arXiv subjects

F. Haardt

Publications and source records attributed to F. Haardt.

At least 55 records · Page 3Linked to original sources

arXiv2008

Ejection of hypervelocity binary stars by a black hole of intermediate mass orbiting Sgr A*

The discovery of hypervelocity binary stars (HVBs) in the Galactic halo would provide definite evidence of the existence of a massive black hole companion to Sgr A*. Here we use an hybrid approach to compute the rate of ejection and the total number of HVBs produced by a hypothetical intermediate-mass black hole (IMBH, M_2<10^5\msun) orbiting Sgr A*. Depending on the mass of M_2 and on the properties of binary stars in the central parsec of the Milky Way, we show that the number of undisrupted HVBs expected to be expelled from the Galactic Center before binary black hole coalescence ranges from zero to a few dozens at most. Therefore, the non-detection of stellar binaries in a complete survey of hypervelocity stars would not rule out the occurrence of an IMBH-Sgr A* in-spiralling event within the last few 10^8 years.

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arXiv2008

Massive black hole binaries in gaseous nuclear discs

We study the evolution of a massive black hole pair in a rotationally supported nuclear disc. The distributions of stars and gas mimic the nuclear region of a gas-rich galaxy merger remnant. Using high-resolution SPH simulations, we follow the black hole dynamics and trace the evolution of the underlying background, until the black holes form a binary. We find that the gravitational perturbation of the pair creates a core in the disc density profile, hence decreasing the gas-dynamical drag. This leads the newly formed binary to stall at a separation of ~5 pc. In the early phases of the sinking, black holes lose memory of their initial orbital eccentricity if they co-rotate with the disc, as rotation of the gaseous background promotes circularization of the black hole orbits. Circularization is efficient until the black holes bind in a binary, though in the latest stages of the simulations a residual eccentricity > 0.1 is still present. Black holes are treated as sink particles, allowing for gas accretion. We find that accretion strongly depends on the dynamical properties of the black holes, and occurs preferentially after circularization.

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arXiv2008

Are the black hole masses in Narrow Line Seyfert 1 galaxies actually small?

Narrow Line Seyfert 1 galaxies (NLS1s) are generally considered peculiar objects among the broad class of Type 1 active galactic nuclei, due to the relatively small width of the broad lines, strong X-ray variability, soft X-ray continua, weak [OIII], and strong FeII line intensities. The mass M_BH of the central massive black hole (MBH) is claimed to be lighter than expected from known MBH-host galaxy scaling relations, while the accretion rate onto the MBH larger than the average value appropriate to Seyfert 1 galaxies. In this Letter, we show that NLS1 peculiar M_BH and L/L_Edd turn out to be fairly standard, provided that the broad line region is allowed to have a disc-like, rather than isotropic, geometry. Assuming that NLS1s are rather ``normal'' Seyfert 1 objects seen along the disc axis, we could estimate the typical inclination angles from the fraction of Seyfert 1 classified as NLS1s, and compute the geometrical factor relating the observed FWHM of broad lines to the virial mass of the MBH. We show that the geometrical factor can fully account for the "black hole mass deficit" observed in NLS1s, and that L/L_Edd is (on average) comparable to the value of the more common broad line Seyfert 1 galaxies.

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arXiv2007

Supermassive black hole binaries in gaseous and stellar circumnuclear discs: orbital dynamics and gas accretion

The dynamics of two massive black holes in a rotationally supported nuclear disc of 10^8 solar masses is explored using N-Body/SPH simulations. Gas and star particles are co-present in the disc. Described by a Mestel profile, the disc has a vertical support provided by turbulence of the gas, and by stellar velocity dispersion. A primary black hole of 4 million solar masses is placed at the centre of the disc, while a secondary black hole is set initially on an eccentric co-rotating orbit in the disc plane. Its mass is in a 1 to 1, 1 to 4, and 1 to 10 ratio, relative to the primary. With this choice, we mimic the dynamics of black hole pairs released in the nuclear region at the end of a gas-rich galaxy merger. It is found that, under the action of dynamical friction, the two black holes form a close binary in ~10 Myrs. The inspiral process is insensitive to the mass fraction in stars and gas present in the disc and is accompanied by the circularization of the orbit. We detail the gaseous mass profile bound to each black hole that can lead to the formation of two small Keplerian discs, weighing ~2 % of the black hole mass, and of size ~0.01 pc. The mass of the tightly (loosely) bound particles increases (decreases) with time as the black holes spiral into closer and closer orbits. Double AGN activity is expected to occur on an estimated timescale of ~10 Myrs, comparable to the inspiral timescale. The double nuclear point-like sources that may appear during dynamical evolution have typical separations of ~10 pc.

