The broad-band X-ray spectrum of the dipping Low Mass X-ray Binary EXO0748--676

We present results of a 0.1-100 keV BeppoSAX observation of the dipping LMXRB EXO 0748-676 performed in 2000 November. During the observation the source exhibited X-ray eclipses, type I X-ray bursts and dipping activity over a wide range of orbital phases. The 0.1-100keV "dip-free"(ie. dipping and eclipsing intervals excluded) spectrum is complex,especially at low-energies where a soft excess is present. Two very different spectral models give satisfactory fits. The first is the progressive covering model, consisting of separately absorbed black body and cut-off power-law components.The second model is an absorbed cut-off power-law together with a moderately ionized absorber with a sub-solar abundance of Fe and a 2.13 keV absorption feature (tentatively identified with Si xiii). This ionized absorber may be the same feature as seen by Chandra during dips from EXO 0748-676.Comment: 7 pages, 5 figures, paper accepted for publication in Astronomy and Astrophysic

Chandra and XMM Observations of the ADC Source 0921-630

We analyze observations of the low mass X-ray binary 2S0921-63 obtained with the gratings and CCDs on Chandra and XMM. This object is a high inclination system showing evidence for an accretion disk corona (ADC). Such a corona has the potential to constrain the properties of the heated accretion disk in this system, and other LMXBs by extension. We find evidence for line emission which is generally consistent with that found by previous experiments, although we are able to detect more lines. For the first time in this source, we find that the iron K line has multiple components. We set limits on the line widths and velocity offsets, and we fit the spectra to photoionization models and discuss the implications for accretion disk corona models. For the first time in any ADC source we use these fits, together with density constraints based on the O VII line ratio, in order to constrain the flux in the medium-ionization region of the ADC. Under various assumptions about the source luminosity this constrains the location of the emitting region. These estimates, together with estimates for the emission measure, favor a scenario in which the intrinsic luminosity of the source is comparable to what we observe.Comment: 40 pages, 11 figures submitted to Ap.

Lattice dynamics and the electron-phonon interaction in Ca2_2RuO4_4

We present a Raman scattering study of Ca$_2$RuO$_4$, in which we investigate the temperature-dependence of the lattice dynamics and the electron-phonon interaction below the metal-insulator transition temperature ({\it T}$_{\rm MI}$). Raman spectra obtained in a backscattering geometry with light polarized in the ab-plane reveal 9 B$_{1g}$ phonon modes (140, 215, 265, 269, 292, 388, 459, 534, and 683 cm$^{-1}$) and 9 A$_g$ phonon modes (126, 192, 204, 251, 304, 322, 356, 395, and 607 cm$^{-1}$) for the orthorhombic crystal structure (Pbca$-$D$_{2h}^{15}$). With increasing temperature toward {\it T}$_{\rm MI}$, the observed phonon modes shift to lower energies and exhibit reduced spectral weights, reflecting structural changes associated with the elongation of the RuO$_6$ octahedra. Interestingly, the phonons exhibit significant increases in linewidths and asymmetries for {\it T} $>$ {\it T}$_{\rm N}$. These results indicate that there is an increase in the effective number of electrons and the electron-phonon interaction strengths as the temperature is raised through {\it T}$_{\rm N}$, suggesting the presence of orbital fluctuations in the temperature regime {\it T}$_{\rm N}$ $<$ {\it T} $<$ {\it T}$_{\rm MI}$.Comment: 6 pages, 4 figure

Nonlinear Spin Dynamics in Ferromagnets with Electron-Nuclear Coupling

Nonlinear spin motion in ferromagnets is considered with nonlinearity due to three factors: (i) the sample is prepared in a strongly nonequilibrium state, so that evolution equations cannot be linearized as would be admissible for spin motion not too far from equilibrium, (ii) the system considered consists of interacting electron and nuclear spins coupled with each other via hyperfine forces, and (iii) the sample is inserted into a coil of a resonant electric circuit producing a resonator feedback field. Due to these nonlinearities, coherent motion of spins can develop, resulting in their ultrafast relaxation. A complete analysis of mechanisms triggering such a coherent motion is presented. This type of ultrafast coherent relaxation can be used for studying intrinsic properties of magnetic materials.Comment: 1 file, LaTex, 23 page

