arXiv subjects

Alberto Sesana

Publications and source records attributed to Alberto Sesana.

At least 19 recordsLinked to original sources

arXiv2026

GUEST: Gravitational Universe Exploration with Satellite Tracking. A passive satellite laser-ranging mission for the dark gravitational Universe

GUEST is a space mission concept whose central objective is the detection of gravitational waves (GWs) in the microhertz band -- a physics-rich frequency window that no other present or planned detector can reach at a significant level. The concept is simple: two dense, passive spheres, covered with cube-corner retroreflectors, deployed in {highly eccentric} Earth orbits ($e \gtrsim 0.7$, period $P \gtrsim 33$ h), tracked continuously by the global network of satellite laser-ranging stations over a minimum observation time of 10 years, with an expected total duration of 30 years. The orbits themselves act as resonant detectors of the oscillating gravitational perturbations, with the microhertz sensitivity emerging from the selected orbital parameters. From the same data stream, GUEST delivers a programme of fundamental and applied science that cuts across particle physics, gravitational-wave astronomy, cosmology, astrophysics, and geodesy: the first coherent search for GWs from supermassive black-hole binaries in the $\mu$Hz band, the exploration of primordial GW backgrounds in the unexplored energy-scale gap between pulsar-timing arrays and LISA, a dedicated probe of ultra-light dark matter in a parameter region untouched by any other experiment, a new way to search for ultra-light bosons, order-of-magnitude-improved tests of new gravitational interactions at astronomical ranges, and a step change in the absolute determination of $GM_\oplus$ that underpins the Global Geodetic Observing System and future navigation and Earth-observation missions. This white paper presents the motivation, scientific reach, and mission concept of GUEST.

astro-ph.CO↗

arXiv2026

Comparing gravitational wave background predictions from cosmological simulations to pulsar timing observations

The recent detection of a gravitational wave background (GWB) by pulsar timing arrays (PTAs) may represent the first evidence of gravitational waves from merging supermassive black hole binaries, opening a new window on the low-frequency end of the gravitational wave spectrum. These inspiralling binaries are expected to dominate the signal, although most theoretical models seem to predict somewhat lower amplitudes than what is observed. We present the first comprehensive statistical framework to quantify the tension between PTA measurements and theoretical predictions, maximising the constraining power of current data and allowing straightforward application to future PTA datasets. We further investigate how different assumptions in the observational inference, particularly the use of a power-law model for the GWB spectrum, can bias tension estimates and potentially overstate discrepancies with theory. We apply our framework to compare predictions from the FABLE cosmological simulation with the NANOGrav 15-year dataset. For our fiducial black hole population, we find tension values of $1\sigma$-$2.5\sigma$, indicating no statistically significant disagreement with the observations. We further explore physically motivated modifications to the merging black hole population, guided by electromagnetic observations and theoretical uncertainties. In particular, scenarios with boosted black hole masses at high redshift and more equal-mass mergers substantially increase the predicted GWB amplitude, improving agreement with PTA data. Finally, we investigate the high-mass end of the black hole mass function and the impact of finite simulation volume. We find that the $(100 \, \mathrm{cMpc} \, h^{-1})^3$ FABLE box is sufficient to robustly predict the GWB signal at the most constraining frequency.

astro-ph.GA↗

arXiv2026

The LISA Astrophysics MBHcatalogues Project: A comparison of predictions of simulated massive black hole binaries

In the hierarchical paradigm of galaxy formation, central massive black holes (MBHs) are expected to coalesce after the merger of their host galaxies. One of the main goals of the Laser Interferometer Space Antenna (LISA) is to constrain the origin and growth of MBHs through their merger rates and mass distribution. Predicting MBH merger rates requires not only tracing their statistical population from large to small physical scales (kpc to sub-pc) but also modelling their formation, accretion, dynamics, mergers, and their galactic physical processes across cosmic time. This project is the result of a large collaborative effort undertaken by the LISA Astrophysics Working Group, bringing together its collective expertise on MBH formation, evolution, and modelling, to build a comprehensive understanding of MBH merger rates across cosmic time. The project compares various theoretical predictions of MBH merger rates, quantifies the spread, and evaluates the global astrophysical uncertainties of the LISA event rates. To build a unique and complete view, our work is based on about 20 semi-analytical models and cosmological simulations from the literature, all employing distinct approaches to modelling MBH and galaxy physics. To compute the merger rates, we also incorporate delays arising from the dynamical phase of MBH hardening to coalescence. We present the expected LISA merger rates given current galaxy formation models and discuss how the merger rate depends on model assumptions, such as the seeding model and the resolution of cosmological simulations.

