X-ray bursters and the X-ray sources of the galactic bulge

Type 1 X-ray bursts, optical, infrared, and radio properties of the galactic bulge sources, are discussed. It was proven that these burst sources are neutron stars in low mass, close binary stellar systems. Several burst sources are found in globular clusters with high central densities. Optical type 1 X-ray bursts were observed from three sources. Type 2 X-ray bursts, observed from the Rapid Burster, are due to an accretion instability which converts gravitational potential energy into heat and radiation, which makes them of a fundamentally different nature from Type 1 bursts

A Density Spike on Astrophysical Scales from an N-Field Waterfall Transition

Hybrid inflation models are especially interesting as they lead to a spike in the density power spectrum on small scales, compared to the CMB, while also satisfying current bounds on tensor modes. Here we study hybrid inflation with $N$ waterfall fields sharing a global $SO(N)$ symmetry. The inclusion of many waterfall fields has the obvious advantage of avoiding topologically stable defects for $N>3$. We find that it also has another advantage: it is easier to engineer models that can simultaneously (i) be compatible with constraints on the primordial spectral index, which tends to otherwise disfavor hybrid models, and (ii) produce a spike on astrophysically large length scales. The latter may have significant consequences, possibly seeding the formation of astrophysically large black holes. We calculate correlation functions of the time-delay, a measure of density perturbations, produced by the waterfall fields, as a convergent power series in both $1/N$ and the field's correlation function $\Delta(x)$. We show that for large $N$, the two-point function is $\,\propto\Delta^2(|{\bf x}|)/N$ and the three-point function is $<\delta t({\bf x})\,\delta t({\bf y})\,\delta t({\bf 0})>\,\propto\Delta(|{\bf x}-{\bf y}|)\Delta(|{\bf x}|)\Delta(|{\bf y}|)/N^2$. In accordance with the central limit theorem, the density perturbations on the scale of the spike are Gaussian for large $N$ and non-Gaussian for small $N$.Comment: 15 pages in double column format, 6 figures. V2: Further clarifications, updated to coincide with version published in Physics Letters

Comparative pelvic development of the axolotl (Ambystoma mexicanum) and the Australian lungfish (Neoceratodus forsteri): Conservation and innovation across the fish-tetrapod transition

Background: The fish-tetrapod transition was one of the major events in vertebrate evolution and was enabled by many morphological changes. Although the transformation of paired fish fins into tetrapod limbs has been a major topic of study in recent years, both from paleontological and comparative developmental perspectives, the interest has focused almost exclusively on the distal part of the appendage and in particular the origin of digits. Relatively little attention has been paid to the transformation of the pelvic girdle from a small unipartite structure to a large tripartite weight-bearing structure, allowing tetrapods to rely mostly on their hindlimbs for locomotion. In order to understand how the ischium and the ilium evolved and how the acetabulum was reoriented during this transition, growth series of the Australian lungfish Neoceratodus forsteri and the Mexican axolotl Ambystoma mexicanum were cleared and stained for cartilage and bone and immunostained for skeletal muscles. In order to understand the myological developmental data, hypotheses about the homologies of pelvic muscles in adults of Latimeria, Neoceratodus and Necturus were formulated based on descriptions from the literature of the coelacanth (Latimeria), the Australian Lungfish (Neoceratodus) and a salamander (Necturus).Results: In the axolotl and the lungfish, the chondrification of the pelvic girdle starts at the acetabula and progresses anteriorly in the lungfish and anteriorly and posteriorly in the salamander. The ilium develops by extending dorsally to meet and connect to the sacral rib in the axolotl. Homologous muscles develop in the same order with the hypaxial musculature developing first, followed by the deep, then the superficial pelvic musculature.Conclusions: Development of the pelvic endoskeleton and musculature is very similar in Neoceratodus and Ambystoma. If the acetabulum is seen as being a fixed landmark, the evolution of the ischium only required pubic pre-chondrogenic cells to migrate posteriorly. It is hypothesized that the iliac process or ridge present in most tetrapodomorph fish is the precursor to the tetrapod ilium and that its evolution mimicked its development in modern salamanders

Model atmospheres and X-ray spectra of iron-rich bursting neutron stars. II. Iron rich Comptonized Spectra

This paper presents the set of plane-parallel model atmosphere equations for a very hot neutron star (X-ray burst source). The model equations assume both hydrostatic and radiative equilibrium, and the equation of state of an ideal gas in local thermodynamic equilibrium (LTE). The equation of radiative transfer includes terms describing Compton scattering of photons on free electrons in fully relativistic thermal motion, for photon energies approaching m_e *c^2. Model equations take into account many bound-free and free-free energy-dependent opacities of hydrogen, helium, and the iron ions, and also a dozen bound-bound opacities for the highest ions of iron. We solve model equations by partial linearisation and the technique of variable Eddington factors. Large grid of H-He-Fe model atmospheres of X-ray burst sources has been computed for 10^7 < T_eff < 3*10^7 K, a wide range of surface gravity, and various iron abundances. We demonstrate that the spectra of X-ray bursters with iron present in the accreting matter differ significantly from pure H-He spectra (published in an earlier paper), and also from blackbody spectra. Comptonized spectra with significant iron abundance are generally closer to blackbody spectra than spectra of H-He atmospheres. The ratio of color to effective temperatures in our grid always remains in the range 1.2 < T_c/T_eff < 1.85. The present grid of model atmospheres and theoretical X-ray spectra will be used to determine the effective temperatures, radii and M/R ratios of bursting neutron stars from observational data.Comment: A&A in prin

A Search for High-Excitation Redshift Systems in the Absorption Spectra of Five Quasars

We have searched the absorption spectra of five quasars for the presence of redshift system dominated by the highly ionized doublets C iv, N v, and O vi, which could be the strongest lines produced by absorbing clouds with collisional ionization temperatures between 10^5 ° and 10^6 °K. There is at most marginal evidence for one such system apiece in the spectra of PHL 957 and 4C 05.34, which are the two quasars with the largest known emission redshifts. Highly ionized redshift systems of this type are not widespread among the five quasars we investigated; the number of redshifts found in the observed spectra is not significantly larger than the number found in similar random-number spectra. Less than 5 percent of the observed absorption lines are identified in a statistically significant way by redshift systems of this type

Fossil imprint of a powerful flare at the galactic center along the magellanic stream

The Fermi satellite discovery of the gamma-ray emitting bubbles extending 50° (10 kpc) from the Galactic center has revitalized earlier claims that our Galaxy has undergone an explosive episode in the recent past. We now explore a new constraint on suc

An upper limit to the dry merger rate at <z> ~ 0.55

We measure the fraction of Luminous Red Galaxies (LRGs) in dynamically close pairs (with projected separation less than 20 $h^{-1}$ kpc and velocity difference less than 500 km s$^{-1}$) to estimate the dry merger rate for galaxies with $-23 < M(r)_{k+e,z=0.2} +5 \log h < -21.5$ and $0.45 < z < 0.65$ in the 2dF-SDSS LRG and QSO (2SLAQ) redshift survey. For galaxies with a luminosity ratio of $1:4$ or greater we determine a $5\sigma$ upper limit to the merger fraction of 1.0% and a merger rate of $< 0.8 \times 10^{-5}$ Mpc$^{-3}$ Gyr$^{-1}$ (assuming that all pairs merge on the shortest possible timescale set by dynamical friction). This is significantly smaller than predicted by theoretical models and suggests that major dry mergers do not contribute to the formation of the red sequence at $z < 0.7$.Comment: 8 pages emulateapj style, 3 figures, accepted by AJ (March 2010