1,114 research outputs found
The Origin of the Spatial Distribution of X-ray luminous AGN in Massive Galaxy Clusters
We study the spatial distribution of a 95% complete sample of 508 X-ray point
sources (XPS) detected in the 0.5-2.0 keV band in Chandra ACIS-I observations
of 51 massive galaxy clusters found in the MACS survey. Covering the redshift
range z=0.3-0.7, our cluster sample is statistically complete and comprises all
MACS clusters with X-ray luminosities in excess of 4.5 x 10^44 erg/s (0.1-2.4
keV, h_0=0.7, LCDM). Also studied are 20 control fields that do not contain
clusters. We find the XPS surface density, computed in the cluster restframe,
to exhibit a pronounced excess within 3.5 Mpc of the cluster centers. The
excess, believed to be caused by AGN in the cluster, is significant at the 8.0
sigma confidence level compared to the XPS density observed at the field edges.
No significant central excess is found in the control fields. To investigate
the physical origin of the AGN excess, we study the radial AGN density profile
for a subset of 24 virialized clusters. We find a pronounced central spike
(r<0.5 Mpc), followed by a depletion region at about 1.5 Mpc, and a broad
secondary excess centered at approximately the virial radius of the host
clusters (~2.5 Mpc). We present evidence that the central AGN excess reflects
increased nuclear activity triggered by close encounters between infalling
galaxies and the giant cD-type elliptical occupying the very cluster center. By
contrast, the secondary excess at the cluster-field interface is likely due to
black holes being fueled by galaxy mergers. In-depth spectroscopic and
photometric follow-up observations of the optical counterparts of the XPS in a
subset of our sample are being conducted to confirm this picture.Comment: ApJ Letters, accepted (4 pages, 3 figures, uses emulateapj
The Weak Scale from BBN
The measured values of the weak scale, , and the first generation masses,
, are simultaneously explained in the multiverse, with all these
parameters scanning independently. At the same time, several remarkable
coincidences are understood. Small variations in these parameters away from
their measured values lead to the instability of hydrogen, the instability of
heavy nuclei, and either a hydrogen or a helium dominated universe from Big
Bang Nucleosynthesis. In the 4d parameter space of ,
catastrophic boundaries are reached by separately increasing each parameter
above its measured value by a factor of , respectively.
The fine-tuning problem of the weak scale in the Standard Model is solved: as
is increased beyond the observed value, it is impossible to maintain a
significant cosmological hydrogen abundance for any values of that
yield both hydrogen and heavy nuclei stability.
For very large values of a new regime is entered where weak interactions
freeze out before the QCD phase transition. The helium abundance becomes
independent of and is determined by the cosmic baryon and lepton
asymmetries. To maintain our explanation of from the anthropic cost of
helium dominance then requires universes with such large to be rare in the
multiverse. Implications of this are explored, including the possibility that
new physics below 10 TeV cuts off the fine-tuning in .Comment: 26 pages plus appendix, 13 figure
Nonlinear theory of resonant slow waves in anisotropic and dispersive plasmas
The solar corona is a typical example of a plasma with strongly anisotropic transport processes. The main dissipative mechanisms in the solar corona acting on slow magnetoacoustic waves are the anisotropic thermal conductivity and viscosity [Ballai et al., Phys. Plasmas 5, 252 (1998)] developed the nonlinear theory of driven slow resonant waves in such a regime. In the present paper the nonlinear behavior of driven magnetohydrodynamic waves in the slow dissipative layer in plasmas with strongly anisotropic viscosity and thermal conductivity is expanded by considering dispersive effects due to Hall currents. The nonlinear governing equation describing the dynamics of nonlinear resonant slow waves is supplemented by a term which describes nonlinear dispersion and is of the same order of magnitude as nonlinearity and dissipation. The connection formulas are found to be similar to their nondispersive counterparts
A Contemporary View of Coronal Heating
Determining the heating mechanism (or mechanisms) that causes the outer
atmosphere of the Sun, and many other stars, to reach temperatures orders of
magnitude higher than their surface temperatures has long been a key problem.