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arXiv2007

Hypervelocity stars and the environment of Sgr A*

Hypervelocity stars (HVSs) are a natural consequence of the presence of a massive nuclear black hole (Sgr A*) in the Galactic Center. Here we use the Brown et al. sample of unbound and bound HVSs together with numerical simulations of the propagation of HVSs in the Milky Way halo to constrain three plausible ejection mechanisms: 1) the scattering of stars bound to Sgr A* by an inspiraling intermediate-mass black hole (IMBH); 2) the disruption of stellar binaries in the tidal field of Sgr A*; and 3) the two-body scattering of stars off a cluster of stellar-mass black holes orbiting Sgr A*. We compare the predicted radial and velocity distributions of HVSs with the limited-statistics dataset currently available, and show that the IMBH model appears to produce a spectrum of ejection velocities that is too flat. Future astrometric and deep wide-field surveys of HVSs should shed unambiguous light on the stellar ejection mechanism and probe the Milky Way potential on scales as large as 200 kpc.

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arXiv2007

Interaction of massive black hole binaries with their stellar environment: II. Loss-cone depletion and binary orbital decay

We study the long-term evolution of massive black hole binaries (MBHBs) at the centers of galaxies using detailed scattering experiments to solve the full three-body problem. Ambient stars drawn from a isotropic Maxwellian distribution unbound to the binary are ejected by the gravitational slingshot. We construct a minimal, hybrid model for the depletion of the loss cone and the orbital decay of the binary, and show that secondary slingshots - stars returning on small impact parameter orbits to have a second super-elastic scattering with the MBHB - may considerably help the shrinking of the pair in the case of large binary mass ratios. In the absence of loss-cone refilling by two-body relaxation or other processes, the mass ejected before the stalling of a MBHB is half the binary reduced mass. About 50% of the ejected stars are expelled ejected in a "burst" lasting ~1E4 yrs M_6^1/4, where M_6 is the binary mass in units of 1E6 Msun. The loss cone is completely emptied in a few bulge crossing timescales, 1E7 yrs M_6^1/4. Even in the absence of two-body relaxation or gas dynamical processes, unequal mass and/or eccentric binaries with M_6 >0.1 can shrink to the gravitational wave emission regime in less than a Hubble time, and are therefore "safe" targets for the planned Laser Interferometer Space Antenna (LISA).

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arXiv2006

Unresolved X-ray background: clues on galactic nuclear activity at z>6

We study, by means of dedicated simulations of massive black hole build-up, the possibility to constraint the existence and nature of the AGN population at z>6 with available and planned X-ray and near infrared space telescopes. We find that X-ray deep-field observations can set important constraints to the faint-end of the AGN luminosity function at very high redshift. Planned X-ray telescopes should be able to detect AGN hosting black holes with masses down to >10^5 Msun (i.e., X-ray luminosities in excess of 10^42 erg s^-1), and can constrain the evolution of the population of massive black hole at early times (6 6 and we discuss the possible role of AGN in the reionization of the Universe at these early epochs, showing that AGN alone can provide enough ionizing photons only in the (improbable) case of an almost completely homogeneous inter-galactic medium. Finally, we show that super-Eddington accretion, suggested by the observed QSOs at z>6, must be a very rare event, confined to black holes living in the highest density peaks.

