789 research outputs found
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.Comment: Minor changes, accepted to MNRAS (11 pags, 14 figs). Movies (.avi)
are available at http://pitto.mib.infn.it/~haardt/MOVIES
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<z<10). We find that this population of AGN should contribute substantially
(~25%) to the unresolved fraction of the cosmic X-ray background in the 0.5-10
keV range, and that a significant fraction (~3-4%) of the total background
intensity would remain unaccounted even after future X-ray observations. As
byproduct, we compute the expected UV background from AGN at z>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.Comment: 9 pages, 7 figures, MNRAS in pres
Blind adaptive constrained reduced-rank parameter estimation based on constant modulus design for CDMA interference suppression
This paper proposes a multistage decomposition for blind adaptive parameter estimation in the Krylov subspace with the code-constrained constant modulus (CCM) design criterion. Based on constrained optimization of the constant modulus cost function and utilizing the Lanczos algorithm and Arnoldi-like iterations, a multistage decomposition is developed for blind parameter estimation. A family of computationally efficient blind adaptive reduced-rank stochastic gradient (SG) and recursive least squares (RLS) type algorithms along with an automatic rank selection procedure are also devised and evaluated against existing methods. An analysis of the convergence properties of the method is carried out and convergence conditions for the reduced-rank adaptive algorithms are established. Simulation results consider the application of the proposed techniques to the suppression of multiaccess and intersymbol interference in DS-CDMA systems
The role of relativistic jets in the heaviest and most active supermassive black holes at high redshift
In powerful radio-quiet active galactic nuclei (AGN), black holes heavier
than one billion solar masses form at a redshift ~1.5-2. Supermassive black
holes in jetted radio-loud AGN seems to form earlier, at a redshift close to 4.
The ratio of active radio-loud to radio-quiet AGN hosting heavy black holes is
therefore a rather a strong function of redshift. We report on some recent
evidence supporting this conclusion, gathered from the Burst Alert Telescope
(BAT, onboard Swift) and by the Large Area Telescope (LAT, onboard Fermi). We
suggest that the more frequent occurrence of relativistic jets in the most
massive black holes at high redshifts, compared to later times, could be due to
the average black hole spin being greater in the distant past, or else to the
jet helping a fast accretion rate (or some combination of the two scenarios).
We emphasize that the large total accretion efficiency of rapidly spinning
black holes inhibits a fast growth, unless a large fraction of the available
gravitational energy of the accreted mass is not converted into radiation, but
used to form and maintain a powerful jet.Comment: 6 pages, 3 figures, accepted for publication in MNRAS, main journa
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