astro-ph.GA↗

arXiv2026

Archival Multiband Gravitational-Wave Signals from Massive Black Hole Binary Mergers

While massive black hole binaries (MBHBs) merge at gravitational-wave frequencies above the pulsar timing array (PTA) sensitivity band, we show that they leave orphaned low-frequency contributions in the PTA pulsar term. Due to the light-propagation time between each pulsar in the array and Earth, the pulsar term acts as a time-delayed probe of a chirping merger with a specific frequency response determined by the direction of origin and intrinsic properties of the MBHB. We provide a detailed consideration of how such a multiband signal would manifest in a full PTA, demonstrate an approach to stack these orphaned pulsar terms across the array, and discuss prospects for an archival, multiband search in conjunction with MBHB mergers observed in astrometric data or spaceborne interferometers like LISA.

astro-ph.HE↗

arXiv2026

Gravitational Waves from the Cosmic Dawn: Tracing Cosmic Black Hole Binaries with ET, LGWA and LISA

Next generation detectors, such as LISA, LGWA, and ET will, for the first time, probe the high redshift Universe, offering unique insight into the birth, growth, and dynamics of the first black holes (BHs) during their earliest stages formation. We aim to predict merger rates and gravitational wave (GW) signatures of "cosmic" binary BHs, forming as a result of galaxy mergers, at z>=4. We investigate how BH seeding, accretion physics and dynamical delays affect their properties and detectability across cosmic epochs. We use the semi-analytic model Cosmic Archaeology Tool (CAT) to trace the evolution and delayed-mergers, driven by dynamical friction, of BH binaries formed from light, medium-weight and heavy seeds, under Eddington-limited (EL) and super-Eddington (SE) accretion prescriptions. We employ the GWFish package to evaluate their GW signals and detectability by LISA, LGWA and ET. Our results show the impact of BH accretion and seeding prescriptions on the properties and distribution of detectable sources. In the EL model, the detected populations are dominated by nearly equal-mass binaries. In contrast, SE growth leads to lower mass ratios for LISA detections and medium ratios for ET and LGWA. We present the total detection rates predicted under the two accretion scenarios. The SE model allows BHs to grow faster, transferring a significant fraction of detectable systems from the ET band to the LISA band, compared to the EL model. As a result, the predicted LISA detection rate increases from ~32 yr^-1 in the EL case to ~64 yr^-1 in the SE scenario, and the ET detection rate reduces from ~64 yr^-1 in the EL model to only ~4 yr^-1 in the SE scenario. LGWA yields comparable detection rates in both scenarios (~21 yr^-1 in EL and ~12 yr^-1 in SE). The combined information encoded in mass ratios, redshift evolution and merger rates emerge as a promising diagnostic of early BH growth.

astro-ph.GA↗

arXiv2026

Reducing cosmological degeneracies by combining multiple classes of LISA gravitational-wave standard sirens

We present the first joint gravitational-wave cosmological inference with LISA extreme mass-ratio inspirals at $z\lesssim1$ (galaxy redshifts) and massive black hole binaries at $z\gtrsim1$ (electromagnetic counterparts). Combining these standard sirens reduces cosmological degeneracies and yields competitive constraints on the Hubble constant $H_0$ and the dark-energy equation-of-state parameter $w_0$. This highlights LISA's potential for late-time cosmology across a broad redshift range with systematics distinct from electromagnetic distance indicators.

