Broadband X-ray emission and the reality of the broad iron line from the Neutron Star - White Dwarf X-ray binary 4U 1820-30

Broad relativistic iron lines from neutron star X-ray binaries are important probes of the inner accretion disk. The X-ray reflection features can be weakened due to strong magnetic fields or very low iron abundances such as is possible in X-ray binaries with low mass, first generation stars as companions. Here we investigate the reality of the broad iron line detected earlier from the neutron star low mass X-ray binary 4U~1820--30 with a degenerate helium dwarf companion. We perform a comprehensive, systematic broadband spectral study of the atoll source using \suzaku{} and simultaneous \nustar{} \& \swift{} observations. We have used different continuum models involving accretion disk emission, thermal blackbody and thermal Comptonization of either disk or blackbody photons. The \suzaku{} data show positive and negative residuals in the region of iron K band. These features are well described by two absorption edges at 7.67\pm0.14\kev and 6.93\pm0.07\kev or partial covering photoionized absorption or by blurred reflection. Though, the simultaneous \swift{} and \nustar{} data do not clearly reveal the emission or absorption features, the data are consistent with the presence of either absorption or emission features. Thus, the absorption based models provide an alternative to the broad iron line or reflection model. The absorption features may arise in winds from the inner accretion disk. The broadband spectra appear to disfavour continuum models in which the blackbody emission from the neutron star surface provides the seed photons for thermal Comptonization. Our results suggest emission from a thin accretion disk (kT_{disk} \sim 1\kev), Comptonization of disk photons in a boundary layer most likely covering a large fraction of the neutron star surface and innermost parts of the accretion disk, and blackbody emission (kT_{bb} \sim 2\kev) from the polar regions.Comment: 12 pages, 8 figures, 5 tables. Accepted for publication in MNRA

XMM-Newton view of a hard X-ray transient IGR J17497-2821

We present spectral and energy-dependent timing characteristics of the hard X-ray transient IGR J17497–2821 based on XMM–Newton observations performed five and nine days after its outburst on 2006 September 17. We find that the source spectra can be well described by a hard ( ∼ 1.50) power law and a weak multicolour disc blackbody with inner disc temperature kTin ∼ 0.2 keV. A broad iron Kα line with FWHM ∼ 27 000 km s−1, consistent with that arising from an accretion disc truncated at large radius, was also detected. The power density spectra of IGR J17497–2821, derived from the high-resolution (30 µs) timing-mode XMM–Newton observations, are characterized by broad-band noise components that are well modelled by three Lorentzians. The shallow power-law slope, low disc luminosity and the shape of the broad-band power density spectrum indicate that the source was in the hard state. The rms variability in the softer energy bands (0.3–2 keV) found to be ∼1.3 times that in 2–5 and 5–10 keV energy bands. We also present the energy-dependent timing analysis of the RXTE/PCA data, where we find that at higher energies, the rms variability increases with energy. Key words: methods: observational – X-rays: binaries – X-rays: individual: IGR J17497

A quest for the origin of the Sagnac effect

In the literature, there is no consensus on the origin of the relativistic Sagnac effect, particularly from the standpoint of the rotating observer. The experiments of Wang et al. (Phys Lett A 312(1–2):7, 2003; Phys Rev Lett 93(14):143901, 2004) has, however, questioned the pivotal role of rotation of the platform in Sagnac effect. Recently, the relative motion between the reflectors which force light to propagate along a closed path and the observer has been ascribed as the cause of the Sagnac effect. Here, we propose a thought experiment on linear Sagnac effect and explore another one proposed earlier to demonstrate that the origin of the Sagnac effect is neither the rotation of frame affecting clock synchronization nor the relative motion between the source and the observer; Sagnac effect originates purely due to asymmetric position of the observer with respect to the light paths. Such a conclusion is validated by analysis of a gedanken Sagnac kind experiment involving rotation

Traversable wormholes supported by dark matter and monopoles with semiclassical effects

We present a new traversable wormhole explication of Einstein’s field equations supported by the profile of Einasto Dark Matter densities (Einasto in Trudy Inst Astrofiz Alma-Ata 51:87, 1965; PTarO 36:414, 1968; Astron Nachr 291:97, 1969) and global monopole charges along with semiclassical effects in the local universe as the galactic halo. The Einasto DM density profile produces a suitable shape function that meets all the requirements for presenting the wormhole geometries. The Null Energy Condition (NEC) is violated by the obtained solution with different redshift functions i.e. the Einasto profile representing DM candidate within the wormholes gives the fuel to sustain these wormhole structures in the galactic halo. Moreover, the reported wormhole geometries are getting asymptotically flat and non-flat depending only on the choices of redshift function whereas all the wormhole structures are maintaining their balance of equilibrium under the action of different forces