↗

arXiv2025

Uncertainties in the supermassive black hole abundance and implications for the GW background

The present-day mass function of supermassive black holes is the most important observable quantity for the prediction and theoretical interpretation of the gravitational wave background (GWB) measured by pulsar timing arrays (PTAs). Due to the limited sample size of galaxies with dynamically inferred SMBH masses, more readily measurable galaxy properties $X$ that correlate with the black hole mass are used as labels (via scaling relations $M_{\bullet}-X$), which can then be counted in a larger galaxy catalog to produce a measurement of the mass function. Estimating the amplitude of the GWB from the local mass function is therefore simpler than general measurements of scaling relations and galaxy mass/luminosity functions for two reasons: the contribution to the characteristic strain is dominated by a narrow range of masses, and the mass proxy $X$ is always marginalized over. While consistent errors in $X$ in both catalogs are irrelevant, relatively small biases between them can produce significant shifts in the predicted SMBH abundance. In this work, we explore measurements of the SMBH mass function using different mass proxies through a set of catalogs with a number of redundant measurements between them. This enables us to investigate internal inconsistencies that lead to discrepancies in the final black hole abundance, while minimizing observational systematic biases induced by combining disparate sets of measurements. We focus on 3 proxies: the velocity dispersion $σ$, K-band luminosity $L$, and a combination of $L$ and radius $R$ defined by the fundamental plane. We show that all three can be reconciled to some degree, but highlight the remaining dependence on poorly-quantified systematic corrections between the scaling relation catalogs and the mass function catalogs, as well as the potential impact of selection effects.

astro-ph.GA↗

arXiv2025

Exploring the spectrum of stochastic gravitational-wave anisotropies with pulsar timing arrays

Anisotropies in the nanohertz gravitational-wave background are a compelling next target for pulsar timing arrays (PTAs). Measurements or informative upper limits to the anisotropies are expected in the near future and can offer important clues about the origin of the background and the properties of the sources. Given that each source is expected (in the simplest scenario of circular inspirals) to emit at a fixed frequency, the anisotropy will most generally vary from one frequency to another. The main result presented in this work is an analytical model for the anisotropies produced by a population of inspiralling supermassive black-hole binaries (SMBHBs). This model can be immediately connected with parametrizations of the SMBHB mass function and can be easily expanded to account for new physical processes taking place within the PTA frequency band. We show that a variety of SMBHB models predict significant levels of anistropy at the highest frequencies accessible to PTA observations and that measurements of anisotropies can offer new information regarding this population beyond the isotropic component. We also model the impact of additional dynamical effects driving the binary towards merger and show that, if these processes are relevant within the PTA band, the detectability of anisotropies relative to the isotropic background will be enhanced. Finally, we use the formalism presented in this work to predict the level anisotropy of the circular and linear polarizations of the SGWB due to the distribution of binary orientation angles with respect to the line of sight.

astro-ph.CO↗

arXiv2025

Evolution of SMBHs in light of PTA measurements: implications for growth by mergers and accretion

We study the growth of supermassive black holes accounting for both accretion and mergers. The former is informed by observations of the quasar luminosity function (QLF) and the latter by the gravitational wave-background (GWB) recently detected by PTAs, while estimates of the present-day black hole mass function provide a boundary condition. The GWB is dominated by the most massive black holes ($\gtrsim10^{9}M_{\odot}$). We show that their evolution can be simplified into a two-step process: mergers dominate at $z\leq1$, while accretion peaks at $1.4\leq z\leq2$. The large amplitude of the observed GWB suggests a significant number of mergers. We show that this generically implies a higher average Eddington ratio for quasars relative to a scenario in which mergers are negligible. In the absence of mergers, matching local estimates of BH abundance to the QLF implies a radiative efficiency $ε_r=0.12$ and Eddington ratio $λ=0.2$. With mergers, a progenitor of mass $M_i$ is boosted to a final total mass $M_f$ and there is a direct relation between the mass gained in mergers and the average Eddington ratio of the quasar population, given by $M_f/M_i\simλ/0.2$. There is thus a tension between the observed GWB, quasar properties, and the BH mass function: estimates of the mass function consistent with Eddington ratios inferred in quasars and $ε_r\sim0.1$ underpredict the GWB; multiple/equal mass mergers can boost the GWB, but lead to a high Eddington ratio. If the local mass function is on the high end of current estimates, the GWB is more readily explained, but requires low efficiencies $ε_r\sim10^{-2}$ not expected in standard luminous accretion models. The significant merger rate implied by the GWB also strongly suggests that the most massive BHs in the local universe have significant spin due to the orbital angular momentum from mergers, perhaps $a\sim0.5$.

