Pau Amaro-Seoane

Low-frequency gravitational-wave science with eLISA/NGO

We review the expected science performance of the New Gravitational-Wave Observatory (NGO, a.k.a. eLISA), a mission under study by the European Space Agency for launch in the early 2020s. eLISA will survey the low-frequency gravitational-wave sky (from 0.1 mHz to 1 Hz), detecting and characterizing a broad variety of systems and events throughout the Universe, including the coalescences of massive black holes brought together by galaxy mergers; the inspirals of stellar-mass black holes and compact stars into central galactic black holes; several millions of ultra-compact binaries, both detached and mass transferring, in the Galaxy; and possibly unforeseen sources such as the relic gravitational-wave radiation from the early Universe. eLISAs high signal-to-noise measurements will provide new insight into the structure and history of the Universe, and they will test general relativity in its strong-field dynamical regime.

Stellar remnants in galactic nuclei: Mass segregation

The study of how stars distribute themselves around a massive black hole (MBH) in the center of a galaxy is an important prerequisite for the understanding of many galactic-center processes. These include the observed overabundance of point X-ray sources at the Galactic center and the prediction of rates and characteristics of tidal disruptions of extended stars by the MBH and of inspirals of compact stars into the MBH, the latter being events of high importance for the future space-borne gravitational wave interferometer LISA. In relatively small galactic nuclei hosting MBHs with masses in the range 10–10 M , the single most important dynamical process is two-body relaxation. It induces the formation of a steep density cusp around the MBH and strong mass segregation, as more massive stars lose energy to lighter ones and drift to the central regions. Using a spherical stellar dynamicalMonte Carlo code, we simulate the long-term relaxational evolution of galactic nucleus models with a spectrum of stellar masses. Our focus is the concentration of stellar black holes to the immediate vicinity of the MBH. We quantify this mass segregation for a variety of galactic nucleus models and discuss its astrophysical implications. Special attention is given to models developed to match the conditions in the MilkyWay nucleus; we examine the presence of compact objects in connection to recent high-resolution X-ray observations. Subject headingg s: black hole physics — galaxies: nuclei — galaxies: star clusters — gravitational waves — methods: n-body simulations — stellar dynamics Online material: color figuresThe study of how stars distribute themselves around a massive black hole (MBH) in the center of a galaxy is an important prerequisite for the understanding of many galactic-center processes. These include the observed overabundance of point X-ray sources at the Galactic center and the prediction of rates and characteristics of tidal disruptions of extended stars by the MBH and of inspirals of compact stars into the MBH, the latter being events of high importanceforthefuturespace-bornegravitationalwaveinterferometerLISA.Inrelativelysmallgalacticnucleihosting MBHs with masses in the range 10 5 –10 7 M� , the single most important dynamical process is two-body relaxation. It induces the formation of a steep density cusp around the MBH and strong mass segregation, as more massive stars loseenergytolighteronesand driftto thecentral regions. Usingaspherical stellardynamicalMonteCarlocode, we simulate the long-term relaxational evolution of galactic nucleus models with a spectrum of stellar masses. Our focus is the concentration of stellar black holes to the immediate vicinity of the MBH. We quantify this mass segregation for a variety of galactic nucleus models and discuss its astrophysical implications. Special attention is given to models developed to match the conditions in the Milky Way nucleus; we examine the presence of compact objects in connection to recent high-resolution X-ray observations. Subject headingg black hole physics — galaxies: nuclei — galaxies: star clusters — gravitational waves — methods: n-body simulations — stellar dynamics

Intermediate and extreme mass-ratio inspirals — astrophysics, science applications and detection using LISA

Black hole binaries with extreme (gtrsim104:1) or intermediate (~102–104:1) mass ratios are among the most interesting gravitational wave sources that are expected to be detected by the proposed laser interferometer space antenna (LISA). These sources have the potential to tell us much about astrophysics, but are also of unique importance for testing aspects of the general theory of relativity in the strong field regime. Here we discuss these sources from the perspectives of astrophysics, data analysis and applications to testing general relativity, providing both a description of the current state of knowledge and an outline of some of the outstanding questions that still need to be addressed. This review grew out of discussions at a workshop in September 2006 hosted by the Albert Einstein Institute in Golm, Germany.