astro-ph.CO↗

arXiv2026

Two-body relaxation in the EMRI-TDE disk model for Quasi Periodic Eruptions

Quasi Periodic Eruptions (QPEs) are luminous bursts of soft X-rays recently discovered in galactic nuclei. They repeat on timescales of hours to weeks, superimposed to an otherwise stable quiescent X-ray level, consistent with emission from a radiatively efficient accretion flow around relatively low-mass MBHs. Although their physical origin is still debated, their quasi-periodicity naturally arises within the 'impact model', in which the X-ray bursts are generated by the interaction between an sBH or a star in a close orbit around the central MBH and the accretion disk formed by a tidal disruption event (TDE). While this model is consistent with the phenomenology of QPEs, it remains unclear whether such specific physical configurations are sufficiently commonto explain the observed QPE number density. We present the first end-to-end quantitative calculation of the expected QPE rate and abundance within the framework of the impact model. To this purpose, we combine the rates of TDEs and extreme mass-ratio inspirals (EMRIs) around MBHs spanning a range of masses masses. We employ the public code \textsc{PhaseFlow} to simulate seven systems with MBH masses between $10^5 M_\odot$ and $10^8 M_\odot$, each sourronded by a three-component population: one composed of $1M_\odot$ stars, and two consisting of sBHs with masses of $10M_\odot$ and $40 M_\odot$. Based on the emission constraints available in the literature, we restrict to sBH EMRIs on prograde orbit with eccentricity $e<0.5$ and inclination $\iota<20^{\circ}$ with respect to the accretion disk. For stellar EMRIs the constraints instead arise from the requirement that the star avoid tidal disruption. We find that the predicted QPE number density spans the range $10^{-12} \rm Mpc^{-3}$ to $10^{-6} \rm Mpc^{-3}$, depending on the assumed orbital period interval and on the adopted eccentricity and inclination thresholds.

astro-ph.HE↗

arXiv2026

LISA and the LISA Science Team

LISA, the Laser Interferometer Space Antenna, due to launch mid-2035, is a large class space mission by the European Space Agency (ESA). In partnership with NASA and ESA-member states, ESA is on track to launch what is expected to be the first space-based gravitational wave detector. By hosting detectors in space, one gains access to a lower frequency band of gravitational wave sources and, with them, a plethora of new science. To maximise this scientific gain, ESA and NASA selected 20 scientists for the LISA Science Team to carry out and/or lead the necessary actions leading up to LISA's launch. We give a short overview and update of the LISA mission, its science objectives and related waveforms, as well as the work of the LISA Science Team as of April 2026.

astro-ph.IM↗

arXiv2025

Nanohertz Gravitational Waves

Evidence of a gravitational wave (GW) signal has emerged in pulsar timing array (PTA) data, opening a new window into the nanoHz GW Universe. We explore the physics of GW signals potentially explaining the data, with a primary focus on GW backgrounds (GWBs), considering both astrophysical and cosmological origins. We describe how: (i) An astrophysical nanoHz GWB emerges as the superposition of individual signals from inspiralling massive black-hole binaries (MBHBs); (ii) Environment coupling, eccentricity, and sparse sampling, affect the MBHB signal spectrum and statistical properties, causing great uncertainty in theoretical predictions, but simultaneously offering a handle to discriminate a potential astrophysical origin; (iii) PTA data offers unprecedented opportunities to constrain high-energy physics beyond the standard model, by probing early Universe GWBs, originated during or after inflation; (iv) Different early Universe GWBs, typically created by non-linear and out-of-equilibrium dynamics, can explain the PTA data, as e.g. from inflation scenarios, first order phase transitions, or topological defects; (v) The PTA detection of GWs opens a new window to explore the Universe, with profound implications for astrophysics and particle physics, probing e.g. the equation of state of the early Universe, the origin of the cosmological perturbations, the nature of the dark matter, or whether exotic objects like primordial black holes or cosmic strings exist.

astro-ph.CO↗

arXiv2025

Data span and frequency coverage requirements for robust detection and inference in PTAs: A case study with EPTA DR2