astro-ph.CO↗

arXiv2024

Linear polarization of the stochastic gravitational-wave background with pulsar timing arrays

Pulsar-timing collaborations have recently reported evidence for the detection of an isotropic stochastic gravitational-wave background consistent with one sourced by a population of inspiralling supermassive black hole binaries. However, a certain degree of anisotropy and polarization may be present. Thus, the characterization of the energy density and polarization of the background at different angular scales is important. In this paper, we describe the signatures of linear polarization in the stochastic gravitational-wave background on the timing residuals obtained with pulsar-timing arrays. We expand the linear polarization map in terms of spin-weighted spherical harmonics and recast it into the $E$-mode (parity even) and $B$-mode (parity odd) basis. We provide expressions for the minimum-variance estimators for the coefficients of that expansion and evaluate the smallest detectable signal as a function of the signal-to-noise ratio with which the isotropic GW signal is detected and the number of pulsars in the survey. We evaluate the covariance between the estimators for the spherical-harmonic coefficients of the linear polarization $E$-modes and those for the intensity anisotropy. We also show that there is no covariance between the spherical-harmonic coefficients for the $B$-modes of the linear polarization and those for the circular polarization, even though both have the same parity. Our approach results in simple, elegant, and easily evaluated expressions for the overlap reduction functions for linear polarization.

astro-ph.CO↗

arXiv2024

The distribution of the gravitational-wave background from supermassive black holes

The recent detection of gravitational waves (GWs) by pulsar timing array (PTA) collaborations spurred a variety of questions regarding the origin of the signal and the properties of its sources. The amplitude of a GW background produced by inspiralling supermassive black holes (SMBHs) can be predicted in a relatively robust manner from the present-day merged remnants, observed as single SMBHs at the centers of galaxies, but falls short of the signal measured by PTAs by a significant amount, requiring equal mass mergers, extremely short delay times, and no accretion in order to achieve a modest consistency. In this work, we revisit NANOGrav's 15-yr data set and reassess the aforementioned discrepancy using the full spectral information captured by PTA data. As previously noted in the literature, the discrete number of point sources contributing to the background may lead to deviations in the observed spectrum relative to the average ($h^2_c \propto f^{-4/3}$) due to Poisson fluctuations, providing additional information about the source population beyond the background amplitude. We derive a simple expression for the characteristic strain distribution given a SMBH model, which is generally applicable regardless of the method used to model the black hole population. We then refit the NANOGrav free spectrum using a minimal model based on the local mass function, showing that the current GW measurement requires roughly $\sim 10$ times more black holes than suggested by local observations and disfavors mass functions dominated by few very heavy sources, with the typical mass that contributes to the background $\lesssim 10^{10}M_{\odot}$. Given the range of SMBH models found to be consistent with the isotropic background, we address what is the typical number sources that would be individually detectable, given the current sensitivity.

astro-ph.CO↗

arXiv2024

Unlocking New Physics with Joint Power Spectrum and Voxel Intensity Distribution Forecasts in Line-Intensity Mapping

The power spectrum and voxel intensity distribution (VID) are two summary statistics that can be applied to condense the information encoded in line-intensity maps. The information contained in both summary statistics is highly complementary, and their combination allows for a major increase in precision of parameter estimation from line-intensity mapping (LIM) surveys. Until recently, combination of these statistics required simulation-based estimations of their covariance. In this work we leverage an analytical model of covariance between these observables to run a joint Fisher forecast focusing on the CO(1-0) rotational line targeted by the COMAP survey and a wider, shallower hypothetical iteration. We consider a generalized phenomenological non-CDM model, models with axion dark matter, and local primordial non-Gaussianity, to highlight where a combined analysis of the power spectrum and VID can be most useful. Our results demonstrate improvements in sensitivity to beyond-$Λ$CDM physics over analyses using either the power spectrum or VID on their own, by factors ranging from 2 to 50, showcasing the potential of joint analyses in unlocking new insights into fundamental physics with LIM surveys.

astro-ph.CO↗

arXiv2023

Where are NANOGrav's big black holes?