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arXiv2006

On the search of electromagnetic cosmological counterparts to coalescences of massive black hole binaries

We explore the nature of possible electromagnetic counterparts of coalescences of massive black hole binaries at cosmological distances detectable by the Laser Interferometer Space Antenna (LISA). An electromagnetic precursor, during the last year of gravitational wave (GW)-driven inspiral, or an afterglow within few years after coalescence, may highlight the position in the sky of galaxies hosting LISA sources. We show that observations of precursors and afterglows are mutually exclusive, depending on the mass of the primary black hole. Precursors are expected to occur in binaries where the primary (more massive) black hole is heavier than ~10^7 \Msun. They may correspond to on-off states of accretion, i.e., to a bright X-ray source decaying into quiescence before black hole coalescence, and are likely associated to disturbed galaxies showing signs of ongoing starbursts. Coalescences of lighter binaries, with masses <5x10^6 \Msun, lack of any precursor, as gas is expected to be consumed long before the GW-driven orbital decay. Such events would not be hosted by (massive) galaxies with an associated starburst, given the slow binary inspiral time compared to the typical time scale of starbursts. By contrast, coalescence, for such light binaries, is followed by an electromagnetic afterglow, i.e., an off-on accretion state rising in <20 yrs. Using a cosmological merger tree algorithm, we show that future X-ray missions such as XEUS will be able to identify, in 20 yrs operation, almost all the massive BH binary detectable by LISA, and, in only 5 yrs, all the LISA sources at z>6.

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arXiv2006

Interaction of massive black hole binaries with their stellar environment: I. Ejection of hypervelocity stars

We use full three-body scattering experiments to study the ejection of hypervelocity stars (HVSs) by massive black hole binaries (MBHBs) at the center of galaxies. Ambient stars drawn from a Maxwellian distribution unbound to the binary are expelled by the gravitational slingshot. Accurate measurements of thermally averaged hardening, mass ejection, and eccentricity growth rates (H, J, and K) for MBHBs in a fixed stellar background are obtained by numerical orbit integration from initial conditions determined by Monte Carlo techniques. Three-body interactions create a subpopulation of HVSs on nearly radial, corotating orbits, with a spatial distribution that is initially highly flattened in the inspiral plane of the MBHB, but becomes more isotropic with decreasing binary separation. The degree of anisotropy is smaller for unequal mass binaries and larger for stars with higher kick velocities. Eccentric MBHBs produce a more prominent tail of high-velocity stars and break planar symmetry, ejecting HVSs along a broad jet perpendicular to the semimajor axis. The jet two-sidedness decreases with increasing binary mass ratio, while the jet opening-angle increases with decreasing kick velocity and orbital separation. The detection of a numerous population of HVSs in the halo of the Milky Way by the next generation of large astrometric surveys like GAIA may provide a unique signature of the history, nature, and environment of the MBH at the Galactic center.

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arXiv2006

Hardening in a Time--Evolving Stellar Background: Hyper--Velocity Stars, Orbital Decay and Prediction for Lisa

We study the long-term evolution of massive black hole binaries (MBHBs) at the centers of galaxies using detailed full three-body scattering experiments. Stars, drawn from a distribution unbound to the binary, are ejected by the gravitational slingshot. We quantify the effect of secondary slingshots -- stars returning on small impact parameter orbits to have a second super-elastic scattering with the MBHB -- on binary separation. Even in the absence of two-body relaxation or gas dynamical processes, very unequal mass binaries of mass M=10^7 solar masses can shrink to the gravitational wave emission regime in less than a Hubble time, and are therefore a target for the planned Laser Interferometer Space Antenna (LISA). Three-body interactions create a subpopulation of hypervelocity stars on nearly radial, corotating orbits, with a spatial distribution that is initially highly flattened in the inspiral plane of the MBHB, but becomes more isotropic with decreasing binary separation. The mass ejected is ~0.7 times the binary reduced mass, and most of the stars are ejected in an initial burst lasting much less than a bulge crossing time.

