ROSAT PSPC detection of soft X-ray absorption in GB 1428+4217: The most distant matter yet probed with X-ray spectroscopy

We report on a ROSAT PSPC observation of the highly-luminous z = 4.72 radio-loud quasar GB 1428+4217 obtained between 1998 December 11 and 17, the final days of the ROSAT satellite. The low-energy sensitivity of the PSPC detector was employed to constrain the intrinsic X-ray absorption of the currently most distant X-ray detected object. Here we present the detection of significant soft X-ray absorption towards GB 1428+4217, making the absorbing material the most distant matter yet probed with X-ray spectroscopy. X-ray variability by 25+-8 per cent is detected on a timescale of 6500 s in the rest frame. The X-ray variation requires an unusually high radiative efficiency of at least 4.2, further supporting the blazar nature of the source.Comment: 6 pages incl. 6 figures, accepted for publication in Monthly Notice

Radio-mode feedback in local AGNs: dependence on the central black hole parameters

Radio mode feedback, in which most of the energy of an active galactic nucleus (AGN) is released in a kinetic form via radio-emitting jets, is thought to play an important role in the maintenance of massive galaxies in the present-day Universe. We study the link between radio emission and the properties of the central black hole in a large sample of local radio galaxies drawn from the Sloan Digital Sky Survey (SDSS), based on the catalogue of Best and Heckman (2012). Our sample is mainly dominated by massive black holes (mostly in the range $10^8-10^9 M_{\odot}$) accreting at very low Eddington ratios (typically $\lambda < 0.01$). In broad agreement with previously reported trends, we find that radio galaxies are preferentially associated with the more massive black holes, and that the radio loudness parameter seems to increase with decreasing Eddington ratio. We compare our results with previous studies in the literature, noting potential biases. The majority of the local radio galaxies in our sample are currently in a radiatively inefficient accretion regime, where kinetic feedback dominates over radiative feedback. We discuss possible physical interpretations of the observed trends in the context of a two-stage feedback process involving a transition in the underlying accretion modes.Comment: accepted for publication in Monthly Notices of the Royal Astronomical Societ

Self-Consistent Cosmological Simulations of DGP Braneworld Gravity

We perform cosmological N-body simulations of the Dvali-Gabadadze-Porrati braneworld model, by solving the full non-linear equations of motion for the scalar degree of freedom in this model, the brane bending mode. While coupling universally to matter, the brane-bending mode has self-interactions that become important as soon as the density field becomes non-linear. These self-interactions lead to a suppression of the field in high-density environments, and restore gravity to General Relativity. The code uses a multi-grid relaxation scheme to solve the non-linear field equation in the quasi-static approximation. We perform simulations of a flat self-accelerating DGP model without cosmological constant. The results of the DGP simulations are compared with standard gravity simulations assuming the same expansion history, and with DGP simulations using the linearized equation for the brane bending mode. This allows us to isolate the effects of the non-linear self-couplings of the field which are noticeable already on quasi-linear scales. We present results on the matter power spectrum and the halo mass function, and discuss the behavior of the brane bending mode within cosmological structure formation. We find that, independently of CMB constraints, the self-accelerating DGP model is strongly constrained by current weak lensing and cluster abundance measurements.Comment: 21 pages; 10 figures. Revised version matching published versio

Band-structure topologies of graphene: spin-orbit coupling effects from first principles

The electronic band structure of graphene in the presence of spin-orbit coupling and transverse electric field is investigated from first principles using the linearized augmented plane-wave method. The spin-orbit coupling opens a gap at the $K(K')$-point of the magnitude of 24 $\mu$eV (0.28 K). This intrinsic splitting comes 96% from the usually neglected $d$ and higher orbitals. The electric field induces an additional (extrinsic) Bychkov-Rashba-type splitting of 10 $\mu$eV (0.11 K) per V/nm, coming from the $\sigma$-$\pi$ mixing. A 'mini-ripple' configuration with every other atom is shifted out of the sheet by less than 1% differs little from the intrinsic case.Comment: 4 pages, 4 figure

Cosmological Simulations of Normal-Branch Braneworld Gravity

We introduce a cosmological model based on the normal branch of DGP braneworld gravity with a smooth dark energy component on the brane. The expansion history in this model is identical to LambdaCDM, thus evading all geometric constraints on the DGP cross-over scale r_c. This model can serve as a first approximation to more general braneworld models whose cosmological solutions have not been obtained yet. We study the formation of large scale structure in this model in the linear and non-linear regime using N-body simulations for different values of r_c. The simulations use the code presented in (F.S., arXiv:0905.0858) and solve the full non-linear equation for the brane-bending mode in conjunction with the usual gravitational dynamics. The brane-bending mode is attractive rather than repulsive in the DGP normal branch, hence the sign of the modified gravity effects is reversed compared to those presented in arXiv:0905.0858. We compare the simulation results with those of ordinary LambdaCDM simulations run using the same code and initial conditions. We find that the matter power spectrum in this model shows a characteristic enhancement peaking at k ~ 0.7 h/Mpc. We also find that the abundance of massive halos is significantly enhanced. Other results presented here include the density profiles of dark matter halos, and signatures of the brane-bending mode self-interactions (Vainshtein mechanism) in the simulations. Independently of the expansion history, these results can be used to place constraints on the DGP model and future generalizations through their effects on the growth of cosmological structure.Comment: 17 pages, 10 figures; v2: minor changes; v3: references added; v4: added appendix on comparison with previous results; matches published version; v5: corrected Eqs. (2.4-2.5) and Fig. 1 following Ref. [28]; all following results unchange

Relativistic Iron Lines in Galactic Black Holes: Recent Results and Lines in the ASCA Archive

Recent observations with Chandra and XMM-Newton, aided by broad-band spectral coverage from RXTE, have revealed skewed relativistic iron emission lines in stellar-mass Galactic black hole systems. Such systems are excellent laboratories for testing General Relativity, and relativistic iron lines provide an important tool for making such tests. In this contribution to the Proceedings of the 10th Annual Marcel Grossmann Meeting on General Relativity, we briefly review recent developments and present initial results from fits to archival ASCA observations of Galactic black holes. It stands to reason that relativistic effects, if real, should be revealed in many systems (rather than just one or two); the results of our archival work have borne-out this expectation. The ASCA spectra reveal skewed, relativistic lines in XTE J1550-564, GRO J1655-40, GRS 1915+105, and Cygnus X-1.Comment: to appear in the proc. of the 10th Annual Marcel Grossmann Meeting on General Relativity, 5 pages, 1 figure, uses specific .cls and .sty file