The X-ray spectral evolution of Cyg X-2 in the framework of bulk Comptonization

We used the newly developed thermal plus bulk Comptonization model comptb to investigate the spectral evolution of the neutron star LMXB Cyg X-2 along its Z-track. We selected a single source in order to trace in a quantitative way the evolution of the physical parameters of the model. We analyzed archival broad-band BeppoSAX spectra of Cyg X-2. Five broad-band spectra have been newly extracted according to the source position in the Z-track described in the colour-colour and colour-intensity diagrams. We have fitted the spectra of the source with two comptb components. The first one, with bulk parameter delta=0, dominates the overall source broad-band spectrum and its origin is related to thermal upscattering (Comptonization) of cold seed photons off warm electrons in high-opacity enviroment. We attribute the origin of these seed photons to the part of the disk which illuminates the outer coronal region (transition layer) located between the accretion disk itself and the neutron star surface. This thermal component is roughly constant with time and with inferred mass accretion rate. The second comptb model describes the overall Comptonization (thermal plus bulk, delta > 0) of hotter seed photons which come from both the inner transition layer and from the neutron star surface. The appearance of this component in the colour-colour or hardness-intensity diagram is more pronounced in the horizontal branch and is progressively disappearing towards the normal branch, where a pure blackbody spectrum is observed. The spectral evolution of Cyg X-2 is studied and interpreted in terms of changes in the innermost environmental conditions of the system, leading to a variable thermal-bulk Comptonization efficiency.Comment: 10 pages, 7 figures, accepted for publication in A&

Variable-Depth Liner Evaluation Using Two NASA Flow Ducts

Four liners are investigated experimentally via tests in the NASA Langley Grazing Flow Impedance Tube. These include an axially-segmented liner and three liners that use reordering of the chambers. Chamber reordering is shown to have a strong effect on the axial sound pressure level profiles, but a limited effect on the overall attenuation. It is also shown that bent chambers can be used to reduce the liner depth with minimal effects on the attenuation. A numerical study is also conducted to explore the effects of a planar and three higher-order mode sources based on the NASA Langley Curved Duct Test Rig geometry. A four-segment liner is designed using the NASA Langley CDL code with a Python-based optimizer. Five additional liner designs, four with rearrangements of the first liner segments and one with a redistribution of the individual chambers, are evaluated for each of the four sources. The liner configuration affects the sound pressure level profile much more than the attenuation spectra for the planar and first two higher-order mode sources, but has a much larger effect on the SPL profiles and attenuation spectra for the last higher-order mode source. Overall, axially variable-depth liners offer the potential to provide improved fan noise reduction, regardless of whether the axially variable depths are achieved via a distributed array of chambers (depths vary from chamber to chamber) or a group of zones (groups of chambers for which the depth is constant)

Spectral variability of quasars from multi-epoch photometric data in the Sloan Digital Sky Survey Stripe 82

We present a new approach to analysing the dependence of quasar variability on rest-frame wavelengths. We exploited the spectral archive of the Sloan Digital Sky Survey (SDSS) to create a sample of more than 9000 quasars in the Stripe 82. The quasar catalogue was matched with the Light Motion Curve Catalogue for SDSS Stripe 82 and individual first-order structure functions were computed. The structure functions are used to create a variability indicator that is related to the same intrinsic timescales for all quasars (1 to 2 yr in the rest frame). We study the variability ratios for adjacent SDSS filter bands as a function of redshift. While variability is almost always stronger in the bluer passband compared to the redder, the variability ratio depends on whether strong emission lines contribute to either one band or the other. The variability ratio-redshift relations resemble the corresponding colour index-redshift relations. From the comparison with Monte Carlo simulations of variable quasar spectra we find that the observed variability ratio-redshift relations are closely fitted assuming that (a) the r.m.s. fluctuation of the quasar continuum follows a power law-dependence on the intrinsic wavelength with an exponent -2 (i.e., bluer when brighter) and (b) the variability of the emission line flux is only about 10% of that of the underlying continuum. These results, based upon the photometry of more than 8000 quasars, confirm the previous findings by Wilhite et al. (2005) from 315 quasars with repeated SDSS spectroscopy. Finally, we find that quasars with unusual spectra and weak emission lines tend to have less variability than conventional quasars. This trend is opposite to what is expected from the dilution effect of variability due to line emission and may be indicative of high Eddington ratios in these unconventinal quasars.Comment: Accepted for publication in Astronomy and Astrophysic

A Runaway Black Hole in COSMOS: Gravitational Wave or Slingshot Recoil?

