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The black hole occupation fraction (focc) defines the fraction of galaxies that harbor central massive black holes (MBHs), irrespective of their accretion activity level. While it is widely accepted that focc is nearly 100% in local massive galaxies with stellar masses M⋆ ≳ 1010 M⊙, it is not yet clear whether MBHs are ubiquitous in less-massive galaxies. In this work, we present new constraints on focc based on over 20 yr of Chandra imaging data for 1606 galaxies within 50 Mpc. We employ a Bayesian model to simultaneously constrain focc and the specific accretion-rate distribution function, p(λ), where the specific accretion rate is defined as λ = LX/M⋆, where LX is the MBH accretion luminosity in the 2─10 keV range. Notably, we find that p(λ) peaks around 1028ergs−1M⊙−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⋆: in high-mass galaxies (M⋆ ≍ 1011−12 M⊙), the occupation fraction is >93% (a 2σ lower limit), and then declines to 66%−7%+8% (1σ errors) between M⋆ ≍ 109−10 M⊙, and to 33%−9%+13% in the dwarf galaxy regime between M⋆ ≍ 108−9 M⊙. 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. 

ABSTRACT The shrinking of a binary orbit driven by the interaction with a gaseous circumbinary disc, initially advocated as a potential way to catalyse the binary merger, has recently been debated in the case of geometrically thick (i.e. with H/R ≳ 0.1) discs. However, a clear consensus is still missing mainly owing to numerical limitations, such as fixed orbit binaries or lack of resolution inside the cavity carved by the binary in its circumbinary disc. In this work, we assess the importance of evolving the binary orbit by means of hydrodynamic simulations performed with the code gizmo in meshless finite mass mode. In order to model the interaction between equal mass circular binaries and their locally isothermal circumbinary discs, we enforce hyper-Lagrangian resolution inside the cavity. We find that fixing the binary orbit ultimately leads to an overestimate of the gravitational torque that the gas exerts on the binary and an underestimate of the torque due to the accretion of material on to the binary components. Furthermore, we find that the modulation of the accretion rate on the binary orbital period is strongly suppressed in the fixed orbit simulation, while it is clearly present in the live binary simulations. This has potential implications for the prediction of the observable periodicities in massive black hole binary candidates. 

ABSTRACT The search for gravitational waves using Pulsar Timing Arrays (PTAs) is a computationally expensive complex analysis that involves source-specific noise studies. As more pulsars are added to the arrays, this stage of PTA analysis will become increasingly challenging. Therefore, optimizing the number of included pulsars is crucial to reduce the computational burden of data analysis. Here, we present a suite of methods to rank pulsars for use within the scope of PTA analysis. First, we use the maximization of the signal-to-noise ratio as a proxy to select pulsars. With this method, we target the detection of stochastic and continuous gravitational wave signals. Next, we present a ranking that minimizes the coupling between spatial correlation signatures, namely monopolar, dipolar, and Hellings & Downs correlations. Finally, we also explore how to combine these two methods. We test these approaches against mock data using frequentist and Bayesian hypothesis testing. For equal-noise pulsars, we find that an optimal selection leads to an increase in the log-Bayes factor two times steeper than a random selection for the hypothesis test of a gravitational wave background versus a common uncorrelated red noise process. For the same test but for a realistic European PTA (EPTA) data set, a subset of 25 pulsars selected out of 40 can provide a log-likelihood ratio that is 89 % of the total, implying that an optimally selected subset of pulsars can yield results comparable to those obtained from the whole array. We expect these selection methods to play a crucial role in future PTA data combinations. 

Abstract Since 2015 the gravitational-wave observations of LIGO and Virgo have transformed our understanding of compact-object binaries. In the years to come, ground-based gravitational-wave observatories such as LIGO, Virgo, and their successors will increase in sensitivity, discovering thousands of stellar-mass binaries. In the 2030s, the space-based LISA will provide gravitational-wave observations of massive black holes binaries. Between the $$\sim 10$$ ∼ 10 –10 3 Hz band of ground-based observatories and the $$\sim 10^{-4}$$ ∼ 1 0 − 4 –10 − 1 Hz band of LISA lies the uncharted decihertz gravitational-wave band. We propose a Decihertz Observatory to study this frequency range, and to complement observations made by other detectors. Decihertz observatories are well suited to observation of intermediate-mass ( $$\sim 10^{2}$$ ∼ 1 0 2 –10 4 M ⊙ ) black holes; they will be able to detect stellar-mass binaries days to years before they merge, providing early warning of nearby binary neutron star mergers and measurements of the eccentricity of binary black holes, and they will enable new tests of general relativity and the Standard Model of particle physics. Here we summarise how a Decihertz Observatory could provide unique insights into how black holes form and evolve across cosmic time, improve prospects for both multimessenger astronomy and multiband gravitational-wave astronomy, and enable new probes of gravity, particle physics and cosmology. 

Abstract The science objectives of the LISA mission have been defined under the implicit assumption of a 4-years continuous data stream. Based on the performance of LISA Pathfinder, it is now expected that LISA will have a duty cycle of $$\approx 0.75$$ ≈ 0.75 , which would reduce the effective span of usable data to 3 years. This paper reports the results of a study by the LISA Science Group, which was charged with assessing the additional science return of increasing the mission lifetime. We explore various observational scenarios to assess the impact of mission duration on the main science objectives of the mission. We find that the science investigations most affected by mission duration concern the search for seed black holes at cosmic dawn, as well as the study of stellar-origin black holes and of their formation channels via multi-band and multi-messenger observations. We conclude that an extension to 6 years of mission operations is recommended.