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arXiv2005

LISA double black holes: Dynamics in gaseous nuclear discs

We study the inspiral of double black holes, with masses in the LISA window of detectability, orbiting inside a massive circum-nuclear disc. Using high-resolution SPH simulations, we follow the black hole dynamics in the early phase when gas-dynamical friction acts on the black holes individually, and continue our simulation until they form a close binary. We find that in the early sinking the black holes lose memory of their initial orbital eccentricity if they co-rotate with the gaseous disc, forming a binary with a low eccentricity, consistent with zero within our numerical resolution limit. The cause of circularization resides in the rotation present in the gaseous background where dynamical friction operates. Circularization may hinder gravitational waves from taking over and leading the binary to coalescence. In the case of counter-rotating orbits the initial eccentricity does not decrease, and the black holes may bind forming an eccentric binary. When dynamical friction has subsided, for equal mass black holes and regardless their initial eccentricity, angular momentum loss, driven by the gravitational torque exerted on the binary by surrounding gas, is nevertheless observable down to the smallest scale probed. In the case of unequal masses, dynamical friction remains efficient down to our resolution limit, and there is no sign of formation of any ellipsoidal gas distribution that may further harden the binary. During inspiral, gravitational capture of gas by the black holes occurs mainly along circular orbits: eccentric orbits imply high relative velocities and weak gravitational focusing. Thus, AGN activity may be excited during the black hole pairing process and double active nuclei may form when circularization is completed, on distance-scales of tens of pcs.

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arXiv2005

Cosmic Backgrounds from Miniquasars

A large population of Intermediate Mass Black Holes (IMBHs) might be produced at early cosmic times as a left over of the evolution of the very massive first stars. Accretion onto IMBHs provides a source of (re)ionizing radiation. We show that the baryon mass fraction locked into IMBHs and their growth is strongly constrained by the observed residual Soft X-ray Background (SXRB) intensity. Thus, unless they are extremely X-ray quiet, miniquasars must be quite rare and/or have a short shining phase. As a byproduct, we find that miniquasars can not be the only source of reionization and that their alleged contribution to the near infrared bands is completely negligible.

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arXiv2005

Low-Frequency Gravitational Radiation from Coalescing Massive Black Holes

We compute the expected low-frequency gravitational wave signal from coalescing massive black hole (MBH) binaries at the center of galaxies. We follow the merging history of halos and associated holes via cosmological Monte Carlo realizations of the merger hierarchy from early times to the present in a LCDM cosmology. MBHs get incorporated through a series of mergers into larger and larger halos, sink to the centre owing to dynamical friction, accrete a fraction of the gas in the merger remnant to become more massive, and form a binary system. Stellar dynamical processes dominates the orbital evolution of the binary at large separations, while gravitational wave emission takes over at small radii, causing the final coalescence of the system. We discuss the observability of inspiraling MBH binaries by a low-frequency gravitational wave experiment such as the planned Laser Interferometer Space Antenna (LISA), discriminating between resolvable sources and unresolved confusion noise. Over a 3-year observing period LISA should resolve this GWB into discrete sources, detecting ~90 individual events above a S/N=5 confidence level, while expected confusion noise is well below planned LISA capabilities.

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arXiv2004

The gravitational wave signal from massive black hole binaries and its contribution to the LISA data stream

Massive black hole binaries, with masses in the range 1E3-1E8 Msun, are expected to be the most powerful sources of gravitational radiation at mHz frequencies, and hence are among the primary targets for the planned Laser Interferometer Space Antenna (LISA). We extend and refine our previous analysis (Sesana et al. 2004), detailing the gravitational wave signal expected from a cosmological population of massive black hole binaries. As done in our previous paper, we follow the merger history of dark matter halos, the dynamics of the massive black holes they host, and their growth via gas accretion and binary coalescences in a LCDM cosmology. Stellar dynamical processes dominates the orbital evolution of black hole binaries at large separations, while gravitational wave emission takes over at small radii, causing the final coalescence of the pairs. We show that the GW signal from this population, in a 3 year LISA observation, will be resolved into approx 90 discrete events with S/N>5, among which approx. 35 will be observed above threshold until coalescence. These "merging events" involve relatively massive binaries, M=10E5 Msun, in the redshift range 2 6) and, although their S/N integrated over the duration of the observation can be substantial, the final coalescence phase is at too high frequency to be directly observable by space-based interferometers such as LISA. LISA will be able to detect a fraction approx. 90% of all the coalescences of massive black hole binaries occurring at z=5. The residual confusion noise from unresolved massive black hole binaries is expected to be at least an order of magnitude below the estimated stochastic noise.

