Spectral variations of the X-ray binary pulsar LMC X-4 during its long period intensity variation and a comparison with Her X-1

We present spectral variations of the binary X-ray pulsar LMC X-4 using the RXTE/PCA observations at different phases of its 30.5 day long super-orbital period. Only out of eclipse data were used for this study. During the high state of the super-orbital period of LMC X-4, the spectrum is well described by a high energy cut-off power-law with a photon index in the range of 0.7-1.0 and an iron emission line. In the low state, the spectrum is found to be flatter with power-law photon index in the range 0.5-0.7. A direct correlation is detected between the continuum flux in 7-25 keV energy band and the iron emission line flux. The equivalent width of the iron emission line is found to be highly variable during low intensity state, whereas it remains almost constant during the high intensity state of the super-orbital period. It is observed that the spectral variations in LMC X-4 are similar to those of Her X-1 (using RXTE/PCA data). These results suggest that the geometry of the region where the iron line is produced and its visibility with respect to the phase of the super-orbital period is similar in LMC X-4 and Her X-1. A remarkable difference between these two systems is a highly variable absorption column density with phase of the super-orbital period that is observed in Her X-1 but not in LMC X-4.Comment: 7 pages, 5 figures, Accepted for publication in Astronomy and Astrophysic

Sonic-Point Model of Kilohertz Quasi-Periodic Brightness Oscillations in Low-Mass X-ray Binaries

Strong, coherent, quasi-periodic brightness oscillations (QPOs) with frequencies ranging from about 300 Hz to 1200 Hz have been discovered with the Rossi X-ray Timing Explorer in the X-ray emission from some fifteen neutron stars in low-mass binary systems. Two simultaneous kilohertz QPOs differing in frequency by 250 to 350 Hertz have been detected in twelve of the fifteen sources. Here we propose a model for these QPOs. In this model the X-ray source is a neutron star with a surface magnetic field of 10^7 to 10^10 G and a spin frequency of a few hundred Hertz, accreting gas via a Keplerian disk. The frequency of the higher-frequency QPO in a kilohertz QPO pair is the Keplerian frequency at a radius near the sonic point at the inner edge of the Keplerian flow whereas the frequency of the lower-frequency QPO is approximately the difference between the Keplerian frequency at a radius near the sonic point and the stellar spin frequency. This model explains naturally many properties of the kilohertz QPOs, including their frequencies, amplitudes, and coherence. We show that if the frequency of the higher-frequency QPO in a pair is an orbital frequency, as in the sonic-point model, the frequencies of these QPOs place interesting upper bounds on the masses and radii of the neutron stars in the kilohertz QPO sources and provide new constraints on the equation of state of matter at high densities. Further observations of these QPOs may provide compelling evidence for the existence of a marginally stable orbit, confirming a key prediction of general relativity in the strong-field regime.Comment: 67 pages, including 15 figures and 5 tables; uses aas2pp4; final version to appear in the Astrophysical Journal on 1 December 199

Correlations in Quasi-Periodic Oscillation and Noise Frequencies Among Neutron-Star and Black-Hole X-ray Binaries

We study systematically the ~0.1-1200 Hz quasi-periodic oscillations (QPOs) and broad noise components observed in the power spectra of non-pulsing neutron-star and black-hole low-mass X-ray binaries. We show that among these components we can identify two, occurring over a wide range of source types and luminosities, whose frequencies follow a tight correlation. The variability components involved in this correlation include neutron-star kilohertz QPOs and horizontal-branch oscillations, as well as black-hole QPOs and noise components. Our results suggest that the same types of variability may occur in both neutron-star and black-hole systems over three orders of magnitude in frequency and with coherences that vary widely but systematically. Confirmation of this hypothesis will strongly constrain theoretical models of these phenomena and provide additional clues to understanding their nature.Comment: 15 pages, 2 figures (one color figure), to appear in the Astrophysical Journa

Scattering and Iron Fluorescence Revealed During Absorption Dips in Circinus X-1

We show that dramatic spectral evolution associated with dips occurring near phase zero in RXTE observations of Cir X-1 is well-fit by variable and at times heavy absorption (N_H > 10^24 cm^-2) of a bright component, plus an underlying faint component which is not attenuated by the variable column and whose flux is ~10% of that of the unabsorbed bright component. A prominent Fe emission line at ~6.5 keV is evident during the dips. The absolute line flux outside the dips is similar to that during the dips, indicating that the line is associated with the faint component. These results are consistent with a model in which the bright component is radiation received directly from a compact source while the faint component may be attributed to scattered radiation. Our results are also generally consistent with those of Brandt et al., who found that a partial- covering model could explain ASCA spectra of a low-to-high transition in Cir X-1. The relative brightness of the two components in our model requires a column density of ~2*10^23 cm^-2 if the faint component is due to Thomson scattering in material that mostly surrounds the source. We find that illumination of such a scattering cloud by the observed direct component would produce an Fe K-alpha fluorescence flux that is in rough agreement with the flux of the observed emission line. We also conclude that if the scattering medium is not highly ionized, our line of sight to the compact source does not pass through it. Finally, we discuss simple pictures of the absorbers responsible for the dips themselves.Comment: Accepted for publication in The Astrophysical Journal (23 pages, including 11 figures

New XMM-Newton analysis of three bright X-ray sources in M31 globular clusters, including a new black hole candidate

