171 research outputs found
The Role of Gas in the Merging of Massive Black Holes in Galactic Nuclei. I. Black Hole Merging in a Spherical Gas Cloud
Using high-resolution SPH numerical simulations, we investigate the effects
of gas on the inspiral and merger of a massive black hole binary. This study is
motivated by both observational and theoretical work that indicate the presence
of large amounts of gas in the central regions of merging galaxies. N-body
simulations have shown that the coalescence of a massive black hole binary
eventually stalls in a stellar background. However, our simulations suggest
that the massive black hole binary will finally merge if it is embedded in a
gaseous background. Here we present results in which the gas is assumed to be
initially spherical with a relatively smooth distribution. In the early
evolution of the binary, the separation dimishes due to the gravitational drag
exerted by the background gas. In the later stages, when the binary dominates
the gravitational potential in its vicinity, the medium responds by forming an
ellipsoidal density enhancement whose axis lags behind the binary axis, and
this offset produces a torque on the binary that causes continuing loss of
angular momentum and is able to reduce the binary separation to distances where
gravitational radiation is efficient. Assuming typical parameters from
observations of Ultra Luminous Infrared Galaxies, we predict that a black hole
binary will merge within yrs; therefore these results imply that in a
merger of gas-rich galaxies, any massive central black holes will coalescence
soon after the galaxies merge. Our work thus supports scenarios of massive
black hole evolution and growth where hierarchical merging plays an important
role. The final coalescence of the black holes leads to gravitational radiation
emission that would be detectable up to high redshift by LISA. We show that
similar physical effects are important for the formation of close binary stars.Comment: 38 pages, 14 figures, submitted to Ap
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
Angular Momentum and the Formation of Stars and Black Holes
The formation of compact objects like stars and black holes is strongly
constrained by the requirement that nearly all of the initial angular momentum
of the diffuse material from which they form must be removed or redistributed
during the formation process. The mechanisms that may be involved and their
implications are discussed for (1) low-mass stars, most of which probably form
in binary or multiple systems; (2) massive stars, which typically form in
clusters; and (3) supermassive black holes that form in galactic nuclei. It is
suggested that in all cases, gravitational interactions with other stars or
mass concentrations in a forming system play an important role in
redistributing angular momentum and thereby enabling the formation of a compact
object. If this is true, the formation of stars and black holes must be a more
complex, dynamic, and chaotic process than in standard models. The
gravitational interactions that redistribute angular momentum tend to couple
the mass of a forming object to the mass of the system, and this may have
important implications for mass ratios in binaries, the upper stellar IMF in
clusters, and the masses of supermassive black holes in galaxies.Comment: Accepted by Reports on Progress in Physic
Primary cutaneous aggressive epidermotropic cytotoxic T-cell lymphomas: reappraisal of a provisional entity in the 2016 WHO classification of cutaneous lymphomas.
Primary cutaneous CD8-positive aggressive epidermotropic T-cell lymphoma is a rare and poorly characterized variant of cutaneous lymphoma still considered a provisional entity in the latest 2016 World Health Organization Classification of Cutaneous lymphomas. We sought to better characterize and provide diagnostic and therapeutic guidance of this rare cutaneous lymphoma. Thirty-four patients with a median age of 77 years (range 19-89 years) presented primarily with extensive annular necrotic plaques or tumor lesions with frequent mucous membrane involvement. The 5-year survival was 32% with a median survival of 12 months. A subset of 17 patients had a prodrome of chronic patches prior to the development of aggressive ulcerative lesions. We identified cases with lack of CD8 or αβ T-cell receptor expression yet with similar clinical and pathological presentation. Allogeneic stem cell transplantation provided partial or complete remissions in 5/6 patients. We recommend the term primary cutaneous aggressive epidermotropic cytotoxic T-cell lymphoma as this more broad designation better describes this clinical-pathologic presentation, which allows the inclusion of cases with CD8 negative and/or αβ/γδ T-cell receptor chain double-positive or double-negative expression. We have identified early skin signs of chronic patch/plaque lesions that are often misdiagnosed as eczema, psoriasis, or mycosis fungoides. Our experience confirms the poor prognosis of this entity and highlights the inefficacy of our standard therapies with the exception of allogeneic stem cell transplantation in selected cases
Stronger security notions for decentralized traceable attribute-based signatures and more efficient constructions
We revisit the notion of Decentralized Traceable Attribute-Based Signatures (DTABS) introduced by El Kaafarani et al. (CT-RSA 2014) and improve the state-of-the-art in three dimensions: Firstly, we provide a new stronger security model which circumvents some shortcomings in existing models. Our model minimizes the trust placed in attribute authorities and hence provides, among other things, a stronger definition for non-frameability. In addition, our model captures the notion of tracing soundness which is important for many applications of the primitive. Secondly, we provide a generic construction that is secure w.r.t. our strong security model and show two example instantiations in the standard model which are more efficient than existing constructions (secure under weaker security definitions). Finally, we dispense with the need for the expensive zero-knowledge proofs required for proving tracing correctness by the tracing authority. As a result, tracing a signature in our constructions is significantly more efficient than existing constructions, both in terms of the size of the tracing proof and the computational cost required to generate and verify it. For instance, verifying tracing correctness in our constructions requires only 4 pairings compared to 34 pairings in the most efficient existing construction
Supermassive Black Hole Binaries: The Search Continues
Gravitationally bound supermassive black hole binaries (SBHBs) are thought to
be a natural product of galactic mergers and growth of the large scale
structure in the universe. They however remain observationally elusive, thus
raising a question about characteristic observational signatures associated
with these systems. In this conference proceeding I discuss current theoretical
understanding and latest advances and prospects in observational searches for
SBHBs.Comment: 17 pages, 4 figures. To appear in the Proceedings of 2014 Sant Cugat
Forum on Astrophysics. Astrophysics and Space Science Proceedings, ed.