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arXiv2003

The Merging History of Massive Black Holes

We investigate a hierarchical structure formation scenario describing the evolution of a Super Massive Black Holes (SMBHs) population. The seeds of the local SMBHs are assumed to be 'pregalactic' black holes, remnants of the first POPIII stars. As these pregalactic holes become incorporated through a series of mergers into larger and larger halos, they sink to the center owing to dynamical friction, accrete a fraction of the gas in the merger remnant to become supermassive, form a binary system, and eventually coalesce. A simple model in which the damage done to a stellar cusps by decaying BH pairs is cumulative is able to reproduce the observed scaling relation between galaxy luminosity and core size. An accretion model connecting quasar activity with major mergers and the observed BH mass-velocity dispersion correlation reproduces remarkably well the observed luminosity function of optically-selected quasars in the redshift range 1<z<5. We finally asses the potential observability of the gravitational wave background generated by the cosmic evolution of SMBH binaries by the planned space-born interferometer LISA.

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arXiv2003

Aborted jets and the X-ray emission of radio-quiet AGNs

We propose that radio-quiet quasars and Seyfert galaxies have central black holes powering outflows and jets which propagate only for a short distance, because the velocity of the ejected material is smaller than the escape velocity. We call them "aborted" jets. If the central engine works intermittently, blobs of material may be produced, which can reach a maximum radial distance and then fall back, colliding with the blobs produced later and still moving outwards. These collisions dissipate the bulk kinetic energy of the blobs by heating the plasma, and can be responsible (entirely or at least in part) for the generation of the high energy emission in radio-quiet objects. This is alternative to the more conventional scenario in which the X-ray spectrum of radio-quiet sources originates in a hot (and possibly patchy) corona above the accretion disk. In the latter case the ultimate source of energy of the emission of both the disk and the corona is accretion. Here we instead propose that the high energy emission is powered also by the extraction of the rotational energy of the black hole (and possibly of the disk). By means of Montecarlo simulations we calculate the time dependent spectra and light curves, and discuss their relevance to the X-ray spectra in radio-quiet AGNs and galactic black hole sources. In particular, we show that time variability and spectra are similar to those observed in Narrow Line Seyfert 1 galaxies.

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arXiv2003

Physical interpretation of the UV/X-ray variability behavior of NGC 7469

We present a re-analysis of the simultaneous 30 day IUE/XTE observation of NGC 7469 done in 1996. Our main progress, in comparison to previous spectral analysis, is to adopt and fit directly to the data a detailed Comptonization model. This firstly allows to fit simultaneously the data from the UV to the hard X-ray band in a self-consistent way and secondly it gives direct constraints on the physical parameters of the disc-corona system. Our results are completely consistent with a slab geometry where all the observed UV emission is supposed to cross the corona but more photon-starved geometries also give acceptable fits. Whatever the geometry is, the UV seed photon emission appears to be dominated by the reprocessing of the X-rays. We also found very interesting correlations between the different model parameters, the most important one being the anticorrelation between the corona temperature and the UV flux. Such an anticorrelation is clearly inconsistent with a fixed disc-corona configuration and suggests a variation of the geometry of the system. We also find a correlation between the corona optical depth and the X-ray flux which may reflect processes linked to the corona formation. During the observations, NGC 7469 appears to accrete near its Eddington limit. This source could then be an example of magnetically dominated disc-corona system as recently proposed by Merloni (2003).

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arXiv2003

The XMM/BeppoSAX observation of Mkn 841

Mkn 841 has been observed simultaneously by XMM and BeppoSAX in January 2001. Due to operational contingency, the 30ks XMM observation was split into two parts, separated by about 15 hours. We first report the presence of a narrow iron line which appears to be rapidly variable between the two pointings, requiring a non-standard interpretation. We then focus on the analysis of the broad band (0.3-200 keV) continuum using the XMM/EPIC, RGS and SAX/PDS data. The Mkn 841 spectrum is well fitted by a comptonization model in a geometry more photon-fed than a simple slab geometry above a passive disk. It presents a relatively large reflection (R>2) which does not agree with an apparently weak iron line. It also show the presence of a strong soft excess wellfitted by a comptonized spectrum in a cool plasma, suggesting the presence of a multi-temperature corona.

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