An XMM-Newton observation of Ton S180: Constraints on the continuum emission in ultrasoft Seyfert galaxies

We present an XMM-Newton observation of the bright, narrow-line, ultrasoft Seyfert 1 galaxy Ton S180. The 0.3-10 keV X-ray spectrum is steep and curved, showing a steep slope above 2.5 keV (Gamma ~ 2.3) and a smooth, featureless excess of emission at lower energies. The spectrum can be adequately parameterised using a simple double power-law model. The source is strongly variable over the course of the observation but shows only weak spectral variability, with the fractional variability amplitude remaining approximately constant over more than a decade in energy. The curved continuum shape and weak spectral variability are discussed in terms of various physical models for the soft X-ray excess emission, including reflection off the surface of an ionised accretion disc, inverse-Compton scattering of soft disc photons by thermal electrons, and Comptonisation by electrons with a hybrid thermal/non-thermal distribution. We emphasise the possibility that the strong soft excess may be produced by dissipation of accretion energy in the hot, upper atmosphere of the putative accretion disc.Comment: 9 pages, accepted for publication in MNRA

Evidence for the Galactic X-ray Bulge II

A mosaic of 5 \ros~PSPC pointed observations in the Galactic plane ($l\sim25^{\circ}$) reveals X-ray shadows in the $0.5-2.0$ keV band cast by distant molecular clouds. The observed on-cloud and off-cloud X-ray fluxes indicate that $\sim15$% and $\sim37$% of the diffuse X-ray background in this direction in the \tq~keV and 1.5 keV bands, respectively, originates behind the molecular gas which is located at $\sim$3 kpc from the Sun. The implication of the derived background X-ray flux beyond the absorbing molecular cloud is consistent with, and lends further support to recent observations of a Galactic X-ray bulge.Comment: 19 pages, 5 figures, 2 table

The North Ecliptic Pole Supercluster

We have used the ROSAT All-Sky Survey to detect a known supercluster at z=0.087 in the North Ecliptic Pole region. The X-ray data greatly improve our understanding of this supercluster's characteristics, approximately doubling our knowledge of the structure's spatial extent and tripling the cluster/group membership compared to the optical discovery data. The supercluster is a rich structure consisting of at least 21 galaxy clusters and groups, 12 AGN, 61 IRAS galaxies, and various other objects. A majority of these components were discovered with the X-ray data, but the supercluster is also robustly detected in optical, IR, and UV wavebands. Extending 129 x 102 x 67 (1/h50 Mpc)^3, the North Ecliptic Pole Supercluster has a flattened shape oriented nearly edge-on to our line-of-sight. Owing to the softness of the ROSAT X-ray passband and the deep exposure over a large solid angle, we have detected for the first time a significant population of X-ray emitting galaxy groups in a supercluster. These results demonstrate the effectiveness of X-ray observations with contiguous coverage for studying structure in the Universe.Comment: Accepted for publication in The Astrophysical Journal; 5 pages with 2 embedded figures; uses emulateapj.sty; For associated animations, see http://www.ifa.hawaii.edu/~mullis/nep3d.html; A high-resolution color postscript version of the full paper is available at http://www.ifa.hawaii.edu/~mullis/papers/nepsc.ps.g

Rotational invariance and order-parameter stiffness in frustrated quantum spin systems

We compute, within the Schwinger-boson scheme, the Gaussian-fluctuation corrections to the order-parameter stiffness of two frustrated quantum spin systems: the triangular-lattice Heisenberg antiferromagnet and the J1-J2 model on the square lattice. For the triangular-lattice Heisenberg antiferromagnet we found that the corrections weaken the stiffness, but the ground state of the system remains ordered in the classical 120 spiral pattern. In the case of the J1-J2 model, with increasing frustration the stiffness is reduced until it vanishes, leaving a small window 0.53 < J2/J1 < 0.64 where the system has no long-range magnetic order. In addition, we discuss several methodological questions related to the Schwinger-boson approach. In particular, we show that the consideration of finite clusters which require twisted boundary conditions to fit the infinite-lattice magnetic order avoids the use of ad hoc factors to correct the Schwinger-boson predictions.Comment: 9 pages, Latex, 6 figures as ps files, fig.1 changed and minor text corrections, to appear in Phys.Rev.

