Probing AGN Accretion Physics through AGN Variability

2021 ◽  
Author(s):  
Vishal Pramod Kasliwal
Keyword(s):  
Accretion Physics

Vertical axis wind turbine operation in icing conditions: A review study

2021 ◽  
pp. 0309524X2110618
Author(s):  
Syed Abdur Rahman Tahir ◽  
Muhammad Shakeel Virk
Keyword(s):  
Wind Turbine ◽  
Wind Turbines ◽  
Vertical Axis ◽  
Electricity Production ◽  
Vertical Axis Wind Turbine ◽  
Vertical Axis Wind Turbines ◽  
Horizontal Axis Wind Turbines ◽  
Promising Solution ◽  
Review Study ◽  
Accretion Physics

Vertical Axis Wind Turbine (VAWT) can be a promising solution for electricity production in remote ice prone territories of high north, where good wind resources are available, but icing is a challenge that can affect its optimum operation. A lot of research has been made to study the icing effects on the conventional horizontal axis wind turbines, but the literature about vertical axis wind turbines operating in icing conditions is still scarce, despite the importance of this topic. This paper presents a review study about existing knowledge of VAWT operation in icing condition. Focus has been made in better understanding of ice accretion physics along VAWT blades and methods to detect and mitigate icing effects.

scholarly journals Cosmological black holes are not described by the Thakurta metric: LIGO-Virgo bounds on PBHs remain unchanged

2021 ◽  
Vol 81 (11) ◽  
Author(s):  
Gert Hütsi ◽  
Tomi Koivisto ◽  
Martti Raidal ◽  
Ville Vaskonen ◽  
Hardi Veermäe
Keyword(s):  
Black Holes ◽  
Black Hole ◽  
Dark Matter ◽  
Compact Object ◽  
Compact Objects ◽  
Primordial Black Hole ◽  
Mass Growth ◽  
Black Hole Binaries ◽  
Physical Conditions ◽  
Accretion Physics

AbstractWe show that the physical conditions which induce the Thakurta metric, recently studied by Bœhm et al. in the context of time-dependent black hole masses, correspond to a single accreting compact object in the entire Universe filled with isotropic non-interacting dust. In such a case, accretion physics is not local but tied to the properties of the whole Universe. We show that radiation, primordial black holes or particle dark matter cannot produce the specific energy flux required for supporting the mass growth of the compact objects described by the Thakurta metric. In particular, this solution does not apply to black hole binaries. We conclude that compact dark matter candidates and their mass growth cannot be described by the Thakurta metric, and thus existing constraints on the primordial black hole abundance from the LIGO-Virgo and the CMB measurements remain valid.

scholarly journals Fuse Studies of Dwarf Novae during Quiescence

2004 ◽  
Vol 194 ◽  
pp. 194-195
Author(s):  
E. M. Sion ◽  
F. H. Cheng ◽  
P. Godon ◽  
P. Szkody
Keyword(s):  
Boundary Layer ◽  
White Dwarf ◽  
White Dwarfs ◽  
Disk Accretion ◽  
Dwarf Novae ◽  
Preliminary Results ◽  
Accretion Physics

AbstractWe have obtained FUSE spectra of several dwarf novae which reveal the properties of their accreting white dwarfs, their rates of accretion during quiescence and the nature of their boundary layer/inner disk regions including accretion belts on the white dwarfs. Among the systems for which preliminary results will be presented are BV Cen, WW Ceti, EY Cyg, SS Aur and VW Hyi. The contribution of these FUSE studies to our understanding of disk accretion physics will be emphasized within the context of the white dwarf and boundary layer.

scholarly journals A parameter survey of Sgr A* radiative models from GRMHD simulations with self-consistent electron heating

2020 ◽  
Vol 494 (3) ◽  
pp. 4168-4186 ◽  
Author(s):  
J Dexter ◽  
A Jiménez-Rosales ◽  
S M Ressler ◽  
A Tchekhovskoy ◽  
M Bauböck ◽  
...  
Keyword(s):  
Faraday Rotation ◽  
Near Infrared ◽  
Good Description ◽  
Galactic Centre ◽  
Electron Heating ◽  
Black Hole Candidate ◽  
Long Baseline ◽  
Rotation Measure ◽  
Accretion Physics ◽  
Sgr A

ABSTRACT The Galactic centre black hole candidate Sgr A* is the best target for studies of low-luminosity accretion physics, including with near-infrared (NIR) and submillimetre wavelength long baseline interferometry experiments. Here, we compare images and spectra generated from a parameter survey of general relativistic MHD simulations to a set of radio to NIR observations of Sgr A*. Our models span the limits of weak and strong magnetization and use a range of sub-grid prescriptions for electron heating. We find two classes of scenarios can explain the broad shape of the submillimetre spectral peak and the highly variable NIR flaring emission. Weakly magnetized ‘disc-jet’ models where most of the emission is produced near the jet wall, consistent with past work, as well as strongly magnetized (magnetically arrested disc) models where hot electrons are present everywhere. Disc-jet models are strongly depolarized at submillimetre wavelengths as a result of strong Faraday rotation, inconsistent with observations of Sgr A*. We instead favour the strongly magnetized models, which provide a good description of the median and highly variable linear polarization signal. The same models can also explain the observed mean Faraday rotation measure and potentially the polarization signals seen recently in Sgr A* NIR flares.

