425 research outputs found
Search of sub-parsec massive binary black holes through line diagnosis
We investigate on the spectral properties of an active black hole, member of
a massive (10^7 - 10^9 Msun) sub-parsec black hole binary. We work under the
hypothesis that the binary, surrounded by a circum-binary disc, has cleared a
gap, and that accretion occurs onto the secondary black hole fed by material
closer to the inner edge of the disc. Broad line emission clouds orbit around
the active black hole and suffer erosion due to tidal truncation at the Roche
Lobe surface, following gap opening and orbital decay. We consider three of the
most prominent broad emission lines observed in the spectra of AGNs, i.e. CIV,
MgII and H{\beta}, and compute the flux ratios between the lines of MgII and
CIV (FMgII/FCIV) and those of MgII and H{\beta} (FMgII/FH{\beta}). We find that
close black hole binaries have FMgII/FCIV up to one order of magnitude smaller
than single black holes. By contrast FMgII/FH{\beta} may be significantly
reduced only at the shortest separations. Peculiarly low values of line flux
ratios together with large velocity offsets between the broad and narrow
emission lines and/or periodic variability in the continuum (on timescales >=
years) would identify genuine sub-pc binary candidates.Comment: 7 pages, 4 figures. Accepted for publication in MNRA
Aligned Spins: Orbital Elements, Decaying Orbits, and Last Stable Circular Orbit to high post-Newtonian Orders
In this article the quasi-Keplerian parameterisation for the case that spins
and orbital angular momentum in a compact binary system are aligned or
anti-aligned with the orbital angular momentum vector is extended to 3PN
point-mass, next-to-next-to-leading order spin-orbit, next-to-next-to-leading
order spin(1)-spin(2), and next-to-leading order spin-squared dynamics in the
conservative regime. In a further step, we use the expressions for the
radiative multipole moments with spin to leading order linear and quadratic in
both spins to compute radiation losses of the orbital binding energy and
angular momentum. Orbital averaged expressions for the decay of energy and
eccentricity are provided. An expression for the last stable circular orbit is
given in terms of the angular velocity type variable .Comment: 30 pages, 2 figures, v2: update to match published versio
Magnetothermal instabilities in magnetized anisotropic plasmas
Using the transport equations for an ideal anisotropic collisionless plasma
derived from the Vlasov equation by the 16-moment method, we analyse the
influence of pressure anisotropy exhibited by collisionless magnetized plasmas
on the magnetothermal (MTI) and heat-flux-driven buoyancy (HBI) instabilities.
We calculate the dispersion relation and the growth rates for these
instabilities in the presence of a background heat flux and for configurations
with static pressure anisotropy, finding that when the frequency at which heat
conduction acts is much larger than any other frequency in the system (i.e.
weak magnetic field) the pressure anisotropy has no effect on the MTI/HBI,
provided the degree of anisotropy is small. In contrast, when this ordering of
timescales does not apply the instability criteria depend on pressure
anisotropy. Specifically, the growth time of the instabilities in the
anisotropic case can be almost one order of magnitude smaller than its
isotropic counterpart. We conclude that in plasmas where pressure anisotropy is
present the MTI/HBI are modified. However, in environments with low magnetic
fields and small anisotropy such as the ICM the results obtained from the
16-moment equations under the approximations considered are similar to those
obtained from ideal MHD.Comment: v3: 16 pages, 2 figures, fixed typos, added references and a final
note on related wor
Probing the nature of the massive black hole binary candidate SDSS J1536+0441
We present an imaging study of the black hole binary candidate SDSS
J1536+0441 (z=0.3893), based on deep, high resolution VzK images collected at
the ESO/VLT. The images clearly show an asymmetric elongation, indicating the
presence of a companion source at ~1" (~5 kpc projected distance) East from the
quasar. The host galaxy of the quasar is marginally resolved. We find that the
companion source is a luminous galaxy, the light profile of which suggests the
presence of an unresolved, faint nucleus (either an obscured AGN or a compact
stellar bulge). The study of the environment around the quasar indicates the
occurrence of a significant over-density of galaxies with a redshift compatible
with z~0.4. This suggests that it resides in a moderately rich cluster of
galaxies.Comment: 4 figures, 1 table. Accepted for publication in ApJ Letter
Properties of Accretion Flows Around Coalescing Supermassive Black Holes
What are the properties of accretion flows in the vicinity of coalescing
supermassive black holes (SBHs)? The answer to this question has direct
implications for the feasibility of coincident detections of electromagnetic
(EM) and gravitational wave (GW) signals from coalescences. Such detections are
considered to be the next observational grand challenge that will enable
testing general relativity in the strong, nonlinear regime and improve our
understanding of evolution and growth of these massive compact objects. In this
paper we review the properties of the environment of coalescing binaries in the
context of the circumbinary disk and hot, radiatively inefficient accretion
flow models and use them to mark the extent of the parameter space spanned by
this problem. We report the results from an ongoing, general relativistic,
hydrodynamical study of the inspiral and merger of black holes, motivated by
the latter scenario. We find that correlated EM+GW oscillations can arise
during the inspiral phase followed by the gradual rise and subsequent drop-off