Pulsar Timing Arrays (PTAs) are approaching the sensitivity required for a $5\sigma$ detection of the nanohertz stochastic gravitational-wave background (GWB). This makes it crucial to deeply understand the behaviour of our analysis pipelines. A counterintuitive feature of the European Pulsar Timing Array (EPTA) second data release is that restricting the dataset to the last 10.3 years (DR2new) increases the inferred GWB significance from $\leq2\sigma$ for the full 25-year dataset (DR2full) to $\geq3.5\sigma$. We investigate whether this behaviour indicates an anomaly or is a possible outcome of the pipeline. Using realistic, DR2-like simulations with varying timespans, we find that the first 10 years contribute little to the GWB evidence due to their limited frequency coverage. This produces substantial overlap between the HD S/N distributions of DR2full and DR2new. Random noise fluctuations therefore yield a higher GWB evidence in DR2new than in DR2full in $15\%$ of cases. Furthermore, $5\%$ of simulations match the HD S/N of the real data, indicating that the observed behaviour is consistent with being a $\sim2\sigma$ outcome due to noise fluctuations. Regardless of significance, DR2new simulations introduce biases in the GWB parameter estimation due to spectral leakage effects that are ignored in standard analyses and which flatten the inferred spectrum. Including leakage removes these biases, demonstrating the reliability of DR2new when the signal is properly modelled. Furthermore, we demonstrate that combining EPTA DR2full with long-baseline data from NANOGrav and PPTA, as well as low-frequency data from LOFAR and NenuFAR, significantly enhances GWB evidence and parameter accuracy. Finally, we examine the impact of the observation timespan and find that short-baseline datasets introduce strong amplitude biases and are ineffective at constraining the GWB.

astro-ph.HE↗

arXiv2025

The first year of LISA Galactic foreground

Galactic white-dwarf binaries play a central role in the inference model for the Laser Interferometer Space Antenna. In this manuscript, we employ the $\texttt{bahamas}$ codebase to characterize, in a global-fit fashion, the reconstruction of the Galactic foreground during the first year of observation. To account for its statistical properties, we represent the data in time--frequency domain, and characterize the effectiveness of multiple approaches, e.g. statistically viable likelihoods, sampling schemes, segmentation widths, and gaps density. Our analysis yields consistent results across, with overwhelming evidence in favor of a non-stationary model in less than a month of data. Moreover, we show robustness against the presence of additional extragalactic foregrounds, and test the suitability of our approximations on the more complex simulated data in the $\textit{Yorsh}$ data challenge.

astro-ph.IM↗

arXiv2025

Suppressed "lump" EM signature in radiation pressure dominated accreting massive black hole binaries

We investigate the impact of radiation pressure on electromagnetic signatures of accreting massive black hole binaries (MBHBs) at milli-parsec separations, using 3D hyper-Lagrangian resolution hydrodynamical simulations. We model binaries embedded in a self-gravitating circumbinary disc that evolves following an adiabatic equation of state, including viscous heating and black-body cooling. Focusing on binaries with a total mass of $10^6 \, M_{\odot}$, eccentricities $e=0,0.45,0.9$ and mass ratios $q=1, 0.7$, we find that radiation pressure significantly affects both the spectral energy distributions (SEDs) and the light curves (LCs). The emission from the mini-discs shifts from the optical towards UV frequencies and with a peak luminosity orders of magnitude higher, while the circumbinary disc becomes colder and dimmer as a result of its geometrically thinner configuration. Temporal variability is affected as well: near UV and soft-X ray fluxes are higher and more variable. Crucially, radiation pressure suppresses the characteristic "lump" formation in equal-mass circular systems, while a lump is formed for higher eccentricities without imprinting any modulation on the flux. In the circular case we still find a modulation on the cavity edge timescale at a frequency $0.36 \, f_{\rm K}$, while in eccentric binaries, only robust orbital period modulations ($f=1,2 \, f_{\rm K}$) are observed, with no modulation associated with the cavity orbital motion. Moreover, the enhanced emission from the mini-discs and streams due to radiation pressure, one redshifted, results in brighter flux in the optical G band, proving detectability of MBHBs signatures even at higher redshift ($z=0.6-1.0$). Our results reveal that radiation pressure plays a crucial role in shaping MBHBs spectral and time-domain features, with implications for their identification in time-domain surveys.

astro-ph.HE↗

arXiv2025

Assessing the performance of future space-based detectors: Astrophysical foregrounds and individual sources