scholarly journals Ambartsumian’s vision and further insight to key puzzles of ultra-high energy astrophysics

2021 ◽  
pp. 236-310
Author(s):  
G. Ter-Kazarian
Keyword(s):  
Microscopic Theory ◽  
Vacuum State ◽  
High Energy ◽  
Spontaneous Breaking ◽  
Ultra High Energy ◽  
High Energy Astrophysics ◽  
Superdense Matter ◽  
Equilibrium Configurations ◽  
Accretion Physics ◽  
Stellar Masses

We review the Ambartsumian’s cosmogony, which involves his fundamental ideas on Stellar Associations and eruptive Activity of Galactic Nuclei, where the creation process is at work. Itis caused by the violent outburst events of transformations of superdense matter in supermassive compact bodies in galaxies, away from the accretion physics. We discuss the pioneering works of V.A. Armbartsumyan and G.S. Saakyan carried out at Byurakan Observatory in the earlier of 1960’s towards the physics of equilibrium configurations of degenerate superdense gas of elementary particles, particularity, the hyperon configurations of stellar masses. These issues have been comprehensively developed later on by G. Ter-Kazarian in the proposed theory of distortion of space-time continuum(DSTC) at huge energies (respectively, at short distances < 0.4fm), which underlies the microscopic theory of black hole (MTBH). The MTBH has further proved to be quite fruitful for ultra-high energy astrophysics. The MTBH explores the most important process of spontaneous breaking of gravitation gauge symmetry at huge energies, and thereof for that of re-arrangement of vacuum state. As a corollary, MTBH has smeared out the central singularities of BHs, and makes room for their growth and merging behavior, with implications of vital interest for high energy astrophysics.

scholarly journals A Multi-Wavelength View of OJ 287 Activity in 2015–2017: Implications of Spectral Changes on Central-Engine Models and MeV-GeV Emission Mechanism

Galaxies ◽  
2020 ◽  
Vol 8 (1) ◽  
pp. 15 ◽  
Author(s):  
Pankaj Kushwaha
Keyword(s):  
High Energy ◽  
Model Parameters ◽  
Bl Lac Objects ◽  
Independent Evidence ◽  
Diverse Range ◽  
Inverse Compton Scattering ◽  
Central Engine ◽  
Multi Wavelength ◽  
Geometrical Effect ◽  
Accretion Physics

A diverse range of observational results and peculiar properties across the domains of observation have made OJ 287 one of the best-explored BL Lac objects on the issues of relativistic jets and accretion physics as well as the strong theory of gravity. We here present a brief compilation of observational results from the literature and inferences/insights from the extensive studies but focus on the interpretation of its ∼12-yr quasi-periodic optical outbursts (QPOOs) and high energy emission mechanisms. The QPOOs in one model are attributed to the disk-impact related to dynamics of the binary SMBHs while alternative models attribute it to the geometrical effect related to the precession of a single jet or double jets. We discuss implications of the new spectral features reported during the 2015–2017 multi-wavelength high activity of the source—a break in the NIR-optical spectrum and hardening of the MeV-GeV emission accompanied by a shift in the location of its peak, in the context of the two. The reported NIR-optical break nicely fits the description of a standard accretion disk emission from an SMBH of mass ∼ 10 10 M ⊙ while the time of its first appearance at the end of May, 2013 (MJD 56439) is in close coincidence with the time of impact predicted by the disk-impact binary SMBH model. This spectral and temporal coincidence with the model parameters of the disk-impact binary SMBH model provides independent evidence in favor of the model over the geometrical models which argue for a total central-engine mass in the range of 10 7 - 9 M ⊙ . On the other hand, the MeV-GeV spectral change is naturally reproduced by the inverse Compton scattering of photons from the broad-line region and is consistent with the detection of broad emission lines during the previous cycles of quasi-periodic outbursts. Combining this with previous SED studies suggests that in, OJ 287, the MeV-GeV emission results from external Comptonization.

Saturation of MRI via parasitic modes

2010 ◽  
Vol 6 (S274) ◽  
pp. 449-452
Author(s):  
Martin E. Pessah
Keyword(s):  
Magnetic Field ◽  
Instantaneous Growth Rate ◽  
Magnetorotational Instability ◽  
Parasitic Mode ◽  
Order Unity ◽  
Tearing Modes ◽  
Physical Mechanisms ◽  
Accretion Physics ◽  
Secondary Instabilities ◽  
The Magnetic Field

AbstractUnderstanding the physical mechanisms that play a role in the saturation of the magnetorotational instability (MRI) has been an outstanding problem in accretion physics since the early 90's. Here, we present the summary of a study of the parasitic modes that feed off exact viscous, resistive MRI modes. We focus on the situation in which the amplitude of the magnetic field produced by the MRI is such that the instantaneous growth rate of the fastest parasitic mode matches that of the fastest MRI mode. We argue that this "saturation" amplitude provides an estimate of the magnetic field that can be generated by the MRI before the secondary instabilities suppress its growth significantly. We show that there exist two regimes, delimited by a critical Elsasser number of order unity, in which saturation is achieved via secondary instabilities that correspond to either Kelvin-Helmholtz or tearing modes.

Accretion physics: It's not U, it's B

2017 ◽  
Vol 1 (4) ◽  
Author(s):  
Jon Miller
Keyword(s):  
Accretion Physics
Sign in / Sign up

Export Citation Format

Share Document