C.Sopuerta (Berlin: Springer-Verlag
The SINS survey of z~2 galaxy kinematics: properties of the giant star forming clumps
We have studied the properties of giant star forming clumps in five z~2
star-forming disks with deep SINFONI AO spectroscopy at the ESO VLT. The clumps
reside in disk regions where the Toomre Q-parameter is below unity, consistent
with their being bound and having formed from gravitational instability. Broad
H{\alpha}/[NII] line wings demonstrate that the clumps are launching sites of
powerful outflows. The inferred outflow rates are comparable to or exceed the
star formation rates, in one case by a factor of eight. Typical clumps may lose
a fraction of their original gas by feedback in a few hundred million years,
allowing them to migrate into the center. The most active clumps may lose much
of their mass and disrupt in the disk. The clumps leave a modest imprint on the
gas kinematics. Velocity gradients across the clumps are 10-40 km/s/kpc,
similar to the galactic rotation gradients. Given beam smearing and clump
sizes, these gradients may be consistent with significant rotational support in
typical clumps. Extreme clumps may not be rotationally supported; either they
are not virialized, or they are predominantly pressure supported. The velocity
dispersion is spatially rather constant and increases only weakly with star
formation surface density. The large velocity dispersions may be driven by the
release of gravitational energy, either at the outer disk/accreting streams
interface, and/or by the clump migration within the disk. Spatial variations in
the inferred gas phase oxygen abundance are broadly consistent with inside-out
growing disks, and/or with inward migration of the clumps.Comment: accepted Astrophys. Journal, February 9, 201
Elliptical Galaxies and Bulges of Disk Galaxies: Summary of Progress and Outstanding Issues
This is the summary chapter of a review book on galaxy bulges. Bulge
properties and formation histories are more varied than those of ellipticals. I
emphasize two advances: 1 - "Classical bulges" are observationally
indistinguishable from ellipticals, and like them, are thought to form by major
galaxy mergers. "Disky pseudobulges" are diskier and more actively star-forming
(except in S0s) than are ellipticals. Theys are products of the slow
("secular") evolution of galaxy disks: bars and other nonaxisymmetries move
disk gas toward the center, where it starbursts and builds relatively flat,
rapidly rotating components. This secular evolution is a new area of galaxy
evolution work that complements hierarchical clustering. 2 - Disks of
high-redshift galaxies are unstable to the formation of mass clumps that sink
to the center and merge - an alternative channel for the formation of classical
bulges. I review successes and unsolved problems in the formation of
bulges+ellipticals and their coevolution (or not) with supermassive black
holes. I present an observer's perspective on simulations of dark matter galaxy
formation including baryons. I review how our picture of the quenching of star
formation is becoming general and secure at redshifts z < 1. The biggest
challenge is to produce realistic bulges+ellipticals and disks that overlap
over a factor of 10**3 in mass but that differ from each other as observed over
that whole range. Second, how does hierarchical clustering make so many giant,
bulgeless galaxies in field but not cluster environments? I argue that we rely
too much on AGN and star-formation feedback to solve these challenges.Comment: 46 pages, 10 postscript figures, accepted for publication in Galactic
Bulges, ed. E. Laurikainen, R. F. Peletier, & D. A. Gadotti (New York:
Springer), in press (2015
Multimessenger astronomy with pulsar timing and X-ray observations of massive black hole binaries
We demonstrate that very massive (>10^8\msun), cosmologically nearby (z<1)
black hole binaries (MBHBs), which are primary targets for ongoing and upcoming
pulsar timing arrays (PTAs), are particularly appealing multimessenger
carriers. According to current models for massive black hole formation and
evolution, the planned Square Kilometer Array (SKA) will collect gravitational
wave signals from thousands of such massive systems, being able to individually
resolve and locate in the sky several of them (maybe up to a hundred). By
employing a standard model for the evolution of MBHBs in circumbinary discs,
with the aid of dedicated numerical simulations, we characterize the gas-binary
interplay, identifying possible electromagnetic signatures of the PTA sources.
We concentrate our investigation on two particularly promising scenarios in the
high energy domain, namely, the detection of X-ray periodic variability and of
double broad K\alpha iron lines. Up to several hundreds of periodic X-ray
sources with a flux >10^-13 erg s^-1 cm^-2 will be in the reach of upcoming
X-ray observatories. Double relativistic K\alpha lines may be observable in a
handful of low redshift (z<0.3) sources by proposed deep X-ray probes, such as
Athena. (Abridged)Comment: 19 pages, 11 figures, submitted to MNRAS, minor revision of the
reference lis
Enhanced Security of Attribute-Based Signatures
Despite the recent advances in attribute-based signatures (ABS), no schemes have yet been considered under a strong privacy definition. We enhance the security of ABS by presenting a strengthened simulation-based privacy definition and the first attribute-based signature functionality in the framework of universal composability (UC). Additionally, we show that the UC definition is equivalent to our strengthened experiment-based security definitions.
To achieve this we rely on a general unforgeability and a simulation-based privacy definition that is stronger than standard indistinguishability-based privacy. Further, we show that two extant concrete ABS constructions satisfy this simulation-based privacy definition and are therefore UC secure. The two concrete constructions are the schemes by Sakai et al. (PKC\u2716) and by Maji et al. (CT-RSA\u2711). Additionally, we identify the common feature that allows these schemes to meet our privacy definition, giving us further insights into the security requirements of ABS
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