For decades the problem has been known as the coronal heating problem, but it
is now clear that `coronal heating' cannot be treated or explained in isolation
and that the heating of the whole solar atmosphere must be studied as a highly
coupled system. The magnetic field of the star is known to play a key role,
but, despite significant advancements in solar telescopes, computing power and
much greater understanding of theoretical mechanisms, the question of which
mechanism or mechanisms are the dominant supplier of energy to the chromosphere
and corona is still open. Following substantial recent progress, we consider
the most likely contenders and discuss the key factors that have made, and
still make, determining the actual (coronal) heating mechanism (or mechanisms)
so difficult
Prospects and Blind Spots for Neutralino Dark Matter
Using a simplified model framework, we assess observational limits and discovery prospects for neutralino dark matter, taken here to be a general admixture of bino, wino, and Higgsino. Experimental constraints can be weakened or even nullified in regions of parameter space near 1) purity limits, where the dark matter is mostly bino, wino, or Higgsino, or 2) blind spots, where the relevant couplings of dark matter to the Z or Higgs bosons vanish identically. We analytically identify all blind spots relevant to spin-independent and spin-dependent scattering and show that they arise for diverse choices of relative signs among M_1, M_2, and μ. At present, XENON100 and IceCube still permit large swaths of viable parameter space, including the well-tempered neutralino. On the other hand, upcoming experiments should have sufficient reach to discover dark matter in much of the remaining parameter space. Our results are broadly applicable, and account for a variety of thermal and non-thermal cosmological histories, including scenarios in which neutralinos are just a component of the observed dark matter today. Because this analysis is indifferent to the fine-tuning of electroweak symmetry breaking, our findings also hold for many models of neutralino dark matter in the MSSM, NMSSM, and Split Supersymmetry. We have identified parameter regions at low tan β which sit in a double blind spot for both spin-independent and spin-dependent scattering. Interestingly, these low tan β regions are independently favored in the NMSSM and models of Split Supersymmetry which accommodate a Higgs mass near 125 GeV
Transverse oscillations of coronal loops
On 14 July 1998 TRACE observed transverse oscillations of a coronal loop generated by an external disturbance most probably caused by a solar flare. These oscillations were interpreted as standing fast kink waves in a magnetic flux tube. Firstly, in this review we embark on the discussion of the theory of waves and oscillations in a homogeneous straight magnetic cylinder with the particular emphasis on fast kink waves. Next, we consider the effects of stratification, loop expansion, loop curvature, non-circular cross-section, loop shape and magnetic twist.
An important property of observed transverse coronal loop oscillations is their fast damping. We briefly review the different mechanisms suggested for explaining the rapid damping phenomenon. After that we concentrate on damping due to resonant absorption. We describe the latest analytical results obtained with the use of thin transition layer approximation, and then compare these results with numerical findings obtained for arbitrary density variation inside the flux tube.
Very often collective oscillations of an array of coronal magnetic loops are observed. It is natural to start studying this phenomenon from the system of two coronal loops. We describe very recent analytical and numerical results of studying collective oscillations of two parallel homogeneous coronal loops.
The implication of the theoretical results for coronal seismology is briefly discussed. We describe the estimates of magnetic field magnitude obtained from the observed fundamental frequency of oscillations, and the estimates of the coronal scale height obtained using the simultaneous observations of the fundamental frequency and the frequency of the first overtone of kink oscillations.
In the last part of the review we summarise the most outstanding and acute problems in the theory of the coronal loop transverse oscillations
Indirect coupling between spins in semiconductor quantum dots
The optically induced indirect exchange interaction between spins in two
quantum dots is investigated theoretically. We present a microscopic
formulation of the interaction between the localized spin and the itinerant
carriers including the effects of correlation, using a set of canonical
transformations. Correlation effects are found to be of comparable magnitude as
the direct exchange. We give quantitative results for realistic quantum dot
geometries and find the largest couplings for one dimensional systems.Comment: 4 pages, 3 figure
A new life for sterile neutrino dark matter after the pandemic
We propose a novel mechanism to generate sterile neutrinos in theearly Universe, by converting ordinary neutrinos in scatteringprocesses . After initial production byoscillations, this leads to an exponential growth in the abundance. Weshow that such a production regime naturally occurs for self-interacting, and that this opens up significant new parameter space where make up all of the observed dark matter. Our results provide strong motivationto further push the sensitivity of X-ray line searches, and to improve onconstraints from structure formation.<br
A new life for sterile neutrino dark matter after the pandemic
We propose a novel mechanism to generate sterile neutrinos in theearly Universe, by converting ordinary neutrinos in scatteringprocesses . After initial production byoscillations, this leads to an exponential growth in the abundance. Weshow that such a production regime naturally occurs for self-interacting, and that this opens up significant new parameter space where make up all of the observed dark matter. Our results provide strong motivationto further push the sensitivity of X-ray line searches, and to improve onconstraints from structure formation.<br
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