Spin wave analysis to the spatially-anisotropic Heisenberg antiferromagnet on triangular lattice

We study the phase diagram at T=0 of the antiferromagnetic Heisenberg model on the triangular lattice with spatially-anisotropic interactions. For values of the anisotropy very close to J_alpha/J_beta=0.50, conventional spin wave theory predicts that quantum fluctuations melt the classical structures, for S=1/2. For the regime J_beta<J_alpha, it is shown that the incommensurate spiral phases survive until J_beta/J_alpha=0.27, leaving a wide region where the ground state is disordered. The existence of such nonmagnetic states suggests the possibility of spin liquid behavior for intermediate values of the anisotropy.Comment: Revised version, 4 pages, Latex (twocolumn), 4 figures as eps files. To appear in PR

Quasiparticle vanishing driven by geometrical frustration

We investigate the single hole dynamics in the triangular t-J model. We study the structure of the hole spectral function, assuming the existence of a 120 magnetic Neel order. Within the self-consistent Born approximation (SCBA) there is a strong momentum and t sign dependence of the spectra, related to the underlying magnetic structure and the particle-hole asymmetry of the model. For positive t, and in the strong coupling regime, we find that the low energy quasiparticle excitations vanish outside the neighbourhood of the magnetic Goldstone modes; while for negative t the quasiparticle excitations are always well defined. In the latter, we also find resonances of magnetic origin whose energies scale as (J/t)^2/3 and can be identified with string excitations. We argue that this complex structure of the spectra is due to the subtle interplay between magnon-assisted and free hopping mechanisms. Our predictions are supported by an excellent agreement between the SCBA and the exact results on finite size clusters. We conclude that the conventional quasiparticle picture can be broken by the effect of geometrical magnetic frustration.Comment: 6 pages, 7 figures. Published versio

The Heisenberg model on the 1/5-depleted square lattice and the CaV4O9 compound

We investigate the ground state structure of the Heisenberg model on the 1/5-depleted square lattice for arbitrary values of the first- and second-neighbor exchange couplings. By using a mean-field Schwinger-boson approach we present a unified description of the rich ground-state diagram, which include the plaquette and dimer resonant-valence-bond phases, an incommensurate phase and other magnetic orders with complex magnetic unit cells. We also discuss some implications of ours results for the experimental realization of this model in the CaV4O9 compound.Comment: 4 pages, Latex, 7 figures included as eps file

Geometrical Distance Determination using Type I X-ray Bursts

With the excellent angular resolution of the Chandra X-ray Observatory, it is possible to geometrically determine the distance to variable Galactic sources, based on the phenomenon that scattered radiation appearing in the X-ray halo has to travel along a slightly longer path than the direct, unscattered radiation. By measuring the delayed variability, constraints on the source distance can be obtained if the halo brightness is large enough to dominate the point spread function (PSF) and to provide sufficient statistics. The distance to Cyg X-3, which has a quasi-sinusoidal light curve, has been obtained with this approach by Predehl et al. Here we examine the feasibility of using the delayed signature of type I X-ray bursts as distance indicators. We use simulations of delayed X-ray burst light curves in the halo to find that the optimal annular region and energy band for a distance measurement with a grating observation is roughly 10-50" and 1-5 keV respectively, assuming Chandra's effective area and PSF, uniformly distributed dust, the input spectrum and optical depth to GX 13+1, and the Weingartner & Draine interstellar grain model. We find that the statistics are dominated by Poisson noise rather than systematic uncertainties, e.g., the PSF contribution to the halo. Using Chandra, a distance measurement to such a source at 4 (8) kpc could be made to about 23% (30%) accuracy with a single burst with 68% confidence. By stacking many bursts, a reasonable estimate of systematic errors limit the distance measurement to about 10% accuracy.Comment: 7 pages, 4 figures; Accepted for publication in Ap