The space mission LISA, scheduled for launch in 2035, aims to detect gravitational wave (GW) signals in the milli-Hz band. In the context of the ESA Voyage 2050 Call for new mission concepts, other frequency ranges are explored by the Gravitational-Wave Space 2050 Working Group to conceive new proposals for a post-LISA space-based detector. In this work, we give a preliminary estimate of the observational potential of three mission designs proposed in the literature, namely $\mu$Ares, AMIGO and the Decihertz Observatory. The analysis framework includes astrophysical GW sources, such as massive black hole binaries and extreme mass-ratio inspirals, and compact binaries, such as stellar black holes and white dwarfs. For each detector, we first present a consistent computation of the unresolved gravitational wave background (GWB) produced by the sum of all anticipated astrophysical populations using an iterative subtraction algorithm. We then investigate which types of systems are the most appealing by measuring the number of GW signals detected and exploring the source properties.

astro-ph.HE↗

arXiv2025

Central Massive Black Holes Are Not Ubiquitous in Local Low-Mass Galaxies

The black-hole occupation fraction ($f_\mathrm{occ}$) defines the fraction of galaxies that harbor central massive black holes (MBHs), irrespective of their accretion activity level. While it is widely accepted that $f_\mathrm{occ}$ is nearly 100% in local massive galaxies with stellar masses $M_\star \gtrsim 10^{10}~M_\odot$, it is not yet clear whether MBHs are ubiquitous in less-massive galaxies. In this work, we present new constraints on $f_\mathrm{occ}$ based on over 20 years of Chandra imaging data for 1606 galaxies within 50 Mpc. We employ a Bayesian model to simultaneously constrain $f_\mathrm{occ}$ and the specific accretion-rate distribution function, $p(λ)$, where the specific accretion rate is defined as $λ=L_\mathrm{X}/M_\star$, and $L_\mathrm{X}$ is the MBH accretion luminosity in the 2-10 keV range. Notably, we find that $p(λ)$ peaks around $10^{28}~\mathrm{erg~s^{-1}}~M_\odot^{-1}$; above this value, $p(λ)$ decreases with increasing $λ$, following a power-law that smoothly connects with the probability distribution of bona-fide active galactic nuclei. We also find that the occupation fraction decreases dramatically with decreasing $M_\star$: in high mass galaxies ($M_\star \approx 10^{11-12}M_\odot$), the occupation fraction is $>93\%$ (a $2σ$ lower limit), and then declines to $66_{-7}^{+8}\%$ ($1σ$ errors) between $M_\star\approx10^{9-10}M_\odot$, and to $33_{-9}^{+13}\%$ in the dwarf galaxy regime between $M_\star\approx10^{8-9}~M_\odot$. Our results have significant implications for the normalization of the MBH mass function over the mass range most relevant for tidal disruption events, extreme mass ratio inspirals, and MBH merger rates that upcoming facilities are poised to explore.

astro-ph.GA↗

arXiv2025

Updated predictions for gravitational wave emission from TDEs for next generation observatories

In this paper, we investigate the gravitational wave (GW) emission from stars tidally disrupted by black holes (TDEs), using a semi-analytical approach. Contrary to previous works where this signal is modeled as a monochromatic burst, we here take into account all its harmonic components. On top of this, we also extend the analysis to a population of repeated-partial TDEs, where the star undergoes multiple passages around the black hole before complete disruption. For both populations, we estimate the rate of individual GW-detections considering future observatories like LISA and a potential deci-Hertz (dHz) mission, and derive the GW background from these sources. Our conclusions, even if more conservative, are consistent with previous results presented in literature. In fact, full disruptions of stars will not be seen by LISA but will be important targets for dHz observatories. In contrast, GWs from repeated disruptions will not be detectable in the near future.

astro-ph.HE↗

arXiv2025

Variability in the supermassive black hole binary candidate SDSS J2320+0024: No evidence of periodic modulation

Supermassive black hole binaries (SBHBs) are a natural outcome of galaxy mergers, and they are expected to be among the loudest gravitational-wave sources at low frequencies. The source SDSS J2320+0024 was recently proposed as a promising SBHB candidate due to a possible periodicity in its light curve and variability in the MgII emission line. In this work, we reanalysed the optical (g, r, and i bands) light curves of J2320+0024 within the framework of Bayesian model selection. When periodicity was searched for together with red noise, analysis of the g-band light curve reveals a peak in the posterior of the period at ~290 days. The posterior profile is too broad to yield a preference for periodic models over models that include only red noise. Furthermore, the same peak is not present in the analysis of the r-band and i-band light curve. A periodic model without red noise identified a different (~1100 days) periodicity, but this model is statistically significantly disfavoured relative to the other models tested. In summary, we find no significant evidence in favour of a true periodic signal over red-noise variability. Our analysis questions the robustness of the previously proposed periodicity and emphasises the importance of rigorous statistical treatment. While our findings challenge the binary interpretation for J2320+0024, they do not rule it out. A statistically robust joint analysis of the photometric light curves and evolving broad-line profiles would shed further light on the true nature of this object.