We present a detailed study of a peculiar source in the COSMOS survey at z=0.359. Source CXOCJ100043.1+020637 (CID-42) presents two compact optical sources embedded in the same galaxy. The distance between the 2, measured in the HST/ACS image, is 0.495" that, at the redshift of the source, corresponds to a projected separation of 2.46 kpc. A large (~1200 km/s) velocity offset between the narrow and broad components of Hbeta has been measured in three different optical spectra from the VLT/VIMOS and Magellan/IMACS instruments. CID-42 is also the only X-ray source having in its X-ray spectra a strong redshifted broad absorption iron line, and an iron emission line, drawing an inverted P-Cygni profile. The Chandra and XMM data show that the absorption line is variable in energy by 500 eV over 4 years and that the absorber has to be highly ionized, in order not to leave a signature in the soft X-ray spectrum. That these features occur in the same source is unlikely to be a coincidence. We envisage two possible explanations: (1) a gravitational wave recoiling black hole (BH), caught 1-10 Myr after merging, (2) a Type 1/ Type 2 system in the same galaxy where the Type 1 is recoiling due to slingshot effect produced by a triple BH system. The first possibility gives us a candidate gravitational waves recoiling BH with both spectroscopic and imaging signatures. In the second case, the X-ray absorption line can be explained as a BAL-like outflow from the foreground nucleus (a Type 2 AGN) at the rearer one (a Type 1 AGN), which illuminates the otherwise undetectable wind, giving us the first opportunity to show that fast winds are present in obscured AGN.Comment: 13 figures; submitted to ApJ. Sent back to the referee after the first interaction and awaiting the final comment

QCD and Light-Front Holography

The soft-wall AdS/QCD model, modified by a positive-sign dilaton metric, leads to a remarkable one-parameter description of nonperturbative hadron dynamics. The model predicts a zero-mass pion for zero-mass quarks and a Regge spectrum of linear trajectories with the same slope in the leading orbital angular momentum $L$ of hadrons and the radial quantum number $N$. Light-Front Holography maps the amplitudes which are functions of the fifth dimension variable $z$ of anti-de Sitter space to a corresponding hadron theory quantized on the light front. The resulting Lorentz-invariant relativistic light-front wave equations are functions of an invariant impact variable $\zeta$ which measures the separation of the quark and gluonic constituents within the hadron at equal light-front time. The result is a semi-classical frame-independent first approximation to the spectra and light-front wavefunctions of meson and baryon light-quark bound states, which in turn predict the behavior of the pion and nucleon form factors. The effects of chiral symmetry breaking increase as one goes toward large interquark separation, consistent with spectroscopic data, and the hadron eigenstates generally have components with different orbital angular momentum; e.g., the proton eigenstate in AdS/QCD with massless quarks has L=0 and L=1 light-front Fock components with equal probability. The soft-wall model also predicts the form of the non-perturbative effective coupling $\alpha_s^{AdS}(Q)$ which agrees with the effective coupling extracted from the Bjorken sum rule. The AdS/QCD model can be systematically improved by using its complete orthonormal solutions to diagonalize the full QCD light-front Hamiltonian or by applying the Lippmann-Schwinger method in order to systematically include the QCD interaction terms. A new perspective on quark and gluon condensates is also reviewed.Comment: Invited talk, presented by SJB at the 50th Crakow School, Zakopane, Poland; final version to appear in proceeding

Dielectric response of modified Hubbard models with neutral-ionic and Peierls transitions

The dipole P(F) of systems with periodic boundary conditions (PBC) in a static electric field F is applied to one-dimensional Peierls-Hubbard models for organic charge-transfer (CT) salts. Exact results for P(F) are obtained for finite systems of N = 14 and 16 sites that are almost converged to infinite chains in deformable lattices subject to a Peierls transition. The electronic polarizability per site, \alpha_{el} = (\partial P/\partial F)_0, of rigid stacks with alternating transfer integrals t(1 +/- \delta) diverges at the neutral-ionic transition for \delta = 0 but remains finite for \delta > 0 in dimerized chains. The Peierls or dimerization mode couples to charge fluctuations along the stack and results in large vibrational contributions, \alpha_{vib}, that are related to \partial P/\partial \delta and that peak sharply at the Peierls transition. The extension of P(F) to correlated electronic states yields the dielectric response \kappa of models with neutral-ionic or Peierls transitions, where \kappa peaks >100 are found with parameters used previously for variable ionicity \rho and vibrational spectra of CT salts. The calculated \kappa accounts for the dielectric response of CT salts based on substituted TTFs (tetrathiafulvalene) and substituted CAs (chloranil). The role of lattice stiffness appears clearly in models: soft systems have a Peierls instability at small \rho and continuous crossover to large \rho, while stiff stacks such as TTF-CA have a first-order transition with discontinuous \rho that is both a neutral-ionic and Peierls transition. The transitions are associated with tuning the electronic ground state of insulators via temperature or pressure in experiments, or via model parameters in calculations.Comment: 10 pages, 9 figures; J.Chem.Phys., in pres