in the light curve at the time of coalescence. While there are indications that
the latter EM signature is a more robust one, a detection of either signal
coincidentally with GWs would be a convincing evidence for an impending SBH
binary coalescence. The observability of an EM counterpart in the hot accretion
flow scenario depends on the details of a model. In the case of the most
massive binaries observable by the Laser Interferometer Space Antenna, upper
limits on luminosity imply that they may be identified by EM searches out to
z~0.1-1. However, given the radiatively inefficient nature of the gas flow, we
speculate that a majority of massive binaries may appear as low luminosity AGN
in the local universe.Comment: Revised version accepted to Class. Quantum Grav. for proceedings of
8th LISA Symposium. 15 pages, 3 figures, includes changes suggested in
referee report
Structure and evolution of circumbinary disks around supermassive black hole (SMBH) binaries
It is generally believed that gaseous disks around supermassive black hole
(SMBH) binaries in centers of galaxies can facilitate binary merger and give
rise to observational signatures both in electromagnetic and gravitational wave
domains. We explore general properties of circumbinary disks by reformulating
standard equations for the viscous disk evolution in terms of the viscous
angular momentum flux F_J. In steady state F_J is a linear function of the
specific angular momentum, which is a generalization of (but is not equivalent
to) the standard constant \dot M disk solution. If the torque produced by the
central binary is effective at stopping gas inflow and opening a gap (or
cavity) in the disk, then the inner part of the circumbinary disk can be
approximated as a constant F_J disk. We compute properties of such disks in
different physical regimes relevant for SMBH binaries and use these results to
understand the gas-assisted evolution of SMBH pairs starting at separations
10^{-4}-10^{-2} pc. We find the following. (1) Pile-up of matter at the inner
edge of the disk leads to continuous growth of the torque acting on the binary
and can considerably accelerate its orbital evolution compared to the
gravitational wave-driven decay. (2) Torque on the binary is determined
non-locally and does not in general reflect the disk properties in the vicinity
of the binary. (3) Binary evolution depends on the past history of the disk
evolution. (4) Eddington limit can be important in circumbinary disks even if
they accrete at sub-Eddington rates at late stages of binary evolution. (5)
Circumbinary disk self-consistently evolved under the action of the binary
torque emits more power and has spectrum different from the spectrum of
constant \dot M disk - it is steeper (\nu F_\nu\propto \nu^{12/7}) and extends
to shorter wavelength, facilitating its detection.Comment: Discussion of overflow across the orbit of the secondary added in
section 5.2.6. 23 pages, 10 figures, submitted to Ap
Raman spectral analysis of the brainstem and responses of neuroglia and cytokines in whole-body gamma-irradiated rats after administration of aminothiol-based radioprotector GL2011
The search for an effective and non-toxic radioprotector is ongoing. We tested a novel, natural aminothiol-based radioprotector, GL2011, that was applied 30 min, 3 h or 6 h after the exposure of male albino Wistar rats to a 6.7 Gy mild dose of gamma radiation. The molecular signatures of radioprotection were investigated with Raman microspectroscopy of brainstem tissue samples. Morphological changes and activation of astrocytes and microglia were assessed by immunohistochemistry. Global markers of neuroinflammation were followed by ELISA to monitor blood plasma levels of proinflammatory (IL-6 and TNF-α) and anti-inflammatory (IL-10) cytokines. A thirty-day follow-up determined survival of unprotected animals 37.5%. A survival increase was observed after radioprotection (75%, irrespective of the time of application). Raman spectra revealed a slightly deleterious effect of radiation on nucleic acids in surviving animals that was mitigated with the radioprotector, as GL2011 preserved the morphology of both astrocytes and microglia, with reduced microglial infiltration. Cytokine assessment revealed an immunomodulatory effect of the novel radioprotector. The overall results point out the positive effects of a single dose of GL2011 applied at different times. The molecular and cellular changes in the brainstem indicate that the radioprotector applied after radiation conferred better protection, which underlines its translation to cure radiation accidents
The role of pressure anisotropy in the turbulent intracluster medium
In low-density plasma environments, such as the intracluster medium (ICM),
the Larmour frequency is much larger than the ion-ion collision frequency. In
such a case, the thermal pressure becomes anisotropic with respect to the
magnetic field orientation and the evolution of the turbulent gas is more
correctly described by a kinetic approach. A possible description of these
collisionless scenarios is given by the so-called kinetic magnetohydrodynamic
(KMHD) formalism, in which particles freely stream along the field lines, while
moving with the field lines in the perpendicular direction. In this way a
fluid-like behavior in the perpendicular plane is restored. In this work, we
study fast growing magnetic fluctuations in the smallest scales which operate
in the collisionless plasma that fills the ICM. In particular, we focus on the
impact of a particular evolution of the pressure anisotropy and its
implications for the turbulent dynamics of observables under the conditions
prevailing in the ICM. We present results from numerical simulations and
compare the results which those obtained using an MHD formalism.Comment: 7 pages, 14 figures, Journal of Physics: Conference Serie
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