Evolution of binary black holes in self gravitating discs Dissecting the torques

Context. Massive black hole binaries, formed in galaxy mergers, are expected to evolve in dense circumbinary discs. Understanding of the disc-binary coupled dynamics is vital to assess both t he final fate of the system and the potential observable featu res that may be tested against observations. Aims. Aimed at understanding the physical roots of the secular evolution of the binary, we study the interplay between gas accretion and gravity torques in changing the binary elements (semi-major axis and eccentricity) and its total angular momentum budget. We pay special attention to the gravity torques, by analysing t heir physical origin and location within the disc. Methods. We analyse three-dimensional smoothed particle hydrodynamics simulations of the evolution of initially quasi-circu lar massive black hole binaries (BHBs) residing in the central hollow (cavity) of massive self-gravitating circumbinary discs. We perfo rm a set of simulations adopting different thermodynamics for the gas within the cavity and for the ’numerical size’ of the black holes. Results. We show that (i) the BHB eccentricity growth found in our previous work is a general result, independent of the accretion a nd the adopted thermodynamics; (ii) the semi-major axis decay depends not only on the gravity torques but also on their subt le interplay with the disc-binary angular momentum transfer due to accretion; (iii) the spectral structure of the gravity torques is predominately caused by disc edge overdensities and spiral arms developing in the body of the disc and, in general, does not reflect direc tly the period of the binary; (iv) the net gravity torque changes sig n across the BHB corotation radius (positive inside vs negative outside) We quantify the relative importance of the two, which appear to depend on the thermodynamical properties of the instreaming gas, and which is crucial in assessing the disc‐binary angular momentum transfer; (v) the net torque manifests as a purely kin ematic (non-resonant) effect as it stems from the low density cavity, where the material flows in and out in highly eccentric orbits. Conclusions. Both accretion onto the black holes and the interaction with gas streams inside the cavity must be taken into account to assess the fate of the binary. Moreover, the total torque exe rted by the disc affects the binary angular momentum by changing all the elements (mass, mass ratio, eccentricity, semimajor axis) of the black hole pair. Commonly used prescriptions equating tidal torque to semi-major axis shrinking might therefore be poor approximations for real astrophysical systems.

A comprehensive Nbody study of mass segregation in star clusters: Energy equipartition and escape

We address the dynamical evolution of an isolated self-gravitating system with two stellar mass groups. We vary the individual ratio of the heavy to light bodies, μ from 1.25 to 50 and alter also the fraction of the total heavy mass M h from 5 to 40 per cent of the whole cluster mass. Clean-cut properties of the cluster dynamics are examined, like core collapse, the evolution of the central potential, as well as escapers. We present in this work collisional N-body simulations, using the high-order integrator NBODY6++ with up to Ν * = 2 x 10 4 particles improving the statistical significancy of the lower-N * simulations by ensemble averages. Equipartition slows down the gravothermal contraction of the core slightly. Beyond a critical value of μ ≈ 2, no equipartition can be achieved between the different masses; the heavy component decouples and collapses. For the first time, the critical boundary between Spitzer-stable and Spitzer-unstable systems is demonstrated in direct N-body models. We also present the measurements of the Coulomb logarithm and discuss the relative importance of the evaporation and ejection of escapers.

Surface brightness profile of the Milky Way’s nuclear star cluster

Context. Although the Milky Way nuclear star cluster (MWNSC) was discovered more than four decades ago, several of its key properties have not been determined unambiguously up to now because of the strong and spatially highly variable interstellar extinction toward the Galactic centre. Aims. In this paper we aim at determining the shape, size, and luminosity/mass of the MWNSC. Methods. To investigate the properties of the MWNSC, we used Spitzer/IRAC images at 3:6 and 4:5 m, where interstellar extinction is at a minimum but the overall emission is still dominated by stars. We corrected the 4:5 m image for PAH emission with the help of the IRAC 8:0 m map and for extinction with the help of a [3:6 4:5] colour map. Finally, we investigated the symmetry of the

ON STRONG MASS SEGREGATION AROUND A MASSIVE BLACK HOLE: IMPLICATIONS FOR LOWER-FREQUENCY GRAVITATIONAL-WAVE ASTROPHYSICS

We present, for the first time, a clear N-body (NB) realization of the strong mass segregation solution for the stellar distribution around a massive black hole (MBH). We compare our NB results with those obtained by solving the orbit-averaged Fokker-Planck (FP) equation in energy space. The NB segregation is slightly stronger than in the FP solution, but both confirm the robustness of the regime of strong segregation when the number fraction of heavy stars is a (realistically) small fraction of the total population. In view of recent observations revealing a dearth of giant stars in the sub-parsec region of the Milky Way, we show that the timescales associated with cusp re-growth are not longer than (0.1 – 0.25) × Trlx (rh ). These timescales are shorter than a Hubble time for black holes masses M • 4 × 106 M ☉ and we conclude that quasi-steady, mass-segregated, stellar cusps may be common around MBHs in this mass range. Since extreme mass ratio inspirals detection rates by Laser Interferometer Space Antenna are expected to peak for M • ~ 4 × 105-106 M ☉, a good fraction of these events should originate from strongly segregated stellar cusps.

Rapid Eccentricity Oscillations and the Mergers of Compact Objects in Hierarchical Triples

Kozai-Lidov (KL) oscillations can accelerate compact object mergers via gravitational wave (GW) radiation by driving the inner binaries of hierarchical triples to high eccentricities. We perform direct three-body integrations of high mass ratio compact object triple systems using Fewbody including post-Newtonian terms. We find that the inner binary undergoes rapid eccentricity oscillations (REOs) on the timescale of the outer orbital period which drive it to higher eccentricities than secular theory would otherwise predict, resulting in substantially reduced merger times. For a uniform distribution of tertiary eccentricity (

Accretion of stars on to a massive black hole: a realistic diffusion model and numerical studies

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Triplets of supermassive black holes: astrophysics, gravitational waves and detection

), ~40% of systems merge within ~1-2 eccentric KL timescales whereas secular theory predicts that only ~20% of such systems merge that rapidly. This discrepancy becomes especially pronounced at low