Absorption features in the spectra of X-ray bursting neutron stars

The discovery of photospheric absorption lines in XMM-Newton spectra of the X-ray bursting neutron star in EXO0748-676 by Cottam and collaborators allows us to constrain the neutron star mass-radius ratio from the measured gravitational redshift. A radius of R=9-12km for a plausible mass range of M=1.4-1.8Msun was derived by these authors. It has been claimed that the absorption features stem from gravitationally redshifted (z=0.35) n=2-3 lines of H- and He-like iron. We investigate this identification and search for alternatives. We compute LTE and non-LTE neutron-star model atmospheres and detailed synthetic spectra for a wide range of effective temperatures (effective temperatures of 1 - 20MK) and different chemical compositions. We are unable to confirm the identification of the absorption features in the X-ray spectrum of EXO0748-676 as n=2-3 lines of H- and He-like iron (Fe XXVI and Fe XXV). These are subordinate lines that are predicted by our models to be too weak at any effective temperature. It is more likely that the strongest feature is from the n=2-3 resonance transition in Fe XXIV with a redshift of z=0.24. Adopting this value yields a larger neutron star radius, namely R=12-15km for the mass range M=1.4-1.8Msun, favoring a stiff equation-of-state and excluding mass-radius relations based on exotic matter. Combined with an estimate of the stellar radius R>12.5km from the work of Oezel and collaborators, the z=0.24 value provides a minimum neutron-star mass of M>1.48Msun, instead of M>1.9Msun, when assuming z=0.35.Comment: 8 pages, 17 figure

Discovery of narrow X-ray absorption features from the dipping low-mass X-ray binary X 1624-490 with XMM-Newton

We report the discovery of narrow X-ray absorption features from the dipping low-mass X-ray binary X 1624-490 during an XMM-Newton observation in 2001 February. The features are identified with the K alpha absorption lines of Fe xxv and Fe xxvi and have energies of 6.72 +/- 0.03 keV and 7.00 +/- 0.02 keV and equivalent widths (EWs) of -7.5 +1.7 -6.3 eV and -16.6 +1.9 -5.9 eV, respectively. The EWs show no obvious dependence on orbital phase, except during a dip, and correspond to a column of greater than 10^17.3 Fe atom /cm2. In addition, faint absorption features tentatively identified with Ni xxvii K alpha and Fe xxvi K beta may be present. A broad emission feature at 6.58 +0.07 -0.04 keV with an EW of 78 +19 -6 eV is also evident. This is probably the 6.4 keV feature reported by earlier missions since fitting a single Gaussian to the entire Fe-K region gives an energy of 6.39 +0.03 -0.04 keV. A deep absorption feature is present during the dip with an energy consistent with Fe xxv K alpha. This is the second dipping LMXRB source from which narrow Fe absorption features have been observed. Until recently the only X-ray binaries known to exhibit narrow X-ray absorption lines were two superluminal jet sources and it had been suggested that these features are related to the jet formation mechanism. It now appears likely that ionized absorption features may be common characteristics of accreting systems with accretion disks.Comment: 6 pages. To appear in A&

Radius-expansion burst spectra from 4U 1728-34: an ultracompact binary?

Recent theoretical and observational studies have shown that ashes from thermonuclear burning may be ejected during radius-expansion bursts, giving rise to photoionisation edges in the X-ray spectra. We report a search for such features in Chandra spectra observed from the low-mass X-ray binary 4U 1728-34. We analysed the spectra from four radius-expansion bursts detected in 2006 July, and two in 2002 March, but found no evidence for discrete features. We estimate upper limits for the equivalent widths of edges of a few hundred eV, which for the moderate temperatures observed during the bursts, are comparable with the predictions. During the 2006 July observation 4U 1728-34 exhibited weak, unusually frequent bursts (separated by <2 hr in some cases), with profiles and alpha-values characteristic of hydrogen-poor fuel. Recurrence times as short as those measured are insufficient to exhaust the accreted hydrogen at solar composition, suggesting that the source accretes hydrogen deficient fuel, for example from an evolved donor. The detection for the first time of a 10.77 min periodic signal in the persistent intensity, perhaps arising from orbital modulation, supports this explanation, and suggests that this system is an ultracompact binary similar to 4U 1820-30.Comment: 9 pages, 6 figures, accepted by Ap

BeppoSAX observation of the eclipsing dipping X-ray binary X1658-298

Results of a 2000 August 12-13 BeppoSAX observation of the 7.1 hr eclipsing, dipping, bursting, transient, low-mass X-ray binary (LMXRB) X1658-298 are presented. The spectrum outside of eclipses, dips and bursts can be modeled by the combination of a soft disk-blackbody and a harder Comptonized component with a small amount (1.3 10E21 atom/cm2) of low-energy absorption. In contrast, an RXTE observation 18 months earlier during the same outburst, measured an absorption of 5.0 10E22 atom/cm2. Such a change is consistent with a thinning of the accretion disk as the outburst progresses. Structured residuals from the best-fit spectral model are present which are tentatively identified with Ne-K/Fe-L and Fe-K shell emission. The spectral changes during dips are complex and may be modeled by a strong (~3 10E23 atom/cm2) increase in absorption of the Comptonized component only, together with reductions in normalizations of both spectral components. This behavior is in contrast to the ``complex continuum'' model for X-ray dip sources, where the softer blackbody component rapidly suffers strong absorption. It is however, similar to that found during recent XMM-Newton observations of the eclipsing, dipping, LMXRB EXO0748-676.Comment: 11 pages. Accepted for publication in A&A