We present detailed analysis of three globular cluster X-ray sources in the XMM-Newton extended survey of M31. The X-ray counterpart to the M31 globular cluster Bo 45 (XBo 45) was observed with XMM-Newton on 2006 December 26. Its combined pn+MOS 0.3--10 keV lightcurve exhibited a r.m.s variability of ~10%, and its 0.3--7.0 keV emission spectrum was well described by an absorbed power law with photon index 1.44$\pm$0.12. Its variability and emission is characteristic of low mass X-ray binaries (LMXBs) in the low-hard state, whether the accretor is a neutron star or black hole. Such behaviour is typically observed at luminosities \la10% Eddington. However, XBo 45 exhibited this behaviour at an unabsorbed, 0.3--10 keV luminosity of 2.5$\pm0.2\times 10^{38}$ erg s$^{-1}$, or{~140%} Eddington for a 1.4 $M_{\odot}$ neutron star accreting hydrogen. Hence, we identify XBo 45 as a new candidate black hole LMXB. XBo 45 appears to have been consistently bright for ~30 years, consistent with theoretical prediction for a globular cluster black hole binary formed via tidal capture. Bo 375 was observed in the 2007, January 2 XMM-Newton observation, and has a two-component spectrum that is typical for a bright neutron star LMXB. Bo 135 was observed in the same field as Bo 45, and could contain either a black hole or neutron star.Comment: Accepted by ApJ, 16 pages, 5 figures. This version includes the final changes made at the request of the refere

Effects of Strong Magnetic Fields on Neutron Star Structure

We study static neutron stars with poloidal magnetic fields and a simple class of electric current distributions consistent with the requirement of stationarity. For this class of electric current distributions, we find that magnetic fields are too large for static configurations to exist when the magnetic force pushes a sufficient amount of mass off-center that the gravitational force points outward near the origin in the equatorial plane. (In our coordinates an outward gravitational force corresponds to $\partial\ln g_{tt}/\partial r>0$, where $t$ and $r$ are respectively time and radial coordinates and $g_{tt}$ is coefficient of $dt^2$ in the line element.) For the equations of state (EOSs) employed in previous work, we obtain configurations of higher mass than had been reported; we also present results with more recent EOSs. For all EOSs studied, we find that the maximum mass among these static configurations with magnetic fields is noticeably larger than the maximum mass attainable by uniform rotation, and that for fixed values of baryon number the maximum mass configurations are all characterized by an off-center density maximum.Comment: Submitted to the Astrophysical Journal. 37 pages, 8 figures, uses aastex macro

Electrodynamics of Magnetars: Implications for the Persistent X-ray Emission and Spindown of the Soft Gamma Repeaters and Anomalous X-ray Pulsars

(ABBREVIATED) We consider the structure of neutron star magnetospheres threaded by large-scale electrical currents, and the effect of resonant Compton scattering by the charge carriers (both electrons and ions) on the emergent X-ray spectra and pulse profiles. In the magnetar model for the SGRs and AXPs, these currents are maintained by magnetic stresses acting deep inside the star. We construct self-similar, force-free equilibria of the current-carrying magnetosphere with a power-law dependence of magnetic field on radius, B ~ r^(-2-p), and show that a large-scale twist softens the radial dependence to p < 1. The spindown torque acting on the star is thereby increased in comparison with a vacuum dipole. We comment on the strength of the surface magnetic field in the SGR and AXP sources, and the implications of this model for the narrow measured distribution of spin periods. A magnetosphere with a strong twist, B_\phi/B_\theta = O(1) at the equator, has an optical depth ~ 1 to resonant cyclotron scattering, independent of frequency (radius), surface magnetic field strength, or charge/mass ratio of the scattering charge. When electrons and ions supply the current, the stellar surface is also heated by the impacting charges at a rate comparable to the observed X-ray output of the SGR and AXP sources, if B_{dipole} ~ 10^{14} G. Redistribution of the emerging X-ray flux at the ion and electron cyclotron resonances will significantly modify the emerging pulse profile and, through the Doppler effect, generate a non-thermal tail to the X-ray spectrum. The sudden change in the pulse profile of SGR 1900+14 after the 27 August 1998 giant flare is related to an enhanced optical depth to electron cyclotron scattering, resulting from a sudden twist imparted to the external magnetic field.Comment: 31 January 2002, minor revisions, new section 5.4.

The Spectrum of the Black Hole X-ray Nova V404 Cygni in Quiescence as Measured by XMM-Newton

We present XMM observations of the black hole X-ray nova V404 Cygni in quiescence. Its quiescent spectrum can be best fitted by a simple power-law with slope 2. The spectra are consistent with that expected for the advection-dominated accretion flow (ADAF). V404 Cyg was roughly equal in luminosity compared to the previous observation of Chandra. We see variability of a factor of 4 during the observation. We find no evidence for the presence of fluorescent or H-like/He-like iron emission, with upper limits of 52 eV and 110 eV respectively. The limit on the fluorescent emission is improved by a factor of 15 over the previous estimate, and the restriction on H-like/He-like emission is lower than predicted from models by a factor of roughly 2.Comment: 6 pages, 7 figures, ApJ accepte

European Nature and Health Network Initiatives

Attention to the importance of nature and human health linkages has increased in the past years, both in science and in policy. While knowledge about and recognition of the importance of nature and human health linkages are increasing rapidly, challenges still remain. Among them are building bridges between relevant but often still somewhat disconnected sectors and topics. There is a need to bring together researchers in the fields of health sciences, ecology, social sciences, sustainability sciences and other interdisciplinary sciences, as well as for cooperation between governments, companies and citizens. In this chapter, we introduce European networking initiatives aimed at building such bridges