astro-ph.GA↗

arXiv2025

Constraints on the early growth of massive black holes from PTA and JWST with L-GalaxiesBH

Recent Pulsar Timing Arrays (PTAs) results provided strong evidence for a stochastic gravitational wave background (sGWB), consistent with a population of merging massive black holes (MBHs) at $z<1$. Meanwhile, JWST observations at $z>5$ suggest a higher number density of accreting MBHs than previously estimated. Together with constraints from local MBHs and high-$z$ quasars, these findings offer a unique opportunity to test MBH seeding and early growth models. We explore this using ${\tt L-Galaxies}\textit{BH}$, a new extension of the galaxy formation model ${\tt L-Galaxies}$, developed to explicitly model all stages of MBH evolution, including seeding, accretion, and binary dynamics. To take advantage of both the high resolution of the ${\tt MillenniumII}$ and the large volume of the ${\tt Millennium}$ simulations, we run ${\tt L-Galaxies}\textit{BH}$ on the former and use its outputs as initial conditions for the latter, via our $\textit{grafting}$ method. We find that reproducing the number density of high-$z$ active MBHs observed by JWST requires either a heavy seed formation rate significantly higher than that predicted by current models ($\gtrsim 0.01 Mpc^{-3}$ at $z \sim 10$), or widespread formation of light seeds undergoing multiple phases of super-Eddington accretion. Furthermore, matching the amplitude of the PTA sGWB signal requires nearly all galaxies with stellar masses $M_{*}> 10^9 M_\odot$ to host central MBHs by $z\sim0$. Given the extreme heavy seed densities required to satisfy both PTA and JWST constraints, our results favor a scenario in which MBHs originate from light seeds that grow rapidly and efficiently in the early universe. This work demonstrates the power of combining multi-messenger data with physical models to probe the origins and evolution of MBHs across cosmic time.

astro-ph.GA↗

arXiv2025

Off-center black hole seed formation? Implications for high and low redshift massive black holes

Recent studies show that light seeds of black holes, which grow into massive black holes (MBHs) over time, often struggle to remain at the centers of their birthplaces in high-redshift galaxies, limiting their ability to accrete gas and merge with other black holes. In this work, we investigate how off-center formation of the first seeds affects the evolution of the MBH and massive black hole binary (MBHB) populations over cosmic history. To this end, we use the $\texttt{L-Galaxies}{\it BH}$ semi-analytical model, which includes multiple seed formation mechanisms, with light Population III remnants being the most significant contributors. To incorporate off-center formation, we modify the model to track the initial seed location, the sinking timescales toward the galactic center, and any growth during this phase. The results indicate that seed formation occurring away from the galactic center has a negligible impact on the MBH population at $z<1$, but causes significant differences at higher redshifts. Particularly, the abundance of $10^5 M_{\odot}$ MBHs at $z>4$ can be up to 2-10 times smaller compared to a nuclear seed formation model. Quasar luminosity functions with $\rm L_{bol}>10^{44} \rm erg/s$ are similarly affected, although they still align with observational constraints. The off-centre formation also alters the galaxy-MBH mass relation. At $z>5$, the amplitude of the relation can be up to 2 dex smaller than in nuclear seed models. These differences fade by $z \sim 2$ for galaxies $>10^{11} M_{\odot}$, and by $z=0$ for smaller galaxies. Notably, the overmassive MBH population recently unveiled by JWST is still present in the model, suggesting they can form independently of the seed dynamics. Finally, the merging rate of MBHs within LISA sensitivity band is strongly impacted. Specifically, there is a suppression of events at high-$z$ and an enhancement at low-$z$.

astro-ph.GA↗