3,203 research outputs found
A magnetohydrodynamic model for multi-wavelength flares from Sagittarius~A (I): model and the near-infrared and X-ray flares
Flares from the supermassive black hole in our Galaxy, Sagittarius~A
(Sgr A), are routinely observed over the last decade or so. Despite
numerous observational and theoretical efforts, the nature of such flares still
remains poorly understood, although a few phenomenological scenarios have been
proposed. In this work, we develop the Yuan et al. (2009) scenario into a
magnetohydrodynamic (MHD) model for Sgr A flares. This model is
analogous with the theory of solar flares and coronal mass ejection in solar
physics. In the model, magnetic field loops emerge from the accretion flow onto
Sgr A and are twisted to form flux ropes because of shear and
turbulence. The magnetic energy is also accumulated in this process until a
threshold is reached. This then results in a catastrophic evolution of a flux
rope with the help of magnetic reconnection in the current sheet. In this
catastrophic process, the magnetic energy is partially converted into the
energy of non-thermal electrons. We have quantitatively calculated the
dynamical evolution of the height, size, and velocity of the flux rope, as well
as the magnetic field in the flare regions, and the energy distribution of
relativistic electrons in this process. We further calculate the synchrotron
radiation from these electrons and compare the obtained light curves with the
observed ones. We find that the model can reasonably explain the main
observations of near-infrared (NIR) and X-ray flares including their light
curves and spectra. It can also potentially explain the frequency-dependent
time delay seen in radio flare light curves.Comment: 17 pages, 13 figures, accepted by MNRA
(De-)activating the growth machine for redevelopment: the case of Liede urban village in Guangzhou
This research investigates the mechanism of urban village redevelopment in south China. Through a revised typology of place entrepreneurs based on the growth machine thesis and a case study of Liede village in central Guangzhou, it illustrates how land-based interests embedded in an imbalanced power relationship can (de-)activate urban village redevelopment. The study reveals that while urban villagers, as represented by the village collective, have entrenched interests in the redevelopment process, the city government – as monopolistic land manager and place entrepreneur – plays the deciding role in forging and halting a growth machine geared towards urban village redevelopment. Although developers are also part of the process, the (de-)activation of redevelopment growth machine/coalition in Guangzhou has largely been dominated by the city government. With a comparative view on the original growth machine model, it is hoped that this study would furnish both theoretical and practical thoughts for future research
Freeze-in Dirac neutrinogenesis: thermal leptonic CP asymmetry
We present a freeze-in realization of the Dirac neutrinogenesis in which the
decaying particle that generates the lepton-number asymmetry is in thermal
equilibrium. As the right-handed Dirac neutrinos are produced non-thermally,
the lepton-number asymmetry is accumulated and partially converted to the
baryon-number asymmetry via the rapid sphaleron transitions. The necessary
CP-violating condition can be fulfilled by a purely thermal kinetic phase from
the wavefunction correction in the lepton-doublet sector, which has been
neglected in most leptogenesis-based setup. Furthermore, this condition
necessitates a preferred flavor basis in which both the charged-lepton and
neutrino Yukawa matrices are non-diagonal. To protect such a proper Yukawa
structure from the basis transformations in flavor space prior to the
electroweak gauge symmetry breaking, we can resort to a plethora of model
buildings aimed at deciphering the non-trivial Yukawa structures.
Interestingly, based on the well-known tri-bimaximal mixing with a minimal
correction from the charged-lepton or neutrino sector, we find that a
simultaneous explanation of the baryon-number asymmetry in the Universe and the
low-energy neutrino oscillation observables can be attributed to the mixing
angle and the CP-violating phase introduced in the minimal correction.Comment: 28 pages and 7 figures; more discussions and one figure added, final
version published in the journa
A Geometric View on Constrained M-Estimators
We study the estimation error of constrained M-estimators, and derive
explicit upper bounds on the expected estimation error determined by the
Gaussian width of the constraint set. Both of the cases where the true
parameter is on the boundary of the constraint set (matched constraint), and
where the true parameter is strictly in the constraint set (mismatched
constraint) are considered. For both cases, we derive novel universal
estimation error bounds for regression in a generalized linear model with the
canonical link function. Our error bound for the mismatched constraint case is
minimax optimal in terms of its dependence on the sample size, for Gaussian
linear regression by the Lasso
Reconciling the Quasar Microlensing Disc Size Problem with a Wind Model of Active Galactic Nucleus
Many analyses have concluded that the accretion disc sizes measured from the
microlensing variability of quasars are larger than the expectations from the
standard thin disc theory by a factor of . We propose a simply model by
invoking a strong wind from the disc to flatten its radial temperature profile,
which can then reconcile the size discrepancy problem. This wind model has been
successfully applied to several microlensed quasars with a wind strength
by only considering the inward decreasing of the mass accretion
rate (where is defined through ).
After further incorporating the angular momentum transferred by the wind, our
model can resolve the disc size problem with an even lower wind parameter. The
corrected disc sizes under the wind model are correlated with black hole masses
with a slope in agreement with our modified thin disc model.Comment: 8 pages, MNRAS in pres
- …