1,093 research outputs found
Renormalization Group Study of Magnetic Catalysis in the 3d Gross-Neveu Model
Magnetic catalysis describes the enhancement of symmetry breaking quantum
fluctuations in chirally symmetric quantum field theories by the coupling of
fermionic degrees of freedom to a magnetic background configuration. We use the
functional renormalization group to investigate this phenomenon for interacting
Dirac fermions propagating in (2+1)-dimensional spacetime, described by the
Gross-Neveu model. We identify pointlike operators up to quartic fermionic
terms that can be generated in the renormalization group flow by the presence
of an external magnetic field. We employ the beta function for the fermionic
coupling to quantitatively analyze the field dependence of the induced spectral
gap. Within our pointlike truncation, the renormalization group flow provides a
simple picture for magnetic catalysis.Comment: 14 pages, 6 figures, typos correcte
Ultraviolet observations of the X-ray photoionized wind of Cygnus X-1 during X-ray soft/high state
(Shortened) Ultraviolet observations of the black hole X-ray binary Cygnus
X-1 were obtained using the STIS on HSTubble. We detect P Cygni line features
show strong, broad absorption components when the X-ray source is behind the
companion star and noticeably weaker absorption when the X-ray source is
between us and the companion star. We fit the P Cygni profiles using the SEI
method applied to a spherically symmetric stellar wind subject to X-ray
photoionization from the black hole. The Si IV doublet provides the most
reliable estimates of the parameters of the wind and X-ray illumination. The
velocity increases with radius according to
, with and
km s.The microturbulent velocity was
km s. Our fit implies a ratio of X-ray luminosity to wind mass-loss rate
of L, measured at = 4.8. Our
models determine parameters that may be used to estimate the accretion rate
onto the black hole and independently predict the X-ray luminosity. Our
predicted L matches that determined by contemporaneous RXTE ASM remarkably
well, but is a factor of 3 lower than the rate according to
Bondi-Hoyle-Littleton spherical wind accretion. We suggest that some of the
energy of accretion may go into powering a jet.Comment: 34 pages, 21 figures, 4 tables, accepted for publication in Ap
Photon propagation in a cold axion background with and without magnetic field
A cold relic axion condensate resulting from vacuum misalignment in the early
universe oscillates with a frequency m, where m is the axion mass. We determine
the properties of photons propagating in a simplified version of such a
background where the sinusoidal variation is replaced by a square wave profile.
We prove that previous results that indicated that charged particles moving
fast in such a background radiate, originally derived assuming that all momenta
involved were much larger than m, hold for long wavelengths too. We also
analyze in detail how the introduction of a magnetic field changes the
properties of photon propagation in such a medium. We briefly comment on
possible astrophysical implications of these results.Comment: 17 pages, 4 figures, revised version includes an extended discussion
on physical implication
Renormalization flow of QED
We investigate textbook QED in the framework of the exact renormalization
group. In the strong-coupling region, we study the influence of
fluctuation-induced photonic and fermionic self-interactions on the
nonperturbative running of the gauge coupling. Our findings confirm the
triviality hypothesis of complete charge screening if the ultraviolet cutoff is
sent to infinity. Though the Landau pole does not belong to the physical
coupling domain owing to spontaneous chiral symmetry breaking (chiSB), the
theory predicts a scale of maximal UV extension of the same order as the Landau
pole scale. In addition, we verify that the chiSB phase of the theory which is
characterized by a light fermion and a Goldstone boson also has a trivial
Yukawa coupling.Comment: 4 pages, 1 figur
Clues about the scarcity of stripped-envelope stars from the evolutionary state of the sdO+Be binary system phi Persei
Stripped-envelope stars (SESs) form in binary systems after losing mass
through Roche-lobe overflow. They bear astrophysical significance as sources of
UV and ionizing radiation in older stellar populations and, if sufficiently
massive, as stripped supernova progenitors. Binary evolutionary models predict
them to be common, but only a handful of subdwarfs (i.e., SESs) with B-type
companions are known. This could be the result of observational biases
hindering detection, or an incorrect understanding of binary evolution. We
reanalyze the well-studied post-interaction binary phi Persei. Recently, new
data improved the orbital solution of the system, which contains a ~1.2 Msun
SES and a rapidly rotating ~9.6 Msun Be star. We compare with an extensive grid
of evolutionary models using a Bayesian approach and find initial masses of the
progenitor of 7.2+/-0.4 Msun for the SES and 3.8+/-0.4 Msun for the Be star.
The system must have evolved through near-conservative mass transfer. These
findings are consistent with earlier studies. The age we obtain, 57+/-9 Myr, is
in excellent agreement with the age of the alpha Persei cluster. We note that
neither star was initially massive enough to produce a core-collapse supernova,
but mass exchange pushed the Be star above the mass threshold. We find that the
subdwarf is overluminous for its mass by almost an order of magnitude, compared
to the expectations for a helium core burning star. We can only reconcile this
if the subdwarf is in a late phase of helium shell burning, which lasts only
2-3% of the total lifetime as a subdwarf. This could imply that up to ~50 less
evolved, dimmer subdwarfs exist for each system similar to phi Persei. Our
findings can be interpreted as a strong indication that a substantial
population of SESs indeed exists, but has so far evaded detection because of
observational biases and lack of large-scale systematic searches.Comment: 11 pages, 5 figures, accepted for publication in A&
Pressure-induced isostructural phase transition of metal-doped silicon clathrates
We propose an atomistic model for the pressure-induced isostructural phase
transition of metal-doped silicon clathrates, Ba8Si46 and K8Si46, that has been
observed at 14 GPa and 23 GPa, respectively. The model explains successfully
the equation of state, transition pressure, change of Raman spectra and
dependence on the doped cations as well as the effects of substituting Si(6c)
atoms with noble metals.Comment: 5 pages, two coumn, 5 figures. See http://www.iitaka.org/down.html
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