Multiple pulsar timing array (PTA) collaborations have recently reported the first detection of gravitational waves (GWs) of nanohertz frequencies. The signal is expected to be primarily sourced by inspiralling supermassive black hole binaries (SMBHBs) and these first results are broadly consistent with the expected GW spectrum from such a population. Curiously, the measured amplitude of the GW background in all announced results is a bit larger than theoretical predictions. In this work, we show that the amplitude of the stochastic gravitational wave background (SGWB) predicted from the present-day abundance of SMBHs derived from local scaling relations is significantly smaller than that measured by the PTAs. We demonstrate that this difference cannot be accounted for through changes in the merger history of SMBHs and that there is an upper limit to the boost to the characteristic strain from multiple merger events, due to the fact that they involve black holes of decreasing masses. If we require the current estimate of the black hole mass density -- equal to the integrated quasar luminosity function through the classic Soltan argument -- to be preserved, then the currently measured PTA result would imply that the typical total mass of SMBHs contributing to the background should be at least $\sim 3 \times 10^{10} M_\odot$, a factor of $\sim 10$ larger than previously predicted. The required space density of such massive black holes corresponds to order $10$ $3 \times 10^{10} M_\odot$ SMBHs within the volume accessible by stellar and gas dynamical SMBH measurements. By virtue of the GW signal being dominated by the massive end of the SMBH distribution, PTA measurements offer a unique window into such rare objects and complement existing electromagnetic observations.

astro-ph.CO↗

arXiv2023

A multi-tracer empirically-driven approach to line-intensity mapping lightcones

Line-intensity mapping (LIM) is an emerging technique to probe the large-scale structure of the Universe. By targeting the integrated intensity of specific spectral lines, it captures the emission from all sources and is sensitive to the astrophysical processes that drive galaxy evolution. Relating these processes to the underlying distribution of matter introduces observational and theoretical challenges, such as observational contamination and highly non-Gaussian fields, which motivate the use of simulations to better characterize the signal. In this work we present SkyLine, a computational framework to generate realistic mock LIM observations that include observational features and foreground contamination, as well as a variety of self-consistent tracer catalogs. We apply our framework to generate realizations of LIM maps from the MultiDark Planck 2 simulations coupled to the UniverseMachine galaxy formation model. We showcase the potential of our scheme by exploring the voxel intensity distribution and the power spectrum of emission lines such as 21 cm, CO, CII, and Lyman-$α$, their mutual cross-correlations, and cross-correlations with galaxy clustering. We additionally present cross-correlations between LIM and sub-millimeter extragalactic tracers of large-scale structure such as the cosmic infrared background and the thermal Sunyaev-Zel'dovich effect, as well as quantify the impact of galactic foregrounds, line interlopers and instrument noise on LIM observations. These simulated products will be crucial in quantifying the true information content of LIM surveys and their cross-correlations in the coming decade, and to develop strategies to overcome the impact of contaminants and maximize the scientific return from LIM experiments.

astro-ph.CO↗

arXiv2023

Seeking dark matter with $γ$-ray attenuation

The flux of high-energy astrophysical $γ$ rays is attenuated by the production of electron-positron pairs from scattering off of extragalactic background light (EBL). We use the most up-to-date information on galaxy populations to compute their contributions to the pair-production optical depth. We find that the optical depth inferred from $γ$-ray measurements exceeds that expected from galaxies at the $\sim2σ$ level. If the excess is modeled as a frequency-independent re-scaling of the standard contribution to the EBL from galaxies, then an excess (an overall $14-30\%$ increase of the EBL) over the null hypothesis of no excess at the $2.7σ$ level. If the frequency dependence of the excess is instead modeled as a two-photon decay of a dark-matter axion, then the excess is favored over the null hypothesis at the $2.1σ$ confidence level. While we find no evidence for a dark-matter signal, the analysis sets the strongest current bounds on the photon-axion coupling over the $8-25$ eV mass range. This work highlights the sensitivity of $γ$-ray optical depth measurements to ALPs, which is expected to improve with new observatories and better EBL determinations from future observations.

astro-ph.CO↗

arXiv2022

The cosmic optical background excess, dark matter, and line-intensity mapping

Recent studies using New Horizons LORRI images have returned the most precise measurement of the cosmic optical background to date, yielding a flux that exceeds that expected from deep galaxy counts by roughly a factor of two. We investigate whether this excess, detected at $\sim 4σ$ significance, is due to axion-like dark matter that decays to monoenergetic photons. We compute the spectral energy distribution from such decays and the contribution to the flux measured by LORRI. Assuming that axion-like particles make up all of the dark matter, the parameter space unconstrained to date that explains the measured excess spans masses and effective axion-photon couplings of 8 - 20 eV masses and 3 - 6 $\times 10^{-11}$ GeV$^{-1}$, respectively. If the excess arises from dark-matter decay to a photon line, there will be a significant signal in forthcoming line-intensity mapping measurements that will allow the discrimination of this hypothesis from other candidates.

