49 research outputs found
The viscosity parameter alpha and the properties of accretion disc outbursts in close binaries
The physical mechanisms driving angular momentum transport in accretion discs
are still unknown. Although it is generally accepted that, in hot discs, the
turbulence triggered by the magneto-rotational instability is at the origin of
the accretion process in Keplerian discs, it has been found that the values of
the stress-to-pressure ratio (the alpha "viscosity" parameter) deduced from
observations of outbursting discs are an order of magnitude higher than those
obtained in numerical simulations. We test the conclusion about the
observation-deduced value of alpha using a new set of data and comparing the
results with model outbursts. We analyse a set of observations of dwarf-nova
and AM CVn star outbursts and from the measured decay times determine the
hot-disc viscosity parameter alpha_h. We determine if and how this method is
model dependent. From the dwarf-nova disc instability model we determine an
amplitude vs recurrence-time relation and compare it to the empirical
Kukarkin-Parenago relation between the same, but observed, quantities. We found
that all methods we tried, including the one based on the amplitude vs
recurrence-time relation, imply alpha_h ~ 0.1 - 0.2 and exclude values an order
of magnitude lower. The serious discrepancy between the observed and the
MRI-calculated values of the accretion disc viscosity parameter alpha is
therefore real since there can be no doubt about the validity of the values
deduced from observations of disc outbursts.Comment: Astronomy and Astrophysics, in press. (In Fig. 3b the upper sequence
of numbers and symbols is an artefact of the compilation on astro-ph) and
should be ignored.
Models of AM CVn star outbursts
Outbursting AM CVn stars exhibit outbursts similar to those observed in
different types of dwarf novae. Their light-curves combine the characteristic
features of SU UMa, ER UMa, Z Cam, and WZ Sge-type systems but also show a
variety of properties never observed in dwarf novae. The compactness of AM CVn
orbits and their unusual chemical composition make these systems valuable
testbeds for outburst models. We aim for a better understanding of the role of
helium in the accretion disc instability mechanism, testing the model for dwarf
novae outbursts in the case of AM CVn stars, and aim to explain the outburst
light-curves of these ultra-compact binaries. We calculated the properties of
the hydrogen-free AM CVn stars using our previously developed numerical code
adapted to the different chemical composition of these systems and supplemented
with formulae accounting for mass transfer rate variations, additional sources
of the disc heating, and the primary's magnetic field. We discovered how
helium-dominated discs react to the thermal-viscous instability and were able
to reproduce various features of the outburst cycles in the light-curves of AM
CVn stars. The AM CVn outbursts can be explained by the suitably adapted
dwarf-nova disc instability model but, as in the case of its application to
hydrogen-dominated cataclysmic variables, one has to resort to additional
mechanisms to account for the observed superoutbursts, dips, cycling states,
and standstills. We show that the enhanced mass-transfer rate, due presumably
to variable irradiation of the secondary, must not only be taken into account
but is a determining factor that shapes AM CVn star outbursts. The cause of the
variable secondary's irradiation has yet to be understood; the best candidate
is the precession of a tilted/warped disc.Comment: Astronomy and Astrophysics - in press; corrected (language) versio
Multi-gluon helicity amplitudes with one off-shell leg within high energy factorization
Basing on the Slavnov-Taylor identities, we derive a new prescription to
obtain gauge invariant tree-level scattering amplitudes for the process g*g->Ng
within high energy factorization. Using the helicity method, we check the
formalism up to several final state gluons, and we present analytical formulas
for the the helicity amplitudes for N=2. We also compare the method with
Lipatov's effective action approach.Comment: 25 pages, quite a few figures, an appendix added, typos correcte
Predictions for Pb Collisions at TeV: Comparison with Data
Predictions made in Albacete {\it et al} prior to the LHC Pb run at
TeV are compared to currently available data. Some
predictions shown here have been updated by including the same experimental
cuts as the data. Some additional predictions are also presented, especially
for quarkonia, that were provided to the experiments before the data were made
public but were too late for the original publication are also shown here.Comment: 55 pages 35 figure
The Evolution of Compact Binary Star Systems
We review the formation and evolution of compact binary stars consisting of
white dwarfs (WDs), neutron stars (NSs), and black holes (BHs). Binary NSs and
BHs are thought to be the primary astrophysical sources of gravitational waves
(GWs) within the frequency band of ground-based detectors, while compact
binaries of WDs are important sources of GWs at lower frequencies to be covered
by space interferometers (LISA). Major uncertainties in the current
understanding of properties of NSs and BHs most relevant to the GW studies are
discussed, including the treatment of the natal kicks which compact stellar
remnants acquire during the core collapse of massive stars and the common
envelope phase of binary evolution. We discuss the coalescence rates of binary
NSs and BHs and prospects for their detections, the formation and evolution of
binary WDs and their observational manifestations. Special attention is given
to AM CVn-stars -- compact binaries in which the Roche lobe is filled by
another WD or a low-mass partially degenerate helium-star, as these stars are
thought to be the best LISA verification binary GW sources.Comment: 105 pages, 18 figure
Precision QCD, Hadronic Structure & Forward QCD, Heavy Ions: Report of Energy Frontier Topical Groups 5, 6, 7 submitted to Snowmass 2021
This report was prepared on behalf of three Energy Frontier Topical Groups of
the Snowmass 2021 Community Planning Exercise. It summarizes the status and
implications of studies of strong interactions in high-energy experiments and
QCD theory. We emphasize the rich landscape and broad impact of these studies
in the decade ahead. Hadronic interactions play a central role in the
high-luminosity Large Hadron Collider (LHC) physics program, and strong
synergies exist between the (HL-)LHC and planned or proposed experiments at the
U.S. Electron-Ion Collider, CERN forward physics experiments, high-intensity
facilities, and future TeV-range lepton and hadron colliders. Prospects for
precision determinations of the strong coupling and a variety of
nonperturbative distribution and fragmentation functions are examined. We also
review the potential of envisioned tests of new dynamical regimes of QCD in
high-energy and high-density scattering processes with nucleon, ion, and photon
initial states. The important role of the high-energy heavy-ion program in
studies of nuclear structure and the nuclear medium, and its connections with
QCD involving nucleons are summarized. We address ongoing and future
theoretical advancements in multi-loop QCD computations, lattice QCD, jet
substructure, and event generators. Cross-cutting connections between
experimental measurements, theoretical predictions, large-scale data analysis,
and high-performance computing are emphasized.Comment: 95 pages (bibliography 30 pages), 28 figures; v.2: minor changes,
authors and references adde
General Overview of Black Hole Accretion Theory
I provide a broad overview of the basic theoretical paradigms of black hole
accretion flows. Models that make contact with observations continue to be
mostly based on the four decade old alpha stress prescription of Shakura &
Sunyaev (1973), and I discuss the properties of both radiatively efficient and
inefficient models, including their local properties, their expected stability
to secular perturbations, and how they might be tied together in global flow
geometries. The alpha stress is a prescription for turbulence, for which the
only existing plausible candidate is that which develops from the
magnetorotational instability (MRI). I therefore also review what is currently
known about the local properties of such turbulence, and the physical issues
that have been elucidated and that remain uncertain that are relevant for the
various alpha-based black hole accretion flow models.Comment: To be published in Space Science Reviews and as hard cover in the
Space Sciences Series of ISSI: The Physics of Accretion on to Black Holes
(Springer Publisher