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 $v$ increases with radius $r$ according to $v=v_\infty(1-r_\star/r)^\beta$, with$\beta\approx0.75$ and $v_\infty\approx1420$ km s$^{-1}$.The microturbulent velocity was $\approx160$ km s$^{-1}$. Our fit implies a ratio of X-ray luminosity to wind mass-loss rate of L$_{X,38}/\dot M_{-6} \approx 0.33$, measured at $\dot M_{-6}$ = 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$_x$ 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 for more informatio