astro-ph.CO↗

arXiv2022

Primordial trispectrum from kSZ tomography

The kinetic Sunyaev Zel'dovich effect is a secondary CMB temperature anisotropy that provides a powerful probe of the radial-velocity field of matter distributed across the Universe. This velocity field is reconstructed by combining high-resolution CMB measurements with galaxy survey data, and it provides an unbiased tracer of matter perturbations in the linear regime. In this paper, we show how this measurement can be used to probe primordial non-Gaussianity of the local type, particularly focusing on the trispectrum amplitude $τ_{\rm NL}$, as may arise in a simple two-field inflation model that we provide by way of illustration. Cross-correlating the velocity-field-derived matter distribution with the biased large-scale galaxy density field allows one to measure the scale-dependent bias factor with sample variance cancellation. We forecast that a configuration corresponding to CMB-S4 and VRO results in a sensitivity of $σ_{f_{\rm NL}} \approx 0.59$ and $σ_{τ_{\rm NL}} \approx 1.5$. These forecasts predict improvement factors of 10 and 195 for $σ_{f_{\rm NL}}$ and $σ_{τ_{\rm NL}}$, respectively, over the sensitivity using VRO data alone, without internal sample variance cancellation. Similarly, for a configuration corresponding to DESI and SO, we forecast a sensitivity of $σ_{f_{\rm NL}} \approx 3.1$ and $σ_{τ_{\rm NL}} \approx 69$, with improvement factors of 2 and 5, respectively, over the use of the DESI data-set in isolation. We find that a high galaxy number density and large survey volume considerably improve our ability to probe the amplitude of the primordial trispectrum for the multi-field model considered.

astro-ph.CO↗

arXiv2022

Analytical covariance between voxel intensity distributions and line-intensity mapping power spectra

The power spectrum and the voxel intensity distribution (VID) are two of the main proposed summary statistics to study line-intensity maps. We reformulate the derivation of the VID in terms of the local overdensities and derive for the first time an analytic covariance between the VID and the line-intensity mapping power spectrum. We study the features of this covariance for different experimental setups and show that we can recover similar results to simulation-based covariances. With this formalism, we also compute the cosmic variance contribution the VID uncertainty, which we find to be subdominant with respect to the standard variance from Poisson sampling. Our results allow for general joint analyses of the VID and the line-intensity mapping power spectrum.

astro-ph.CO↗

arXiv2021

Pulsar-timing measurement of the circular polarization of the stochastic gravitational-wave background

Pulsar-timing arrays (PTAs) are in the near future expected to detect a stochastic gravitational-wave background (SGWB) produced by a population of inspiralling super-massive black hole binaries. In this work, we consider a background that can be anisotropic and circularly polarized. We use the expansion of the intensity and the circular polarization in terms of spherical harmonics and the overlap reduction functions for each term in this expansion. We propose an unbiased real-space estimator that can separate the intensity and circular-polarization contributions of the SGWB to pulsar-timing-residual correlations and then validate the estimator on simulated data. We compute the signal-to-noise ratio of a circular-polarization component that has a dipole pattern under different assumptions about the PTA. We find that a nearly-maximal circular-polarization dipole may be detectable, which can aid in determining whether or not the background is dominated by a handful of bright sources.

astro-ph.CO↗

arXiv2020

Kinetic Sunyaev-Zel'dovich tomography with line-intensity mapping

The kinetic Sunyaev-Zel'dovich (kSZ) effect is a secondary cosmic microwave background (CMB) anisotropy induced by the scattering of CMB photons off intervening electrons. Through cross-correlations with tracers of large-scale structure, the kSZ effect can be used to reconstruct the 3-dimensional radial-velocity field, a technique known as kSZ tomography. We explore the cross-correlation between the CMB and line-intensity fluctuations to retrieve the late-time kSZ signal across a wide redshift range. We focus on the CII emission line, and predict the signal-to-noise ratio of the kSZ tomography signal between redshifts $z=1-5$ for upcoming experiments. We show that while instruments currently under construction may reach a low-significance detection of kSZ tomography, next-generation experiments will achieve greater sensitivity, with a detection significance of $\mathcal{O}(10^2-10^3)$. Due to sample-variance cancellation, the cross-correlation between the reconstructed velocity field from kSZ tomography and intensity fluctuations can improve measurements of %the scale-dependent bias contributions from new physics to the power spectrum at large scales. To illustrate this improvement, we consider models of the early Universe that induce primordial local-type non-gaussianity and correlated compensated isocurvature perturbations. We show that with CMB-S4 and an AtLAST-like survey, the uncertainty on $f_{\rm NL}$ and $A_{\rm CIP}$ can be reduced by a factor of $\sim 3$, achieving $σ(f_{\rm NL}) \lesssim 1$. We further show that probing both low and high redshifts is crucial to break the degeneracy between the two parameters.

astro-ph.CO↗